measurements of ignition times, oh time … rates in jet fuel and surrogate oxidation systems a...
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MEASUREMENTS OF IGNITION
TIMES, OH TIME-HISTORIES, AND
REACTION RATES IN JET FUEL AND
SURROGATE OXIDATION SYSTEMS
A DISSERTATION
SUBMITTED TO THE DEPARTMENT OF MECHANICAL ENGINEERING
OF STANFORD UNIVERSITY
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
Subith S. Vasu
August 2010
http://creativecommons.org/licenses/by-nc/3.0/us/
This dissertation is online at: http://purl.stanford.edu/sb917nt0373
© 2010 by Subith Vasu Sumathi. All Rights Reserved.
Re-distributed by Stanford University under license with the author.
This work is licensed under a Creative Commons Attribution-Noncommercial 3.0 United States License.
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I certify that I have read this dissertation and that, in my opinion, it is fully adequatein scope and quality as a dissertation for the degree of Doctor of Philosophy.
Ronald Hanson, Primary Adviser
I certify that I have read this dissertation and that, in my opinion, it is fully adequatein scope and quality as a dissertation for the degree of Doctor of Philosophy.
Craig Bowman
I certify that I have read this dissertation and that, in my opinion, it is fully adequatein scope and quality as a dissertation for the degree of Doctor of Philosophy.
David Golden
Approved for the Stanford University Committee on Graduate Studies.
Patricia J. Gumport, Vice Provost Graduate Education
This signature page was generated electronically upon submission of this dissertation in electronic format. An original signed hard copy of the signature page is on file inUniversity Archives.
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Abstract Fossil-based hydrocarbon fuels account for over 80% of the primary energy consumed
in the world - it is still expected to be about 70% in year 2050 - and nearly 60% of that
amount is used in the transport sector. The basis for globalization is transportation and a
driving force has been the growth in global air traffic. The current climate crisis
magnifies the need for improving the performance of jet engines by introducing scientific
designs in which the use of chemical kinetics will be essential and critical for better
performance and reducing pollutant emissions. Most aviation fuels are jet fuels
originating from crude oil and there are major gaps in our knowledge of the high-
temperature chemistry of real liquid carbon-based fuels. There is a critical need for
experimental kinetic databases that can be used for the validation and refinement of jet
fuel surrogate mechanisms. To fill this need, experiments were performed using shock
tube and laser absorption methods to investigate jet fuel and surrogate oxidation systems
under engine-relevant conditions. Ignition times and OH species time-histories were
measured and low-uncertainty measurements of the reactions of OH with several stable
intermediates were carried out. The work presented in this study can be broken into three
categories: 1) jet fuel oxidation, 2) surrogate oxidation, and 3) OH radical reactions with
several stable combustion intermediates.
Ignition delay times were measured for gas-phase jet fuel oxidation (Jet-A and JP-8) in
air behind reflected shock waves in a heated high-pressure shock tube. Initial reflected shock
conditions were as follows: temperatures of 715-1229 K, pressures of 17-51 atm, equivalence
ratios () of 0.5 and 1, and oxygen concentrations of 10 and 21 % in synthetic air. Ignition
delay times were measured using sidewall pressure and OH* emission at 306 nm. The new
experimental results were modeled using several kinetic mechanisms using various jet fuel
surrogate mixtures.
Normal and cyclo alkanes are the two most important chemical classes found in jet fuels.
Ignition delay time experiments were conducted during high-pressure oxidation of two
commonly used representative components for normal and cyclo alkanes in jet fuel
surrogates, i.e., n-dodecane and methylcyclohexane (MCH), respectively. Fuel/air ignition
was studied for the following shock conditions: temperatures of 727-1177 K, pressures of 17-
v
50 atm, ’s of 0.5 and 1. OH concentration time-histories during high-pressure n-dodecane,
n-heptane and MCH oxidation were measured behind reflected shock waves in a heated,
high-pressure shock tube. Experimental conditions covered temperatures of 1121 to 1422 K,
pressures of 14.1-16.7 atm, and initial fuel concentrations of 500 to 1000 ppm (by volume),
and an equivalence ratio of 0.5 with O2 as the oxidizer in argon as the bath gas. OH
concentrations were measured using narrow-linewidth ring-dye laser absorption near the R-
branchhead of the OH A-X (0,0) system at 306.47 nm. Detailed comparisons of these data
with the predictions of various kinetic mechanisms were made. Sensitivity and pathway
analyses for these reference fuel components were performed, leading to reaction rate
recommendations with improved model performance.
Reactions of OH radical with two alkenes (ethylene and propene) and a diene (1,3-
butadiene) were studied behind reflected shock waves. Measurements were conducted in the
range of temperatures from 890-1438 K and pressures from 1.99-10.18 atm for three initial
concentrations of fuels (500ppm, 751.1ppm and 1000ppm). OH radicals were produced by
shock-heating tert-butyl hydroperoxide, (CH3)3- CO-OH, and monitored by narrow-line
width ring dye laser absorption of the well characterized R1(5) line of the OH A-X (0, 0)
band near 306.7 nm. OH time-histories were modeled by using a modified oxidation
mechanism and rate constants for the reactions of OH with ethylene, propene, and 1,3-
butadiene were extracted by matching modeled and measured OH concentration time
histories in the reflected shock region. Detailed error analyses yielded an uncertainty estimate
of ± 22.8% (OH+ethylene at 1201 K), ±16.5% (OH+propene at 1136 K), and ± 13%
(OH+1,3-butadiene at 1200K). Canonical and variational transition state theory
calculations using recent ab initio results gave excellent agreement with our experimental
measurements and data outside our range and hence the resulting expressions can be used
directly in combustion models. In the current studies, a rate measurement for the
decomposition of TBHP has been obtained in the range 745-1014 K using both incident and
reflected OH data.
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Acknowledgements I owe a great deal of gratitude to my advisor, Professor Ronald Hanson, for the
opportunities, support, encouragement and guidance he has provided me during my time
at Stanford. His work ethic, creative ideas, and critical thinking are unmatchable in this
world and I feel like I still have a lot to learn from him. I would like to thank my reading
committee, Professors David Golden and Craig Bowman for their valuable suggestions
regarding this thesis. I have been fortunate enough to work with Dr. David Davidson who
is not only an outstanding scientific researcher, but also an incredible advisor regarding
life. I am particularly grateful to Matt Oehlschlaeger, Venkatesh Vasudevan and Rob
Cook for teaching me early in my research work. I would like to thank Prof. Heinz Pitsch
and Dr. Jay Jeffries for their support and help. I would like to give special thanks to all
my friends and colleagues in the Hanson research group (current and former). I am also
grateful to everyone and my friends who have encouraged and helped me develop as a
researcher. Finally, I would like to acknowledge the constant support my family has been
providing me.
This research was sponsored by the U.S. Army Research Office (ARO) and the U.S.
Department of Energy.
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Contents
Abstract .................................................................................................................................... iv
Acknowledgements .................................................................................................................. vi
Contents .................................................................................................................................. vii
List of Tables ............................................................................................................................ x
List of Figures ......................................................................................................................... xii
Chapter 1 : Introduction ......................................................................................................... 1
1.1 Motivation ..................................................................................................... 1
1.2 Background ..................................................................................................... 3
1.2.1 Jet Fuel Ignition Times ........................................................................... 3
1.2.2 Surrogate Ignition Times ........................................................................ 5
1.2.3 High-pressure OH Time-histories During Surrogate Oxidation ............. 7
1.2.4 Reaction of OH with Alkenes ................................................................. 9
1.2.5 OH+1,3-Butadiene ................................................................................ 10
1.3 Scope and Organization of Thesis ................................................................ 11
Chapter 2 : Method ................................................................................................................ 14
2.1 Shock Tube Facilities .................................................................................... 14
2.1.1 High-pressure Shock Tube .................................................................... 14
2.1.2 Low-pressure Shock Tube .................................................................... 16
2.2 OH Laser Absorption Diagnostics ................................................................ 16
2.3 Modeling Shock Tube Data .......................................................................... 19
Chapter 3 : High-pressure Jet Fuel Ignition Times ............................................................ 20
3.1 Jet Fuel/Air Mixture Preparation .................................................................. 20
3.2 Measuring Ignition Delay Times .................................................................. 22
3.3 Jet Fuel Ignition: Results and Discussion ..................................................... 23
3.3.1 Comparison with Previous Measurements............................................ 23
3.3.2 Comparison of Different Fuel Types .................................................... 25
3.3.3 Pressure Scaling .................................................................................... 26
3.3.4 Jet Fuel Surrogate Modeling Prediction ............................................... 27
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3.3.5 Effect of Variation of Equivalence Ratio and Oxygen Concentration . 31
3.3.6 Low-temperature (NTC Region) Ignition Delay Time Data ................ 32
Chapter 4 : High-pressure Single-Component Surrogate Ignition Times ........................ 39
4.1 Experimental Method.................................................................................... 39
4.2 n-Dodecane/Air Ignition: Results and Discussion ........................................ 41
4.2.1 High-pressure n-Dodecane Ignition Data ............................................. 41
4.2.2 Kinetic Modeling Prediction ................................................................. 42
4.2.3 Effect of Pressure-time Histories .......................................................... 43
4.2.4 Comparison of n-Dodecane Ignition Times with Jet-A and Other n-
Alkanes 44
4.3 MCH/Air Ignition: Results and Discussion .................................................. 46
4.3.1 High-pressure MCH Ignition Data........................................................ 46
4.3.2 Comparison of Various Cyclo-alkanes ................................................. 49
4.3.3 Kinetic Modeling .................................................................................. 50
4.3.4 Effect of Pressure-time Histories .......................................................... 52
4.4 Comparison of Jet Fuel and Surrogate Component Ignition ........................ 56
Chapter 5 : OH Time-histories During Surrogate Oxidation at High-Pressure .............. 61
5.1 Experimental Method.................................................................................... 61
5.2 Kinetic Modeling Details .............................................................................. 62
5.3 OH Time-Histories Results ........................................................................... 64
5.3.1 n-Heptane Results ................................................................................. 64
5.3.2 MCH Results ......................................................................................... 66
5.3.3 n-Dodecane Results .............................................................................. 68
5.4 Discussion ..................................................................................................... 70
5.4.1 n-Heptane Kinetic Analysis .................................................................. 71
5.4.2 MCH Kinetic Analysis .......................................................................... 74
5.4.3 n-Dodecane Kinetic Analysis ............................................................... 79
Chapter 6 : Alkenes Reaction With OH: OH+C2H4 and OH+C3H6 ................................. 85
6.1 Method .......................................................................................................... 85
6.2 Kinetic Measurements .................................................................................. 86
6.2.1 OH+C2H4, k1 ......................................................................................... 87
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6.2.2 OH+C3H6, k2 ......................................................................................... 90
6.2.3 TBHP Decomposition, k3...................................................................... 92
6.3 Comparison with Previous Data ................................................................... 97
6.4 Theoretical Calculations of OH+C2H4 and OH+C3H6 ................................ 100
Chapter 7 : 1,3-Butadiene+OHProducts ........................................................................ 109
7.1 Method ........................................................................................................ 109
7.2 Kinetic Measurements ................................................................................ 109
7.3 TST Calculations ........................................................................................ 115
Chapter 8 : Conclusions ...................................................................................................... 122
8.1 Summary of Results .................................................................................... 122
8.1.1 Jet Fuel Oxidation ............................................................................... 122
8.1.2 Surrogate Fuels Oxidation .................................................................. 123
8.1.3 Reactions of OH with Alkenes ........................................................... 126
8.1.4 OH+1,3-Butadiene .............................................................................. 127
8.1.5 Archival Publications .......................................................................... 127
8.2 Recommendations for Future Work ............................................................ 128
8.2.1 Multi-species Measurements in Multi-component Mixtures .............. 128
8.2.2 Measurements of Reactions in the NTC Regime Using OH Lasers ... 129
8.2.3 Kinetics of Biobutanol ........................................................................ 130
Appendix A: Modes of Ignition in High-Pressure Fuel/Air Mixture Ignition ................ 132
Appendix B: Influence of Impurities, Particles, and Wall Effects on Ignition ............... 135
Appendix C: Shock Tube Boundary Layer and Facility Effects on Ignition ................. 139
Appendix D: TST Theory and Calculations ...................................................................... 142
Input Files to Multiwell-2010.1 Code .................................................................... 145
References ............................................................................................................................. 150
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List of Tables Table 1.1 High-temperature OH+ethylene and OH+propene studies in literature. ......... 13
Table 3.1 Summary of Jet-A/air ignition data at phi=1.0. Jet-A Composite Blend
#04POSF4658. (Dr. = driver gas mixture; Vshock = incident shock velocity at the
endwall)..................................................................................................................... 36
Table 3.2 Summary of jet fuel ignition data indicating variation with fuel type (jet-A and
JP-8), equivalence ratio, and fuel sample. (Dr. denotes driver gas mixture; Vshock is
the incident shock velocity at the endwall). .............................................................. 37
Table 3.3 Jet fuel surrogate mechanisms and their associated surrogate mixtures. ......... 38
Table 3.4 Jet fuel surrogate mixtures used with the Ranzi [48] and the Zhang et al. [19]
mechanisms. .............................................................................................................. 38
Table 4.1 Summary of current shock tube experimental results in n-dodecane/air, phi=0.5.
................................................................................................................................... 58
Table 4.2 Summary of current shock tube experimental results in n-dodecane/air, phi=1.0.
................................................................................................................................... 59
Table 4.3 Summary of current high-pressure ignition time results in MCH/air (=1.0).
Mixture: MCH=1.96%, O2=20.60%, N2=77.44%. Vshock is the incident shock
velocity at the endwall. ............................................................................................. 60
Table 5.1 Mechanisms used in this study of surrogate components. ............................... 82
Table 5.2 Summary of current high-pressure OH absorption experiments in MCH and n-
heptane. aSat.= saturated signal (transmission~0) and in this case, ign was obtained
from raw absorption signals instead of XOH profiles. *Off=Offline absorption
measurement. bNL=No laser was used. ign was obtained from pressure traces as the
midpoint of pressure jump during ignition for both Off and NL cases. ................... 83
Table 5.3 Summary of OH absorption data (fuel=n-dodecane). aSat., saturated
signal;transmission~0. .............................................................................................. 84
Table 6.1 OH+ethylene rate coefficient data. ................................................................ 105
Table 6.2 Uncertainty analysis for OH+alkenes reactions. OH+ethylene (k1): initial
incident shock conditions: 1201 K, 1.99 atm, and 500 ppm ethylene/Ar.
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OH+propene (k2): initial incident shock conditions: 1136 K, 2.02 atm, and 299 ppm
ethylene/Ar. ............................................................................................................. 106
Table 6.3 OH+propene rate coefficient data. ................................................................. 107
Table 6.4 TBHP decomposition rate data. Superscripts denotes mixtures: a500ppm
ethylene; b1000ppm ethylene; c300 ppm propene. .................................................. 108
Table 7.1 Overall OH+1,3-butadiene rate coefficient, k1, data. ..................................... 119
Table 7.2 Uncertainty analysis for overall OH+1,3-butadiene rate coefficient, k1. ....... 120
Table 7.3 Energies of the products and the transition states in kcal mol-1 units relative to
the reactants trans-1,3-butadiene + OH, without and with zero-point energy. Q1
represents the Q1 diagnostic of the QCISD(T)/cc-pVQZ calculations at the
B3LYP/6-311++G(d,p) geometry. The values are very close to the ones obtained at
the MP2 geometry (not shown). .............................................................................. 121
Table 7.4 Energies The equilibrium constant and Keff of the 1,3-butadiene + OH
CH2CHCHCH2OH reaction. For details, see text. .................................................. 121
Table 8.1 Laser-based quantitative absorption diagnostics developed in our lab [194]. 131
xii
List of Figures Figure 1.1 Comparison of rate constants for C2H4+OH used in several current
mechanisms. Variations of a factor of ~5.5 are evident at high temperatures near
1000 K. Shown are rates used in JetSurF 1.0 [46], Westbrook et al. [47], Ranzi [48],
Galway natural gas mechanism III [51], and Chaos et al. [52]. ................................ 10
Figure 2.1 Experimental set-up for HPST experiments. .................................................. 18
Figure 3.1 Example JP-8/air ignition delay time data (driver gas: He). Initial reflected
shock conditions, 1019 K, 22.2 atm, = 1. .............................................................. 22
Figure 3.2 Ignition delay times including previous data for Jet-A/air for phi=1. Solid
circles: current work for Jet-A POSF4658; open circles: Dean et al. [9]; open
squares: current work for Dean et al. [9] Jet-A fuel. ................................................ 24
Figure 3.3 Ignition delay times for Jet-A/air and JP-8/air, phi=1. ................................... 25
Figure 3.4 Pressure scaling of (phi=1) τign: left- Jet-A/air; right- Jet-A/air and JP-8/air. 26
Figure 3.5 Jet fuel ignition delay times for phi=1 (data from Figure 3.3), and mechanisms
prediction. The Violi #3 surrogate was used with the Ranzi and the Zhang et al.
mechanisms. See text for details. .............................................................................. 29
Figure 3.6 Jet fuel ignition delay times for phi=1, and the Ranzi mechanism predictions
for different surrogate fuel mixtures listed in Table 3.4. .......................................... 30
Figure 3.7 Jet fuel ignition fuel ignition delay times: the effect of phi and XO2. ............. 32
Figure 3.8 Left: low-temperature pressure and emission traces (Driver gas: He 70%, N2
30 %). Right: ignition delay times including NTC region data, (Jet-A/air, phi=1). . 33
Figure 4.1 Example ignition data from HPST. Left: n-Dodecane/air. Right: MCH/air. 40
Figure 4.2 n-Dodecane/air τign data (20atm) and predictions for =0.5 (left) and1.0 (right).
................................................................................................................................... 42
Figure 4.3 Left: n-Dodecane /air τign results for phi=1.0. CHEMSHOCK modeling was
conducted using the You et al. mechanism with an extreme-case linear pressure rise
(dP5/dt) of 10% per milli-second. Right: Measured n-dodecane/air pressure-time
histories near 20 atm, phi=1.0. .................................................................................. 44
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Figure 4.4 Comparison of high-pressure n-dodecane ignition delay times at 20 atm. Left:
with Jet-A/air at two equivalence ratios. Right: with n-heptane/air (current work) at
=1.0. Dashed lines are fit through data. Solid lines are Westbrook et al. [47] (LLNL)
modeling prediction. ................................................................................................. 46
Figure 4.5 Left: High-pressure MCH/air ignition delay time results (=1.0, MCH=1.96%,
O2=20.60%, N2=77.44%). High-pressure data scaled to 20, or 45 atm using τign ~ P-
0.87. Low-pressure data scaled to 1.5atm using Vasu et al. [121] correlation (=1.0,
MCH=1.962 %). Modeling shows τign predictions at 20 atm. Right: High-pressure
MCH/air ignition delay time results near 45 atm (=1.0, MCH=1.96%, O2=20.60%,
N2=77.44%) and pressure scaling. Data (solid symbols) scaled to 45 atm using τign ~
P -0.87. Grey solid line is a fit through data. Constant U,V modeling results at 20 and
45 atm are shown using Orme et al., Ranzi et al., and Pitz et al. mechanisms. ........ 47
Figure 4.6 Current high-pressure MCH/air (=1.0, MCH=1.96%, O2=20.60%,
N2=77.44%) τign results. Left: Current data scaled to 20, or 45 atm using τign ~ P (-
0.87). RCM data (unheated) from Pitz et al. [22]. Lines are constant U,V modeling
predictions using the Pitz et al. [22] mechanism: 1) 10 atm; 2) 20 atm; and 3) 45 atm.
Right: HPST data scaled to 20, or 45 atm using τign ~ P (-0.87); RCM data (unheated)
from Pitz et al. [22]; Solid lines are fit through data at respective pressures. Low-
pressure data scaled to 1.5atm using Vasu et al. [121] correlation (=1.0,
MCH=1.962 %). ....................................................................................................... 48
Figure 4.7 High-pressure cyclo-alkanes/air (=1.0) τign results. Mixtures: MCH=1.96%,
O2=20.60%, N2=77.44%; cyclo-hexane=2.28%, O2=20.53%, N2=77.19%; cyclo-
pentane=2.72%, O2=20.44%, N2=76.84%. Current MCH data scaled to 45 atm using
τign ~ P (-0.87). Cyclo-hexane (scaled using τign ~ P -1.1 and cyclo-pentane (scaled
using τign ~ P-0.9) data from Daley et al. [124]. Solid lines are fit through data. ....... 50
Figure 4.8 Left: Measured and predicted (Ranzi et al. [48], Pitz et al. [22]) P(t) histories.
Right: Computed pressure-time histories for MCH/air ignition at 20atm, =1.0, Pitz
et al. [22] mechanism. ............................................................................................... 54
Figure 4.9 HPST pressure-time histories for MCH/air (=1.0) ignition. Left: near 45atm.
Right: near 20atm. ..................................................................................................... 54
xiv
Figure 4.10 High-pressure ignition times data from HPST in Jet-A and major single-
component surrogate fuels at similar conditions (=1.0). HPST data scaled to 20 atm
using respective pressure scaling for individual fuels (see text). Solid lines are fit
through data at 20 atm: iso-octane and toluene data from Davidson et al. [115] and
Vasu et al. [143], respectively; n-dodecane, Jet-A, MCH, and n-heptane are from
current work. ............................................................................................................. 57
Figure 5.1 High-pressure OH absorption data for n-heptane; initial Xfuel=1000ppm,
XO2=0.022, XAr=0.977, =0.5. Data 1: 1271K, 15atm; data 2: 1236K, 15.28atm; data
3: 1230K, 15.81atm; data 4: 1121K, 14.1atm. .......................................................... 65
Figure 5.2 High-pressure OH absorption profile data and modeling predictions for n-
heptane; initial Xfuel=1000ppm, XO2=0.022, XAr=0.977, =0.5, 1271K, 15atm. 1:
Current experiment, 2: Curran et al. [149], 3: Seiser et al. [150], 4: Tsang [151], 5:
Patel et al. [153], 6: Ranzi et al. [48], 7: SanDiego [154], 8: Golovichev [152], 9:
Gokulakrishnan et al. [157], 10: Chaos et al. [52], 11: Biet et al. [156], 12: You et al.
[118]. ......................................................................................................................... 66
Figure 5.3 (A): OH absorption data for MCH; initial XMCH=1000 ppm, XO2=0.021,
XAr=0.978, =0.5. Data 1: 1285 K, 15.16 atm; data 2: 1269 K, 15.8 atm; data 3:
1213 K, 14.44 atm; data 4: 1205 K, 14.47 atm. (B): OH absorption data for MCH;
initial XMCH=750 ppm, XO2=0.01575, XAr=0.9835, =0.5. Data 1: 1304 K, 15.92 atm;
data 2: 1303K, 15.63 atm; data 3: 1266 K, 15.83 atm. ............................................. 67
Figure 5.4 High-pressure OH absorption data and modeling predictions (using Ranzi [48],
Orme et al. [36], Pitz et al. [22] mechanisms) for MCH/O2/Ar. Initial XMCH=1000
ppm, XO2=0.021, XAr=0.978, 1262K, 15.45 atm, =0.5. .......................................... 68
Figure 5.5 High-pressure OH absorption data: n-dodecane; initial Xdodecane=1000ppm, O2,
Ar; =0.5. Left: Data 1: 1422K, 15.5atm; data 2: 1230K, 16.73atm; data 3: 1217K,
16.07atm; data 4: 1196K, 15.77atm; data 5: 1158K, 15.19atm. Right: Comparison
of measured and modeled OH time-histories at 1217 K, 16.1 atm. .......................... 69
Figure 5.6 High-pressure OH data variation with temperature. Initial reflected shock
conditions: 16 atm, 1000 ppm n-dodecane/O2/Ar, = 0.5. Left: peak XOH vs T5.
Solid line is a linear fit through data. Right: High-temperature, low-concentration
xv
ignition delay times in n-dodecane. Solid black line is least-squares fit through data.
................................................................................................................................... 70
Figure 5.7 Major oxidation pathways prediction using the Chaos et al. [52] mechanism
integrated ROP approach. Xheptane=1000ppm, XO2=0.022, balance=Ar. 1275K, 16atm,
=0.5. Details of molecular structures and pathways can be found in [52]. ............. 71
Figure 5.8 OH sensitivity for n-heptane oxidation using the Chaos et al. [52] mechanism.
Xheptane=1000ppm, XO2=0.022, balance=Ar. 1271K, 15atm, =0.5. ......................... 72
Figure 5.9 Influence of higher rate for CH3+HO2=CH3O+OH, new rate= 6.8 x1013
cm3/mole/s, on the OH predictions by Chaos et al. [52] for n-heptane oxidation.
Xheptane=1000ppm, XO2=0.022, balance=Ar, =0.5, 1271K, 15atm. ......................... 73
Figure 5.10 Major MCH oxidation pathways using the Orme et al. [36] mechanism
(integrated ROP), 1275 K, 16 atm, 1000 ppm MCH/O2/Ar, = 0.5. Refer to Orme et
al. [36] for IUPAC nomenclature of species and for detailed molecular structures
and pathways. ............................................................................................................ 75
Figure 5.11 OH Sensitivity for MCH oxidation using the Orme et al. [36] mechanism.
XMCH=1000 ppm, O2 (=0.5), balance=Ar. 1262 K, 15.45 atm. (a): during ignition,
(b): at early times. ..................................................................................................... 76
Figure 5.12 Effect of modifications to the Orme et al. [36] mechanism predictions for
MCH oxidation (combining the Libby et al. [68] recommendations (1 and 3) and the
6.788x1013 cm3/mole/s value for CH3+HO2=CH3O+OH reaction). Initial
XMCH=1000ppm, O2 (=0.5), balance=Ar. 1262K, 15.45atm. See text for details of
the Libby et al. [68] recommendations. .................................................................... 78
Figure 5.13 n-Dodecane oxidation pathways using the You et al. [118] mechanism.
XNC12H26=1000ppm, O2, =0.5, balance=Ar, 1275K, 16atm. ................................... 81
Figure 5.14 Normalized OH sensitivity results for n-dodecane oxidation using the You et
al. [118] mechanism. XNC12H26=1000ppm, =0.5, balance=Ar. 1275K, 16atm. ....... 81
Figure 6.1 Example OH+ethylene rate measurement at 1201K, 1.992 atm. Model
predictions using the best-fit predictions for the rate and a factor of two variations
from the measured rate are also shown. .................................................................... 88
Figure 6.2 OH sensitivity plot for rate measurement of OH+ethylene at 1201 K, 1.992. 89
xvi
Figure 6.3 Example OH+propene rate measurement at 1136 K, 2.024 atm. Model
predictions using the best fit predictions for the rate and 50% variation from the
measured rate are also shown. .................................................................................. 91
Figure 6.4 OH sensitivity plot for rate measurement of OH+propene rate measurement at
1136 K, 2.024 atm. .................................................................................................... 91
Figure 6.5 (CH3)3-CO-OH OH+ (CH3)3CO, k3, rate measurement at 890 K, 2.615 atm.
Model predictions using the best fit predictions for the rate and 50% variation from
the measured rate (k3) are also shown. ...................................................................... 93
Figure 6.6 Early-time OH sensitivity plot during OH+propene rate measurement at 890
K, 2.615 atm showing the dominance of TBHP decomposition, (CH3)3-CO-OH
OH+ (CH3)3CO, k3. ................................................................................................... 93
Figure 6.7 Example low-temperature OH+propene, k2, rate measurement at 890 K, 2.615
atm. Adjustments to k2 using the 25% uncertainty in (CH3)3-CO-OH OH+
(CH3)3CO are also shown. ........................................................................................ 94
Figure 6.8 OH sensitivity plot during incident shock for (CH3)3-CO-OH OH+
(CH3)3CO, k3, rate measurement at 795 K, 0.558 atm. ............................................. 96
Figure 6.9 Example incident shock (CH3)3-CO-OHOH+(CH3)3CO, k3, rate
measurement at 795 K, 0.558 atm. Model predictions using the best fit predictions
for the rate and 50% variation from the measured rate (k3) are shown. Influence on
k3 due to an assumed factor of 2 uncertainties in k1 is also shown. .......................... 96
Figure 6.10 Arrhenius plot for OH+ethylene (k1) at temperatures greater than 400K. ... 98
Figure 6.11 Arrhenius plot for OH+propene (k2) at temperatures greater than 800 K. ... 99
Figure 6.12 Arrhenius plot for TBHP(CH3)3CO+OH. ............................................... 100
Figure 6.13 Arrhenius plot for OH+ethylene (k1) at temperatures greater than 400K.
Comparison with theory and evaluations is shown. ................................................ 102
Figure 6.14 Arrhenius plot for OH+ethylene (k1) at temperatures greater than 600K.
Comparison with current TST calculations is shown. ............................................ 103
Figure 6.15 Arrhenius plot for OH+propene (k2) at temperatures greater than 650K.
Comparison with theoretical calculations is shown. ............................................... 104
Figure 7.1 Example OH+ 1,3-butadiene rate measurement. Initial reflected shock
conditions: 1200K, 2.20 atm, 356 ppm 1,3-butadiene, 18 ppm TBHP in argon.
xvii
Model predictions using the best fit predictions (k1 = 5.92x1012 cm3/mol/s) for the
target rate coefficient and 50% variation from the measured rate are also shown. 110
Figure 7.2 OH sensitivity plot for conditions of Figure 7.1. 1200K, 2.20 atm. ............. 112
Figure 7.3 Example OH+ 1,3-butadiene rate measurement at 1026K, 2.13 atm. Model
predictions using the best fit predictions for the rate and 50% variation from the
measured rate are also shown. ................................................................................ 113
Figure 7.4 OH sensitivity plot for rate measurement of OH+1,3-butadiene at 1026K, 2.13
atm (conditions of Figure 7.3). ............................................................................... 113
Figure 7.5 Comparison of measured high-temperature OH+1,3-butadiene rate data with
previous measurements (Liu et al. [64]) and a rate coefficient estimation (Laskin et
al. [56]). ................................................................................................................... 114
Figure 7.6 Variational transition-state theory (V-TST) and measured rate coefficients in
the 1000-1406 K temperature range. Curves neglecting tunneling and variational
effects are also presented. ....................................................................................... 117
Figure 8.1 OH+1-butanol rate used in latest mechanisms (Black et al. [205], Sarathy et al.
[201], Moss et al. [202]). ........................................................................................ 130
Figure A.1 Pressure time-histories from Davidson et al. [115] study near 20atm. ........ 133
Figure D.1 3-D potential energy surfaces for the collinear reaction A+BCAB+C. ... 144
1
Chapter 1: Introduction
1.1 Motivation
Fossil fuels account for over 80% of the primary energy consumed in the world (it is
still expected to be about 70% in 2050), and nearly 60% of that amount is used in the
transport sector. Hence, it is widely concluded that fossil fuels are responsible for the
emission of a significant amount of pollutants in the atmosphere, including GHG
(greenhouse gases). The current fuel/energy crisis magnifies the need for improving the
performance of combustion engines by introducing scientific designs in which the use of
chemical kinetics will be essential and critical for reducing pollutant emissions (such as,
soot, CO2 etc). There are major gaps in our knowledge of the high-temperature chemistry
of real liquid carbon-based fuels. These gaps can be partially filled by reasonable
theoretical chemical estimates, but benchmark experiments are needed to validate these
estimations, to provide insight into chemistry which is not yet accessible by theory, and
to provide highly accurate values for the most crucial reactions. One of the key emerging
technologies focuses on designing an engine type, such as those based on homogeneous
charge compression ignition concept (HCCI), where the chemical kinetic combustion
process is controlled by temperature, pressure, and composition of the in-cylinder charge.
Efforts to improve combustion efficiency and to reduce the formation of pollutants
include improved engine designs, advances in emissions control technologies and the use
of cleaner burning fuel formulations, all of which require detailed understanding of fuel
oxidation chemistry.
In this thesis, research work has focused on using state-of-the-art shock tube and laser
absorption methods to investigate jet fuel and surrogate oxidation systems. Most of these
studies were conducted at practical engine-relevant conditions (high pressure, low to
intermediate temperatures). Some details of the relevance of these studies are given
below. The development of surrogate fuels for the optimization of engine performance
and the attendant chemical kinetics represents one of the important research fronts of
combustion science and technology. These kinetics mechanisms accurately describe the
2
burning characteristics of surrogates of practical fuels such as those used in gasoline,
diesel, or jet engines, and those derived from organic sources such as petroleum, biomass,
coal, and/or natural gas. Studies of large realistic fuel molecules include several notable
challenges in both experiment and modeling.
Ignition delay time (τign) is an important parameter in the combustor design of most
engines, including ram-jets, scramjets, pulsed detonation engines, gas turbines, and
advanced combustor concepts designed for low NOx emissions. The performance of
kerosene-fueled conventional ram-jet engines has been a subject of investigation for the
last 40 years, and recently, interest in the capabilities of hydrocarbon-fueled scramjet
technology has increased [1,2]. In scramjets, rapid spontaneous ignition and complete
reaction of fuel are required to achieve efficient combustion. Ignition characteristics also
affect heat release rates, and if too rapid, can promote dynamic instabilities or choking [3].
Dependence of ignition delay time on temperature, pressure and composition is critical in
describing the combustion of liquid fuels in diesel engines and combustion chambers of
various types, as well as in optimizing external combustion.
While ignition delay times are necessary to validate the overall behavior of reaction
mechanisms, species time-histories, in particular those of important radicals such as OH,
are needed to impose stronger constraints on mechanisms so that deficiencies can be
identified with greater confidence. Shock tubes are nearly ideal devices for studying
ignition phenomena as they provide well-controlled step changes in temperature (T) and
pressure (P), well-defined time zero and τign, and for moderate or large diameter tubes,
are generally not significantly affected by surface or transport phenomena. Volatile
impurities in the shock tubes (prior to the introduction of the test gas) are also not
typically a problem because of the good high vacuum characteristics and cleanliness of
modern shock tubes. Laser-based diagnostic studies are non-intrusive, provide in-situ
measurements (e.g., concentration of individual species including trace species,
temperature, pressure and velocity), and have fast-time response (sub microsecond). The
combination of shock-heating and laser detection provide a state-of-the-art test platform
for combustion chemistry.
Using this platform, we have conceived, defined and performed gas-phase ignition
delay time, OH species time-history and rate measurement experiments. High-pressure
3
experiments were performed in a heated, high-pressure shock tube at Stanford (modified
to deal with low-vapor pressure fuels). These studies were performed under practical
engine conditions for distillate gas-phase fuels, Jet-A and JP-8, and their major surrogate
components including: n-dodecane and methylcyclohexane (MCH). Ignition delay times
were measured using classical shock tube methods and were combined with laser
absorption measurements of species concentration time-histories. These species time-
history measurements provided important additional information about the oxidation
kinetics that is not available from ignition delay times. This work was challenging, in that
OH absorption experiments are difficult to conduct at high pressures due to various
factors such as gas dynamic interactions and spectroscopic considerations. Detailed
comparisons of these experimental data were then made using several kinetic
mechanisms. The performance of the various models varied notably, even though all
tested models have been validated against a variety of experimental data in other reacting
configurations. This strongly illustrated the utility of and need for shock tube species
time-history (and reaction rate data) for testing reaction mechanisms. Strategies were
developed to improve these mechanisms and to measure some of the key OH reactions.
Specifically, reactions of several stable intermediates and OH were measured and
theoretical calculations were performed to support the experimental findings.
1.2 Background
1.2.1 Jet Fuel Ignition Times
Distillate fuels such as kerosenes are defined by broad composition guidelines, hence
ignition delay times vary with fuel composition, among other variables, and this provides
a challenge in dealing with most practical fuels. Jet fuels such as JP-4, JP-5, JP-7, Jet-A,
JP-8, T-6, TS-1, and others, often contain thousands of compounds. JP-8 fuel is a military
equivalent to the commercial fuel Jet-A and differs primarily by the addition of trace
amounts of additives such as lubricity improvers, corrosion inhibitors, icing inhibitors,
and antistatic additives [4-6].
Although the ignition characteristics of aviation fuel mixtures have been a subject of
investigation for many years, there remains a critical need for experimental data on
4
ignition delay times of Jet-A and JP-8 fuels in air. Recent modeling efforts have been
focused on developing reduced mechanisms and finding suitable surrogate mixtures for
jet fuels. Challenges still remain in modeling real jet fuel behavior and in finding a
suitable physical surrogate (a mixture that has generally the same physical properties as
the jet fuel [6]) and a chemical surrogate (a mixture that has generally the same chemical-
class composition and average molecular weight as the jet fuel [6]) that can accurately
duplicate the real fuel behavior and ignition times for a wide range of conditions. Past
experimental work can be divided into two groups: flow reactors and bombs, and heated
and unheated shock tubes [7,8]. Nearly all shock tube studies of kerosene/jet fuel ignition
have been performed with droplets or liquid films. Ignition delay time was reported to be
influenced by the following parameters: oxygen concentration, incident shock speed,
droplet size and spacing, initial droplet temperature. However, droplet transport problems
cause significant uncertainties in those studies.
To our knowledge, there has been only one shock tube study of gas-phase jet fuel
ignition. In that recent work by Dean et al. [9], ignition delay times of gas-phase Jet-
A/Air mixtures were studied behind reflected shock waves in a heated shock tube. The
Jet-A ignition delay time data from Dean et al. range over temperatures of 1000 to 1800
K and fall into two pressure groups: 10 and 20 atm. Two different heated shock tubes and
optical arrangements were used in the study. A comparison of the ignition delay times of
Dean et al. and the current study is shown later. Due to the high boiling points of large
hydrocarbon components of Jet-A and JP-8 fuels, heating of the shock tube and
associated mixing facilities is necessary to prepare a homogeneous gas phase mixture for
ignition delay time studies, and this largely explains why so few studies have been
conducted with gas-phase jet fuel using the shock tube method. This method, however, is
generally preferred over other methods as it can provide reliable, reproducible
measurements of gas-phase ignition delay times, amenable to a simple constant-volume
modeling constraint, over a wide range of temperatures, pressures and fuel mixtures.
Commercial jet fuels may contain in excess of 1000 components [6,10]. Due to the
complexity of such jet fuels, it is necessary to establish simple multi-component
surrogate mixtures for Jet-A, JP-8 and kerosene fuels, which can reproduce the
combustion behavior of the complex fuels. The use of a simple surrogate blend,
5
containing a relatively small number of high-purity hydrocarbons and blended to simulate
the combustion performance of a practical fuel, has the advantage of allowing fuel
composition to be accurately controlled and monitored for research purposes. There is a
critical need for experimental kinetic databases that can be used for the validation and
refinement of jet fuel surrogate mechanisms. Shock tube studies can contribute to these
databases by providing ignition delay time measurements, which describe the overall
performance of mechanisms, and species concentration time-history measurements,
which constrain modeling of the reaction pathways and the rates involved in these
mechanisms. The jet fuel surrogate mixtures used in these detailed mechanisms
approximate the chemical behavior of distillate jet fuels that contain hundreds of
compounds (such as Jet-A, JP-8, and other related kerosene-based fuels), with a smaller
discrete subset of compounds.
The literature contains a wide variety of surrogate mixtures for jet fuels, of varying
complexity, that are valid for different applications [7]. However, none of the
calculations using current mechanisms and these surrogate mixtures successfully predict
the combustion behavior of real fuels at all conditions. Challenges still remain in the
modeling of real fuel behavior, particularly at high pressures and low temperatures,
where data are needed for jet fuel as well as for surrogate mixtures and single fuel
components. Based on their recent detailed review work, Dagaut and Cathonnet [8] cited
the key need for high-pressure ignition measurements of kerosene-based fuels. In a very
recent article, Colket et al. [11] proposed a roadmap for future development of surrogate
fuels. This team of authors concluded that the main limitations in developing
fundamental chemical kinetic models applicable to jet fuel surrogates have been the lack
of reliable validation data sets and studies that compare real fuels and surrogate mixtures
for fundamental target conditions [11].
1.2.2 Surrogate Ignition Times
Large n-alkanes constitute more than 50 % by volume of jet fuels and generally
likewise, the surrogate mixtures [4]. Detailed kinetic mechanisms of these larger
hydrocarbon components of jet fuels are not well understood, although the development
of experimental kinetic databases for surrogate jet fuels has been a focus of much recent
6
research [11]. n-Dodecane is widely used as the primary representative for n-alkanes in
jet fuel surrogates [5], and thus there is a critical need for experimental characterization
of n-dodecane oxidation. Currently even ignition delay time data are not readily available,
and this has been a major factor impeding the development of an accurate chemical
kinetic mechanism for n-dodecane combustion [12]. Dependence of ignition delay time
on temperature, pressure and composition is critical in describing the combustion of
liquid fuels in practical engines (e.g., internal combustion and gas turbine combustors)
and advanced combustion chambers of various types, as well as in optimizing external
combustion. Only a limited number of previous n-dodecane oxidation studies have been
performed. Kadota et al. reported ignition delays of single n-dodecane droplets in
quiescent air [13]. Eigenbrod et al., using a static system, observed similar ignition times
for n-dodecane and kerosene droplets [14]. Segawa et al. found that the ignition delay
time depended on the size of droplets and that τign increased monotonically with an
increase in the initial n-dodecane droplet diameter [15]. Recently, Holley et al. [16]
measured ignition temperatures and extinction strain rates of n-dodecane/air in a
counterflow configuration under non-premixed flame conditions. We have, however,
found no studies of gas-phase shock tube ignition delay time data.
Cyclo-alkanes (naphthenes) are an important chemical class present not only in jet
fuels, but also in other practical fuels such as gasoline and diesel, and may constitute up
to 20 % by volume of jet fuels such as Jet-A/Jet-A1/JP-8, around 60% by volume of RP-1
and more than 40 % by wt. of diesel fuels [4,6,17,18]. Cyclo-alkanes in diesel fuels have
been found to influence particulate matter (PM) emissions [18], and the soot precursor
production potential of cyclo-alkanes (via formation of polycyclic aromatics) is much
higher than that of normal- and iso-paraffin compounds [19]. For many years, the
combustion and kinetic behavior of straight chain and branched alkanes has received
significant attention, while cyclo-alkanes, on the other hand, have received, until recently,
only scant attention [20].
Methylcyclohexane (MCH) is one of the simplest cyclo-alkanes and is widely used to
represent the cyclo-alkane fraction of jet fuel surrogates [5,21-24]. It has also been
proposed as a fuel for scramjets, because in the presence of a catalyst MCH can be
endothermically dehydrogenated to form toluene and hydrogen, thus providing a
7
significant heat sink (~2190 kJ/kg) for cooling the engine [25]. Very few studies
concerning MCH combustion exist which can be utilized for surrogate fuel development,
though recently there have been a variety of experimental and kinetic modeling studies
conducted for other types of cyclo-alkanes [20,26]. The kinetics of cyclo-alkanes is
regarded as critical to properly representing the reactivity of gasoline and kerosene-based
fuels [27]. Previous researchers studied some aspects of the kinetics of MCH in a variety
of different experimental facilities, such as, low-temperature oxidation in a flow tube [28],
unimolecular decomposition in a pyrolysis apparatus [29], oxidation in a single-cylinder
production-type engine [30], high-temperature (1050-1200 K) pyrolysis and oxidation in
a turbulent flow reactor [31], oxidation in a pressurized flow reactor [21], ignition delay
times (ign) in rapid compression machines (RCM) [22,32,33], decomposition in laminar
diffusion flames [34], and ignition in shock tubes [35,36]. Granata et al. [37] reported a
semi-detailed model of MCH pyrolysis (validated using the measurements of Zeppieri et
al. [31]) and included cyclo-alkanes as reference components to broaden their surrogate
model’s capabilities for heavy practical fuels such as jet fuels, kerosene, and diesel oils.
Granata et al. [37] pointed out that their modeling study was specifically constrained by
the lack of experimental data on the oxidation of MCH. High-pressure (above 4 atm)
shock tube ignition delays of MCH have not been reported so far.
1.2.3 High-pressure OH Time-histories During
Surrogate Oxidation
Species time-history measurements of OH would provide strong kinetic targets for the
validation and refinement of detailed models and will lead to more accurate ignition time
predictions, which are intimately tied to the actual radical pool kinetics. Hydrocarbon
ignition is, to a large extent, controlled by the chemistry of the small transient radical
pool (H, OH, CH3, etc.), and in particular, very few or no data are available for these
species in shock tube studies of relevant surrogate components. At higher pressures, most
current mechanisms have been validated only against the measured yields of the more
stable intermediates, and hence the importance and role of the small radical pool has not
been tested. As well, at higher temperatures, where the initial removal of hydrocarbons is
8
strongly affected by decomposition as well as H-atom abstraction reactions, species time-
history measurements of OH can provide information about both these pathways. In the
current study, we have measured OH concentrations during MCH, n-dodecane and n-
heptane oxidation near 15 atm in argon.
n-Heptane is one of the most widely researched higher-order hydrocarbons, mainly
because of its application as a primary reference fuel, as a homogeneous charge
compression ignition (HCCI)-relevant fuel, and as a diesel fuel. Numerous experimental
and modeling studies of high-pressure n-heptane oxidation and pyrolysis can be found in
the literature [20,38-40]. Starting with the early kinetic model of n-heptane combustion
developed by Coats and Williams [41], well-established kinetic reaction mechanisms
(some of which will be examined later in this thesis) for this fuel are widely available.
However, the only OH concentration measurements in n-heptane/O2/Ar systems were
conducted by Davidson et al. [42] for pressures between 2-3.8 atm and temperatures from
1540 to 1784 K in a low-pressure shock tube at our laboratory. Davidson et al. [42]
examined some of the n-heptane mechanisms and noted that mechanisms were only
moderately successful in duplicating the OH time-histories. By comparing with the
ethylene yield measured (using a microwave-lamp absorption diagnostic at 174nm) in the
shock tube studies of n-heptane pyrolysis by Horning et al. [43] at atmospheric conditions,
Davidson et al. [42] noted that the predicted ignition times in n-heptane oxidation
appeared to follow the predicted time scale of ethylene removal in n-heptane pyrolysis
systems. This suggested that the OH time histories are strongly linked to ethylene
populations, and Davidson et al. [42] found that the ability of the n-heptane mechanisms
to model OH is directly linked to their ability to model ethylene concentrations. Hence,
accurate OH measurements can be used to place constraints on the initial fuel
decomposition processes, the size of the radical pool that exists during the induction
regime before ignition, and the reaction rates of major product species such as ethylene.
Nonetheless, there exist no OH measurements for high-pressure n-heptane oxidation
systems.
Various researchers from our laboratory have measured OH time-histories during fuel
oxidation for different hydrocarbons, however, OH absorption experiments are difficult
to conduct at high-pressures due to various factors such as gas dynamic interactions and
9
spectroscopic considerations. Consequently, most of those studies, with the exception of
Petersen et al. [44] (in methane), were carried out near atmospheric pressures or at
pressures less than 10 atm using low-pressure shock tubes. Current data are the first OH
species time-history measurements in MCH and n-dodecane oxidation and the first OH
species time-history measurements in high-pressure n-heptane oxidation systems.
1.2.4 Reaction of OH with Alkenes
Simplest alkenes such as, ethylene (C2H4) and propene (C3H6), are the most important
intermediates in the oxidation of hydrocarbons fuels [45] and are formed in large
quantities during combustion of hydrocarbons from methane to real fuels in all practical
engines. Also, ethylene and propene are present in practical fuels and are also emitted
into the atmosphere through anthropogenic and natural sources. Alkane fuels rapidly
decompose to simple olefins (before ignition) during high-temperature oxidation [45].
The oxidation kinetics of ethylene and propene are very important to the hierarchical
development of the kinetic mechanisms of real fuels [45-48]. Alkenes contribute to soot
production (and other pollutant formation) and therefore strategies for mitigating
pollutant formation in advanced combustion systems depend on alkenes oxidation
chemistry. Knowledge of alkene oxidation reactions is critical to the development of
accurate modeling of combustion processes such as ignition, heat release, etc., at high
temperatures. Accurate determinations of ignition times and species time-histories for
hydrocarbon fuels are strongly sensitive to olefin concentration levels (mostly ethylene
and propene) and oxidation rates [45].
Hydroxyl (OH) radical is an important transient reactive radical during combustion
and in atmospheric chemistry, and it is also widely accepted that OH radical oxidation of
ethylene and propene is the major oxidation route for these molecules under atmospheric
and combustion conditions [45,49,50]. Ethylene and propene react with OH to form
various products including water (the final product): C2H4+OHProducts, and
C3H6+OHProducts.
Because it is the most important (and the simplest alkene) stable intermediate during
combustion of higher hydrocarbons, extensive experimental and theoretical studies have
been conducted on OH+ethylene. Table 1.1 lists all studies above 500 K along with some
10
of the low temperature studies of OH+ethylene. Despite these measurements, relatively
large variations in the rates (magnitude and activation energy) for C2H4+OH exist in
recent combustion kinetic mechanisms for practical fuels (see Figure 1.1). Shown are
rates used in recent hydrocarbon mechanisms such as, JetSurF 1.0 [46], Westbrook et al.
[47], Ranzi [48], Galway natural gas mechanism III [51], and Chaos and Dryer [52]. We
believe this lack of consensus among modeling community arises from the scatter in the
experimental studies listed in Table 1.1. Relatively very few studies have been conducted
on the reaction of OH with propene (see Table 1.1). To the best of our knowledge, there
has been no experimental study of OH+propene above 1210 K. Hence investigations at
higher temperatures appear to be warranted for both OH+ethylene and OH+propene.
0.50 0.75 1.00 1.25 1.50 1.75 2.001E10
1E11
1E12
1E13 1333 K 667 K 500 K
5
4
3
2
1000 K/T
k 1 [c
c/m
ol/s
]
1) JetSurF 1.0, 20092) Westbrook et al., 20093) Natural gas III (Galway), 20104) Ranzi, 20065) Chaos et al., 2007
1
1000 K
Figure 1.1 Comparison of rate constants for C2H4+OH used in several current
mechanisms. Variations of a factor of ~5.5 are evident at high temperatures near 1000 K.
Shown are rates used in JetSurF 1.0 [46], Westbrook et al. [47], Ranzi [48], Galway
natural gas mechanism III [51], and Chaos et al. [52].
1.2.5 OH+1,3-Butadiene
1,3-Butadiene (1,3-C4H6) is an important stable intermediate formed during the
combustion of hydrocarbons, from methane to real fuels, in practical engines. It is also a
hazardous, carcinogenic, toxic pollutant and genotoxic (in humans and other mammals)
chemical, which is widely used in petroleum and rubber industries, and emitted into the
11
atmosphere from sources including tobacco smoke, forest fires, automobile exhaust, and
gasoline evaporative emissions [53-55]. Because of this, there is a need to include the
oxidation kinetics of 1,3-butadiene in the hierarchical development of the kinetic
mechanisms of real fuels [26,31,36,45,56]. A better knowledge of the oxidation kinetics
of 1,3-butadiene (as the simplest conjugate olefin) would help improve the understanding
of the role of complex olefins in combustion, soot formation and toxic emissions [57,58].
There have been many studies of the most important of the 1,3-butadiene oxidation
reactions, OH+1,3-C4H6=products. But these have been limited mainly to low
temperatures [53-55,59-67]. There also have been several high-temperature studies of
1,3-butadiene oxidation and pyrolysis in flames, flow reactors, and shock tubes
[56,57,68,69], but these studies have not directly addressed determination of the rate
coefficient of OH+1,3-butadiene. The only experimental study (> 500K) for the overall
rate of this reaction we have found was conducted in a flow system using pulsed
radiolysis coupled with microwave resonance absorption to monitor the OH
concentration in the range 313-1203 K [64]. To the best of our knowledge, there has been
neither an experimental nor a theoretical study of this reaction above 1203 K.
1.3 Scope and Organization of Thesis
The primary objective of this work was to conduct high-quality experiments using
state-of-the-art shock tube and laser absorption methods to investigate jet fuel and
surrogate oxidation systems. Ignition times and species time-histories were measured
under engine-relevant conditions and low-uncertainty measurements of the reactions of
OH with several stable intermediates were carried out. Current data were used as
validation targets and improvements to several surrogate mechanisms were provided
using detailed kinetic analysis. Theoretical calculations of the measured rates were
provided which supports our experimental findings.
Chapter 2 describes the experimental apparatus, OH laser diagnostics and methods
used to model the shock tube data. Chapter 3 presents ignition delay time measurements
in jet fuels (Jet-A/air and JP-8/air) at high pressures using the heated HPST. Predictions
of several jet fuel kinetic mechanisms using surrogate mixtures were analyzed. Chapter 4
contains high-pressure ignition delay time measurements in two n-dodecane/air and
12
MCH/air, which are the two most important single-component jet fuel surrogates.
Available kinetic mechanisms for these two fuels were analyzed for their predictive
capabilities. Chapter 5 describes high-pressure OH time-histories measured during
oxidation of three single-component surrogates (n-dodecane, MCH, and n-heptane)
diluted in argon using heated HPST. Detailed comparisons of experimental data with
predictions of available kinetic mechanisms were made and procedure was shown to
improve their predictive qualities.
Chapter 6 presents measurements of the reactions of OH radicals with two important
alkenes (ethylene and propene) behind reflected shock waves. Also discussed in this
chapter are canonical transition state theory (TST) calculations for these reactions.
Chapter 7 describes measurements of the reaction of OH with 1,3-butadiene. Theoretical
results of the rate coefficient and the branching fractions for the H-abstraction channels
of the target reaction were also presented. Chapter 8 summarizes the thesis and suggests
future work. Appendix section details modes of ignition, shock tube nonideal effects, and
TST theory.
13
Table 1.1 High-temperature OH+ethylene and OH+propene studies in literature.
Reference Temperature Range Experimental Method and Comments OH+ethylene Bott and Cohen [50] 1197 K Reflected Shock Tube (S.T.), 1.04atm, OH resonance
microwave absorption at 309nm Smith [70] 1220 K Laser pyrolysis/laser fluorescence Bradley et al. [71] ~1300 K Incident S.T., OH UV Lamp, rate was determined
relative to methane + OH Baldwin et al. [72] 813 K Reaction vessel, pressure and gas sample analysis
using method similar to gas chromatograph Bhargava and Westmoreland [73]
1455-1740 K Laminar flames, molecular-beam mass spectrometry (MBMS)
Westenberg and Fristrom [74]
1250-1400 K Flames, electron spin resonance (ESR) spectroscopy
Tully [75] 291-591 K Flash photolysis, laser induced fluorescence (LIF) Tully [76] 650-901 K Laser photolysis, LIF Liu et al. [64,77] 343-1173 K Pulse radiolysis/ OH resonance microwave absorption,
1atm Westbrook et al. [78] 1003-1253 K Jet-stirred reactor (JSR) Greiner [79] 299-497 K Flash photolysis/ kinetic spectrograph Fulle et al. [80] 300-814 K Laser flash photolysis/ saturated LIF Srinivasan et al. [81] 1463-1931 K Reflected S.T., OH UV Lamp Baldwin et al. [82] 773 K Reaction vessel, pressure and gas sample analysis
using method similar to gas chromatograph (GC) Diau and Lee [83] 544-673 K Laser photolysis/ LIF Zellner and Lorenz [84] 296-524 K Laser photolysis/resonance fluorescence Hoare and Patel [85] 734-798 K Reaction vessel/GC Avramenko and Lorentso [86]
350-451 K Discharge flow system. OH generated from H2O vapor. OH measured by U.V. absorption. Source of OH at fault
OH+propene
Bott and Cohen [50] 1204 K Reflected S.T., 1.04atm, OH resonance microwave absorption at 309nm
Tully and Goldsmith [87] 293-896 K Laser photolysis, LIF Smith et al. [88] 960-1210 K Laser pyrolysis, LIF Baldwin et al. [89] 773 K Reaction vessel, pressure and gas sample analysis
using method similar to gas chromatograph. Relative rate method using OH+tetramethylbutane reaction
Atkinson and Pitts Jr. [90] 297-425 K Flash photolysis, LIF Yetter and Dryer [91] 1020 K Flow reactor (1atm), GC
14
Chapter 2: Method This chapter describes the shock tubes and the OH laser diagnostics used in this study.
2.1 Shock Tube Facilities
2.1.1 High-pressure Shock Tube
All high-pressure (above 10 atm) ignition delay times and OH time-histories were
measured behind reflected shocks using Stanford’s heated, high-pressure shock tube
(HPST), which was designed to attain pressures as high as 600 atm. The shock tube
driver section is 3 m long with a 7.5 cm internal diameter. Helium was used as the driver
gas, except for the low-temperature cases (< 900 K) where a tailored driver gas mixture
(20-30% N2 in He) was used. Diaphragms were made of aluminum of 1.25 to 2 mm
thickness with cross-scribing. The stainless steel driven section of the HPST is 5 m long
with a 5 cm internal diameter and was wrapped with thin copper sheets and heated to 100
C using 13 separate heating zones to produce uniform temperature along its length; this
prevents condensation of the fuel/air mixture. Uniform temperature (±3 ºC) along the
length of the shock tube is achieved through a series of heaters controlled using
Watlow™ controls. Before introduction of the test mixture, ultimate pressures of less
than 10-5 torr and leak-outgasing rates of less than 10-4 torr/min are regularly achieved
using VarianTM SD 250 roughing pumps and a turbo-molecular pump (VarianTM Turbo-V
250). End-wall movements during experiments were limited to less than 1 mm by
attaching the last section of the driven tube to a 2 ton concrete and steel mass. A fuller
description of the shock tube can be found in Petersen [92].
The incident shock speeds were determined using six fast-response piezo-electric
pressure transducers (PCB 113A), spaced at approximately 30 cm intervals over the last 2
meters of the shock tube, and five time-interval counters (Phillips PM6666). Uncertainty
in the time interval measurement is ±0.3 μs. The velocity of the incident shock at the end
wall is then determined by extrapolation. Typical shock attenuation rates, defined as the
normalized slope of axial velocity extrapolated to the end wall (in %/meter), ranged from
0.2 to 2 %/m for the current experiments. Fill pressure (P1) is monitored using two static
15
pressure transducers (Setra Model 280E). A K-type thermocouple protruding slightly
from the sidewall midway along the driven section was used to determine pre-shock test
mixture temperature (T1). Using P1 and T1, conditions behind the reflected shock wave
(temperature T5, pressure P5) were determined by the one-dimensional normal shock
equations and the Sandia thermodynamic database of Kee et al. [93] including additional
thermodynamic properties information on liquid Jet-A, OH and surrogates as
recommended by Burcat and Ruscic [94]. Ignition pressure was monitored using a piezo-
electric pressure transducer (Kistler model 603B1) located 10 mm from the end-wall. For
ignition experiments, the emission from OH* (using a Schott Glass UG-5 filter with >95%
transmission at 306 nm), detected using a lens/slit setup with a Thorlabs PDA55 detector,
was monitored at an observation window located at the same axial location. The axial
spatial resolution of the detector system (<5 mm) was determined from the optical design
calculations of Flower [95]. A schematic of the emission set-up is given in Figure 2.1.
For the current reflected shock temperatures and pressures, real gas corrections to the
temperatures were insignificant and within the experimental uncertainty for the HPST
[96]. Vibrational relaxation in air was also taken into consideration, when computing the
reflected shock temperatures, since the vibrational relaxation state of O2 and N2 was
found to have significant effect on the predicted temperatures and pressures using the
shock code. Temperatures and pressures calculated in this study assume full vibrational
relaxation of the shocked gases in both incident and reflected regime. The experimentally
measured pressures were found to be consistent with the predicted pressures when full
vibrational relaxation was assumed, as was also seen by Gauthier et al. [39].
Care was taken to minimize the uncertainties caused by reflected shock bifurcation on
time-zero definition, following the recommendations from Petersen [92]. Strong ignition
was observed in all ignition time experiments conducted owing to the large fuel
concentrations employed. As a consequence, the combustion wave may transition to a
detonation wave, thereby affecting the ignition delay time measurement further down the
side wall by shortening the distance and arrival times between the reflected shock wave
and the combustion front. However, for the present diagnostics located at 10 mm from
the endwall, and for the reflected shock temperatures and pressures studied, the estimated
16
discrepancy was less than 10 μs for all cases, and was simply included in the uncertainty
analysis.
2.1.2 Low-pressure Shock Tube
All reaction rate experiments were performed behind reflected shock waves in a
stainless steel, high-purity, helium-driven, unheated low-pressure shock tube (LPST) with
inner diameter of 14.13 cm. Lexan polymer diaphragms were used and the shock tube test
section and mixing manifold were routinely evacuated below 1 Torr using two turbo-
molecular pumps. The shock tube leak-plus-outgassing rate was less than 5 Torr/min.
Incident shock velocities were measured using five piezoelectric transducers (PCB 113A)
spaced axially along the last meter of the tube and linearly extrapolated to the endwall.
Average incident shock wave attenuation rates were between 0.6% and 2% per meter.
Ideal shock relations and the thermodynamic database from Burcat and Ruscic [94] were
used to calculate reflected shock temperature and pressure (T5 and P5). Uncertainties in
the calculation of the initial reflected-shock temperature and pressure were typically less
than 0.7% and 1%, respectively, and arose primarily from the 0.3% uncertainty in the
incident shock velocity determination. This shock tube has been extensively used for
reaction rate measurements in our lab (further details can be found in [97-99]).
2.2 OH Laser Absorption Diagnostics
Absorption spectroscopy of the OH radical near 306.5 nm is well-established at
combustion temperatures and near-atmospheric pressures. The OH absorption
measurements, at the same axial sidewall location where P5 was measured, were made
using narrow-linewidth ring-dye laser absorption close to the R-branchhead (overlap of
the R1(8), R1(9) and R1(10) lines at high-pressures) near 306.47 nm (32630 cm-1) of the
OH A2Σ+-X2Π (0,0) system in high-pressure experiments. This particular peak
wavelength was selected as it has the strongest high-pressure OH absorption feature at
high pressures. In low-pressure experiments (1-10 atm), OH radicals were monitored
using the R1(5) absorption line near 306.7 nm as it has the highest peak and was well
isolated from other peaks. Visible light at 612-614 nm (25-30 mW) was generated in a
Spectra-Physics 380A ring-dye laser cavity by pumping Rhodamine 6G dye with a 5 W,
17
532 nm, cw beam produced by a Coherent Verdi laser. The visible beam was intra-cavity
frequency-doubled using a temperature-tuned AD*A crystal, generating 1-2 mW of UV
light. A small part of the visible output from the laser (the laser has an instantaneous
linewidth of a few MHz) at the fundamental wavelength was used for peaking the laser
power as well as for monitoring laser mode quality in a scanning interferometer and
reading the absolute laser wavelength using a Burleigh WA-1000 wavemeter. High-
pressure absorption measurements require attention to two phenomena not generally
important at lower pressures: stress-induced anisotropy within the observation windows,
and laser propagation through flow-induced perturbations. The current high-pressure
optical setup incorporated recommendations from Petersen [92] to deal with these effects
(choice of the optical components and their relative positioning can be optimized to
minimize these effects). The low-pressure optical setup schematic was same that used in
[99].
Briefly, UV light was separated into two beams: a reference beam Io, and a transmitted
beam I, which passed through the sidewall measurement location, which was at 10 mm
and 20 mm from endwall, for HPST and LPST, respectively. Two sapphire windows
provided optical access to the test section for the UV absorption experiments. Windows
were pre-stressed in a separate rig to reduce stress-induced anisotropy. Common-mode
rejection of laser intensity fluctuations was performed by balancing the two beams prior
to each run using neutral density filters. Thorlabs PDA 55 UV detectors, specially
modified to accommodate large area Hamamatsu S1722-02 detectors (with an effective
active area of 13.2 mm2), monitored both the UV beams. A spectral filter (Newport FSR-
UG11, Schott Glass, UV Bandpass) was used with the detector to reduce interference
from broadband emission during ignition. All data were recorded using a high-resolution
data acquisition system at a sampling rate of 2MHz. The lens/detector setup was
optimized to minimize flow-induced perturbations (beam steering) by insuring that the
beam waist in the shock tube was positioned away from the boundary layer. Additional
details of the two-beam method and the OH ring dye laser absorption diagnostic may be
found in [92,97,99].
18
Pump laser
Tunable ring-dye laser
Powermeter
Scanningetalon
Si Detector
VisibleUV
Wave-meter
I0
I
532 nm
306.47 nm612.94 nm
S-P 380
Coherent Verdi
Laser source
PDA 55*
Laser beam quality
HPST
ND
PDA 55*
IR
IRL
L
Computer Data Acquisition
LL
IR
Absorption Set-up
M
M
MBS
BS
BS
L = lens; ND = neutral density filter; BS = beam splitter; IR = iris; M = mirror; PDA 55* = UV enhanced photodiode; I = diagnostic beam; Io = reference beam
Emission Set-up
PDA 55Kistler PZT
Computer Data Acquisition SHOCKTUBE
SlitL
Figure 2.1 Experimental set-up for HPST experiments.
OH mole fractions (XOH) were determined from the measured absorption using
the Beer-Lambert relation: I/Io=exp{-kνP5XOHL), which assumes attenuation of incident
radiation by a non-saturating, linearly-absorbing medium. P5 (atm) is the total reflected
shock pressure and L is the path length (5 cm and 14.13 cm, for HPST and LPST,
respectively). The well-characterized absorption coefficient, kν, was calculated
incorporating the collision-broadening and collision-shift parameters measured by
Herbon [97] for the R1(5) line and by Davidson et al. [100] at high-pressures for R-
branchhead. Laser-off measurements were also taken to confirm the absence of any
significant contribution of emission to the absorption signal for the range of current
19
experiments. Consequently, off-line and emission corrections were not needed or applied.
Several uncertainties contribute to the determination of XOH with uncertainties in
temperature (T5) having the largest effect. The other main uncertainty comes from
uncertainty in determining the laser wavelength. However, kν values are relatively
constant for small wavelength fluctuations at current conditions. The overall estimated
uncertainties in the inferred values of XOH are ±5% and ±3% at high- and low- pressures,
respectively.
2.3 Modeling Shock Tube Data
In comparing modeled and experimental ignition, OH time-history and rate coefficient
data, the model calculations are done for homogeneous, adiabatic conditions behind
reflected shock waves, with a constant-volume constraint using CHEMKIN 4.1.1 [101].
Shock tubes produce homogeneous mixtures, and behave like near-ideal constant-volume
reactors up to the time of ignition. Hence, the constant-volume, the constant-internal-
energy constraint (constant U,V) is a good assumption for the purpose of ignition delay
time calculations (typically less than 1-2 ms); details about the implications of this
assumption for the later stages of ignition process are described in Davidson and Hanson
[102].
However, at long test times, even in the absence of reaction, the reflected shock
pressure (P5) typically increases slowly (approximately linearly with time in our facility.)
This rise is caused by non-ideal effects such as incident shock attenuation, boundary layer
growth, and interaction of the reflected shock wave with the side-wall boundary layer
(see Petersen [92] for details about the non-idealities in HPST). We developed a model in
our lab, CHEMSHOCK [103], which combines CHEMKIN with an isentropic
compression of the test gas mixture consistent with the actual pressure measured during
the experiment. The influence of these non-ideal effects (if present) on the
measured/modeled ignition data can be estimated using the CHEMSHOCK model.
20
Chapter 3: High-pressure Jet Fuel
Ignition Times This chapter describes ignition delay time measurements in Jet-A/air and JP-8/air at
high pressures using the heated HPST. Initial reflected shock conditions were as follows:
temperatures of 715-1229 K, pressures of 17-51 atm, equivalence ratios of 0.5 and 1, and
oxygen concentrations of 10 and 21 % in synthetic air. As a key step toward defining or
validating a surrogate composition for jet fuels, comparisons with the predictions of
several current kinetic mechanisms are also made.
3.1 Jet Fuel/Air Mixture Preparation
Jet fuel can vary widely in its chemical properties. A recent gas chromatograph results
(Dean et al. [9]) for a Jet-A sample provided by U.S. Oil and Refining Co. identified 167
compounds and suggests an approximate formula C10.84H19.65. Many heavy components
were not recognized in the study. In another study by Gueret et al. [104], the general
formula, obtained using a mass-detector after separation on a capillary column, was
found to be C11H22. Throughout this study we have assumed the widely specified
chemical formula of C11H21 and a density of 0.81 gm/cm3 for all the jet fuels tested [6,9].
The Jet-A Composite Blend used in this study (#04POSF4658) was analyzed by Shafer et
al. [105] using the ASTM D2425 GC-MS [106] technique and had the following main
components by volume: paraffins (normal + iso) 55.2%; monocycloparaffins 17.2%;
dicycloparaffins 7.8%; alkyl benzenes 12.7%; indans+tetralins 4.9%; substituted
naphthalenes 1.3%. The JP-8 fuel used in this study had the following approximate
characteristics: cetane number of 43.3, density of 0.8 kg/l, hydrogen content of 13.9
(wt. %), aromatics composition of 13.9 (vol %), olefins composition of 0.5 (vol %), and
saturates composition of 84.2 (vol %).
It is important to vaporize the jet fuels as completely as possible to reduce any errors
in the fuel-air mixture calculations. A new liquid fuel mixing facility was designed for
this purpose and was attached to the shock tube. Fuel/air mixtures were prepared in a
12.8 liter, magnetically-stirred, thermally insulated, stainless-steel mixing tank. For all
21
experiments, the mixing tank and connecting gas lines to shock tube were heated (using a
custom-made heating blanket provided by HTS/Amptek) to avoid condensation of the
fuel and to achieve higher fuel concentrations. The mixing tank was well-insulated to
avoid any spatial temperature non-uniformity, thereby avoiding surface temperature
variation and local condensation points inside the tank. In all the current experiments,
quantities of liquid Jet-A (JET-A Composite Blend #04POSF4658 supplied by T.
Edwards, AFRL-WP) and JP-8 (supplied by P. Schihl, ARL) were measured and added
volumetrically into the mixing tank using a gas-tight syringe (Hamilton 1010TLL) and
allowed to evaporate until a steady pressure (monitored by a MKS 690A Baratron
pressure transducer) was achieved. For the entire study, air refers to synthetic (dry) air
consisting of 21 % O2 and 79 % N2 (i.e., XN2=3.76 XO2). Research grade synthetic air
(Praxair N2, O2,) was added slowly in accordance with a procedure outlined by Horning
et al. [43] to inhibit condensation of fuel that could lead to inaccuracies in reported fuel
composition. The mixture was stirred using a magnetically driven vane assembly,
typically for 3 hours, before the first shock wave experiment.
Before use in this study, the heated mixing facility was characterized using two single
component fuels, iso-octane and n-heptane, whose vapor pressure are known accurately
(close to 35 torr for both fuels at 20 C, as given in Lide [107]). For iso-octane and n-
heptane, a fixed volume of the liquid fuel (up to 10 cc) was added to the mixing tank at
85 C. The measured pressure using the Baratron, and the ideal-gas predicted value
(assuming complete vaporization) were found to be in very close agreement for these
fuels at this temperature (implying no significant wall adsorption or incomplete
vaporization or other loss mechanisms). For both Jet-A and JP-8, the vapor pressure is
very low (less than 2 torr at 20 C), when compared to the vapor pressures for iso-octane
and n-heptane. With the objective of finding the temperature to which the mixing tank
and fuel lines should be heated so as to avoid any condensation of the jet fuels, 1 cc of the
jet fuel was added to the tank at different temperatures. Comparison of the measured and
the predicted (assuming complete vaporization) pressures showed that the mixing tank
and the connecting lines had to be heated to 125 C to ensure that there was no further
impact on fuel vaporization. A discussion about the effect of different mixing times for
22
the fuel/air mixtures inside the tank and different shock tube temperatures on the ignition
delay time measurements is provided later.
3.2 Measuring Ignition Delay Times
The ignition delay time is defined in this study as the time interval between the
arrival of the reflected shockwave and the onset of ignition at the sidewall observation
location. The arrival of the reflected shockwave was determined by the step rise in
pressure, and the onset of ignition was determined by monitoring both the pressure
history and the emitted light corresponding to OH* emission. The onset of ignition from
the pressure history and OH* emission were defined by locating the time of steepest rise
and linearly extrapolating back in time to the pre-ignition baseline (for details regarding
emission measurements, see Hall et al.[108]). The two methods give very similar results
(± 3%) and ignition delay times are readily identifiable with both diagnostics; example
data are shown in Figure 3.1. The overall uncertainty in the ignition delay time
measurements was 15%, which was dominated primarily by uncertainties in the
temperature (less than 1.8 %, and due mainly to uncertainties in fuel composition and in
the endwall velocity). Larger uncertainties may exist in the post-shock temperatures of
the tailored gas mixtures, and are discussed in a later section.
0 200 400 600 800 1000
0
20
40
60
80
ign
=615 s
Pressure
OH* Emission
JP-8/air, =1.01019 K, 22.2 atm
Pre
ssur
e
[at
m]
Time [s]
Figure 3.1 Example JP-8/air ignition delay time data (driver gas: He). Initial reflected
shock conditions, 1019 K, 22.2 atm, = 1.
23
3.3 Jet Fuel Ignition: Results and Discussion
The results for ignition delay times versus inverse temperature are discussed in this
section and are presented in the following order: comparison with previous measurements;
comparison of different fuel types (Jet-A and JP-8); pressure scaling; surrogate modeling
prediction; effect of variation of equivalence ratio and oxygen concentration; and low-
temperature (NTC region) ignition delay time data. All current experimental results are
summarized in Table 3.1 and Table 3.2.
3.3.1 Comparison with Previous Measurements
The ignition delay time data for Jet-A/air (Jet-A Composite Blend #04POSF4658)
over a range of temperatures for reflected shock pressures (P5) from 18 to 36 atm have
been normalized to 20 atm assuming τign ~ 1/P and are presented Figure 3.2 (solid circles).
These measurements show relatively good agreement with the one earlier (Dean et al. [9],
10-20 atm) shock tube study for gaseous Jet-A (open circles). Over the temperature and
pressure range (10-36 atm) of the current and the Dean et al. [9] data, a normalizing
pressure scaling of 1/P correlates the data very well. However, it must be noted that
different sources of jet fuel were used in current experiments and in those of Dean et al.
[9]; see earlier section for additional fuel information. Hence, to perform a comparative
study of τign measurements from different facilities for same Jet-A fuel, we also tested the
Jet-A fuel used by Dean et al. [9]; results are presented as open square symbols in Figure
3.2. The HPST was heated to 80 C (same temperature as in the Dean et al. [9] study) for
these experiments. The data generated in the current study and those recorded by Dean et
al. [9] are in very good agreement for this Jet-A fuel, which in turn are in good agreement
with the ignition delay time data for Jet-A Composite Blend #04POSF4658. Hence, it
could be concluded that under the current experimental conditions, Jet-A ignition delay
times do not differ between these two Jet-A fuel sources. For all the subsequent Jet-A/air
discussions in the current chapter, only data for the Jet-A Composite Blend
#04POSF4658 is presented.
24
0.6 0.7 0.8 0.9 1.0 1.1 1.21
10
100
1000
833 K
Igni
tion
Del
ay T
ime
[
s]
1000/T [1/K]
Jet-A/air, =1.0
Data scaled to 20 atm using P-1
1666 K
Figure 3.2 Ignition delay times including previous data for Jet-A/air for phi=1. Solid
circles: current work for Jet-A POSF4658; open circles: Dean et al. [9]; open squares:
current work for Dean et al. [9] Jet-A fuel.
As a further point of comparison, there does not appear to be a significant difference
in the ignition delay time measurements when the shock tube was heated to T1 = 80 C or
150 C (both were employed in Dean et al. [9]) or T1=100 C (current work). This
similarity may be attributed to the fact that the ignition process is not strongly affected by
the presence or absence of small amounts of heavier (~C20) fuel components that may be
volatilized (T1=100 C and T1=150 C) or may not be totally volatilized (T1=80 C) in the
experiments. Larger non-volatized hydrocarbons are present only in trace amounts, and
may not be a factor in determining the overall ignition process. The possibility of fuel
cracking from long mixing times at elevated temperatures is a concern in kinetics studies
of jet fuels (Holley et al. [109]). However, in the current experiments, no measurable
change in ignition delay time data was observed for fuel/air mixing times between 2.5 to
8 hours.
25
3.3.2 Comparison of Different Fuel Types
A comparison of the measured ignition delay times for Jet-A/Air and JP-8/Air
mixtures are shown in Figure 3.3 for an equivalence ratio of 1.0. The low scatter of the
data shown in Figure 3.3 enables discernment of small differences in ignition delay times
in fuel types. There is very good agreement between ignition delay times for both fuels at
the high and low temperatures. JP-8 ignition delay times appear to be approximately 10%
shorter than those of Jet-A for temperatures near 1000K. However, owing to our
experimental uncertainties, caution must be applied when utilizing this finding. Since jet
fuels meet only a very broad range of fuel specifications, this apparent difference in the
ignition delay time behavior with respect to fuel type or source is very hard to quantify.
The combustion characteristics such as extinction and autoignition of Jet-A and JP-8,
based on measurements in laminar non-premixed flows, were found to be very similar
[23]. But, [110] found that variations in cetane index, aromatic and naphthene content,
and fuel additives do impact auto-ignition behavior in a single-cylinder engine. In
summary, these comparison experiments suggest that the combustion performance of jet
engines could be affected by variations in the fuel source or type, and more detailed
studies of tightly controlled fuel and surrogate mixtures are clearly needed.
0.85 0.90 0.95 1.00 1.05 1.10 1.15100
1000
10000
=1.0
Jet-A/air JP-8/air
870 K
Ign
itio
n D
ela
y T
ime
[s
]
1000/T [1/K]
1176 K
Data scaled to 20 atm using P-1
Figure 3.3 Ignition delay times for Jet-A/air and JP-8/air, phi=1.
26
3.3.3 Pressure Scaling
When experimental conditions vary over a wide range of pressures, assuming a power-
law dependence to the pressure scaling leads to a more uniform graphic presentation of
the ignition delay time data. The pressure dependence of ignition delay times in Jet-A/air
mixtures is shown in Figure 3.4. A simple 1/P dependence was found for ignition delay
times from 850K to 1250K for the pressure range 20-50 atm based on the scaling of the
data points. This 1/P dependence has been observed in many previous studies with
different experimental facilities [7]. A regression analysis of all the current data for Jet-
A/air and JP-8/air is also shown, which shows τign ~ P-0.98, which is very close to the 1/P
behavior cited previously. However, in all the results presented in the current chapter a
simple 1/P dependence to scale ignition delay times was used. In the current data, there is
slight evidence of negative temperature coefficient (NTC) type ignition delay time roll-
off below 1000K. NTC behavior is attributed to the formation of peroxy radicals, and
large n-alkanes present in the jet fuels have a stronger tendency to show NTC behavior
than small or branched alkanes [111]. Stronger pressure dependence is expected in the
NTC region.
0.8 0.9 1.0 1.1 1.210
100
1000
1250 K 833 K
50 atm
Ign
itio
n D
ela
y T
ime
[s
]
1000/T [1/K]
Jet-A/air = 1.0
22 atm
32 atm
0.8 0.9 1.0 1.1 1.210
100
1000
10000
Ig
nitio
n D
elay
Tim
e [
s]
All data scaled to
20 atm using P-0.98
1250K 833K
1000/T [1/K]
Figure 3.4 Pressure scaling of (phi=1) τign: left- Jet-A/air; right- Jet-A/air and JP-8/air.
27
3.3.4 Jet Fuel Surrogate Modeling Prediction
The current data set provides critical validation targets for jet fuel kinetic mechanisms
and surrogate models. Predictions of the following four available kerosene-based kinetic
mechanisms are analyzed:
1) Ranzi [48], Politecnico di Milano, Italy, JP-8, 280 species, 7800 rxns;
2) Zhang et al. [19], University of Utah, USA, JP-8, 208 species, 1087 rxns;
3) Dagaut and Cathonnet [8], CNRS, France, Kerosene, 209 species, 1673 rxns;
4) Lindstedt and Maurice [112], Imperial College, UK, Kerosene, 154 species, 947
rxns.
The surrogate mixtures suggested by [8] and [112] were used to model the ignition
delay times with their respective mechanisms (see Table 3.3). The Ranzi [48] and the
Zhang et al. [19] mechanisms did not specify a particular surrogate mixture. Hence, the
most widely accepted JP-8 surrogate mixtures from Violi et al. [5] were used to model
the ignition delay times predictions with the Ranzi [48] and the Zhang et al. [19]
mechanisms (Table 3.4 To further investigate the influence of various representative
components of jet fuel compositions on the ignition delay times, two modified Violi #3
mixtures were used: Stanford A, with less n-alkanes and more branched alkanes
compared to Violi #3; and Stanford B, with less n-alkanes and more aromatic
components compared to Violi #3.
Figure 3.5 presents the modeling results for ignition delay time using all the
mechanisms for Jet-A/air at 20 atm and Ф=1.0. In Figure 3.5, the Violi #3 surrogate was
used with the Ranzi and the Zhang et al. mechanisms. The ignition process in heavy fuels
is a multi-step process, starting with the rapid decomposition of the fuel followed by the
slow decomposition of the intermediate products and the exponential growth of the
radical pool. The Lindstedt and Maurice mechanism predictions for the three surrogates
(using different aromatic components) show no marked difference in ignition delay time.
At high temperatures (greater than 1000 K), this mechanism and mixtures yield relatively
good agreement with the current data. However, both these mechanisms fail to show any
roll-off trend in activation energy below about 1050 K. Similarly, the Dagaut and
Cathonnet mechanism and surrogate fail to predict ignition delay times for the
28
temperature range of current experiments at 20 atm pressure. It should be noted that both
the Dagaut and Cathonnet and the Lindstedt and Maurice surrogate mixtures and their
mechanisms contain species only up to C10. The reasons for the failure of these two
mechanisms could be due either to the chosen surrogate or the mechanism itself. It may
be necessary to include higher molecular weight components, such as n-dodecane, which
calls for significant modifications to be made in these mechanisms. Earlier, Holley et al.
[109] suggested that the inclusion of small hydrocarbons in a surrogate will result in a
fuel that may not mimic satisfactorily the flame behavior of real jet fuels.
The Zhang et al. JP-8 mechanism predicts higher ignition delay times compared to the
data for the temperature range studied at 20 atm, when modeled using the Violi #3
surrogate composition, and failed to capture the high-temperature (greater than 1000 K)
trends in the activation energy. This mechanism was not designed for very low
temperature ignition applications. Also, due to the absence of peroxy chemistry in this
model, the mechanism does not show any roll-off behavior at low temperatures as
observed in the data. The role of peroxy radicals in low-temperature combustion is
significant and is described in detail by Miller et al. [111]. The Zhang et al. mechanism
predictions using different surrogate fuel mixtures were analyzed. Predictions with neat
n-dodecane were the fastest, since the n-alkane sub-mechanism used in this mechanism
leads to shorter ignition delay times than the aromatic sub-mechanism. The Stanford B
and Violi #3 surrogate mixture predictions did not differ at all when modeled using the
Zhang et al. mechanism, suggesting that the aromatic sub-mechanism of the Zhang et al.
mechanism is not complete. The Stanford A surrogate predictions showed the slowest
ignition process, which suggested that adding more branched alkanes to the Violi #3
surrogate was not an option to improve the predictive abilities of the Zhang et al.
mechanism. In general, both the Zhang et al. mechanism and the surrogate mixtures
studied, fail to capture both the high temperature trends in ignition delay times and the
low temperature behavior.
29
0.8 0.9 1.0 1.1 1.210
100
1000
10000
1000K
Jet-A/airJP-8/air
All data scaled to
20 atm using P-1
Igni
tion
Del
ay
Tim
e [s
]
1250K
1000/T [1/K]
833K
Ranzi et al. Lindstedt et al. Zhang et al. Dagaut and Cathonnet
Figure 3.5 Jet fuel ignition delay times for phi=1 (data from Figure 3.3), and mechanisms
prediction. The Violi #3 surrogate was used with the Ranzi and the Zhang et al.
mechanisms. See text for details.
The Ranzi mechanism gives the closest agreement with data under current conditions,
when compared to other mechanisms (Figure 3.5). When modeled using the Violi #3
surrogate composition, predictions at 20 atm are slightly higher than the experiment at
temperatures above 1000K. Below this temperature, the model shows strong NTC
behavior due to the large fraction of n-dodecane in the surrogate fuel (details provided in
[111]). In Figure 3.6, a comparison of the data with different surrogate mixtures using the
Ranzi mechanism is shown. Peroxy chemistry plays a more important role in longer
straight chain alkanes, and large n-alkanes ignite faster under milder conditions than
small or branched chain alkanes. Thus, neat n-dodecane shows the strongest roll-off as
expected (Figure 3.6). The prediction for the Violi #1 surrogate mixture shows the
weakest low-temperature roll-off, more similar to the gradual roll-off trend seen in the
data. However, the Violi #1 surrogate is relatively complex (6 components). Moreover,
the ignition delay times predicted using the Violi #3 surrogate agree more closely with
the data when compared to those using the Violi #1 surrogate at higher temperatures (>
1000 K). There is no significant difference between the predictions for the Stanford A
and Stanford B surrogate mixtures. In general, the Ranzi mechanism captures the high-
temperature trend, but fails to predict the low temperature behavior of ignition delay
30
times. Due to the simplicity of the Violi #3 surrogate mixture and the closeness to data
when used with the Ranzi mechanism (compared to other surrogates at high T), this
combination will be used in the later sections of this chapter in evaluating trends in
experimental data.
0.8 0.9 1.0 1.1 1.210
100
1000
10000
833K
All data scaled to
20 atm using P-1
Igni
tion
De
lay
Tim
e [
s]
1000/T [1/K]
Jet-A/air JP-8/air Violi et al. #3 Violi et al. #1 Stanford A Stanford B neat n-Dodecane
1250K
Figure 3.6 Jet fuel ignition delay times for phi=1, and the Ranzi mechanism predictions
for different surrogate fuel mixtures listed in Table 3.4.
Further experimental work including key species time-history measurements is needed
to provide more comprehensive kinetic targets for refining the mechanisms and surrogate
mixtures. The current comparison study suggests that a better surrogate composition
and/or improved model are particularly needed for simulating combustion behavior of jet
fuels at low temperatures. With regard to improving the surrogate model, adding more
aromatic compounds to the surrogate mixture might be one approach to slow down the
chemistry in the low temperature region, so the mechanism predictions will not roll-off so
early when compared to the data. Previously, many researchers have theoretically
explained and experimentally confirmed the fact that addition of aromatics has an
inhibiting effect on the chemistry of alkanes [21]. However, aromatics contribute
significantly to pollutant formation, including soot, and thus have to be modeled with
care. Moreover, it is important to note that pollutant or soot emissions often depend on
trace fuel species and other additives present in the real fuel, and will not, in general, be
reproduced by a simple chemical surrogate [6].
31
3.3.5 Effect of Variation of Equivalence Ratio and
Oxygen Concentration
Ignition delay time correlations (τign as a function of T, P, XO2, and Ф) are widely
used in codes for modeling engines, and by experimentalists to enable comparison of
findings from different studies that might have been conducted with different reaction
conditions [102]. However, due to the complexities associated with variations in jet fuel
compositions, such an attempt will not be made in the present work. Instead, the results
of jet fuel ignition delay time experiments with varying equivalence ratio and oxygen
concentrations are presented separately in this section.
Jet fuel experiments with equivalence ratios of Φ=1.0 and Φ=0.5 are compared in
Figure 3.7. A clear trend of longer ignition delay times for the lean (Φ=0.5) as compared
to the stoichiometric case (Φ=1.0) is noticed. Gauthier et al. [39] observed similar trends
in gasoline/air mixtures for the same equivalence ratios. The reason for this trend with Φ
is explained in detail by Curran et al. [113] in a comprehensive modeling study of
oxidation using iso-octane. In effect, the higher the concentration of fuel in the mixture
(for nearly constant O2 concentration), the faster the ignition process become. Curran et
al. [113] found that at temperatures below approximately 1150K, increasing Φ, i.e.
increasing the fuel concentration, increases the alkyl-hydroperoxide radical pool
production and results in shorter ignition delay times. After the initial fuel decomposition
steps, the increase in the radical pool concentration is responsible for the rapid reactions
associated with ignition. The Ranzi mechanism predictions using the Violi #3 surrogate
mixture are also presented in Figure 3.7. The mechanism with this surrogate model
consistently overpredicts the ignition delay times for both the lean and stoichiometric
cases, but the trend with equivalence ratio is captured at higher temperatures (T >1000 K).
Results of experiments in jet fuel/air mixtures with different oxygen concentrations
(XO2=0.10 and 0.207) are presented in Figure 3.7; the balance of gas mixture is N2 in
both cases. The low scatter data for 10 % O2 clearly show longer ignition delay times,
compared to the 20.7 % O2 case. In both the low and high O2 concentration cases, the
fuel/air mixtures are stoichiometric. In effect, the lower the O2 concentration the lower
the fuel concentration, and following Curran et al. [113], the longer the ignition delay
32
times. The Ranzi et al. mechanism predictions using the Violi et al. #3 surrogate mixture
(Figure 3.7) is able to capture the trend in the data for the effect of oxygen concentration
on the ignition delay time, and the model predictions are in good quantitative agreement
with the limited data for the 10% O2 case. However, as mentioned in the previous section,
the simulations do not recover the roll-off in ignition time observed at low temperatures
for the 20.7% O2 case.
0.8 0.9 1.0 1.1 1.2
100
1000
10000833 K
Data scaled to
20 atm using P-1
Igni
tion
Del
ay T
ime
[s]
1000/T [1/K]
Jet-A/air, =1.0 JP-8/air, =1.0 Jet-A/air, =0.5 Ranzi et al. =1.0 Ranzi et al. =0.5
1250 K
0.8 0.9 1.0 1.1 1.2
10
100
1000
10000
20.7% O2
833 K
Data scaled to
20 atm using P-1
Igni
tion
Del
ay T
ime
[s]
1000/T [1/K]
Jet-A/air (20.7% O2)
JP-8/air (20.7% O2)
Jet-A/ 10% O2/ N
2
Ranzi et al.
1250 K
10% O2
Figure 3.7 Jet fuel ignition fuel ignition delay times: the effect of phi and XO2.
3.3.6 Low-temperature (NTC Region) Ignition
Delay Time Data
The conventional shock tube driver gas is helium, since the strong incident shock
waves desired for high test gas temperatures (T5) are easily achievable with helium. But
when helium is used as a driver gas, the available test times are typically limited to about
2 ms in the HPST. Available test time is defined as the time interval between the arrival
of the reflected shock at the sidewall location (10 mm from endwall) and the arrival at
that same location of a significant pressure disturbance, usually developed from an
internal shock reflection from the contact surface or from a rarefaction wave propagating
from the end of the driver section. Low-temperature ignition delay time data requires
longer test times than are normally available with conventional shock tube operation.
To increase our shock tube test times, the driver and driven gas mixtures were
“tailored” to allow study of the combustion chemistry at low-to-intermediate
33
temperatures (see, for example, Amadio et al. [114]). Using pure N2 as the driven gas, a
series of experiments were performed with different He/N2 driver gas mixtures (20-30 %
N2 in He) in the low temperature region from 700-900 K in the HPST (for P5 of
approximately 20 atm). Test times in excess of 4 ms were readily achieved. Sample
pressure traces for a non-reactive (N2) case and ignition traces for a reactive (Jet-A/air)
case are shown in Figure 3.8, along with the OH* record.
0 1000 2000 3000 4000
0
20
40
60
80
pressure
Pure N2
770 K, 19.0 atm
Pre
ssu
re
[atm
]
Time [s]
jet-A/air, =1.0774 K, 19.4 atm
Ignition time = 2570 s
OH*
0.8 0.9 1.0 1.1 1.2 1.3 1.410
100
1000
10000
jet-A/air, =1.0 Ranzi (Violi 3 surrogate)
909K 769K
Igni
tion
De
lay
Tim
e [
s]
1000/T [1/K]
1110K
P~20atm
Figure 3.8 Left: low-temperature pressure and emission traces (Driver gas: He 70%, N2
30 %). Right: ignition delay times including NTC region data, (Jet-A/air, phi=1).
The ignition delay time variation with initial reflected shock temperature, normalized
to 20 atm (assuming τign ~ 1/P), is presented in Figure 3.8. At these low temperatures,
ignition delay times are usually long (> 2 ms) and commonly show NTC type behavior,
in which the ignition delay time remain nearly constant or become shorter as the
temperature decreases. The Ranzi mechanism prediction using the Violi et al. #3
surrogate mixture, rolls off much earlier, but later catches up, retaining the primary
feature of the data. At very low temperatures (below 700K), ignition delay times again
increase with decreasing temperature, as seen both in the data and the model results.
The pressure-time history data below 800 K in the NTC region show mild pre-ignition
heat release in Jet-A/air mixtures at pressures close to 20 atm. This is evident in the
pressure and emission traces for the case shown in Figure 3.8 (seen as slow, exponential
growth to strong ignition). Since the test gas behind reflected shock waves behave like a
constant volume reactor for weak to moderate energy release, pre-ignition pressure rises
are related to rapid changes in chemistry with relatively small concomitant energy release
34
(Davidson and Hanson [102]). In the current experiments, all the pressure-time history
data look similar to that in Figure 3.1 for temperatures higher than 800 K.
Modeled pressure traces, by the Ranzi mechanism using the Violi et al. #3 surrogate
mixture (not shown here), do not show this slow, exponential growth in pressure.
Nevertheless, for temperatures below 900 K, the Ranzi mechanism predictions show a
two-stage ignition behavior (pressure smoothly rises to an intermediate plateau before
ignition). In the case of iso-octane/air mixtures, Davidson et al. [115] suggested the
importance of using accurate values of the heats of formation and rate coefficients of
various peroxy species, which dominate the pre-ignition heat release stage of combustion.
For long test times, at a fixed location from the end-wall of the shock tube, and in the
absence of reaction, the reflected shock pressure gradually increases, approximately
linearly with time. This is caused by non-ideal effects such as incident shock attenuation,
boundary layer growth, and interaction of the reflected shock wave with the side wall
boundary layer. Petersen and Hanson [116] confirmed that an isentropic relation between
pressure and temperature can be used to estimate the temperature increase from pressure-
time histories in the same facility. Comparison between pressure increases in non-
reactive (pure N2) and reactive (Jet-A/air, Φ=1.0) mixtures, shows a higher pressure
increase with time in the latter case due to the pre-ignition energy release caused by
chemical reactions. In the non-reactive case, for the temperature range from 700-850 K at
a pressure close to 20 atm, a simple relation could be established for the reflected shock
pressure increase and the incident shock attenuation. At the current side-wall location (10
mm from end-wall), a linear relation between change in reflected pressure with time
(dP/dt) and the incident shock attenuation rate (%/m). This relation could be used to
estimate the total temperature increase due to non-ideal effects from the arrival of
reflected shock wave till the onset of ignition, even in the case of reactive mixtures. In the
current experiments for Jet-A/air ignition delay times below about 4 ms, the maximum
increase in temperature was estimated to be only 21 K (using the isentropic assumption).
However, as seen in Figure 3.8, it is to be noted that ignition delay times are relatively
insensitive to small variations in the temperature in the NTC region as the ignition delay
times in this region are nearly constant. Hence the constant U, V modeling is justified in
this special case to model data.
35
It is clear that additional studies of surrogate blends, as well as studies of the effects of
single reference components on the mixture ignition, are needed to support the jet fuel
surrogate development. In the subsequent chapters of this thesis, major single reference
components present in jet fuels will be studied to understand their ignition chemical
kinetics and the measurement of species concentration time-histories will be conducted,
particularly, for important radicals such as OH that can be used to validate assumptions
made about the internal structure and sub-mechanisms of these large reaction
mechanisms.
36
Table 3.1 Summary of Jet-A/air ignition data at phi=1.0. Jet-A Composite Blend
#04POSF4658. (Dr. = driver gas mixture; Vshock = incident shock velocity at the endwall).
Jet-A/air, phi=1.0, Jet-A=1.276 %, O2=20.74 %, N2=77.98 %, Dr.= He P1, (psi) T1, (ºC) Vshock,(mm/us) T5,(K) P5, (atm) ign, (us)
17.19 99.6 0.792 874 23.3 3109 17.06 90.2 0.801 880 25.1 2355 19.39 99.8 0.836 934 31.4 1230 13.91 97.5 0.850 952 24.0 1358 12.79 96.9 0.854 958 22.5 1287 12.19 99.8 0.857 963 21.7 1412 17.46 94.6 0.866 974 32.5 777 11.34 98.5 0.874 987 21.3 984 16.79 99.8 0.880 997 32.1 641 16.93 92.0 0.901 1022 36.1 400 9.83 89.8 0.910 1035 21.9 539 9.97 93.9 0.914 1042 22.0 486 12.37 100.1 0.931 1073 28.3 278 9.38 96.6 0.971 1130 24.8 138 11.00 99.9 0.979 1145 29.4 101 9.13 98.7 1.010 1192 27.0 52 9.40 99.5 1.027 1220 29.2 34 28.51 101.1 0.854 961 49.3 528 25.98 101.0 0.888 1009 50.9 286 21.31 100.9 0.942 1090 50.5 115 18.73 101.0 0.965 1124 47.8 80 Jet-A/air, phi =1.0, Jet-A=0.615 %, O2=10 %, N2=89.38 %, Dr.= He 11.28 101.0 0.880 1025 18.9 1820 9.67 100.5 0.901 1057 17.5 1340 10.67 101.0 0.907 1067 19.7 1109 9.45 100.6 0.911 1072 17.7 1137 9.54 99.4 0.917 1081 18.3 1005 10.21 98.4 0.924 1092 20.2 818 8.70 101.0 0.935 1111 17.7 732 8.29 101.0 0.943 1124 17.3 646 8.46 101.0 0.945 1127 17.7 569
Jet-A/air, phi =1.0, Jet-A=1.276 %, O2=20.74 %, N2=77.98 %, Dr.= He/N2 28.77 100.8 0.669 715 21.7 3286 24.66 101.0 0.699 753 21.8 2584 23.46 100.9 0.707 763 21.5 2423 20.30 101.0 0.715 774 19.4 2566 21.15 100.6 0.724 785 21.1 2484 19.70 100.6 0.725 786 19.7 2742 19.16 100.8 0.742 808 20.8 2628
37
Table 3.2 Summary of jet fuel ignition data indicating variation with fuel type (jet-A and
JP-8), equivalence ratio, and fuel sample. (Dr. denotes driver gas mixture; Vshock is the
incident shock velocity at the endwall).
JP-8/air, phi =1.0, JP-8=1.276 %, O2=20.74 %, N2=77.98 %, Dr.= He
P1, (psi) T1, (ºC) Vshock,(mm/us) T5,(K) P5, (atm) ign, (us) 20.49 100.2 0.818 910 30.9 1532 12.12 100.4 0.846 949 20.4 1504 11.87 96.7 0.856 960 21.0 1305 10.91 94.7 0.892 1011 22.3 685 15.29 100.4 0.894 1017 30.7 475 9.63 100.5 0.895 1019 19.4 674 10.91 98.6 0.896 1019 22.4 616 8.35 100.5 0.931 1073 19.1 373 7.76 97.7 0.953 1104 19.3 259 7.19 99.7 0.965 1124 18.4 191 7.29 99.4 0.968 1128 18.9 190 7.06 99.2 0.980 1146 19.0 145 Jet-A/air, phi =0.5, Jet-A=0.642 %, O2=20.87 %, N2=78.48 %, Dr.= He (Jet-A Composite Blend #04POSF4658) 13.20 101.4 0.841 970 19.6 1961 12.08 101.4 0.882 1032 21.0 917 11.31 101.5 0.886 1039 19.9 873 10.27 101.8 0.913 1081 19.9 530 11.45 101.6 0.913 1081 22.2 463 9.20 101.7 0.978 1187 22.1 110 Jet-A/air, phi =1.0, Jet-A=1.276 %, O2=20.74 %, N2=77.98 %, Dr.= He (Jet-A fuel same as Dean et al. [9]) 12.41 83.2 0.890 1000 26.4 973 9.86 83.8 0.957 1100 26.2 208 8.85 83.2 0.981 1137 25.5 123 7.44 83.0 0.986 1145 21.8 131 8.43 83.2 1.004 1172 26.1 72 6.93 84.1 1.013 1187 21.9 70 6.94 84.1 1.017 1194 22.2 64 6.92 84.1 1.039 1229 23.7 28
38
Table 3.3 Jet fuel surrogate mechanisms and their associated surrogate mixtures.
Kinetic Mechanism, Surrogate Mixture
Lindstedt and Maurice [112] Dagaut and Cathonnet [8]
Surrogate Mixture Composition
n-decane 89% benzene, (or) toluene, (or)ethyl benzene
(11%)
n-decane 74% n-propylbenzene 15% n-propylcyclohexane 11%
Table 3.4 Jet fuel surrogate mixtures used with the Ranzi [48] and the Zhang et al. [19] mechanisms.
Kinetic Mechanism
Ranzi [48], Zhang et al. [19]
Surrogate Mixture
Violi #1 (Violi et al. [5])
Violi #3 (Violi et al. [5])
Stanford A
Stanford B
Surrogate Mixture Composition
MCH 20% m-xylene 15% tetralin 5% iso-octane 10% n-dodecane 30% tetradecane 20%
MCH 10% toluene 10% benzene 1% iso-octane 5.5% n-dodecane 73.5%
MCH 10% toluene 10% benzene 1% iso-octane 25% n-dodecane 54%
MCH 10% toluene 29.5% benzene 1% iso-octane 5.5% n-dodecane 54%
39
Chapter 4: High-pressure Single-
Component Surrogate Ignition Times This chapter describes ignition delay time measurements in single-component jet fuel
surrogates using the heated HPST. Normal and cyclo alkanes are the two most important
chemical classes found in jet fuels as mentioned in the previous chapter. Accordingly,
high-pressure ignition experiments are conducted in two commonly used representative
components for normal and cyclo alkanes in jet fuel surrogates (i.e., n-dodecane and
MCH, respectively). n-Dodecane/air ignition was studied for the following shock
conditions: temperatures of 727-1177 K, pressures of 18-34 atm, equivalence ratios of 0.5
and 1. Experimental conditions in MCH/air (phi=1.0) were in the range T=795-1100 K
and P=17-49.1 atm. Detailed comparisons of experimental data with predictions of
available n-dodecane and MCH mechanisms are also presented.
4.1 Experimental Method
All experiments were carried out in the reflected shock region of the HPST. The
method and diagnostics were similar to that described in previous chapters. Helium was
used as the driver gas in most cases except to access the low-temperature regime (700-
850 K), where tailored driver mixtures of 20-30% N2 in He were used. In the case of n-
dodecane, mixtures of research grade (≥ 99%) n-dodecane (Sigma-Aldrich) and high-
purity grade(> 99.995%) gases (Praxair N2, O2, and Ar) were prepared. The shock tube
was heated to 105 C for experiments to measure ignition times in synthetic air (79% N2,
21% O2), and the fuel/oxidizer mixing facility was kept at 135 C in both cases. This
heating prevented condensation of the fuel during the mixing and shock tube filling
procedure. The shock tube can be heated to lower temperatures as the partial pressure of
n-dodecane is lower than in the mixing tank and care was taken to verify that this method
has no effect on kinetics findings. Mixtures of research grade (≥ 99.5% pure) MCH (from
Sigma-Aldrich) and high-purity synthetic air (Praxair 79% N2 and 21% O2, >99.999%)
were prepared. In the MCH experiments, the mixing tank and connecting gas lines to the
shock tube were heated to 110 C (where the fuel is at a higher partial pressure) and the
40
shock tube was used both in an unheated and heated (105 C) configuration (where the
pre-shock partial pressure of the fuel is lower) to check the influence of shock tube
heating on the measured ignition delay times.
The onset of ignition from the pressure history and OH* emission were defined by
locating the time of steepest rise and linearly extrapolating back in time to the pre-
ignition baseline. The two methods give very similar results (± 1%) and ignition delay
times are readily identifiable with both diagnostics (see Figure 4.1 for a sample ignition
data from HPST for n-dodecane and MCH). The overall uncertainty in τign was ±10%,
dominated by uncertainties in the determination of T5 (±1%).
0 400 800 1200 1600 2000 2400
0
20
40
60
80
100
120
OH*emission
Pressure
You et al.
Ranzi et al.
ign
=1245s
n-dodecane/air=0.5T
5=996K
P5=20.3 atm
Pre
ssur
e, [a
tm]
Time, [s]
-200 0 200 400 600 800 1000 1200
0
40
80
120
160
200
Pressure
OH* emission
ign
=1185 s
MCH/air, =1.0T=918 K, P=46.4 atm
Time, [s]
Pre
ssur
e, [
atm
]
Figure 4.1 Example ignition data from HPST. Left: n-Dodecane/air. Right: MCH/air.
In the case of n-dodecane, comparisons with predictions of four recent detailed kinetic
mechanisms, one n-dodecane mechanism and three JP-8 mechanisms where n-dodecane
is an important surrogate component are provided: Ranzi [48] JP-8 semi-detailed
mechanism (280 species, 7800 rxns.); Zhang et al. [19] JP-8 detailed mechanism (208
species, 1087 rxns.); Montgomery et al. [117] reduced JP-8 mechanism (94 species, 675
rxns.); and the You et al. [118] (which is now part of the JetSurF 1.0 [46] surrogate
mechanism) n-dodecane detailed mechanism (177 species, 1318 rxns.). Similarly, in the
case of MCH, comparisons are made with the predicted ign using three recent
mechanisms: 1) the Pitz et al. [22] MCH mechanism, 1001 species, 4436 rxns., 2) the
Orme et al. [36] MCH mechanism, 190 species, 904 rxns., 3) the Ranzi [48] JP-8
surrogate mechanism, 280 species, 7800 rxns. The kinetic implications of these
41
comparisons, particularly in the low-temperature negative-temperature-coefficient (NTC)
region where the role of non-ideal facility effects can be significant, are discussed.
4.2 n-Dodecane/Air Ignition: Results and Discussion
4.2.1 High-pressure n-Dodecane Ignition Data
The ignition delay time measurements and modeling for n-dodecane/air for phi=1 and
0.5 are shown in Figure 4.2. All data are summarized in Table 4.1 and Table 4.2. As the
current study is over a limited pressure range (mainly 18-23 atm), data are normalized to
20 atm using a τign ~ 1/P scaling in all figures (unless mentioned otherwise). As discussed
in the previous chapter, this is a good first approximation for jet fuels and surrogate
components for similar fuel concentrations, temperature and pressure. However, the
actual pressure dependence of τign is expected to vary with temperature, particularly at the
lower temperatures, and further studies are needed to more completely characterize this
dependence. All data points are plotted in Figure 4.2 at the value of T pertaining
immediately after reflected shock heating.
The data are characterized by small scatter and show a non-monotonic variation with
temperature. At high temperatures (above ~ 1000 K) ignition delay times were observed
to decrease as temperature is increased. However, at low temperatures, τign shows NTC-
type behavior, in which τign remains nearly constant or decreases as the temperature
decreases. A distinct variation of ignition delay time with equivalence ratio (phi) is also
seen. Ignition delay times for the lean (=0.5) case are consistently longer than for the
stoichiometric measurements (=1.0).
42
0.9 1.0 1.1 1.2 1.3 1.4100
1000
100001176 K 1000 K 833 K 714 K
1000/T, [1/K]
Igni
tion
Del
ay T
ime,
s
]
=0.5 =1.0 You et al. Ranzi et al. Montgomery et al. Zhang et al.
n-Dodecane/air20 atm
0.9 1.0 1.1 1.2 1.3 1.4
100
1000
10000
Current Study You et al. (2007) Zhang et al. (2007) Ranzi et al. (2006) Montgomery et al. (2007)
1176 K 1000 K 833 K 714 K
1000/T, [1/K]
n-Dodecane/air20 atm, =1.0
Ign
ition
Del
ay T
ime,
s
]
Figure 4.2 n-Dodecane/air τign data (20atm) and predictions for =0.5 (left) and1.0 (right).
4.2.2 Kinetic Modeling Prediction
The current data set provides critical validation targets for n-dodecane and jet fuel
surrogate mechanisms. Ignition time predictions of the detailed You et al. [118]
mechanism along with the two semi-detailed (partially lumped) JP-8 mechanisms of
Ranzi [48] and Zhang et al. [19] (Zhang et al. mechanism was not optimized for low
temperature modeling), and an optimized reduced mechanism of Montgomery et al. [117],
are presented in Figure 4.2 using the constant U,V calculation method (CHEMKIN 4.1.1
[101]).
Both the Ranzi and the Montgomery et al. mechanisms predict similar results, and
give relatively close agreement in magnitude with data (at both phi’s), and are able to
qualitatively capture the negative temperature coefficient-type (NTC) roll-off seen in data.
As discussed in the previous chapter, the Ranzi mechanism applied to the Violi et al. [5]
JP-8 surrogate mixture (73.5% by volume of this surrogate is n-dodecane) predicts a roll-
off trend similar to that seen in our measured ignition delay times near 20 atm for Jet-
A/air mixtures. However, in that case, the Ranzi mechanism predicts stronger roll-off
than observed in Jet-A/air data, consistent with the current situation for n-dodecane
(Figure 4.2). The You et al. detailed n-dodecane mechanism is less successful overall but
provides reasonably good agreement with the data at T above 840K for both equivalence
ratios. Note that all four mechanisms show little difference in simulated ignition delay
times between the two phi’s at the highest temperatures of the current study (near 1200
K), for the 20 atm pressure condition.
43
Sensitivity analysis at 20 atm for stoichiometric n-dodecane/air using the You et al.
mechanism confirms the importance of RO2 chemistry below about 1000K during pre-
ignition and the importance of n-dodecane peroxy and hydro-peroxy radical
isomerization chemistry during the initial fuel breakdown steps during the induction time
(specifically at 833K). At high temperatures (1200K), H2O2 chemistry plays a dominant
role in n-dodecane/air ignition.
4.2.3 Effect of Pressure-time Histories
To simulate the potential effect of facility-related pressure rise on measured and
modeled τign, a CHEMSHOCK [103] calculation with the You et al. mechanism using an
extreme case of a facility-dependent pressure rise of 10%/ms (see Figure 4.3). For the
current HPST experiments, the observed dP5/dt was typically between 1 to 10% /ms
(which may also include the effect of pre-ignition energy release due to chemistry). From
Figure 4.3, it is evident that there is no significant difference between the constant U, V
model calculation and the CHEMSHOCK model calculation, which includes this facility-
dependent dP5/dt, for temperatures above 1000 K. At temperatures between about 1000
and 850 K, the fluid-mechanically more accurate CHEMSHOCK model predicts ignition
times that are up to 30% longer than the constant U, V model and results in a model
prediction that is in better agreement with current data. The disagreement between the
CHEMSHOCK (and constant U,V) simulations and the measured ignition delay times
below 800K is currently attributed to uncertainties and limitations of the kinetic
mechanism. Specifically, below 800 K, improved knowledge of the NTC chemistry (such
as the peroxy isomerizations) is needed.
Evidence of pre-ignition pressure rise is seen at low temperatures in the data set. In
Figure 4.3, the measured pressure profiles below 900 K show a perceptible pre-ignition
induction time, then a step change to an intermediate plateau, and then finally, a second
large rise at the point of actual ignition. This pre-ignition induction time appears to be
longer at both high (869 K) and low (727 K) temperatures and has a minimum in the
range 773-818 K. Additionally, the relative magnitude of this pre-ignition pressure step
increases (almost linearly) as temperature decreases. This behavior differs from the
measured pressure profiles of Jet-A (previous chapter), which show only mild pre-
44
ignition heat release at temperatures below 800 K. Modeled pressure traces using the You
et al. mechanism (not shown here), also show this pre-ignition behavior at temperatures
below 975K for Ф=1 at 20 atm for n-dodecane/air.
0.9 1.0 1.1 1.2 1.3 1.4100
1000
10000
Current Study You et al. CHEMSHOCK You et al. Const U,V
1176 K 1000 K 833 K 714 K
1000/T, [1/K]
n-Dodecane/air=1.0
Ign
itio
n D
ela
y T
ime
, s
]
0 200 400 600 800 1000 1200
0
10
20
30
40
50
957K
822K
869K
727K
Pre
ssu
re,
[atm
]
Time, [s]
n-dodecane/air, =1
773
K
Figure 4.3 Left: n-Dodecane /air τign results for phi=1.0. CHEMSHOCK modeling was
conducted using the You et al. mechanism with an extreme-case linear pressure rise
(dP5/dt) of 10% per milli-second. Right: Measured n-dodecane/air pressure-time histories
near 20 atm, phi=1.0.
4.2.4 Comparison of n-Dodecane Ignition Times
with Jet-A and Other n-Alkanes
The current τign measurements for n-dodecane/air are compared with the τign data for
Jet-A/air acquired in the previous chapter and are shown in Figure 4.4. At temperatures
above about 950 K and for both phi’s, τign of these fuels do not show significant
differences. Below this temperature, the n-dodecane ignition times are shorter (compared
to Jet-A), and this difference is largest for the case of phi=1. Thus a multi-component jet
fuel surrogate (instead of neat n-dodecane) would likely be needed to accurately
reproduce the τign data in the NTC region (< 950K), as suggested in the previous chapter.
However, these results also imply that using n-dodecane as a single component surrogate
for jet fuels may be adequate to represent high-temperature combustion conditions. This
similarity between n-dodecane and jet fuels has been observed earlier in other types of
experimental studies: Eigenbrod et al. [14] observed the similarity between n-dodecane
45
and kerosene droplet induction times, and Holley et al. [16] found that extinction strain
rates of n-dodecane and jet fuel non-premixed flames are very close.
Comparison of 3 different n-alkane ignition delay times are also presented in Figure
4.4 (all data scaled to 20 atm using τign ~1/P dependence) for phi=1.0 in air. n-Heptane
ignition times in the NTC region near 20 atm were measured in the range 690-950 K in
this work in order facilitate comparisons between n-alkanes for a range of temperatures.
Note that Figure 4.4 includes measurements in n-decane by Pfahl et al. [119] (which was
at 12.8 atm and was not from the HPST) and previous high temperature n-heptane data
from Gauthier et al. [39]. Omitting the fact that the n-decane measurements near 850 K
are close to those of n-heptane, in the entire temperature regime, n-heptane ignition times
are consistently longer than those of n-decane (which in turn is longer than those of n-
dodecane). A general trend is clearly seen: there is a slight trend where the ignition delay
time increases as the length of the n-alkane chain decreases at post-shock temperatures in
the entire temperature regime from (650-1300 K).
Within the experimental uncertainties, the above observation is not consistent with the
modeling conclusions made recently by Westbrook et al. [47] (also shown in Figure 4.4).
Westbrook et al. [47] concluded that in a shock tube at stoichiometric conditions, any of
these n-alkanes from n-heptane to n-hexadecane can be used to substitute for any other,
with equivalent results. They also cited the success of n-heptane as a model diesel
surrogate (despite its smaller size than conventional diesel components); n-heptane
ignition rate is close enough to those of real diesel fuels, whose ignition rate and cetane
number are established by its large n-alkane components, that diesel ignition is quite well
reproduced by n-heptane. However, current work (Figure 4.4) indicates the following: n-
dodecane is not a good surrogate for jet fuels in the NTC region, n-heptane similarity
with diesel might be fortuitous, and calls for caution while making kinetic conclusions
using simple surrogate to represent practical fuels.
46
0.8 0.9 1.0 1.1 1.2 1.3 1.410
100
1000
10000
833 K
1000/T, [1/K]
Igni
tion
De
lay
Tim
e, s
]
Jet-A/air, =0.5 Jet-A/air, =1.0 n-dodecane/air, =0.5 n-dodecane/air, =1.0
1250 K 714 K1000 K
Figure 4.4 Comparison of high-pressure n-dodecane ignition delay times at 20 atm. Left:
with Jet-A/air at two equivalence ratios. Right: with n-heptane/air (current work) at
=1.0. Dashed lines are fit through data. Solid lines are Westbrook et al. [47] (LLNL)
modeling prediction.
4.3 MCH/Air Ignition: Results and Discussion
4.3.1 High-pressure MCH Ignition Data
All high-pressure τign data for MCH/air (Mixture: MCH=1.96%, O2=20.60%,
N2=77.44%, =1.0) in the range T=795-1098 K and P=17.2-49.2 atm are summarized in
Table 4.3. The high-pressure τign data for two pressures near 20atm and 45atm, both for
an equivalence ratio of 1, are plotted in Figure 4.5. In all figures, data labeled as heated
refers to experiments when the shock tube was kept at 105 ºC, otherwise the shock was
kept at room temperature (T1 = 22.5 ºC). There does not appear to be a significant
difference in τign measurements when the shock tube was heated to T1 = 105 ºC or when
shock tube was kept at room temperature (T1 = 22.5 ºC). This also supports our
observation from the previous chapter, where ignition delay time variation with T1 in the
case of jet fuel was found to be negligible. Current data are characterized by small scatter
and we have used the experimentally found (τign ~ P -0.87) relation to scale τign to nominal
pressures in Figure 4.5. It should be noted that the pressure dependence of the ignition
delay time is expected to vary with temperature and researchers have used different
pressure scaling in different temperature regimes for similar hydrocarbons [120].
Accordingly, the current data follows τign ~ P -0.75 dependence above 912K (without
47
including the NTC region data points). Due to the changing activation energy in the data,
a standard Arrhenius regression expression of current data is not possible, however, in the
high-temperature region above 912K, an overall activation energy of 24.9 kcal/mol was
obtained using the Arrhenius expression. In Figure 4.5, at low temperatures (less than
about 880 K), ignition delay times show negative-temperature-coefficient-type (NTC)
behavior.
0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.310
100
1000
10000
769K1000K
MCH/air 45atm 45atm (heated HPST) 20atm (heated HPST)
Igni
tion
De
lay
Tim
e, [s
]
1000/T, [1/K]
Modeling, 20atm Orme et al. Ranzi et al. Pitz et al.
MCH/air, =1.0
1.5atm data
1667K
0.8 0.9 1.0 1.1 1.2 1.3 1.4100
1000
10000
714K833K1000K1250K
Orme et al. Ranzi et al. Pitz et al.
MCH/air, 45atm, =1.0
1000/T, [1/K]
Igni
tion
De
lay
Tim
e, [
s]
Figure 4.5 Left: High-pressure MCH/air ignition delay time results (=1.0, MCH=1.96%,
O2=20.60%, N2=77.44%). High-pressure data scaled to 20, or 45 atm using τign ~ P-0.87.
Low-pressure data scaled to 1.5atm using Vasu et al. [121] correlation (=1.0,
MCH=1.962 %). Modeling shows τign predictions at 20 atm. Right: High-pressure
MCH/air ignition delay time results near 45 atm (=1.0, MCH=1.96%, O2=20.60%,
N2=77.44%) and pressure scaling. Data (solid symbols) scaled to 45 atm using τign ~ P -
0.87. Grey solid line is a fit through data. Constant U,V modeling results at 20 and 45 atm
are shown using Orme et al., Ranzi et al., and Pitz et al. mechanisms.
Also shown in Figure 4.5 are the low-pressure data from the Vasu et al. [121] study
extrapolated to the conditions of 1.96% MCH, phi = 1.0, 1.5 atm. Current data have
significantly extended the range of high-temperature MCH ignition delay times. Also, the
near-unity pressure dependence (P-1) of the lower-pressure data (P -0.98 from Vasu et al.
[121] empirical correlation) and the current higher-pressure data (P -0.87) extends up to
1560 K. Additionally, current results complement earlier ignition time studies of MCH by
Pitz et al. [22] in a RCM (see Figure 4.6) and the combined data sets provide kinetic
48
targets in the range from 650-1560K. The RCM τign are less reproducible (have more
scatter than the current high-pressure data) when time scales reach near 50ms; see, for
example, the observations of Dooley et al. [122]. By examining all the data from 45 to
10 atm, it is evident that in the intermediate temperature range near 850 K, the pressure
appears to have the most pronounced influence on the measured ignition delay times.
However based on the RCM measurements, at low temperatures near 700 K, ignition
times are almost independent of pressure.
0.8 1.0 1.2 1.4 1.6
100
1000
10000
100000
3
2
625K714K833K1000K
Igni
tion
Del
ay T
ime
[s]
1000/T [1/K]
current data 20 atm (heated) 45 atm 45 atm (heated)
Pitz et al. RCM 10 atm 15 atm 20 atm
1250K
MCH/air, =1.0
1
0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.610
100
1000
10000
100000
769K1000K
=1.0 625K
Ign
itio
n D
ela
y T
ime
[s]
1000/T [1/K]
current data 20 atm (heated) 45 atm 45 atm (heated)
Pitz et al. RCM 10 atm 15 atm 20 atm
1667K
MCH/air1.5atm
Figure 4.6 Current high-pressure MCH/air (=1.0, MCH=1.96%, O2=20.60%,
N2=77.44%) τign results. Left: Current data scaled to 20, or 45 atm using τign ~ P (-0.87).
RCM data (unheated) from Pitz et al. [22]. Lines are constant U,V modeling predictions
using the Pitz et al. [22] mechanism: 1) 10 atm; 2) 20 atm; and 3) 45 atm. Right: HPST
data scaled to 20, or 45 atm using τign ~ P (-0.87); RCM data (unheated) from Pitz et al. [22];
Solid lines are fit through data at respective pressures. Low-pressure data scaled to
1.5atm using Vasu et al. [121] correlation (=1.0, MCH=1.962 %).
It should be noted that the Pitz et al. [22] RCM measurements used 3 different diluents
(100% N2, 50%N2:50% Ar, and 100% Ar) to access their temperature range. According
to a recent study by Würmel et al. [123], the choice of diluent gas affects RCM τign
measurements. Würmel et al. reported that the effect of adding argon to nitrogen
(changing the diluent gases is a standard procedure used to achieve various compressed
temperatures in most RCM facilities) is to decelerate ignition in a RCM at the same
compressed temperature and fuel and oxygen concentration compared to pure N2 as
49
diluent. This increased ignition time is due to the extreme cooling of Ar in the post-
compression period and could be attributed to the effect of the heat capacity of the bulk
carrier gas (see Würmel et al. [123] and Davidson and Hanson [102]). Such an effect
could explain the slightly higher activation energy at high-temperatures (above 833 K)
shown by the RCM data at 10 atm than the current data at 20 atm and 45 atm (see Figure
4.6). Another observation from Figure 4.6 is that as the temperature is decreased,
occurrence of NTC behavior (roll-over) is only slightly delayed, but is stronger, at lower
pressures near 10atm than at 45atm.
4.3.2 Comparison of Various Cyclo-alkanes
Daley et al. [124] recently measured τign in cyclo-pentane/air and cyclo-hexane/air
mixtures at high pressures and high temperatures in a shock tube (their conditions are
similar to current experiments in MCH). A comparison of the high-pressure shock tube
τign results for three cyclo-alkanes/air are shown in Figure 4.7 (for 45atm and =1.0).
Current MCH pressure scaling of τign ~ P -0.87 and Daley et al. [124] pressure scaling of
τign ~ P -1.1 for cyclo-hexane and τign ~ P -0.9 for cyclo-pentane were used in Figure 4.7. It
is interesting to note that the activation energies for these three cyclo-alkanes are nearly
the same, i.e, 23.9 kcal/mol for cyclo-pentane; 24.9 kcal/mol for MCH; and 27.6 kcal/mol
for cyclo-hexane. τign in MCH falls between the data for the other two cyclo-alkanes, and
cyclo-hexane τign values are approximately half of those for cyclo-pentane. This suggests
that reactivity of these fuels is in the order cyclo-hexane>MCH>cyclo-pentane, which
could be attributed to the relative stability of primary cyclo-alkyl radicals formed from
these fuels and their propensity to yield H-atoms (see Sirjean et al. [125] for details on
the high-temperature reactivity of cyclo-alkanes). However, it should be noted that the
above observation is true only in the high-temperature region (above 900K), where the
influence of NTC chemistry is minimal. It is clear from Figure 4.7 that MCH shows
NTC-type behavior at higher temperatures (τign data roll-off at a higher temperature) than
compared to cyclo-hexane at 45 atm. This indicates that propensity for RO2 radical
isomerization (and thereby NTC behavior) is higher in MCH and that MCH would be
more reactive in the NTC region than cyclo-hexane. Similar conclusions in the NTC
50
region can be reached by comparing the RCM ignition delays of MCH (Pitz et al. [22])
and cyclo-hexane (Lemaire et al. [126]), both at pressures near 10atm.
Additionally, at high temperatures and low pressures the activation energy obtained in
argon-dilute mixtures (see Vasu et al. [121]) is almost two times that observed at high
pressures and moderate temperatures (such as results in this section) with air as the
oxidizer. Daley et al. [124] observed a similar difference in activation energy, consistent
with most hydrocarbon fuels in the literature (such as toluene, n-heptane, iso-octane, and
cyclo-pentane), and attributed this difference due to influence from the NTC chemistry in
high-pressure, moderate-temperature studies.
0.8 0.9 1.0 1.1 1.2 1.310
100
1000
10000
Igni
tion
De
lay
Tim
e, [s
]
1000/T, [1/K]
MCH (current) cyclohexane (Daley et al.) cyclopentane (Daley et al.)
45 atm, =1.0fuel/air
Figure 4.7 High-pressure cyclo-alkanes/air (=1.0) τign results. Mixtures: MCH=1.96%,
O2=20.60%, N2=77.44%; cyclo-hexane=2.28%, O2=20.53%, N2=77.19%; cyclo-
pentane=2.72%, O2=20.44%, N2=76.84%. Current MCH data scaled to 45 atm using τign
~ P (-0.87). Cyclo-hexane (scaled using τign ~ P -1.1 and cyclo-pentane (scaled using τign ~ P-
0.9) data from Daley et al. [124]. Solid lines are fit through data.
4.3.3 Kinetic Modeling
Model predictions using the mechanisms of Ranzi [48], Pitz et al. [22] and Orme et al.
[36] are presented in Figure 4.5 for 20 atm and 45 atm, respectively. The Ranzi
mechanism gives the closest agreement and the Orme et al. mechanism predictions are
the farthest from data. In general, all mechanisms predict longer ignition delay times than
experimental results, and none of the mechanisms except the Pitz et al. mechanism
51
exhibits the NTC roll-off behavior. However, at the peak near 870 K (before the NTC
roll-off starts in both the 45 atm data and the Pitz et al. model), ignition delay times
predicted by the Pitz et al. mechanism are approximately 5 times larger than data. At
temperatures higher than 912 K, the global activation energies for ignition and pressure
dependence according to various mechanisms are as follows: 34.0 kcal/mol and τign ~ P -
0.77 (Orme et al.), 29.5 kcal/mol and τign ~ P -0.87 (Pitz et al.), and 32.9 kcal/mol and τign ~ P
-0.76 (Ranzi), whereas the experimental values are 24.9 kcal/mol and τign ~ P -0.75.
Compared to the other two mechanisms, the Pitz et al. mechanism provides slightly better
agreement with the experimental pressure dependence and activation energy. However,
the Pitz et al. mechanism does not predict their RCM measurements very well (see Figure
4.6) using the constant U,V approach, which was used also by Pitz et al. to develop their
mechanism by validating against their RCM data. It should be kept in mind that the
constant U,V approach may be incorrect for RCM environments and various other
approaches (according to Würmel et al. [123], there is currently no off-the-shelf
simulation tool available that allows the realistic description of combustion in a RCM)
have been used in the literature to model RCM experiments mainly due to the heat loss
and compression stroke effects in RCMs (see [26,123] for details on this topic).
Since peroxy chemistry is the dominant channel in the NTC region, current
observations (Figure 4.5) suggest that the MCH sub-mechanism included in the current
detailed Ranzi [48] JP-8 mechanism needs to be improved (in order to accurately model
the current MCH and Jet-A data in the NTC region) by adding or modifying reactions for
the peroxy reaction channels of the MCH oxidation. Of considerable importance is the
finding by Pitz et al. [22] that the RCM ign data in the NTC regime for MCH, where
MCH oxidation proceeds via the peroxy channels, was found to be greatly influenced by
the calculated RO2 (here methylcyclohexylperoxy radical) isomerization rates.
Specifically, the Pitz et al. computations using n- and iso-alkane-based estimates of
methylcyclohexylperoxy radical isomerization rate constants predicted ignition delay
times too long compared to their experiments.
Ignition delay time sensitivity analyses were performed using both the Orme et al. [36]
and the Pitz et al. [22] mechanisms. The key reactions that influence the ignition times
both at 910K and at 1110K (for P=20atm) are nearly the same. Specifically, the formation
52
(via HO2+HO2=H2O2+O2) and decomposition (via H2O2+M=OH+OH+M) are the
dominant reactions influencing τign. Efforts to adjust the mechanisms by modifying these
two reactions k1 and k2 within their uncertainty limits were not conducted; however, the
Tsang and Hampson [127] review estimated an uncertainty factor of 5 for the first
reaction, and very few experimental data exist for the latter one. Additionally, H2O2
formation via H-abstraction reactions from MCH by HO2 radical, i.e.,
MCH+HO2=H2O2+ methylcyclohexyl isomers (3 isomers formed at primary, secondary,
and tertiary sites), have considerable impact on the predictions of mechanisms, and these
rate constants only have been estimated and not measured. Hence, the above mentioned
reactions should be measured directly in order to enable accurate modeling of MCH
ignition times at engine-relevant conditions (such as those presented in the current
chapter). The low-pressure limit of the H2O2+M=OH+OH+M reaction has been recently
measured in our lab [128], however, adopting this value into the Pitz et al. [22]
mechanism did not improve τign predictions at 45 atm.
4.3.4 Effect of Pressure-time Histories
The measured pressure profiles at some conditions, including Figure 4.8, show a weak,
approximately linear pressure ramp starting just before ignition, which is defined as the
ratio DP/Dt in Figure 4.8 (note that this definition of pressure rise in different from dp/dt,
which was defined in the previous chapter). This type of behavior has been observed by
others in various other fuels in high-pressure shock tube ignition studies
[38,40,115,119,129-131] and in RCM ignition studies [132,133] and is currently a topic
of intense research. Note that there is no pre-ignition rise in measured OH* emission
signal (see Figure 4.1) for this case. Various hypotheses [115,129,132,134-141] for this
pressure increase (DP/Dt) have been advanced in literature including chemical heat
release before ignition (either at the measurement location or elsewhere in the reflected
shock region), different modes or regimes of ignition, multi-dimensionality of ignition,
etc. The pre-ignition phenomena (or DP/Dt) may or may not be a result of homogeneous
chemistry alone (here homogeneous refers to uniformity at a given axial location, not
necessarily the entire reflected shock zone), and could be a result of various non-uniform
phenomena (such as axial or even 3-D inhomogeneities which might arise in temperature,
53
radical concentration, or particles in the test gas) acting in addition to chemical heat
release [131,135]. In the shock tube, the main cause of temperature inhomogeneities in
the reflected shock region is likely to be the interaction of the reflected shock wave with
the boundary layer arising from the incident shock [129,142]. Also, in the current
experiments at high-pressures and all temperatures, ignition events could be classified as
“strong ignition” [134,138], which is characterized by large pressure peaks immediately
after the main ignition event and by the following large-amplitude pressure oscillations
seen in Figure 4.8. These observations are consistent with “strong ignition” observations
in n-heptane/air [38] and in iso-octane/air [129] in other shock tube ignition studies and is
not unique to current facility. A detailed discussion on this topic is provided in Appendix.
However, simple constant U,V calculations do not capture the experimentally measured
pressure oscillations caused by the blast wave after ignition (Figure 4.8).
For the current MCH experiments, some qualitative observations can be made about
the nature and magnitude of DP/Dt. Figure 4.9 provides pressure profiles from HPST
experiments for different temperatures (T5) and 2 pressure regions (20 and 45 atm),
illustrating that the DP/Dt values do not show any strong dependency on pressure (i.e., no
variation with P5 when T5 is constant). In addition, it was observed that DP/Dt values are
slightly lower for heated shock tube experiments for the same P5 and T5 compared to
room temperature experiments (not shown here). As T5 increases from 795 K, DP/Dt
values remain approximately constant until about T5=950 K, and increase after that until
T5=1100 K. Consequently, variation of DP/Dt with τign follows the opposite trend, i.e.,
with increasing τign, DP/Dt decreases gradually until τign = 600s and remains nearly
independent of τign thereafter till τign = 2ms (the highest τign for current experiments).
However, at 795K (the lowest temperature near 45 atm, which is labeled as 5 in Figure
4.9) the pressure starts to behave more like a two-stage ignition than showing a linear
DP/Dt as defined in Figure 4.8. It should be also noted that above 950 K, the duration (Dt)
of pressure ramp decreases with increasing temperature in MCH/air mixtures (current
experiments), and this trend is consistent with observations made by Pfahl et al. [119] in
their high-pressure shock tube experiments performed in stoichiometric α-
methylnaphthalene/air mixtures at 12.8 atm.
54
0 1000 2000 3000 4000 5000
0
150
300
450
600
750
0
150
300
450
600
750
Pitz et al.
Ranzi et al.
HPST data
800 1000 1200
30
60
90
DP
Pre
ssu
re [
atm
]Time [s]
Dt
MCH/air, =1.0, T5=918 K, P
5=46.4 atm
Pre
ssu
re [a
tm]
Time [s] 0 10 20 30 4010
20
30
40
50
60
70
80
90725 K
825 K
875 K
775 KPre
sure
, [at
m]
Time, [ms]
MCH/air, =1.020atm
Pitz et al. mechanism
Figure 4.8 Left: Measured and predicted (Ranzi et al. [48], Pitz et al. [22]) P(t) histories.
Right: Computed pressure-time histories for MCH/air ignition at 20atm, =1.0, Pitz et al.
[22] mechanism.
0 400 800 1200 16000
20
40
60
80
100
541
3
Pre
ssu
re, [
atm
]
Time, [s]
1) 1029K2) 1019K3) 962K4) 918K5) 795KMCH/air, =1.0
[A]
2
0 500 1000 1500 20000
10
20
30
40
50
60
MCH/air, =1.0
Time, [s]
Pre
ssur
e, [a
tm]
[B]
912K
937K
1060K
Figure 4.9 HPST pressure-time histories for MCH/air (=1.0) ignition. Left: near 45atm.
Right: near 20atm.
Comparisons of an experimental pressure profile with the Ranzi [48] and the Pitz et al.
[22] model results are presented in Figure 4.8 for a 46.4 atm test condition. These
comparisons serve to confirm that up to the point of ignition, the constant U,V
assumption is a good representation of the shock tube behavior (Davidson and Hanson
[102]). Because of the nature of the constant U,V assumption, predictions by both
mechanisms achieve the same final pressure plateau (at ~ 5 ms).
55
Modeled pressure-time histories for MCH/air at 20 atm using the Pitz et al. [22]
mechanism (Figure 4.8) do not duplicate the experimental DP/Dt; however, a two-stage
pressure increase at low-temperatures (725 K and 775 K) and an approximately linear
increase in pressure P5 at high-temperatures (825 K, and 875 K) are observed. As seen
from Figure 4.8, in modeled pressures, typically even with the simple constant U,V
assumption, the P5 rises to about 25 atm (starting from 20atm) before ignition due to pre-
ignition chemical heat release alone.
The effect of DP/Dt on τign may need to be taken into consideration when comparing
modeled and experimental results especially at longer ignition delay times [119]. Though
developed to deal principally with facility-dependent non-idealities in shock tubes, the
CHEMSHOCK [103] program from our laboratory can be applied along with the
measured pressure profiles to assess the effect of DP/Dt on the modeled τign. A similar
approach was used by Pang et al. [141] and they concluded that the use of the
experimental pressure trace and the CHEMSHOCK model more accurately modeled the
reflected-shock ignition process in hydrogen than the traditional approach using
CHEMKIN with a constant U,V constraint. This new approach allows for combined
facility-dependent effects and energy release phenomena in the reflected shock
environment. It should be noted that CHEMSHOCK assumes stationary, homogeneous
conditions within the test gas volume monitored but does not require homogeneity
throughout the reflected shock region. The CHEMSHOCK ignition delay times predicted
by the Orme et al. [36] mechanism using the measured pressure profile are only 15%
higher than the experimentally measured τign value at P5 = 20.46 atm and T5 = 912 K (note
that this P5 and T5 represent an extreme case where τign is long). In the current simulations,
the pressure was approximated by assuming a linear extrapolation beyond the
experimental ignition point (with the same slope, i.e., DP/Dt) until simulation showed
ignition. As CHEMSHOCK is a zero-dimensional approach, this latter assumption may
force the modeled gases to artificially ignite faster due to the extended increase in
pressure (and temperature) beyond the actual experimental ignition point. Since the
experimental pressure trace does not differentiate between a rise (i.e., DP/Dt) due to non-
ideal gasdynamics or to energy release from chemical reaction (somewhere in the
reflected shock region), caution should be applied in drawing kinetic conclusions about
56
predictability of MCH kinetic mechanisms when used with the CHEMSHOCK modeling
approach. Efforts are currently underway in our laboratory to develop and validate more
advanced versions (one-dimensional and above) of the CHEMSHOCK program to model
shock tube experiments.
4.4 Comparison of Jet Fuel and Surrogate Component
Ignition
Shown in Figure 4.10 is a comparison of high-pressure shock tube ignition delay times
(all from the Stanford HPST) for =1.0 in air (synthetic: 79% N2 and 21% O2) at 20atm
for jet fuel and important jet fuel surrogate components used in the previous chapter.
Apart from current MCH data (scaled using τign ~ P -0.87), Figure 4.10 includes data for the
following fuels: Jet-A from previous chapter (scaled using τign ~ P -1), iso-octane from
Davidson et al. [115] (scaled using τign ~ P -0.79) and toluene from Vasu et al. [143]
(scaled using τign ~ P -1, respectively), n-heptane (high-temperature data Gauthier et al.
[39] and NTC data in the range 690-900 K from current work), and n-dodecane from
current chapter (scaled using τign ~ P -1). In the high-temperature region (above 960K),
toluene shows the longest ignition time (aromatics are known to be least reactive), and
Jet-A, n-dodecane and MCH have the shortest ignition time. Ignition time results for iso-
octane and n-heptane, which are also primary reference fuels for gasoline and surrogates,
lie in between (note that τign results in iso-octane and n-heptane are very close to those of
gasoline measured by Gauthier et al. [39] in the HPST). n-Dodecane, which is the main
n-alkane component of most jet fuel and has physical and chemical properties very
similar to that of jet fuels, is very reactive for the conditions shown. It should be noted
that the starting temperature for the NTC-type roll-off of MCH is very similar to that of
jet fuels above 800K, and among all the single-component fuels considered here, MCH
has the closest ignition times to that of Jet-A at high pressures. Large n-alkanes (here n-
dodecane), which are main components of jet fuel surrogate mixtures, ignite faster under
mild conditions than small or branched chain alkanes in the NTC region and roll-off
much earlier and exhibit stronger NTC behavior than Jet fuel (see Figure 4.10).
57
Results in Figure 4.10 support conclusions from the previous chapter that in the high-
temperature region, using a single-component surrogate (such as MCH above 800K or n-
dodecane above 1000K) may be adequate to represent certain combustion characteristics
(such as ignition delay time) of jet fuels. However, it may be necessary to use multi-
component surrogates in simple surrogate mixtures for jet fuels that can accurately
reproduce ign data in the entire temperature regime (especially in the NTC region below
800K). It should be noted that detailed mechanisms containing a large number of species
and reactions (such as in a multi-component surrogate mixture and mechanism) cannot
easily be applied to the modeling of combustion behavior of jet fuels in computational
fluid dynamics (CFD) calculations. In such cases, modelers can use MCH as a single
component surrogate to simulate global combustion kinetic properties of jet fuels.
0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4
100
1000
10000
6
4
32
1) Iso-octane2) Toluene3) Jet-A4) n-Dodecane5) MCH6) n-Heptane
1000/ T, [1/K]
Fuel/air, =1.0, 20 atm
15
Ign
ition
Del
ay
Tim
es,
[s]
Figure 4.10 High-pressure ignition times data from HPST in Jet-A and major single-
component surrogate fuels at similar conditions (=1.0). HPST data scaled to 20 atm
using respective pressure scaling for individual fuels (see text). Solid lines are fit through
data at 20 atm: iso-octane and toluene data from Davidson et al. [115] and Vasu et al.
[143], respectively; n-dodecane, Jet-A, MCH, and n-heptane are from current work.
Hydrocarbon ignition is, to a large extent, controlled by the chemistry of the small
transient radical pool (H, OH, CH3 etc.), and in particular, very little or no information is
available for these species. At higher pressures, most current mechanisms have been
58
validated only against the measured yields of the more stable intermediates or against
ignition delay times alone, and small radical species time-history data are needed for
complete mechanism validation. Hence, OH concentration time-histories using laser
absorption techniques to this end, and the results of these experiments are presented in
the next chapter.
Table 4.1 Summary of current shock tube experimental results in n-dodecane/air, phi=0.5.
n-dodecane/air, phi=0.5, T1=100.5C,
n-dodecane=0.565 %, O2=20.89 %, N2=78.55 %
T5 (K) P5 (atm) τign (μs)
747 26.0 2243
806 25.0 1522
873 23.1 1918
910 22.7 2134
943 18.1 2276
996 20.3 1245
1039 21.0 826
1049 18.9 839
1054 20.1 788
1082 18.4 566
1087 19.6 527
1117 30.9 266
1118 20.3 346
1149 21.0 261
1163 21.2 213
1177 20.8 165
59
Table 4.2 Summary of current shock tube experimental results in n-dodecane/air, phi=1.0.
n-dodecane/air, phi=1.0, T1=103 C,
n-dodecane=1.123 %, O2=20.77 %, N2=78.10 %
T5 (K) P5 (atm) τign (μs)
727 27.0 809
773 28.7 556
818 26.9 881
822 23.3 805
855 25.9 875
869 22.5 1040
907 23.4 1081
942 21.0 1116
953 20.0 1141
957 19.8 1064
976 20.5 800
978 22.9 912
987 22.0 699
991 21.8 645
992 20.4 739
1008 22.4 570
1015 22.3 508
1036 22.1 432
1087 19.4 403
1102 20.8 268
1107 23.3 298
1109 23.0 278
1118 33.7 183
1123 24.0 262
1125 18.7 188
1135 20.6 178
60
Table 4.3 Summary of current high-pressure ignition time results in MCH/air (=1.0).
Mixture: MCH=1.96%, O2=20.60%, N2=77.44%. Vshock is the incident shock velocity at
the endwall.
T5,(K) T1, (C) Vshock,(mm/us) Atten (%/m)
T5, (K) P5, (atm) ign, (μs)
29.57 22.4 0.763 1.755 795 48.1 1620 27.37 22.4 0.786 1.905 827 49.1 1384 26.42 22.3 0.794 1.832 837 48.8 1337 25.32 21.9 0.806 1.906 854 49.2 1393 24.76 21.9 0.809 1.873 857 48.6 1509 23.24 22.8 0.822 1.958 876 47.8 1484 22.77 22 0.834 1.835 893 49.2 1238 20.22 22.7 0.851 1.974 918 46.4 1185 19.53 22.6 0.854 1.93 922 45.4 1084 17.52 22.7 0.882 2.05 962 44.8 664 16.01 22.5 0.886 2.183 968 41.6 722 15.27 22.3 0.920 1.916 1019 44.6 345 14.37 22.8 0.926 2.082 1029 42.7 318 13.58 21.9 0.938 1.993 1046 42.2 223 12.07 22.45 0.945 2.216 1057 38.2 226 12.24 21.9 0.961 2.167 1081 40.8 133 11.42 22.5 0.963 2.504 1085 38.2 136 14.97 103.6 0.811 0.807 912 20.5 1995 13.95 104 0.820 0.74 926 19.8 1940 12.82 104.15 0.828 0.977 937 18.8 1485 12.55 103.5 0.843 1.119 958 19.5 1226 11.25 103.8 0.868 1.039 994 19.2 842 10.04 104.2 0.873 2.4 1002 17.5 637 8.83 104.3 0.905 2.341 1049 17.2 411 11 104.2 0.913 0.863 1060 22.1 358 7.89 103.7 0.938 2.36 1098 17.3 273 29.5 104 0.836 0.931 948 44.5 670 25.48 104 0.839 0.828 953 39.0 696 25.97 103.7 0.864 0.922 988 43.7 440 25.54 103.5 0.867 0.857 992 43.5 418 25.46 104.15 0.877 0.877 1007 44.8 337
61
Chapter 5: OH Time-histories During
Surrogate Oxidation at High-Pressure This chapter describes high-pressure OH time-histories measured in three single-
component surrogates (n-dodecane, MCH, and n-heptane) using a heated HPST.
Experimental conditions covered temperatures of 1121 to 1422 K, pressures of 14.1-16.7
atm, and initial fuel concentrations of 500 to 1000 ppm (by volume), and an equivalence
ratio of 0.5 with O2 as the oxidizer and argon as the bath gas. Detailed comparisons of
experimental data with predictions of available kinetic mechanisms were made and the
procedure was shown to improve predictive qualities.
5.1 Experimental Method
All experiments were carried out in the reflected shock region of the HPST. The
method and diagnostics are described earlier in a previous chapter. Research grade n-
dodecane (≥ 99%), MCH (≥ 99.5% pure) and n-heptane (≥ 99.5% pure) fuel vapors (all
supplied by Sigma-Aldrich) and high-purity gases (Praxair O2, Ar, > 99.999%) were
prepared similar to that described in previous chapters. The shock tube was heated to 60
C and the fuel/oxidizer mixing facility and connecting lines were kept at a higher
temperature (135 C for n-dodecane, and up to 110 C depending on the fuel-loading
requirements in the case of MCH and n-heptane) to avoid condensation. The amounts of
fuel in mixtures (500 to 1000 ppmv) for the current study were large enough to achieve
sufficient absorption signals (and also high signal-to-noise ratios), but small enough to
avoid significant energy release during ignition. This ensured uniform temperature and
pressure conditions behind the reflected shock wave during reaction.
While all data acquired in this study were with the optical access port located 10mm
from the endwall, our past experience (Petersen et al. [44]) has shown that there is little
sensitivity of the data to small changes in the measurement location. In that study
conducted in the HPST (for similar mixtures in methane/O2/Ar) no difference was
observed between measured peak (time at peak value of XOH for post ignition) for two
measurement locations (10 mm and 20 mm from the endwall). A key implication is that
62
potential combustion-related gas dynamic effects reported in the previous chapter had
minimal impact on our current experiments.
There are numerous ways of defining ignition delay time. ign was defined here as the
time to reach 50% of the peak XOH, with zero time being defined as the arrival of the
reflected shock at the side-wall location. This definition was found to correspond very
well with ignition delay defined as the time to the first rise in pressure after arrival of the
reflected shock front [144] and has been widely used [42,145]. Additionally, model
calculations done by Davidson et al. [42] indicated that this time coincided with the
maximum of the CH concentration that occurred during the most rapid formation of
radicals in the final stages of ignition for n-alkanes. Example OH absorption traces are
shown later in this chapter, however, it is to be noted that the spike in the OH trace at
time zero corresponds to arrival of the reflected front at the diagnostic location. The
diagnostic beam is temporarily steered (due to the density gradient across the reflected
shock wave) off the detector surface resulting in the observed schlieren spike. The arrival
of the reflected shockwave at the sidewall location (and therefore time zero) was
determined both from the step rise in pressure signals and from the OH traces as the
midpoint of this above-mentioned spike.
5.2 Kinetic Modeling Details
In comparing OH experimental data with predictions of available mechanisms, the
calculations were done for homogeneous, adiabatic conditions behind reflected shock
waves, with the common constant-internal-energy, constant-volume constraint (constant
U,V) using CHEMKIN 4.1.1 [101]. For short ignition delay times (typically less than 1-2
ms) in dilute fuel-oxidizer mixtures, the constant U, V constraint is a good assumption for
the purpose of ignition delay time calculations [102,103]. In the modeling, an identical
ign definition to experiments was used, i.e., the time to reach 50% of the peak XOH.
We have compared our OH species measurements with the predictions of three MCH
mechanisms, 11 n-heptane mechanisms, and 4 n-dodecane mechanisms as listed in Table
5.1. The mechanisms discussed here have been developed and optimized to address
particular needs and applications, and have not, in particular, been optimized to
accurately simulate OH time-histories. However, the importance of the small radical pool
63
(H, O, HO2, OH, and CH3) to all ignition processes implies that these mechanisms should
realistically be expected to model OH profiles accurately. It is important to look at the
details of kinetic mechanisms used for comparison in order to get a better sense of the
chemistry and the similarities and differences in predictions.
The MCH mechanism of Orme et al. [36] was built on the reaction scheme developed
by Laskin et al. [56], which described the oxidation of 1,3-butadiene, although the H2/O2
submechanism was based on that of Ó Conaire et al. [146]. The Orme et al. mechanism
was validated against shock tube ignition delays and flow reactor experiments. The Pitz
et al. [22] mechanism contained C1-C6 chemistry of Curran et al. [113] with sub-
mechanisms for toluene, benzene, and cyclo-pentadiene developed by the same authors.
It is important to note that the comprehensive detailed kinetic mechanism for MCH
oxidation from Pitz et al., which adapted the high-temperature MCH reactions from Orme
et al. [36], was validated using RCM ignition delay time measurements but has not
previously been validated for temperatures above 1100 K.
The Ranzi [48] (semi-lumped) JP-8 surrogate mechanism contains n-dodecane, MCH
and n-heptane. The You et al. [118] n-dodecane mechanism was a recently developed
kinetic mechanism for the combustion of n-alkanes up to n-dodecane, where n-heptane is
an integral sub-mechanism component and is currently part of the JetSurF 1.0 [46]
surrogate mechanism. The Montgomery et al. [117] quasi-steady-state reduced
mechanism, created using an automatic genetic optimization scheme, is based on the
parent Violi et al. [5] mechanism which has the same origin as the current Ranzi [48]
mechanism. The semi-detailed JP-8 mechanism of Zhang et al. [19] was not optimized
for low temperature modeling.
The high-temperature Chaos et al. [52] n-heptane mechanism, which was developed
based on the baseline H2/O2 and C1-C4 chemistry of Li et al. [147] and Zhao et al. [148],
respectively, was validated against a variety of experiments including shock tube ignition
delays, premixed flame speeds, and species measurements from a variable pressure flow
reactor (VPFR) and a jet-stirred reactor (JSR). The large n-heptane mechanism of Curran
et al. [149], also known as LLNL (detailed), was validated against shock tube, RCM, and
flow and stirred reactor data. The Seiser et al. [150] mechanism, also referred to as LLNL
(reduced), was reduced from [149] using data for n-heptane extinction and autoignition in
64
counterflow configuration. The n-heptane mechanism of Tsang [151] (denoted as the
NIST mechanism) was based on a variety of other mechanisms and was validated against
burning velocity, ignition delays and low-pressure OH time-histories (by Davidson et al.
[42]) in a shock tube.
The Golovitchev [152] mechanism was validated against a variety of data including
shock tube ign. The highly reduced Patel et al. [153] mechanism used [152] as a starting
point and was validated using single-cylinder engine data. The San Diego [154] n-
heptane mechanism was built by adding lumped n-heptane chemistry to the widely
validated C3 mechanism developed by Petrova and Williams [155]. The Biet et al. [156]
n-heptane mechanism (not validated at high-temperature conditions) has been generated
using the EXGAS software, a computer package developed to perform the automatic
generation of detailed kinetic models for the gas-phase oxidation and combustion of
alkanes. The Gokulakrishnan et al. [157] kerosene mechanism includes the n-heptane
skeletal mechanism derived from [149] and was validated for ignition delay times.
5.3 OH Time-Histories Results
All new experimental data for XOH obtained in this study are summarized in Table 5.2
and Table 5.3.
5.3.1 n-Heptane Results
The Measured OH profiles during n-heptane oxidation are shown in Figure 5.1. All
data show approximately the same behavior, i.e., there is no significant OH formation at
early times (during initial fuel decomposition), and during ignition the OH concentration
rapidly increases, reaches a peak value and slowly falls off. This behavior of OH is
different from the behavior observed in low-pressure experiments, where at early times
an OH plateau (simultaneous with the initial fuel decomposition of the fuel molecule) is
observed, after which OH rises to a higher post-ignition plateau in most fuels [42,68,145].
A low temperature result at 1121K, a case where n-heptane did not ignite during the time
interval of interest in the current study (< 2 ms), is also shown, indicating that there is no
pre-ignition formation of OH even at longer times for the current experiments. As
expected, peak XOH values decrease and ign increases as temperature decreases.
65
The ability of the n-heptane mechanisms investigated to model the current n-heptane
OH profile data is shown in Figure 5.2 for the case of 1271K. Evidently, there is
significant difference also in the shape of OH profiles predicted by various mechanisms.
Wide variations exist in predicted OH profiles for the conditions shown at 1271K.
Notably, the shape of OH profiles of Patel et al. [153], Ranzi et al. [48], SanDiego [154],
Chaos et al. [52], Biet et al. [156], and You et al. [118] mechanisms are very similar to
that of the HPST data. Profiles predicted by Tsang [151] and both the LLNL mechanisms
[149,150] are similar (there is little difference between predictions of the detailed and
reduced LLNL mechanisms) while the Golovichev [152] mechanism predicts a very
different shape compared to the data. Note that because the model predictions are
performed assuming constant U,V constraints, the final plateau value (after decay)
obtained is same for every mechanism.
0 400 800 1200
0
40
80
120
160
200
240
XO
H, [
ppm
]
Time, [s]
1
2
3
4
Figure 5.1 High-pressure OH absorption data for n-heptane; initial Xfuel=1000ppm,
XO2=0.022, XAr=0.977, =0.5. Data 1: 1271K, 15atm; data 2: 1236K, 15.28atm; data 3:
1230K, 15.81atm; data 4: 1121K, 14.1atm.
The data show no significant early formation of OH, and all the n-heptane
mechanisms predict this correctly except for You et al. [118] where there is an
approximately 10ppm early OH plateau at a temperature of 1271K, and pressure of 15
atm, for an initial Xn-heptane=1000ppm. Only the You et al. mechanism predicts shorter
ignition delay times than experiment, and this fact is consistent with the predicted
formation of early-time OH concentrations. Some mechanisms (Ranzi, Patel et al., Biet et
al., Chaos et al.) predict the peak XOH values within 20%, while over-predicting ign by a
significant amount. Patel et al. predicts a very long ignition time, which is understandable
66
considering that it is a highly reduced mechanism developed for fast multi-dimensional
diesel HCCI engine simulations and may not be suitable for modeling OH radical
concentrations. The San Diego [154] mechanism gives the closest agreement by
predicting all parameters within 20% for the current experiments. Both the LLNL
mechanisms [149,150] under-predict peak XOH and over-predict ign by 50 %. The
Horning [43] empirical correlation for n-heptane from our lab, which was developed by
regression analysis of ignition time measurements of n-heptane conducted over a range of
temperature, pressure (1-6atm) and mixture composition, predicts ign within 10% of the
current data.
0 600 1200 1800 2400 3000 3600
0
50
100
150
200
250
300
2,3
4
567
XO
H, [
ppm
]
Time, [s]
1
0 600 1200 1800 2400
0
50
100
150
200
250
300
11
8
10
91
XO
H, [
ppm
]
Time, [s]
12
Figure 5.2 High-pressure OH absorption profile data and modeling predictions for n-
heptane; initial Xfuel=1000ppm, XO2=0.022, XAr=0.977, =0.5, 1271K, 15atm. 1: Current
experiment, 2: Curran et al. [149], 3: Seiser et al. [150], 4: Tsang [151], 5: Patel et al.
[153], 6: Ranzi et al. [48], 7: SanDiego [154], 8: Golovichev [152], 9: Gokulakrishnan et
al. [157], 10: Chaos et al. [52], 11: Biet et al. [156], 12: You et al. [118].
5.3.2 MCH Results
The Example measured OH profiles during oxidation of MCH are shown in Figure 5.3
for two initial fuel concentrations at different temperatures. All data show approximately
the same behavior and the characteristics are similar to those of n-heptane. High
reproducibility of the current experiments (as a result of accurate mixture concentration
and shock-speed determinations, and consistency of the diagnostic technique) and
sensitivity of the measured OH profiles to temperature are evident from Figure 5.3.
Figure 5.4 shows a comparison of the OH profile data in MCH at 1262 K with the
67
predictions of all three MCH models. None of the mechanisms predict OH production
consistent with the early-time data. The semi-lumped mechanism of Ranzi [48] recovers
both the shape and peak value of OH, while the Orme et al. [36] mechanism matches only
the decay rate. The Pitz et al. [22] mechanism predicts a slower rise during ignition and a
near flat peak (also much smaller) compared to the sharp peak observed during
experiments. The measured peak XOH follows an approximately linear dependence on
temperature. The OH peak concentrations for the 1000 ppm mixtures are observed to be
1.7 times the OH peak concentrations in the 750 ppm mixtures. All three mechanisms
recover the temperature dependence of the observed peak OH. The Ranzi et al.
mechanism is in very good agreement with the values from the experiment, while the
other two mechanisms under-predict the peak value at all conditions investigated.
-200 0 200 400 600 800 1000 1200 1400 1600
0
50
100
150
200A
-100 0 100 200 300 400 500 600 700 800 900
0
20
40
60
80
100
120
140
3) 1266 K
2) 1303 K
XO
H,
[pp
m]
Time, [s]
1) 1304 K
B
Time, [s]
XO
H, [
ppm
]
1) 1285 K
2) 1269 K
3) 1213 K
4) 1205K
Figure 5.3 (A): OH absorption data for MCH; initial XMCH=1000 ppm, XO2=0.021,
XAr=0.978, =0.5. Data 1: 1285 K, 15.16 atm; data 2: 1269 K, 15.8 atm; data 3: 1213 K,
14.44 atm; data 4: 1205 K, 14.47 atm. (B): OH absorption data for MCH; initial
XMCH=750 ppm, XO2=0.01575, XAr=0.9835, =0.5. Data 1: 1304 K, 15.92 atm; data 2:
1303K, 15.63 atm; data 3: 1266 K, 15.83 atm.
Comparison of high-temperature, low-concentration ignition delay time measurements
and modeling predictions show that at fixed equivalence ratios, ignition delay times
increase with decreases in either temperature or initial fuel mole concentration. In general,
all three mechanisms do a reasonable job of predicting the ignition delay times, with the
Ranzi and the Orme et al. mechanisms giving closer agreement with our data than the
68
Pitz et al. mechanism, for both initial fuel concentrations. The following activation
energy for ignition predicted by all three mechanisms is slightly higher than the value (38
kcal/mol) obtained during current experiments: 45.18 kcal/mol by Ranzi; 45.54 kcal/mol
by Pitz et al.; and 48.59 kcal/mol by Orme et al. This experimental activation energy for
ignition of MCH is lower than those observed in most of the n-alkanes up to C10 [43].
0 400 800 1200 1600 2000 2400
0
40
80
120
160
200
Orme et al.
Time, [s]
XO
H, [
ppm
]
Pitz et al.
HPSTRanzi et al.
Figure 5.4 High-pressure OH absorption data and modeling predictions (using Ranzi [48],
Orme et al. [36], Pitz et al. [22] mechanisms) for MCH/O2/Ar. Initial XMCH=1000 ppm,
XO2=0.021, XAr=0.978, 1262K, 15.45 atm, =0.5.
5.3.3 n-Dodecane Results
The measured OH profiles for various temperatures for an initial Xfuel of 1000ppm are
plotted in Figure 5.5. At the highest temperature (1422K), there is clear evidence of
early-time radical chemistry (see inset, Figure 5.5), i.e., there is the rapid formation of
OH and attainment of an intermediate plateau level (XOH~20ppm) simultaneous with the
initial decomposition of the fuel molecule. All four mechanisms predict this behavior at
1422 K, but the intermediate plateau level is different for each mechanism: Ranzi
[48](16ppm), Montgomery et al. [117] (18ppm), Zhang et al. [19] (10ppm) and You et al.
[118] (57ppm). At lower temperatures, there is no measurable OH at early time (nor is a
significant OH plateau seen in any of the models.). Figure 5.5 also shows a comparison of
the 1217K data with all four model prediction. The You et al. and the Montgomery et al.
mechanisms recover the shape of the OH peak at ignition, while there is a sharp
overshoot evident in the Ranzi and Zhang et al. predictions.
69
0 200 400 600 800 1000 1200 1400 16000
200
400
600
800
0 10 20 300
20
40
60
1
XO
H, [
pp
m]
Time, s]
23
4
XO
H,
[ppm
]
Time, s]
5
0 400 800 1200 1600 20000
200
400
600
8001000ppm n-dodecane,1217 K, 16. 1 atm,=0.5
XO
H, [
ppm
]
Time, s]
You et al. Ranzi et al. Zhang et al. Montgomery et al. Current Study
Figure 5.5 High-pressure OH absorption data: n-dodecane; initial Xdodecane=1000ppm, O2,
Ar; =0.5. Left: Data 1: 1422K, 15.5atm; data 2: 1230K, 16.73atm; data 3: 1217K,
16.07atm; data 4: 1196K, 15.77atm; data 5: 1158K, 15.19atm. Right: Comparison of
measured and modeled OH time-histories at 1217 K, 16.1 atm.
Figure 5.6 shows a comparison of the measured peak OH concentration and ignition
delay times predicted by different mechanisms. The measured peak XOH follows an
approximately linear dependence on temperature and an approximately linear variation
on initial fuel concentration (Xfuel) as well. The measured OH peak concentrations for the
1000 ppm mixtures are 1.7 times the OH peak concentrations observed in the 515 ppm
mixtures (not shown here). All four mechanisms recover the temperature dependence of
the observed peak OH. Ignition delay times also can be determined from the high-
temperature, low-concentration OH concentration time-histories. Figure 10 shows a
comparison of τign (the time to reach 50% of the peak post-ignition XOH), with the
predictions of the four models for the 1000 ppm n-dodecane, =1 data. τign values depend
strongly on the initial Xfuel concentration, i.e., τign for initial Xfuel=515 ppm mixtures are
2.7 times those for initial Xfuel=1000 ppm mixtures. The You et al. model simulations
gives the closest agreement (within 10%), and the Zhang et al. mechanism gives the
poorest. The activation energies predicted by the Montgomery et al. gives slightly better
(within 40%) agreement to the data than the Ranzi mechanism predictions.
70
0.75 0.80 0.85 0.900
200
400
600
800
10001176K 1111K1250K1333K
1000ppm n-dodecane/O2/Ar
16 atm, =0.5
Pea
k O
H M
ole
Fra
ctio
n
[pp
m]
1000/T, [1/K]
Ranzi et al. You et al. Zhang et al. Montgomery et al. Current Study
0.70 0.75 0.80 0.85 0.9010
100
1000
10000 Current Study You et al. Zhang et al. Montgomery et al. Ranzi et al. Horning et al.
1111K1250K1429K
1000/T, [1/K]
Ign
ition
De
lay
Tim
es,
[us]
1000ppm n-dodecane/O2/Ar
=0.5
Figure 5.6 High-pressure OH data variation with temperature. Initial reflected shock
conditions: 16 atm, 1000 ppm n-dodecane/O2/Ar, = 0.5. Left: peak XOH vs T5. Solid
line is a linear fit through data. Right: High-temperature, low-concentration ignition
delay times in n-dodecane. Solid black line is least-squares fit through data.
Figure 5.6 also shows the predictions of the ignition time correlation of Horning et al.
[43], developed at low pressures for a variety of n-alkanes. This correlation was
developed for n-alkanes from n-propane to n-decane and accounts for the fuel size
through the number of carbon atoms in the molecule. While over-predicting the n-
dodecane ignition delay time at high pressures, it does accurately capture the activation
energy of the current data set. The activation energy for current n-dodecane data is 46
kcal/mol, which is very close to that obtained by Horning et al. [43] (46.5 kcal/mol) and
You et al. [118] (45.5 kcal/mol). Montgomery et al. [117] (43.5kcal/mol), Ranzi [48]
(42.2 kcal/mol) and Zhang et al.[19] (52.9 kcal/mol) do not agree as well.
5.4 Discussion
In order to understand fuel oxidation chemistry and to identify key reactions which
affect OH formation with a view to improve the surrogate modeling predictive
capabilities, kinetic analysis was performed. Rate of production (ROP), reaction pathway,
and OH sensitivity analysis were performed and are discussed. The OH sensitivity
coefficient is defined as: S(XOH, ki, t)= {dXOH(t)/dki}{ki/XOH(t)}.
71
5.4.1 n-Heptane Kinetic Analysis
The mechanism of Chaos et al. [52] was chosen to perform integrated reaction flow
analysis because of its compactness, thorough and extensive validation against a large
experimental data set covering a broad range of applications, and its ability to predict
current values of peak XOH. The results of the flow analysis are summarized in Figure 5.7,
which identifies dominant reaction pathways for n-heptane oxidation (1275K, 16atm).
Approximately 80% of n-C7H16 is removed by H-abstraction reactions, primarily by
reaction with OH. The heptyl (C7H15) radicals which are formed further undergo C-C
bond cleavage to form four main product channels. The decomposition channels of C7H15
are 1-butene (1-C4H8) and n-propyl radical (n-C3H7) (48%), 1-pentene (1-C5H10) and
ethyl radical (C2H5) (35%), and two minor channels (~8% each) producing C3H6, C2H4,
C5H11 (pentyl radical), and p-C4H9 (1-butyl radical). Continued decomposition of these
species forms the relatively stable intermediates CH3, C3H6, and C2H4.
Figure 5.7 Major oxidation pathways prediction using the Chaos et al. [52] mechanism
integrated ROP approach. Xheptane=1000ppm, XO2=0.022, balance=Ar. 1275K, 16atm,
=0.5. Details of molecular structures and pathways can be found in [52].
Based on the results of sensitivity analyses and published uncertainties of several of
these reactions, likely reaction candidates for mechanism adjustment can be selected. As
expected, sensitivity analysis for OH (shown in Figure 5.8) using the Chaos et al. [52]
72
mechanism reveals that the OH concentration both at early times and during ignition is
most sensitive to the branching reaction:H+O2=O+OH. However, this reaction has been
extensively studied over the years, and recent publications place an uncertainty of less
than 10% on this rate coefficient [158]. In addition, OH time histories are strongly
impacted by the following reactions: CH3+HO2=CH3O+OH, and C2H4+OH=C2H3 + H2O.
Note that CH3+HO2 reaction has two product channels and they are in direct competition
with each other (see Figure 5.8).The substantial amount of reactive H radicals produced
from CH3O through the subsequent decomposition reaction CH3O+M=CH2O+H+M
explains the very strong positive sensitivity of this channel. However, even though
CH3+HO2=CH3O+OH has a strong influence on τign, there exists no direct measurement
of this reaction and differences exist in the rate used by the modeling community. Based
on the value (1.81x1013 cm3/mole/s) suggested by Colket et al. [159], the Baulch et al.
[160] review suggested an uncertainty factor of 10 for CH3+HO2=CH3O+OH and another
review by Tsang and Hampson [127] estimated a temperature-independent value of
1.99x1013 cm3/mole/s, with an uncertainty factor of 3.0.
600 700 800 900
-20
-10
0
10
20
30
40
50
60
E
C
B
A) H+O2<=>O+OH B) HO2+OH<=>H2O+O2 C) OH+C2H4<=>C2H3+H2O D) HO2+CH3<=>O2+CH4 E) HO2+CH3<=>OH+CH3O F) C3H6+OH<=>aC3H5+H2O
Nor
ma
lized
OH
Se
nsi
tivity
Time, [s]
A
D, F
Figure 5.8 OH sensitivity for n-heptane oxidation using the Chaos et al. [52] mechanism.
Xheptane=1000ppm, XO2=0.022, balance=Ar. 1271K, 15atm, =0.5.
The CH3+HO2=CH3O+OH values (in units of 1013cm3/mole/s) used by n-heptane
mechanisms are 1.8 in Biet et al. [156], 1.48 in Chaos et al. [52], and 1.1 in Seiser et al.
[150], while the most popular and extensively validated, GRI 3.0 [161] optimized
methane mechanism uses 3.78. The recent OH time-history measurements during iso-
73
octane oxidation in a RCM for pressures between 8.5-15 atm by He et al. [162] obtained
good agreement between data and predictions by a detailed iso-octane mechanism
(Curran et al. [113]) when a value of 7.7x1013 cm3/mole/s was used. We find that using a
value of 6.8x1013 cm3/mole/s (the theoretical rate constant of CH3+HO2=CH3O+OH
calculated by Zhu and Lin [163] is only 50% lower than this value) with the Chaos et al.
[52] mechanism significantly improves the ign predictive capability for the current n-
heptane measurement at 1271K (see Figure 5.9).The values of the peak XOH changes only
minimally by this alteration. Our observations highlight the necessity of further, more
direct study of this reaction in order to obtain more accurate values and hence reliable
predictions for ign. At long times (after about 690 s as in Figure 5.9), there is
disagreement between modified Chaos et al. [52] mechanism predictions and data, but
these differences may be due to post-ignition waves or flow disturbances inside the shock
tube, which cannot be simulated using the current modeling approach, i.e., using constant
U,V CHEMKIN [101].
0 200 400 600 800 1000 1200
0
50
100
150
200
250Chaos et al. mechanism
modified unmodified
current data
XO
H, [
ppm
]
Time, [s]
Figure 5.9 Influence of higher rate for CH3+HO2=CH3O+OH, new rate= 6.8 x1013
cm3/mole/s, on the OH predictions by Chaos et al. [52] for n-heptane oxidation.
Xheptane=1000ppm, XO2=0.022, balance=Ar, =0.5, 1271K, 15atm.
The OH+C2H4 reaction influences OH concentrations not only in n-heptane oxidation
but in MCH and n-dodecane oxidation as well. This reaction accounts for 28% (Chaos et
al. [52]), 37% (Orme et al. [36]), and 50% (You et al. [118]) of the C2H4 removal during
74
n-heptane, MCH, and n-dodecane oxidation, respectively, at 1275 K and 16 atm.
Uncertainty factors of 5.0 and 3.16 are listed for this reaction from reviews conducted by
Tsang and Hampson [127] and Baulch et al. [160], respectively. Increasing the rate of
this reaction, which produces vinyl radicals, also improves (i.e., shortens ign without
affecting peak XOH) the predictions by the Chaos et al. [52] n-heptane mechanism (not
shown here). This reaction was measured in this work and the results are presented in a
later chapter.
5.4.2 MCH Kinetic Analysis
The Orme et al. [36] mechanism was used to perform integrated flow analysis for
MCH oxidation at 1275 K and 16 atm (similar conditions to those presented in the case of
n-heptane in Figure 5.7), and the simplified oxidation pathways are presented in Figure
5.10. For clarity, chemistry below the C2 level is not shown, and symbols are used to
represent some complex species without affecting the purpose of the current discussion.
The Orme et al. mechanism was chosen due to its simplicity and the relatively
encouraging results (peak shape) in Figure 5.4. As shown in Figure 5.10, the importance
of 1,3-butadiene chemistry in MCH oxidation is clearly evident as nearly all of MCH
decomposes through 1,3-butadiene. Also, typical modeled species concentrations during
MCH oxidation using the Orme et al. mechanism indicate that 1,3-butadiene is the
second largest intermediate species following ethylene. 1,3-butadiene reacts with H
atoms (40%) to form ethylene and vinyl (C2H3) radicals, and about 30% of 1,3-butadiene
goes to form allyl radicals. It is important to note that allylic radicals determine the
concentrations of many important aromatic precursors in oxidative systems. Here, allyl
forms C3H6 through +H (+M) reactions, 1-butene through +CH3(+M) reactions, and vinyl
plus formaldehyde through reacting with HO2. These analyses highlight the importance
of 1,3-butadiene, HO2 and C2H4 chemistry during MCH oxidation. Additionally, note
that two of the major stable intermediates during MCH oxidation, C2H4 and C3H6, are
consumed through reactions with OH radicals, just as in the case of n-heptane oxidation
presented earlier, i.e., 40% of C3H6 and 37% of C2H4. Ethylene is also consumed (39%)
by its reaction with O atom to produce methyl and formyl (HCO) radicals.
75
Figure 5.10 Major MCH oxidation pathways using the Orme et al. [36] mechanism
(integrated ROP), 1275 K, 16 atm, 1000 ppm MCH/O2/Ar, = 0.5. Refer to Orme et al.
[36] for IUPAC nomenclature of species and for detailed molecular structures and
pathways.
Normalized OH sensitivity analysis for MCH oxidation is shown in Figure 5.11 using
the Orme et al. [36] mechanism for the OH experimental conditions shown in Figure 5.4.
Besides the reactions mentioned in the case of n-heptane oxidation, OH sensitivity is
affected by the MCH decomposition through the cyclo-hexyl radical
channel:MCH=cyclo-hexyl + methyl. Immediately after the arrival of the reflected shock
(at very early times), the influence of this reaction (labeled A in Figure 5.11) on the OH
concentrations is higher than that of H+O2=O+OH. The influence of the decomposition
channel decreases subsequently and the methyl radical recombination reaction becomes
dominant along with H+O2=O+OH (see Figure 5.11). Hence, modeling the early time
MCH decomposition could be improved by adjusting the above-mentioned rates within
their uncertainty bounds. OH scavenging reaction by HO2 radical has the strongest
negative sensitivity values during ignition in both the cases of n-heptane and MCH.
However, adjustments to this reaction (HO2+OH=H2O+O2) were not studied here,
because this reaction is well-known, and very recent measurements of this reaction [158]
yield an estimated uncertainty of less than 40%.
76
The most important 1,3-butadiene reaction occurring in the MCH sensitivity analysis
of OH near ignition is the reaction (which is also the main 1,3-butadiene decomposition
channel):1,3-butadiene+H=C2H4+C2H3. Tsang and Hampson [127] estimated an
uncertainty factor of 5.0 for the reverse reaction. There exist no direct measurements and
the rate used by Orme et al. [36] is the same as that used in the original Laskin et al. [56]
1,3-butadiene mechanism. However, Libby et al. [68] suggested a 3-times lower rate for
this reaction, than that estimated by Laskin et al., based on fitting their low-pressure OH
time-history measurements with the predictions made by the Laskin et al. 1,3-butadiene
mechanism.
600 700 800 900 1000
-40
0
40
80
120
160
D) CH3+HO2=CH3O+OHE) C2H4+OH=C2H3+H2OF) 1,3-butadiene+H=C2H4+C2H3
C
E
D
No
rma
lize
d O
H S
en
sitiv
ity
Time, [s]
A) Cyclo-hexyl+CH3=MCHB) H+O2=O+OHC) HO2+OH=H2O+O2
a
B
A,F
0 20 40 60 80 100-10
-5
0
5
10
15
Nor
mal
ized
OH
Se
nsiti
vity
Time, [s]
A) Cyclo-hexyl+CH3=MCHB) H+O2=O+OHG) CH3+CH3(+M)=C2H6(+M)
A
B
G
b
Figure 5.11 OH Sensitivity for MCH oxidation using the Orme et al. [36] mechanism.
XMCH=1000 ppm, O2 (=0.5), balance=Ar. 1262 K, 15.45 atm. (a): during ignition, (b): at
early times.
Because of the importance of 1,3-butadiene in MCH oxidation systems, a comparison
of the current MCH mechanisms to predict OH time-histories during 1,3-butadiene (with
Libby et al. [68] data) oxidation was performed. It was found that none of the MCH
mechanisms predict ign or the early time OH plateau before ignition, and only the Ranzi
et al. [48] predicted the plateau of OH value after ignition. The Ranzi et al. JP-8
mechanism does not distinguish between isomers of 1,3-butadiene and all butadiene
isomers along with butyne are treated as a single lumped species. The Orme et al. [36]
ignition time predictions are approximately 10% shorter than the Laskin et al. [56]
predictions, which are about 36% shorter than those of the Libby et al. [68] data.
In order to improve the predictive capabilities of Laskin et al. [56] for high-
temperature 1,3-butadiene oxidation, Libby et al. [68] suggested the following 3
77
recommendations: 1) the GRI 3.0 [161] recommended rate coefficient for H+O2=O+OH,
2) a lower heat of formation of OH, ΔfH°298(OH)=8.92 kcal/mole, as measured recently
by Herbon [97] and Ruscic et al. [164], and 3) a 3-times lower rate for 1,3-
butadiene+H=C2H4+C2H3. The modified OH profile predictions (not shown here, see
Libby et al. [68]), using the Laskin et al. [56] 1,3-butadiene mechanism, after
implementing these 3 recommendations, very closely matched with the Libby et al. data.
The effect of the second recommendation is that predictions of the plateau values were
improved, i.e., the OH plateau of Laskin et al. matched that of Libby et al. The Orme et al.
[36] mechanism already uses this latest value for heat of formation of OH and the plateau
predictions are better for Orme et al. compared to Laskin et al. It must be necessary to
include the Libby et al. recommendations 1 and 3 in the Orme et al. mechanism as well.
ign and early time OH plateau predictions by Orme et al. during 1,3-butadiene oxidation
are improved, but prediction of the shape of the OH profile is destroyed, which showed
early and much slower rise to the experimental plateau value after ignition. Note that
modifications were applied in an incremental fashion, i.e., each modification is applied
step by step to see the effect of each recommendation. Overall, it was found that adding
MCH chemistry to the Laskin et al. 1,3-butadiene oxidation reactions without validating
the modified mechanism against 1,3-butadiene experiments, i.e., the procedure used to
develop the Orme et al. MCH mechanism, affects the predictive capability of the MCH
mechanism to model 1,3-butadiene oxidation.
When the Libby et al. [68] recommendations 1 and 3 were included in the Orme et al.
[36] mechanism in an incremental fashion, the ign predictions for MCH oxidation are
considerably longer than the unmodified Orme et al. predictions (see Figure 5.12). Also,
the shape of the OH profile shows a slower rise to the peak XOH and a lower peak XOH,
which is now much below the experimental peak value. However, if we include the
modifications from Figure 5.9 that were applied in the case of n-heptane oxidation, i.e.,
the new 6.8x1013 cm3/mole/s value for CH3+HO2=CH3O+OH reaction, ign predictions
are now within a few percent of the experimental value. It is clear that, just as in the case
of n-heptane, using the new recommendations affects the peak XOH only minimally;
however predictions of the shape of the rise of OH are slightly distorted due to this
procedure. Finally, the observed effect of increasing the rate of OH+ethylene is similar to
78
that observed by increasing CH3+HO2=CH3O+OH values for both MCH and n-heptane.
The reasons for the remaining discrepancies in the modeled and measured peak OH
concentration are not clear.
0 400 800 1200 1600 2000 2400
0
50
100
150
200 Modifications to Orme et al. C) Libby et al. (1) D) Libby et al. (1+3) E) Libby et al. (1+3)+
new k2 value
A) Current dataB) Orme et al (unmodified)
Time, [s]
X
OH, [
ppm
] A
B CD
E
Figure 5.12 Effect of modifications to the Orme et al. [36] mechanism predictions for
MCH oxidation (combining the Libby et al. [68] recommendations (1 and 3) and the
6.788x1013 cm3/mole/s value for CH3+HO2=CH3O+OH reaction). Initial XMCH=1000ppm,
O2 (=0.5), balance=Ar. 1262K, 15.45atm. See text for details of the Libby et al. [68]
recommendations.
The reason for different OH profile shapes and significantly different peak OH
concentrations (see Figure 5.4) predicted by the Ranzi [48] and Pitz et al. [22]
mechanisms was investigated. For these two mechanisms, the ROP results (for conditions
in Figure 5.4) indicate that the most influential reactions that produce OH are similar and
the reaction CO+OH=CO2+H is the significant OH scavenging reaction. Also, ROP
shows that the main OH precursors or scavengers are H, O, HO2 and CO for both
mechanisms. At this condition (as in Figure 5.4), both these mechanisms predict nearly
the same maximum amounts of CO (~ 5000ppm) and HO2 (~ 30ppm), however, the
maximum H, O, and OH predicted by the Ranzi et al. mechanism (note from Figure 5.2
and Figure 5.4 that the maximum OH predictions by the Ranzi mechanism are very close
to the current experimental results in n-heptane and MCH) are approximately 10, 6 and
3.5 times, respectively, of those predicted by the Pitz et al. mechanism. Naturally, this
would suggest that either the Ranzi mechanism uses more accurate rate values for
79
important OH reactions (all involving small species) or the Pitz et al. mechanism
assumptions regarding larger hydrocarbon chemistry (involving these small radicals)
need to be re-examined. However, replacing the Pitz et al. values of several of the
important OH reactions (i.e., H+O2=O+OH, CO+OH=CO2+H, H+HO2=OH+OH, and
O+H2O=OH+OH) with those of the Ranzi mechanism results in only negligible
improvements in the Pitz et al. mechanism predictions, which are not sufficient (also
supported by our analysis below) to explain the large differences in OH predictions seen
in Figure 5.4.
5.4.3 n-Dodecane Kinetic Analysis
The You et al. [118] mechanism was used to perform integrated flux analysis for n-
dodecane oxidation at 1275K and 16atm. The n-dodecane oxidation pathways according
to the You et al. mechanism are shown in Figure 5.13. Rates of production analyses at
1275K and 16atm indicate that H abstraction (by reaction of the fuel with H-atom and to
a lesser extent OH) dominates the initial removal of n-dodecane and forms 6 dodecyl
radical isomers. The alkyl radicals formed from n-dodecane subsequently undergo
reactions producing various lower alkyl and alkenyl radicals, and alkenes, where β-
scission dominates the later decomposition channels, forming mainly ethylene and
propene along the way. Ethylene and propene hence are two of the most stable
intermediate species during n-dodecane oxidation at these conditions (the rapid removal
of ethylene coincides with the sharp rise in OH during ignition). The
C2H4+OH=C2H3+H2O, accounts for 50% of ethylene removal under these conditions.
These conditions are similar to those presented in the case of n-heptane and MCH in
Figure 5.7 and Figure 5.10. Other n-dodecane kinetic mechanisms are either very large,
lumped or do not predict the XOH peak shape profiles accurately well (Figure 5.5).
The You et al. [118] mechanism reasonably predicts τign, τpeak, and peak XOH. You et al.
state that kinetic parameters of their mechanism for higher order hydrocarbon chemistry
(greater than C4) were derived from the analogous C4 reactions from their base model
without ad hoc parameter tuning. However (drawing from the case of MCH presented
80
earlier), accurately predicting the entire OH time-histories may require more detailed and
rigorous treatment of the n-dodecane decomposition process to C4 or lower hydrocarbons.
We have not adjusted the You et al. [118] mechanism or the included rate constants,
but we can comment on the reactions in these mechanisms most deserving of further
scrutiny. An OH sensitivity analysis was performed using the You et al. [118] mechanism
for the highest temperature (1422K) case. At the earliest times when n-dodecane begins
to decompose (within 7-10 μs), OH production is most sensitive to the rate of the reaction
H+O2=O+OH. However, the intermediate OH plateau level seen at this temperature is
maintained by a balance between the production of OH through H+O2=O+OH and
HO2+H=OH+OH, and removal of OH through C2H4+OH=C2H3+H2O and
CH2O+OH=HCO+H2O. The intermediate plateau level and subsequent ignition delay
time are thus strongly sensitive to decomposition pathways that form C2H4 and CH2O.
Sensitivity analyses conducted at 1275K, 16atm is shown in Figure 5.14, which clearly
indicates the importance of reactions mentioned earlier (in the case of n-heptane and
MCH).
Current results for MCH and n-heptane showed that it is possible to achieve excellent
agreement between measurements and data by following the procedure outlined in this
section. Improved understanding of intermediate chemistry, especially that of CH3, HO2
and alkenes (ethylene and propene) for all three fuels and 1,3-butadiene for MCH, is
critically needed to better understand the overall fuel oxidation and to improve
predictions at high-temperatures and pressures. Such work calls for direct and accurate
measurements of the rate constants of key major reactions and these results are presented
next.
81
Figure 5.13 n-Dodecane oxidation pathways using the You et al. [118] mechanism.
XNC12H26=1000ppm, O2, =0.5, balance=Ar, 1275K, 16atm.
0 40 80 120 160
0
10
20
30
3
4
5
2
1) H+O2=O+OH2) OH+HO2=H2O+O23) CH3+HO2=CH3O+OH4) C2H3+O2=CH2CHO+O5) C2H4+OH=C2H3+H2O
Nor
mal
ized
OH
Se
nsiti
vity
Time, [s]
1
Figure 5.14 Normalized OH sensitivity results for n-dodecane oxidation using the You et
al. [118] mechanism. XNC12H26=1000ppm, =0.5, balance=Ar. 1275K, 16atm.
82
Table 5.1 Mechanisms used in this study of surrogate components.
Mechanism Applicable fuels Year Species Rxns.
Orme et al. [36] MCH 2006 190 904
Pitz et al. [22] MCH 2007 1001 4436
Ranzi et al. [48] MCH, n-heptane, n-dodecane 2006 280 7800
You et al. [118], (alias: JetSurF 1.0 [46]) n-heptane, n-dodecane 2008 177 1318
Chaos et al. [52] n-heptane 2007 107 723
Seiser et al. [150] n-heptane 2000 160 1540
Curran et al. [149] n-heptane 1998 560 2539
Tsang [151] n-heptane 2004 197 2926
Patel et al. [153] n-heptane 2004 29 52
SanDiego [154] n-heptane 2005 51 272
Biet et al. [156] n-heptane 2008 119 970
Gokulakrishnan et al. [157] n-heptane 2007 554 1398
Golovitchev [152] n-heptane 2000 102 520
Montgomery et al. [117] n-dodecane 2007 94 675
Zhang et al. [19] n-dodecane, MCH 2007 208 1087
83
Table 5.2 Summary of current high-pressure OH absorption experiments in MCH and n-
heptane. aSat.= saturated signal (transmission~0) and in this case, ign was obtained from
raw absorption signals instead of XOH profiles. *Off=Offline absorption measurement. bNL=No laser was used. ign was obtained from pressure traces as the midpoint of
pressure jump during ignition for both Off and NL cases.
T5 (K)
P5 (atm)
kν (atm-1cm-1)
ign (50% peak XOH), (μs)
peak (μs)
XOH (peak) (ppm)
XMCH=1000ppm, XO2=0.021, Ar=0.978, =0.5 1332 15.64 55.23 301 sat.a 1285 15.16 50.67 529 590 200 1271 15.32 54.12 643 691.5 182 1269 15.80 59.95 643 699 196 1265 15.22 61.33 718 776.5 198 1262 15.45 61.01 722 781 193 1213 14.44 63.73 1284 1350 167 1205 14.47 62.73 1372 1420.5 156 XMCH=750ppm, XO2=0.01575, Ar=0.9835, =0.5
1304 15.92 59.70 500 530.5 142 1303 15.63 59.87 499 566.5 137 1276 15.77 59.88 690 743 119 1266 15.83 59.83 764 838 112 1283 15.43 *Off 617 Xheptane=1000ppm, XO2=0.022, Ar=0.977, =0.5 1271 15.00 59.07 377 423.5 230 1236 15.28 60.20 652 723 167 1230 15.81 58.12 698 769.5 156 1229 15.13 bNL 708 1121 14.10 61.42 No ignition
84
Table 5.3 Summary of OH absorption data (fuel=n-dodecane). aSat., saturated
signal;transmission~0.
T5 (K)
P5 (atm)
kν (atm-1cm-1)
ign (50% peak XOH), (μs)
peak (μs)
XOH (peak) (ppm)
Xfuel=1000ppm, XO2=0.03596, Ar=0.96304, = 0.5 1230 16.73 45.65 429 475 448 1217 16.07 45.63 508 549 420 1196 15.77 46.18 701 759 389 1186 15.38 24.93 822 880 374 1158 15.19 55.26 1181 1245 306 Xfuel=514.6ppm, XO2=0.01911, balance=Ar, = 0.5 1322 15.52 55.39 293 339 350 1258 15.31 55.68 682 744 269 1252 15.08 54.34 713 798 252 Xfuel=1000ppm, XO2=0.03725, Ar=0.96175, = 0.5
1219 16.10 56.59 485 545 sat.a 1211 16.13 58.05 534 594 sat. 1179 15.93 58.06 847 938 sat. Xfuel=750ppm, XO2=0.02786, balance=Ar, = 0.5 1222 15.84 52.16 625 689 sat. 1221 15.45 52.44 685 742 323 1192 14.98 55.83 1056 1116 sat. Xfuel=1000ppm, XO2=0.037, balance=Ar, = 0.5 1422 15.50 51.20 30 49 sat. 1280 16.57 56.86 195 226 526
85
Chapter 6: Alkenes Reaction With OH:
OH+C2H4 and OH+C3H6
This chapter describes measurements of the reactions of hydroxyl (OH) radicals with
two important alkenes (ethylene and propene) behind reflected shock waves. Highly
sensitive measurements of the overall rates were performed in dilute mixtures of
alkene/TBHP/Ar. Rate constants for the OH reactions with alkenes were extracted by
varying the overall rate to achieve a match between computed and measured OH
concentration time-histories behind reflected shocks. Ethylene and propene react with
OH to form various products including water (the final product):
C2H4+OHProducts (1)
C3H6+OHProducts (2)
Also presented in this chapter are canonical transition state theory (TST) calculations
using recently published high level ab initio geometries and energy barriers. Small
adjustments to theory predictions are made in order to get very good agreement with
experimental data over a wide range of temperatures.
6.1 Method
All experiments were performed in the LPST (described in Chapter 2). Research grade
argon (99.999%), ethylene (99.999%), and propene (99.8%) were supplied by Praxair Inc.
A commercially available solution of 70% TBHP in water was used (supplied by Sigma-
Aldrich). Mixtures were prepared manometrically in a stainless steel mixing chamber
equipped with a magnetic stirrer assembly and mixed for about 6.5 hrs to ensure
homogeneity and consistency. Details of the shock tube and mixing procedure can be
found elsewhere [97,99] and a brief description is given here. Experiments were
conducted with both the mixing assembly and shock tube driven section at room
temperature. Pre-shock GC analysis for such TBHP mixtures showed that less than
0.3ppm of TBHP decomposes in the mixing tank to form acetone [99]. Modeling the
reaction system with the decomposition taken into account showed that this has no
discernible effect on our rate measurements. Also, kinetic simulations show that the small
86
amount of water vapor in the initial reactant mixture has no effect on the rate coefficient
measurements. Uncertainties in the calculation of the initial reflected-shock temperature
and pressure were typically less than 0.7% and 1%, respectively, and arose primarily
from the 0.3% uncertainty in the incident shock velocity determination [97]. The
description of the OH absorption spectroscopy is given in an earlier chapter.
6.2 Kinetic Measurements
A total of 28 kinetic measurements were carried out to ascertain rate coefficients of
OH with alkenes (ethylene and propene). A total of 12 OH+ethylene measurements were
conducted in the range of temperatures from 973-1438 K and pressures from 1.99-10.18
atm for three initial concentrations of ethylene (500ppm, 751.1ppm and 1000ppm). The
experimental range for OH+propene spanned temperatures of 890-1366 K and pressures
of 2.024-2.615 atm (average 2.3 atm) at an initial propene concentration near 300 ppm.
Nominal mixtures with 96-150 ppm TBHP (and water) dilute in argon were prepared. A
kinetic mechanism was constructed to model the experimental OH profiles. The widely
popular GRI 3.0 [161] natural gas oxidation mechanism was chosen as the basis
mechanism for ethylene.
TBHP, the OH precursor used in this study, decomposes almost instantaneously to
form an OH radical and a tert-butoxy radical (CH3)3CO. The tert-butoxy radical
immediately falls apart to form acetone and a methyl radical. The resulting acetone and
methyl radicals can also react with OH. Above decomposition can be represented as:
(CH3)3-CO-OH OH+ (CH3)3CO (3)
(CH3)3CO CH3+ CH3COCH3 (4)
CH3COCH3+OH CH3COCH2+H2O (5)
Reactions 3-5 were added to the GRI 3.0 mechanism; rate coefficients measured by
Vasudevan [99] were used for k3 and k5 , respectively, while Benson and O’Neal [165]
evaluation was used for k4. Also, acetone decomposition reaction, CH3COCH3
CH3CO+CH3 was added with rates taken from Vasudevan [99]. For all species, GRI 3.0
thermodynamic data were used except for the following species. TBHP and (CH3)3CO
data were taken from [94], OH from [97,164], CH3COCH3 from [46], and CH3COCH2
from [47,166]. Note that the thermodynamic data of CH3COCH3 and CH3COCH2 had
87
zero sensitivity to our experimental method. To the above modified GRI 3.0 mechanism,
propene sub-mechanism containing 53 reactions (for propene and products formed from
propene) and 14 species (and associated thermo data) from JetSurF 1.0 [46] were added
to model our propene experiments. All model simulations were conducted assuming
homogeneous adiabatic conditions behind reflected shock waves, with the common
constant-internal-energy, constant-volume constraint (constant U; V) using CHEMKIN
4.1.1 [101].
Even though we cannot distinguish between the various product channels of reactions
1 and 2, the total rate of OH and alkenes measured is independent of the branching ratios.
The choice of products has no discernible effect on our overall rate determination. The
product of C2H4+OH for modeling purpose was taken as C•HCH2+H2O (as given in GRI
3.0 [161]) and the products of C3H6+OH were taken to be water and the three abstraction
products CH3C•CH2, CH3CHC•H, C•H2CHCH2 (allyl). Detailed discussion about the
mechanism of OH+alkenes is provided later.
6.2.1 OH+C2H4, k1
A sample OH absorption trace at the middle of our ethylene temperature range is
shown in Figure 6.1. TBHP and water mixture is not an ideal mixture, which makes it
difficult to know the partial pressure of TBHP. Since wall adsorption and condensation of
TBHP is possible, initial TBHP mole fraction had to be inferred from the OH data.
Previously, GC analyses indicated that there is no decomposition of TBHP in the gas
phase in the mixing chamber [99]. Before preparing the first TBHP mixture, mixing tank
was cleaned thoroughly using acetone and was pumped down for 3 days using Turbo.
The mixing tank and connecting lines were passivated and pumped down (procedure
repeated multiple times) in order to avoid any impurities and to achieve consistent OH
peak yield. It is therefore important that experiments are conducted under pseudo-first-
order conditions in order to minimize influence from secondary chemistry arising out of
TBHP decomposition. For the conditions of the experiment shown (1201 K and 1.992
atm) for 500 ppm of initial ethylene, the measured peak OH yield is ~ 23 ppm. In model
simulations, an initial TBHP mole fraction that resulted in the measured peak OH yield
was used. For example, for the experiment presented in Figure 6.1, an initial TBHP mole
88
fraction of 23 ppm was used. For the experiment shown in Figure 6.1, an overall rate
coefficient of 2.6 x 1012 cc/mol/s was obtained using the best-fit. Model predictions for a
variation of 50% of the measured rates are also shown. In current ethylene experiments,
to confirm that our modeling is consistent, experiments were conducted with TBHP
concentration between 20-40 ppm and higher ethylene concentrations (751.1ppm and
1000ppm). Table 6.1 summarizes the current measurements of the rate coefficient of
OH+ethylene reaction.
0 20 40 60 80 100 120 140 1600
5
10
15
20
25
Time [s]
OH
[pp
m]
1201K, 1.992atm500 ppm ethyleneAr bath gas~23 ppm TBHP (fit)
Experiment
kOH+ethylene
= 2.6 x 1012 cm3mol-1s-1 (best fit)
(k+50%) (k-50%)
Figure 6.1 Example OH+ethylene rate measurement at 1201K, 1.992 atm. Model
predictions using the best-fit predictions for the rate and a factor of two variations from
the measured rate are also shown.
An OH radical sensitivity analysis (for the conditions in Figure 6.1) is provided in
Figure 6.2. In this work, sensitivity is defined as S=(dXOH/dki)/(ki/XOH), where XOH is
the local OH molefraction and ki is the rate coefficient of reaction i. Sensitivity analysis
clearly shows that reaction between OH and ethylene is the only dominant sensitive
reaction over the entire time frame of the experiment with slight interference from the
following reactions:
CH3COCH3+OH CH3COCH2+H2O (5)
CH3+OH1CH2+H2O (6)
CH3+OH(+M) CH3OH(+M) (7)
CH3+ CH3 (+M) C2H6 (+M) (8)
89
0 40 80 120 160-3.75
-3.50
-3.25
-3.00
-2.75
-2.50
-2.25
-2.00
-1.75
-1.50
-1.25
-1.00
-0.75
-0.50
-0.25
0.00
0.25
Time [s]
OH
Sen
sitiv
ity
OH+ethylene CH
3COCH
3+OH<=>CH
3COCH
2+H
2O
OH+CH3(+M)<=>CH
3OH(+M)
OH+CH3<=>1CH
2+H
2O
2CH3(+M)<=>C
2H
6(+M)
Figure 6.2 OH sensitivity plot for rate measurement of OH+ethylene at 1201 K, 1.992.
Because the initial concentration ratio of the reactants ([ethylene]0/[TBHP]0) ratio is
larger than 20, the chemistry is very nearly first-order and hence rate coefficients can also
be determined without using a detailed mechanism fit (see Cook et al. [167]). By
measuring the slope of the initial ln[OH] versus time (which is almost linear) in some of
our experiments, pseudo-first-order rate coefficients can be obtained for total
OH+[ethylene, CH3, and other OH removing species] reaction. Similarly, by conducting
experiments in similar TBHP and argon (without ethylene) concentrations, a pseudo-first-
order rate coefficient for OH+(CH3, and other OH removing species) were obtained. The
difference of these two pseudo-first-order rates give k1 (OH+ethylene) and are also
tabulated in Table 6.1. It is clear from Table 6.1 that k1 determined using the pseudo-first-
order method is typically within 10% of the k1 determined using detailed modeling fit (a
similar conclusion was reached by Cook et al. in their OH+dimethylether experiments).
This further reflects the sensitivity of the current experiments to the target reaction (k1).
A detailed error analysis was carried out to set uncertainty limits on the measured rate
coefficients (using detailed modeling fit). Typical contributions to uncertainties in the
rate coefficients considered are from sources of uncertainties in: temperature (±0.7%),
absorption cross-sections (± 3%), fitting the data to computed profiles, and uncertainties
resulting from secondary chemistry. The individual contributions of each error source
90
were determined by perturbing each error source to the estimated positive and negative
bounds of its 2-σ uncertainty and re-fitting the experimental traces by adjusting the rate
coefficient of interest [98]. The resulting uncertainty in the rate coefficient for each
individual error source were combined using the root-sum-squares (RSS) method, which
assumes that the uncertainty contributions are uncorrelated and are estimated to similar
probabilities (e.g., 2-σ probability that the true value falls within the ± error limit). The
main uncertainty categories and their effect on the target reaction rate of OH+ethylene, k1,
(following the approach used in [98]), for the experiment at 1201K, 1.992 atm are shown
in Table 3. The effect of each of the above uncertainty categories on the rate coefficient
of OH+ethylene (k1) is ascertained and combined to yield an overall uncertainty estimate
of ± 23 % at 1201 K and 1.992 atm.
6.2.2 OH+C3H6, k2
The approach for propene data reduction is the same as that employed in the case of
ethylene. A sample OH absorption trace at the middle of the current propene temperature
range is shown in Figure 6.3. For the conditions of the experiment shown (1136 K and
2.024 atm) for 299.33 ppm of initial propene, the measured peak OH yield is ~ 20 ppm.
For the experiment shown in Figure 6.3, an overall rate coefficient of 7.84 x 1012 cm3mol-
1s-1 was obtained using the best fit. The most sensitive product channel was abstraction
leading to resonance stabilized allyl channel and this rate was varied to achieve the
overall rate. Model predictions for a variation of 50% of the measured OH+propene (k2)
rate are also shown in Figure 6.3. Table 6.3 summarizes the current measurements of the
rate coefficient of OH+propene reaction. An OH radical sensitivity analysis (for the
conditions in Figure 6.3) is provided in Figure 6.4. Sensitivity analysis clearly shows that
reaction between OH and propene is the dominant reaction over the entire time frame of
the experiment with slight interference from reactions 5-8 (as in the case of ethylene). A
detailed error analysis was carried out to set uncertainty limits on the measured
OH+propene rate coefficients. The main uncertainty categories and their effect on the
target reaction rate of OH+propene, k2, (following the approach used in [98]), for the
experiment at 1136 K, 2.024 atm are also shown in Table 6.2. The effect of each of the
91
uncertainty categories on the rate coefficient of OH+propene is ascertained and combined
to yield an overall uncertainty estimate of ± 17 % at 1136 K and 2.024 atm.
-20 0 20 40 60 80 100 1200
5
10
15
20
OH
[ppm
]
Time [s]
Experiment
kOH+propene
=7.84 x 1012 cm3mol-1s-1 (best fit)
(k+50%) (k-50%)
1136 K, 2.024 atm299.33 ppm propeneAr bath gas~20.4 ppm TBHP (fit)
Figure 6.3 Example OH+propene rate measurement at 1136 K, 2.024 atm. Model
predictions using the best fit predictions for the rate and 50% variation from the measured
rate are also shown.
0 40 80 120
-4
-3
-2
-1
0
1136 K, 2.024 atm299.33 ppm propeneAr bath gas~20.4 ppm TBHP (fit)
Time [s]
OH
Sen
sitiv
ity
OH+propene CH
3COCH
3+OH<=>CH
3COCH
2+H
2O
OH+CH3(+M)<=>CH
3OH(+M)
OH+CH3<=>1CH
2+H
2O
2CH3(+M)<=>C
2H
6(+M)
Figure 6.4 OH sensitivity plot for rate measurement of OH+propene rate measurement at
1136 K, 2.024 atm.
92
6.2.3 TBHP Decomposition, k3
At low temperatures (890-1014K), an OH radical sensitivity analysis revealed that at
very early times (<20us), TBHP decomposition (k3), is the most sensitive reaction. This
suggested the possibility of extracting the rate coefficient for reaction 3 by fitting the
early-time, modeled OH traces with the experimental time histories. This approach was
successfully used by Vasudevan [99] in TBHP and formaldehyde (in Ar) experiments to
measure k3 in the range 934-972 K. A sample early-time OH absorption trace and the
corresponding sensitivity plots are shown in Figure 6.5 and Figure 6.6, respectively. For
the conditions of the experiment shown (890 K and 2.615 atm) for 300 ppm of initial
propene, the measured peak OH yield is ~ 21 ppm (for all propene experiments, initial
TBHP concentration was kept constant). For the experiment shown in Figure 6.5, using
the early-time fit, (CH3)3-CO-OH OH+ (CH3)3CO, k3=9.5 x 104 s-1 was obtained using
the best-fit method. Using the procedure outlined in shown in Table 6.2, overall
uncertainty bars at 890 K and 2.615 atm of ± 15.6% are estimated for k3.
Note that the overall rate coefficient of OH+propene, k2=4.14 x 1012 cm3mol-1s-1, was
obtained using the best-fit method for the latter part of the profile, where the dominant
reaction is OH+propene (the procedure to obtain k2 was discussed in previous section). In
these low temperature experiments, a slightly higher uncertainty bars are estimated for k1
and k2 due to the contribution from reaction 3. The uncertainty contributions are small
because we used the measured rate from Vasudevan et al. [99] for reaction 3 (which had
an uncertainty of ± 25%), while fitting the latter part of the OH profile (to estimate k1 and
k2) in these low-temperature experiments. Figure 6.7 shows the influence of reaction 3 on
the determination of k2 at 890 K and 2.615 atm (for the conditions shown in Figure 6.5).
Using the root-sum-squares procedure (as described in Table 6.2), in these low
temperature experiments, the combined uncertainty in k2 will increase by less than 2%
due to uncertainty contribution from reaction 3.
93
0 20 40 60 80 1000
2
4
6
8
10
12
Experiment
best fit: k3=9.5 x 104 s-1, k
OH+propene=4.14 x 1012 cm3mol-1s-1
k3+50%
k3-50%
890 K, 2.615 atm300 ppm propeneAr bath gas~20.7 ppm TBHP (fit)
OH
, [p
pm]
Time, [s]
Figure 6.5 (CH3)3-CO-OH OH+ (CH3)3CO, k3, rate measurement at 890 K, 2.615 atm.
Model predictions using the best fit predictions for the rate and 50% variation from the
measured rate (k3) are also shown.
0 5 10 15 20 25
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
890 K, 2.615 atm300 ppm propeneAr bath gas~20.7 ppm TBHP (fit)
OH
Sen
sitiv
ity
Time [s]
TBHP<=>(CH3)
3CO+OH
OH+Propene
OH+CH3<=>1CH
2+H
2O
OH+CH3(+M)<=>CH
3OH(+M)
Figure 6.6 Early-time OH sensitivity plot during OH+propene rate measurement at 890
K, 2.615 atm showing the dominance of TBHP decomposition, (CH3)3-CO-OH OH+
(CH3)3CO, k3.
94
0 20 40 60 80 1000
2
4
6
8
10
12
Experiment
best fit: k3=9.5 x 104 s-1, k
OH+propene=4.14 x 1012 cm3mol-1s-1
(k3-25%), k
OH+propene=4.54 x 1012 cm3mol-1s-1
(k3+25%), k
OH+propene=3.84 x 1012 cm3mol-1s-1
890 K, 2.615 atm300 ppm propeneAr bath gas~20.7 ppm TBHP (fit)
OH
, [pp
m]
Time, [s]
Figure 6.7 Example low-temperature OH+propene, k2, rate measurement at 890 K, 2.615
atm. Adjustments to k2 using the 25% uncertainty in (CH3)3-CO-OH OH+ (CH3)3CO
are also shown.
At reflected shock temperatures greater than about 1300K, OH radicals are formed
behind the incident shock front itself where temperature behind incident shock, T2, is
high enough to cause some TBHP decomposition (T2=745-795 K). These OH traces
behind the incident shock region does not affect the measurement of OH+alkene
reactions behind the reflected shock region since in such high-temperature reflected
shocks, OH+alkenes rate coefficients extracted in both modeling cases (incorporating the
incident shock OH or without it) are the same. However, incident shock OH can be
modeled to extract rate coefficients for reaction 3 because the OH sensitivity shows that
this reaction dominantly influences OH formation under incident shock condition. Figure
6.8 shows a sample sensitivity at 795K and 0.558 atm. Figure 6.9 shows a sample
incident shock measurement of reaction 3 at 795K and 0.558 atm. Note that the
laboratory time scales in these cases had to be converted into the actual reaction time
(particle time) by multiplying with the ideal density ratio (ρ2/ρ1 = 2.65 in the case of
Figure 6.9). Even though the boundary layer growth behind an incident shock complicate
the actual motion of a particle (from ideal theory), our measurements are close to the
95
region immediately behind the incident shock (i.e., short time scales) and in a large
diameter shock tube (14.13cm), where this assumption is justifiable (see for details [168-
170]) and has been successfully used previously in our laboratorary by Oehlslaeger [98]
in his alkane decomposition measurements.
In Figure 6.9, model predictions using the best-fit predictions for the rate and 50%
variation from the measured rate (k3) are shown. Note that the GRI 3.0 [161] value for
reaction 1 was used at this condition since our measurements for k1 does not go to such
low-temperatures. However, owing to the large sensitivity of reaction 3, a factor of 2
uncertainty in k1 (a conservative estimate) does not have much influence (see Figure 6.9)
on the measurement of k3. For the experiment shown in Figure 6.9, using the early-time
fit (~75 s), (CH3)3-CO-OH OH+ (CH3)3CO, k3, =7.05 x 103 s-1 was obtained using
the best fit method. Using the procedure shown in Table 6.2, overall uncertainty bars at
795 K and 0.558 atm of ± 20% are estimated for k3 in incident shock experiments, which
is higher than the k3 uncertainty in reflected shock experiments. It should be noted that
majority of the uncertainty contribution comes from the wavemeter uncertainty (± 0.01
cm-1) because at current incident shock experimental conditions, the OH absorption is
line is not optimized for incident conditions (lies away from the peak) and small changes
in wavenumber induces large changes in absorption coefficient. Table 6.4 summarizes all
the current measurements of the rate coefficient of (CH3)3-CO-OH OH+ (CH3)3CO, k3,
reaction in the range 745-1014 K.
96
0 20 40 60 80 100 120
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
795 K, 0.558 atm1000 ppm ethyleneAr bath gas~21.4 ppm TBHP (fit)
Time, [s]
OH
Sen
sitiv
ity
TBHP=OH+(CH3)
3CO
OH+ethylene
OH+CH3<=>1CH
2+H
2O
Figure 6.8 OH sensitivity plot during incident shock for (CH3)3-CO-OH OH+
(CH3)3CO, k3, rate measurement at 795 K, 0.558 atm.
0 20 40 60 80 100 1200
2
4
6
8
10
Experiment
best fit: k3=7.05 x 103 s-1, k
1=GRI 3.0
Sensitivity to k3
(k3+50%), k
1=GRI 3.0
(k3-50%), k
1=GRI 3.0
Sensitivity to k1
k3=7.05 x 103 s-1, k
1=2*GRI 3.0
(k3+10.6%), k
1=2*GRI 3.0
k3=7.05 x 103 s-1, k
1=0.5*GRI 3.0
(k3-5%), k
1=0.5*GRI 3.0
Time, [s]
795 K, 0.558 atm1000 ppm ethyleneAr bath gas~21.4 ppm TBHP (fit)
OH
, [p
pm]
Figure 6.9 Example incident shock (CH3)3-CO-OHOH+(CH3)3CO, k3, rate
measurement at 795 K, 0.558 atm. Model predictions using the best fit predictions for the
rate and 50% variation from the measured rate (k3) are shown. Influence on k3 due to an
assumed factor of 2 uncertainties in k1 is also shown.
97
6.3 Comparison with Previous Data
Figure 6.10 presents a comparison of the current measurements of k1 with past data.
Evident in Figure 6.10 are the following observations regarding current data: the
relatively low scatter, good agreement between rate coefficients measured using detailed
modeling and those measured using pseudo-first-order method, and the lack of pressure
dependence as expected for such reactions at high-temperatures. In the overlapping
temperature range, our measurements agree very well with shock tube measurements
from Bott and Cohen [50] (reflected shock, OH resonance absorption), and Bradley et al
[71] (incident shock, UV lamp, relative rate method) and also with the pulse radiolysis
measurements from Liu et al. [64,77]. However, current data are lower than the flame
measurements by Westenberg and Fristrom [74] and is a factor of two higher than the
Smith [70] (laser pyrolysis/LIF) data. Even though there is agreement near 1000K
between current data and the Westbrook et al. [78] flow reactor measurements, above this
temperature the Westbrook et al. [78] measurements are consistently lower. Near 1450K,
current highest temperature measurements and the lowest temperatures measurements
from both Srinivasan et al. [81] (reflected shock, UV Lamp) and Bhargava and
Westmoreland [73] are consistent within the uncertainty bounds of these measurements.
Current lowest temperature measurements are consistent with the lower temperature
measurements of Tully [76] (laser photolysis/ LIF) abstraction measurements.
Note that the overall reaction k1 shows significant non-Arrhenius behavior over the
entire range shown in Figure 6.10. This behavior can be related to the change in
mechanism as a function of temperature in the alkene + OH reactions [171]. At low
temperatures (typically below ~500 K) hydrogen abstraction is very slow, and the only
experimentally measurable reaction is the OH addition to the double bond(s). Addition
typically has negative temperature dependence due to the topography of the entrance
channel of the addition process. As the temperature is increased, new channels open up:
Back-dissociation of the adduct to the reactants, which decreases the apparent rate of loss
of OH, and isomerization channels to form bimolecular products. This increases the
apparent rate of loss of OH via H-abstraction, which also increases the apparent rate of
loss of OH. The order in which these processes “switch on” as a function of temperature
98
depends on the underlying potential energy surface, and is expected to vary among the
various alkenes; note also that the problem is further complicated by pressure dependence.
One consequence of this complex behavior is that at high enough temperatures back-
dissociation becomes so rapid that it completely masks the addition channel, and only the
H-abstraction and the non-abstraction bimolecular channels can be experimentally
observed. Experiments in the intermediate temperature range contain the convoluted
effect of all four (addition, backdissociation, abstraction and isomerization) processes.
0.5 1.0 1.5 2.0 2.51E11
1E12
1E13
k 1, (
Cm
3/m
ol/s
)
1000/T, (1/K)
Srinivasan et al. 2007, S.T.,UV lamp Smith 1987, Laser Pyrolysis/ LIF Baldwin et al. 1966, Vessel, abstraction Baldwin et al. 1984, Vessel Bradley et al. 1976, incident S.T., UV Lamp, (abstraction) Westenberg & Fristrom 1965, Flames, ESR/mass spec. Bhargava & Westmoreland 1998, Flames, MBMS Tully83, Flash Photolysis/LIF Tully88, Laser Photolysis/LIF, total rate Liu et al. 1987/1988, pulse radiolysis/UV lamp (total rate) Westbrook et al. 1988, Jet-stirrred reactor Vasu Pseudo-first order method (k) Vasu (Current Experiments 2.3atm) Vasu (Current Experiments 9atm) Bott and Cohen 1991, S.T., UV Lamp Fulle et al. 1997, Laser flash Photo/SLIF, 0.98atm Greiner 1970, flash photolysis/kinetic spectrograph
Figure 6.10 Arrhenius plot for OH+ethylene (k1) at temperatures greater than 400K.
The reaction between OH and propene (k2) has not been extensively studied at high
temperatures as in the case of the reaction between OH and ethylene. Figure 6.11 presents
a comparison of the current measurements of k2 with past data. The relatively low scatter
in the current data is very evident in this case (Figure 6.11). Very good agreement
between current data and that of Bott and Cohen [50] is seen. However, current data lay
between those measurements from Yetter and Dryer [91] (flow reactor) and Smith et al.
[88] (laser pyrolysis/LIF), and are consistent with the lower temperature measurements of
Tully and Goldsmith [87]. Both the Yetter and Dryer [91] and the Smith et al. [88]
measurements have large uncertainties owing the nature of their experiments. Figure 12
99
shows that we have extended the OH+propene measurements to higher temperatures than
currently exist in the literature. In the case of the propene + OH reaction it was shown
experimentally by Tully and Goldsmith [87] that back-dissociation becomes
instantaneous on the experimental timescales above ~700 K and that hydrogen
abstraction is the dominant channel.
0.5 1.0 1.5
1E13
OH
+C
3H6=
pro
duct
s, [
cm3 /m
ol/s
]
1000/T, [1/K]
JetSurf 1.0 Bott & Cohen, 1991, S.T., 1.05 atm Smith et al. 1985 , Laser Pyrolysis/LIF, ~ 0.1atm Yetter and Dryer 1992 , Flow reactor, 1atm Current data Tully and Goldsmith1985, laser photolysis/LIF, 0.03-0.8atm
Figure 6.11 Arrhenius plot for OH+propene (k2) at temperatures greater than 800 K.
Figure 6.12 summarizes our and earlier measurements of reaction (3). There is good
agreement between all the studies on the temperature dependence of this rate coefficient
(400-1050 K). Our data have very low scatter and are in excellent agreement with earlier
measurements by Vasudevan et al. [99] (reflected shock, OH laser absorption) and
Benson and Spokes [172] (very low pressure pyrolysis/mass spectrometry, high-pressure
limit) in our temperature range (745-1014 K). Current data are consistent with lower
temperature measurements by Kirk and Knox [173] (flow system/GC), Sahetchian et al.
[174] (flow system/GC) and Mulder and Louw [175] (flow reactor, GC). Eliminating the
lowest temperature data at 572K from Benson and Spokes [172], all the data in Figure
6.12 can be fit to a 3-parameter form applicable over 400-1050 K:
k3=8.13 x 10-12 (T)7.827 exp(-14598/T) (s-1) (Eqn. 1)
where T is in K. This expression (R2=0.996) is also shown in Figure 6.12 and can be
safely used in the entire T range. Pressure dependence is expected for this reaction
100
eventhough current expression is close to the high-pressure limit [172]. Equation 1 is
approximately 40% lower (near 1000 K) than the only review recommendation of this
reaction by Benson and O’Neal review [165].
1.0 1.5 2.0 2.51E-5
1E-4
1E-3
0.01
0.1
1
10
100
1000
10000
100000
1000000
1E7
TB
HP
=(C
H3) 3
CO
+O
H, [
1/s
]
1000/T [1/K]
Benson & O'Neal 1970, review Benson & Spokes 1968, VLPP/mass spec Current data (0.5-2.5atm), S.T., UV absorption Vasudevan et al. 2004, S.T., laser absorption Sahetchian et al. 1992, Flow system/GC Kirk & Knox 1960, Flow system/GC Mulder and Louw 1984, flow reactor, GC Current fit
Figure 6.12 Arrhenius plot for TBHP(CH3)3CO+OH.
6.4 Theoretical Calculations of OH+C2H4 and
OH+C3H6
Figure 6.13 shows current and some of the previous k1 data in comparison with recent
theoretical calculations and evaluations. The GRI 3.0 [161] value and the recent Baulch
et al. [176] review are approximately 30% lower (near 1000 K) and the Warnatz [177]
101
review value is 5 times larger at 1000 K than the current data. Liu et al. [178] TST
calculations at the G2//QCISD/6-31G(d,p) level for abstraction are 46% higher and
shows a different activation energy than current k1 data at 1000 K, while the Hippler and
Viskolcz [179] QCISD(T) calculations for the total rate are 50% lower. Liu et al.
calculations also showed that variational and tunneling effects are negligible for this
reaction. We feel that the Hippler and Viskolcz [179] calculations are wrong because
their study concluded that the direct abstraction is not important above 600 K. Very
recent high-level theoretical calculation for abstraction from Senosiain et al. [171] at the
RQCISD(T)/cc-pV∞Z//UQCISD/6-311++G(d,p) level is shown in Figure 6.13 and lie
approximately 40% lower than our data. The Senosiain et al. calculations for the total rate
between 1-10 atm are in good agreement with current data (see Figure 6.13).
We conducted canonical TST calculations using the MultiWell-2010.1 code (by
Barker et al. [180-183]) and using the transition state parameters from the Senosiain et al.
[171] theoretical study (abstraction) and experimentally observed parameters for
reactants (see Appendix D). We used the asymmetric 1-D Eckhart barriers [184] to
incorporate the effects of tunneling and non-classical reflection. Torsional vibration
modes were treated as 1-D hindered rotor approach using the Pitzer-Gwinn approach
[185], which is the most common practice for treating anharmonic motions in quantum
chemistry [186]. The barrier to abstraction are increased by 0.6 kcal/mol (Senosiain et al.
[171] calculations had an estimated uncertainty of ~ 2 kcal/mol) in order to get the best fit
over a wide range of temperatures for k1 in our calculations. Our TST calculation results
are shown in Figure 6.14 which shows excellent agreement with data from a wide range
including current, Tully [76], Bott and Cohen [50], Bradley et al [71], Srinivasan et al.
[81] and Bhargava and Westmoreland [73]. The resulting expression in the range 600-
2000 K can be incorporated directly into combustion models and is given as:
k1=2.23 x 104 (T)2.745 exp(-1115/T) (cm3mol-1s-1) (Eqn. 2)
Caution should be applied while using this expression outside our experimental range
(950-1450 K) to represent the total rate of OH+ethylene.
102
0.5 1.0 1.51E11
1E12
1E13
1E14
k 1(C
m3 /m
ol/s
)
1000/T,(1/K)
GRI: C2H4+OH (abstraction) Liu et al. 2002, theory, G2//QCISD/6-31G(d,p), TST (abstraction) Hippler and Viskolcz 2000, theory, TST, QCISD(T),total rate BaulchReview1992/2005 (abstraction) WarnatzReview(1984),abstraction Senosiain et al. 2006 (abstraction channel) Senosiain et al. 2006 (1 atm) Srinivasan et al. 2007, S.T.,UV lamp Bradley et al. 1976, incident S.T., UV Lamp, (abstraction) Bhargava and Westmoreland 1998, Flames, MBMS, abstraction Tully88, Laser Photolysis/LIF, total rate Vasu (Current Experiments 2.3atm) Vasu (Current Experiments 9atm) Bott and Cohen 1991, S.T., UV Lamp
Figure 6.13 Arrhenius plot for OH+ethylene (k1) at temperatures greater than 400K.
Comparison with theory and evaluations is shown.
Figure 6.15 compares shows current and the previous k2 data with recent theoretical
calculations. Zhou et al. [187] calculations at the PMP2/aug-cc-pVQZ//MP2/cc-pVTZ
level using variational RRK are a factor of 10 lower than our data, which we suspect
might be due to the harmonic treatment of torsional vibration modes. Both the Huynh et
al. [188] (CCSD(T)/cc-pVTZ) and the Zádor et al. [49] (RQCISD(T)/cc-
pVinfZ//B3LYP/6-311++G(d,p)) calculations are in excellent agreement with current,
Bott and Cohen [50], and Tully and Goldsmith [87] data. In the case of reaction (2) there
are 4 different hydrogen abstraction channels (the allyl radical channel is the dominant
one due to resonance stabilization) as described in detail by Zádor et al. [49]; in Figure
6.15, the sum of total abstraction rates is shown. Note that the only high temperature
evaluation of this rate by Tsang [189] was used in the JetSurF 1.0 [46] mechanism and it
is approximately 30% lower than our data near 1200 K (see Figure 6.11). Our TST
calculations using MultiWell-2010.1 code employing Zádor et al. [49] frequencies and
energies for the transition state and experimental values for the reactants (see Appendix
D) are also shown in Figure 6.15. Our TST results show excellent agreement with data
from a wide range of studies and can be fit to a 3-parameter form as:
103
k2=1.94 x 106 (T)2.229 exp(-540/T) (cm3mol-1s-1) (Eqn. 3)
The equation 3 is recommended for use with combustion models in the range 700-1500 K
with the quoted uncertainty applying in our T range 850-1400 K.
0.6 0.8 1.0 1.2 1.4 1.61E11
1E12
1E13
k 1,
C2H
4+O
H,
(cm
3/m
ol/s
)
1000/T, [1/K]
cTST+Tunneling (barrier adjusted to 5.5kcal) cTST+Tunneling (un adjusted = 4.9kcal) Srinivasan et al. 2007, S.T.,UV lamp Baldwin et al. 1984, Vessel Bradley et al. 1976, incident S.T., UV Lamp, (abstraction) Bhargava and Westmoreland 1998, Flames, MBMS, abstraction Tully88, Laser Photolysis/LIF, total rate Current Experiments 9atm Current Experiments 2.3atm Bott and Cohen 1991, S.T., UV Lamp Smith 1987, Laser Pyrolysis/ LIF
Best fit: Current TST
Figure 6.14 Arrhenius plot for OH+ethylene (k1) at temperatures greater than 600K.
Comparison with current TST calculations is shown.
104
0.50 0.75 1.00 1.25 1.501E11
1E12
1E13
Tully and Goldsmith1985, laserphotolysis/LIF, 0.03-0.8atm Vasu et al., current (~2.3 atm) propene+OH Vasu TST (Zador frequencies) Yetter and Dryer 1992 , Flow reactor, 1atm Smith et al. 1985 , Laser Pyrolysis/LIF, ~ 0.1atm Bott & Cohen, 1991, S.T., 1.05 atm
OH
+C
3H6=
prod
ucts
, [c
m3 /m
ol/s
]
1000/T, [1/K]
Zhou et al. 2009, ab initio,variational RRK, (PMP2/aug-cc-PVQZ//MP2/cc-PVTZ (highT)) Zador et al. 2010 (Abstraction channel) Huynh et al.2009, ab-initio, CCSD(T)/cc-pVTZ
Figure 6.15 Arrhenius plot for OH+propene (k2) at temperatures greater than 650K.
Comparison with theoretical calculations is shown.
105
Table 6.1 OH+ethylene rate coefficient data.
T (K) P (atm)
kOH+ethylene (1012 cm3mol-1s-1)
kOH+ethylene (1012 cm3mol-
1s-1) (pseudo-first-order)
1000 ppm ethylene, 20-40 ppm TBHP in Ar 973 2.52 1.16 1.03 1014 2.46 1.4 1.51 1078 2.45 1.7 1.82 1321 2.19 3.5 3.30 1438 2.09 4.69 1301 2.19 3.1 3.39 1232 2.31 2.9 2.98 500 ppm ethylene, ~ 21 ppm TBHP in Ar 1007 2.56 1.55 1085 2.28 1.8 1201 1.99 2.6 751 ppm ethylene, ~ 24 ppm TBHP in Ar 1055 10.18 1.5 1102 7.00 2.0
106
Table 6.2 Uncertainty analysis for OH+alkenes reactions. OH+ethylene (k1): initial
incident shock conditions: 1201 K, 1.99 atm, and 500 ppm ethylene/Ar. OH+propene (k2):
initial incident shock conditions: 1136 K, 2.02 atm, and 299 ppm ethylene/Ar.
Error source 2-σ uncertainty in error source
Uncertainty in (k1, k2) from
positive perturbation
Uncertainty in (k1, k2)
from negative
perturbation
2-σ average uncertainty
contribution in (k1, k2)
Experimental uncertainties
Temperature ± 0.7%, [98] (< 0.1%, < 0.1%)
(< 0.1%, < 0.1%)
(< 0.1%,< 0.1%)
Fitting (signal/noise)
± 5% (5%, 5%) (-5%, -5%)
(5%, 5%)
OH absorption
coefficient (kν) ± 3% , [98] (4%, 2.7%) (-4%, -
6%) (4%, 4.3%)
Mixture concentration
± 0.43 ppm ethylene, [98]
(< 0.1%, < 0.1%)
(< 0.1%, < 0.1%)
(< 0.1%, < 0.1%)
Wavemeter reading ±0.0.01cm-1, [99] (< 0.1%, <0.1%)
(< 0.1%, -1.8%)
(< 0.1%, 0.9%)
Secondary chemistry uncertainties CH3COCH3+OH
CH3COCH2+H2O ± 30%, [99] (0.1%, <
0.1%) (0.1%, < 0.1%)
(0.1%, < 0.1%)
CH3+OH 1CH2+H2O
(uncert. factor =2), [99]
(-27%,-17.7%)
(16%, 11.6%)
(21.5%, 14.6%)
CH3+OH(+M) CH3OH(+M)
(uncert. factor =2), [99]
(-5.8%, -4.3%)
(2%, 2.7%)
(3.9%, 3.5%)
CH3+CH3(+M) C2H6(+M)
(uncert. factor =2) (1.9%, 1%) (-1.1%, -1%)
(1.5%, 1%)
Total 2-σ R.S.S. uncertainty in (k1, k2):
(± 23 %, ±17 %)
107
Table 6.3 OH+propene rate coefficient data.
T (K) P (atm) kOH+propene (1012 cm3mol-1s-1)
299.33 ppm propene, ~ 20ppm TBHP in Ar
1136 2.02 7.84
1225 2.27 9.93
1248 2.21 9.94
1287 2.19 10.53
1142 2.15 7.88
1302 2.09 11.10
1366 2.10 12.34
1137 2.34 7.86
1107 2.36 7.44
1044 2.43 6.46
1364 2.08 12.33
300 ppm propene, ~ 20ppm TBHP in Ar
1033 2.47 6.17
997 2.43 5.85
980 2.51 5.59
932 2.53 4.65
890 2.62 4.14
108
Table 6.4 TBHP decomposition rate data. Superscripts denotes mixtures: a500ppm
ethylene; b1000ppm ethylene; c300 ppm propene.
T (K) P (atm) TBHP decomposition rate
(s-1)
1007 2.56 6.82 x 105 a
745 0.61 9.00 x 102 b
795 0.56 7.05 x 103 b
1014 2.46 1.05 x 106 b
764 0.57 2.30 x 103 c
763 0.57 1.90 x 103 c
997 2.43 6.90 x 105 c
980 2.51 4.65 x 105 c
932 2.53 2.20 x 105 c
890 2.62 9.50 x 104 c
109
Chapter 7: 1,3-Butadiene+OHProducts This chapter describes measurements of the reactions of hydroxyl (OH) radicals with
1,3-butadiene behind reflected shock waves. Highly sensitive measurements of the
overall rates were performed in dilute mixtures of 1,3-butadiene/TBHP/Ar. Rate
constants for the OH reactions with 1,3-butadiene were extracted by varying the overall
rate to achieve a match between computed and measured OH concentration time-histories
behind reflected shocks. 1,3-Butadiene reacts with OH to form various products
including water (the final product):
1,3-C4H6+OHProducts (1)
The rate coefficient and the branching fractions for the H-abstraction channels of the
target reaction were also calculated over the temperature range 250-2500 K using
variational transition-state theory based on QCISD(T)/cc-pVZ//B3LYP/6-311++G(d,p)
quantum chemistry.
7.1 Method
All experiments were performed behind reflected shock waves in the LPST. Mixtures
were made using argon (99.999%, Praxair), 70% tert-butyl hydroperoxide (TBHP) in
water and 1,3-butadiene (99+%) (Sigma-Aldrich). The TBHP method used is described
in the previous chapter.
7.2 Kinetic Measurements
A total of 16 shock tube experiments were performed over a temperature range of
1011 to 1406 K and pressure range of 1.91 to 2.44 atm to determine the overall reaction
rate coefficient k1 of OH+1,3-butadiene at near-pseudo-first-order conditions. Test gas
mixtures were comprised of nominally 100 ppm 70% TBHP in H2O and 356 or 514 ppm
of 1,3-butadiene dilute in argon. A sample OH absorption trace is shown in Figure 7.1.
For the conditions of the experiment shown (1200 K, 2.20 atm, 356 ppm 1,3-
butadiene/argon), the measured peak OH yield is 18 ppm. Because wall adsorption and
condensation of TBHP is possible, the initial TBHP mole fraction was inferred directly
110
from the OH data. In all experiments, the initial OH yield (and thus nominally the initial
TBHP mole fraction) was ~ 18 ppm.
0 20 40 60 80 1000
4
8
12
16
20
24
2.20 atm, 1200 K356 ppm 1,3-butadieneAr bath gas18ppm TBHP (fit)
OH
[p
pm
]
Time [s]
Experiment k
1(best fit)
1.5k1
0.5k1
Figure 7.1 Example OH+ 1,3-butadiene rate measurement. Initial reflected shock
conditions: 1200K, 2.20 atm, 356 ppm 1,3-butadiene, 18 ppm TBHP in argon. Model
predictions using the best fit predictions (k1 = 5.92x1012 cm3/mol/s) for the target rate
coefficient and 50% variation from the measured rate are also shown.
The reaction of OH and 1,3-butadiene can lead to two C4H5 isomers:
CH2CHCHCH2 + OH CH2CHCHC•H + H2O (1a)
CH2CHC•CH2 + H2O (1b)
In our experiments, only the total rate coefficient (k1 = k1a + k1b) is determined as OH
decay is measured and not the product formation. At the low concentrations involved in
the present experiments, the total rate coefficient, k1, is independent of the above
branching ratios and the choice of products has no discernible effect on the overall rate
determination. The tert-butoxy radical from TBHP decomposition immediately falls apart
to form acetone and a methyl radical. The resulting acetone and methyl radicals can also
react with OH.
(CH3)3-CO-OH OH+ (CH3)3CO (2)
(CH3)3CO CH3+ CH3COCH3 (3)
CH3COCH3+OH CH3COCH2+H2O (4)
111
A reaction mechanism for 1,3-butadiene oxidation and TBHP pyrolysis was
constructed based on the 1,3-butadiene mechanism of Laskin et al. [56]. Reactions 2-4
were added to the Laskin et al. mechanism; rate coefficients measured by Vasudevan [99]
were used for k2 and k4, while the Benson and O’Neal [165] evaluation was used for k3.
Rates from the GRI 3.0 mechanism were used for all three channels of the CH3+OH
reaction [161]. The acetone decomposition reaction, CH3COCH3 CH3CO+CH3, was
added with rates taken from Vasudevan [99]. For all species, the Laskin et al. [56]
thermochemical data were used except for the following species: TBHP and (CH3)3CO
data were taken from [94], OH from [97,164], and CH3COCH2 from [47,166]. All model
simulations were conducted assuming homogeneous adiabatic conditions behind reflected
shock waves, with the common constant-internal-energy, constant-volume constraint
(constant U,V) using CHEMKIN 4.1.1 [101].
An OH radical sensitivity analysis (for the conditions in Figure 7.1) is provided in
Figure 7.2. Here sensitivity is defined as S=(dXOH/dki)/(ki/XOH), where XOH is the local
OH mole fraction and ki is the rate coefficient of reaction i. Sensitivity analysis clearly
shows that reaction between OH and 1,3-butadiene is the only dominant sensitive
reaction over the entire time frame of the experiment. Because the initial concentration
ratio of the reactants ([1,3-butadiene]0/[TBHP]0) ratio is very large (~ 20 and 28 times
respectively, for the two 1,3-butadiene concentrations used in current experiments), the
chemistry is almost pseudo-first-order with slight interference from reactions (4)-(7):
CH3COCH3+OH CH3COCH2+H2O (4)
CH3 + OH 1CH2 + H2O (5)
CH3+OH(+M) CH3OH(+M) (6)
CH3+ CH3 (+M) C2H6(+M) (7)
112
0 20 40 60 80 100
-4
-3
-2
-1
0
OH+1,3-butadiene CH
3COCH
3+OH=CH
3COCH
2+H
2O
OH+CH3=1CH
2+H
2O
CH3+CH
3=C
2H
6
OH
Se
nsi
tivity
Time [s]
Figure 7.2 OH sensitivity plot for conditions of Figure 7.1. 1200K, 2.20 atm.
At the lowest temperatures (i.e. 1026K case shown in Figure 7.3 and Figure 7.4), there
is minor influence from Rxn. 2 (TBHP decomposition) at early times (< 3s), however,
this has a negligible influence on the decay of OH. OH sensitivities at all other conditions
are similar to that seen in Figure 7.2. To confirm that our assumption of near-pseudo-
first-order behavior is correct, experiments were conducted with a higher 1,3-butadiene
concentration (514 ppm). Similar rate coefficient results were obtained for both fuel
concentrations. Table 7.1 summarizes the current measurements of the rate coefficient of
OH+1,3-butadiene reaction.
113
0 20 40 60 800
4
8
12
16
20
2.13 atm, 1026 K356 ppm 1,3-butadieneAr bath gas~18.1ppm TBHP (fit)
OH
[ppm
]
Time [s]
Experiment
kOH+1,3-butadiene
=4.51 x 1012 cm3mol-1s-1 (best fit)
2k k/2
Figure 7.3 Example OH+ 1,3-butadiene rate measurement at 1026K, 2.13 atm. Model
predictions using the best fit predictions for the rate and 50% variation from the measured
rate are also shown.
0 20 40 60 80 100-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
OH
Sen
sitiv
ity
Time [s]
OH+1,3-butadiene CH3COCH3+OH<=>CH3COCH2+H2O TBHP<=>TBUTOXY+OH OH+CH3(+M)<=>CH3OH(+M) OH+CH3<=>CH2 (S)+H2O CH3+CH3(+M)=C2H6(+M)
Figure 7.4 OH sensitivity plot for rate measurement of OH+1,3-butadiene at 1026K, 2.13
atm (conditions of Figure 7.3).
Figure 7.5 presents the current data for OH + 1,3-butadiene along with earlier
estimations and measurements at temperatures greater than 500 K. The current
114
measurements have very low uncertainties (less than 15%) and lie higher than the Liu et
al. [64] data for temperatures of 1150-1203 K, and lower for temperatures between less
than 1050K. Liu et al. [64] did not report data between 1050 and 1150 K. Also shown in
Figure 7.5 is the estimated rate used in Laskin et al. [56] 1,3-butadiene mechanism. This
estimation is in very good agreement with current data for all temperatures and lies
within our experimental uncertainties.
0.7 0.8 0.9 1.0 1.1 1.21
10
Current experiments Liu et al. Laskin et al.
k 1
[10
12 c
c/m
ol/s
]
1000/T [1/K]
1000K1250K 833K
Figure 7.5 Comparison of measured high-temperature OH+1,3-butadiene rate data with
previous measurements (Liu et al. [64]) and a rate coefficient estimation (Laskin et al.
[56]).
A detailed error analysis was carried out to set uncertainty limits on the measured rate
coefficients. Typical contributions to uncertainties in the rate coefficients considered are
from sources of uncertainties in: temperature (±0.7%), absorption cross-sections (± 3%),
fitting the data to computed profiles, and uncertainties resulting from secondary
chemistry. The individual contributions of each error source were determined by
perturbing each error source to the estimated positive and negative bounds of its 2-σ
uncertainty and re-fitting the experimental traces by adjusting the rate coefficient of
interest [98]. The resulting uncertainty in the rate coefficient for each individual error
source were combined using the root-sum-squares (RSS) method. The main uncertainty
categories and their effect on the target reaction rate of OH+1,3-butadiene (following the
115
approach used in [98]), for the experiment at 1200K, 2.20 atm are shown in Table 7.2.
An overall uncertainty estimate of ± 12.8 % at 1200 K and 2.20 atm was obtained.
7.3 TST Calculations
Li et al. [65] conducted theoretical calculations for OH+1,3-butadiene and provided
relative energies based on a single-point calculation at the PMP4/6-311++G(dp,d) level
of theory plus zero point energy (ZPE) correction using the geometry optimized at the
MP2/6-311++G(d,p) level of theory. We conducted canonical transition state theory
(TST) calculations using the MultiWell-2010.1 code (by Barker et al. [180-183]) and the
results of these calculations are a factor of ~ 20 below the current measurements. Possible
reasons for this under prediction of theory include: using a low-level of theory, not
treating the low frequency modes as hindered rotation modes in transition states, errors in
calculated energies by Li et al. [65].
In order to support the experimental findings and to estimate the branching ratio,
variational transition-state theory calculations for these reactions were conducted by Vasu
et al. [190]. Relevant stationary points on the OH+1,3-butadiene surface were
characterized at the QCISD(T)/cc-pVZ//B3LYP/6-311++G(d,p) and at the
QCISD(T)/cc-pVZ//MP2/6-311++G(d,p) levels of theory using the Gaussian 03 suite
of programs [191] and the MOLPRO 2006 package [192]. Other details of the
calculations are provided in Vasu et al. [190] and Zádor et al. [49].
The 1,3-butadiene molecule has three distinct sites for H-abstraction in the
experimental temperature range (rotation around the double bond is strongly hindered,
Ebarrier >> kT). However, when branching ratios are discussed, the channels to abstract
hydrogen atoms from the terminal carbon atoms (marked I and II later) are treated
together, because the resulting products are essentially the same, separated only by a
small barrier along an in-plane H-atom motion. The product of reaction (1b), i-C4H5, is a
resonance stabilized radical and thus has a nearly linear carbon backbone section. Also
note that the symmetry number, 1,3-butadiene = 2, while all other ’s are 1. The results of
the quantum chemical calculations (energies) are presented in Table 7.3. Our results are
in good agreement with the thermochemical data of Burcat and Ruscic [94] who report
reaction enthalpies for Rxns. 1a and 1b of -6.6 and -18.1 kcal mol-1, respectively, but
116
disagree with the PMP4/6-311++G(d,pd) values of Li et al. [65] of -15.7 and -29.3 kcal
mol-1. We believe that this discrepancy originates from a possible error in the latter
calculations. Variational effects can be significant in H-abstraction reactions. For this
purpose the energies and frequencies along the MEP were calculated at the B3LYP/6-
311++G(d,p) level of theory. When zero-point energies are included along the MEP, the
effective barriers are shifted towards the reactants and increase by ~1 kcal mol-1.
Figure 7.6 shows the calculated rate coefficients using the B3LYP geometries with the
scaled frequencies applying VTST and including tunneling. Using the calculated high
level energies (see Table 7.3) results in lower rate coefficient at all temperatures. In order
to get good agreement with the experiments, all three barrier heights had to be reduced by
1.6 kcal mol-1 (to 3.5, 2.9 and 1.7 kcal mol-1, respectively), which is within the
uncertainty limits of the calculations. Results without taking into account variational
effects (but using the effectively increased barrier height) and without tunneling are also
presented. Theoretical double Arrhenius expressions for abstraction in the 250-2500 K
temperature range for k1a and k1b are as follows:
The branching fraction between reactions (1a) and (1b) are close to 50% in the current
experimental temperature region. Below ~1000 K the formation of i-C4H5 is more
favored; however, above ~1000 K the differences in the barrier heights become less
important (kT>>Ebarrier), and the fact that there are twice as many hydrogen atoms
attached to terminal carbon atoms than to non-terminal atoms drives the branching ratio
in favor of the n-C4H5 isomer. Note that our determination of the branching fraction is
somewhat more uncertain than the overall rate coefficient, k1 = k1a + k1b.
)smol cm (K)/(1062.1K)/(10 54 . 1 11 3 /K10821.996/K7252.07 6 1b
TT eTe T k
)smol cm (K)/(1049.3K)/(1053 . 1 11 3 /K21602.046/K9592.07 6 1a
TT eTeT k
117
0.7 0.8 0.9 1.0
2
4
6
8
10
12
current experiments V-TST TST, no variational effects V-TST, no tunneling
k 1
[101
2 cc
/mo
l/s]
1000 / T (K)
1000K1250K 1111K
Figure 7.6 Variational transition-state theory (V-TST) and measured rate coefficients in
the 1000-1406 K temperature range. Curves neglecting tunneling and variational effects
are also presented.
As seen in Figure 7.6, the experimental and calculated values agree very well above
1200 K; however, below 1200 K the model systematically underpredicts the overall rate
coefficient. This behavior can be related to the change in mechanism as a function of
temperature in the alkene + OH reactions that is not included in the current theoretical
model. At lower temperatures, hydrogen abstraction is very slow, and the only important
reaction channel of butadiene and OH is OH addition to the double bonds. Addition
reactions typically have a negative temperature dependence due to the topography of the
entrance channel of the addition process [49]. As the temperature is increased, new
channels open up, such as back-dissociation of the adduct to the reactants, isomerization
channels to form bimolecular products and H-abstraction.
Like in the case of alkenes+OH reactions (previous chapter), the order in which these
processes switch on as a function of temperature and pressure depends on the underlying
potential energy surface. One consequence of this complex behavior is that at high
enough temperatures, back-dissociation becomes so rapid that it completely masks the
addition channel, and only the H-abstraction and the non-abstraction bimolecular
channels can be experimentally observed. Experiments in the intermediate temperature
118
range contain the convoluted effect of all possible processes. In the case of the propene +
OH reaction it was shown experimentally by Tully and Goldsmith [87] that back-
dissociation becomes instantaneous on the experimental timescales above ~700 K. In the
case of 1,3-butadiene the preferred OH adduct is the terminal one, which also yields a
resonance-stabilized radical, CH2CHC•HCH2(OH). Due to its resonance stabilization, the
well relative to the reactants is deeper by ~10 kcal mol-1 compared to the other, non-
resonance stabilized adduct CH2CHCH(OH)C•H2. Francisco-Márquez et al. [193] have
calculated the energy of this adduct and obtained -37 kcal mol-1 at the BHandHLYP/6-
311G(d,p) level of theory. This means that this well is also ~10 kcal/mol deeper than the
wells in the case of propene + OH (~-27 kcal mol-1) [49]; therefore, the temperature
region where backdissociation is not yet instantaneous is expected to extend to
temperatures higher than ~700 K.
To check the effect of the increased well-depth we calculated the equilibrium constant
K, of the 1,3-butadiene + OH CH2CHCHCH2OH reaction at various temperatures (see
Table 7.4). Keff =K [C4H6]0 yields the adduct-to-OH ratio at equilibrium; Keff > 1
indicating that the adduct is more preferential. At 1000 K Keff is not small (0.26), and
decreases to near zero only at 1200 K. Thus, it is likely that at the lower end of the
temperature range, the experimental OH concentration decays include OH removal due to
the addition channel and the observed loss of OH will be faster than that predicted by the
abstraction rate alone. The present experimental study are the first measurements of the
reaction of OH with 1,3-butadiene above 1200K and the present theoretical calculations
are the first ones on the kinetics of the H-abstraction this reaction.
119
Table 7.1 Overall OH+1,3-butadiene rate coefficient, k1, data.
T (K) P (atm) kOH+1,3-butadiene (1012 cm3mol-1s-1)
356 ppm 1,3-butadiene, ~ 18ppm TBHP in Ar
1236 2.15 6.20
1318 2.21 7.00
1200 2.20 5.92
1191 2.38 6.06
1073 2.10 4.93
1130 2.25 5.72
1365 2.08 8.16
1026 2.13 4.51
1145 2.39 5.50
514 ppm 1,3-butadiene, ~ 18ppm TBHP in Ar
1101 2.31 5.17
1100 2.41 5.26
1067 2.44 5.00
1011 2.29 4.35
1406 2.08 9.59
1346 1.91 8.01
1052 2.44 4.73
120
Table 7.2 Uncertainty analysis for overall OH+1,3-butadiene rate coefficient, k1.
Initial incident shock conditions: 1200 K, 2.20 atm, and 356 ppm 1,3-butadiene/Ar.
Error source 2-σ uncertainty in error source
Uncertainty in k1
from
positive perturbation
Uncertainty in k1
from
negative perturbation
2-σ average uncertainty
contribution in k1
Experimental uncertainties
Temperature ± 0.7%, Ref. [98] < 0.1% < 0.1% < 0.1% Fitting
(signal/noise) ± 5% 5% -5% 5%
OH absorption
coefficient (kν) ± 3% , Ref. [99] +6% -3.4% 4.7%
Mixture concentration
± 0.4ppm 1,3-butadiene, Ref. [98]
< 0.1% < 0.1% < 0.1%
Wavemeter reading
±0.0.01cm-1, Ref. [99] < 0.1% < 0.1% < 0.1%
Secondary chemistry uncertainties CH3COCH3+OH
CH3COCH2+H2O ± 30%, Ref. [99] 0% 0% 0%
CH3+OH 1CH2+H2O
(uncert. factor =2), Ref. [99]
-15% 6.7% 10.8%
CH3+OH(+M) CH3OH(+M)
(uncert. factor =2), Ref. [99]
-1.7% 0.8% 1.2%
CH3+CH3(+M) C2H6(+M)
(uncert. factor =2) < 0.1% < 0.1% < 0.1%
Total 2-σ R.S.S. uncertainty in k1:
± 12.8 %
121
Table 7.3 Energies of the products and the transition states in kcal mol-1 units relative to
the reactants trans-1,3-butadiene + OH, without and with zero-point energy. Q1
represents the Q1 diagnostic of the QCISD(T)/cc-pVQZ calculations at the B3LYP/6-
311++G(d,p) geometry. The values are very close to the ones obtained at the MP2
geometry (not shown).
species QCISD(T)//B3LYP Q1 QCISD(T)//MP2E E+ZPE E E+ZPE
trans-1,3-butadiene + OH 0 0 0.0117 0 0 trans-1,3-butadiene + OHn-C4H5 + H2O (I) 7.3 5.1 0.0249 8.2 7.4 trans-1,3-butadiene + OHn-C4H5 + H2O (II) 6.6 4.5 0.0254 7.3 6.4 trans-1,3-butadiene + OHi-C4H5 + H2O 5.3 3.3 0.0264 5.4 4.4 n-C4H5 + H2O (I) -6.3 -6.8 0.0148 -4.9 -3.5 n-C4H5 + H2O (II) -6.8 -7.4 0.0149 -5.3 -3.9 i-C4H5 + H2O -16.5 -17.7 0.0230 -16.0 -15.3
Table 7.4 Energies The equilibrium constant and Keff of the 1,3-butadiene + OH
CH2CHCHCH2OH reaction. For details, see text.
T (K) K Keff , [1,3-butadiene]0=356 ppm Keff , [1,3-butadiene]0=514 ppm 600 1.25E-11 136000 175000 700 1.23E-13 1020 1470 800 3.87E-15 28.1 40.6 900 2.65E-16 1.71 2.47 1000 3.13E-17 0.18 0.26 1100 5.49E-18 0.03 0.04 1200 1.30E-18 0.01 0.01 1300 3.87E-19 0.00 0.00
122
Chapter 8: Conclusions
8.1 Summary of Results
8.1.1 Jet Fuel Oxidation
Ignition delay times in jet fuels (Jet-A and JP-8), were studied using the Stanford high-
pressure shock tube facility. Since practical fuels have low vapor pressures, heated
research facilities were designed to fully vaporize the jet fuel. Ignition delay time data for
jet fuels were presented for temperatures of 715-1229 K, pressures of 17.3-50.9 atm,
oxygen concentrations of 10 and 21% and equivalence ratios of 0.5 and 1.0. High-quality
pressure and OH* emission measurements permitted a clear, unequivocal definition of
ignition delay time. The Jet-A ignition delay times have high reproducibility and low
scatter and are in good agreement with previous high-temperature measurements by Dean
et al. [9]. No measurable change in ignition delay time data was observed for different
mixture heating times at 125 C and for the initial temperature of the mixture (80-150 C).
To our knowledge, these are the first gas-phase shock tube ignition delay time data for
JP-8, and our measurements for Jet-A represent an extension of previously studied
conditions. Under the current experimental conditions, ignition delay times of Jet-A fuel
from different sources were found to show no significant difference. We have found also
that ignition delay times for JP-8 and Jet-A fuels are in close agreement at higher
temperatures.
A simple 1/P dependence was found for ignition delay times from 874 to 1220K for
the pressure range studied for both fuels, and can be used for comparison of data from
various experimental facilities. A clear trend of longer ignition delay times for lean
(Φ=0.5) mixtures when compared to the stoichiometric case (Φ=1.0) was evident, and
also observed for 10 % O2 when compared to the 20.7 % O2 case. Test times in excess of
4 ms were achieved, using traditional He/N2 driver gas-tailoring methods to study the
NTC region (near 700 K). At low temperatures (700-850 K), ignition delay times were
longer (> 2 ms) and showed negative temperature coefficient (NTC) type behavior.
Detailed examination of the pressure-time history data and OH* emission traces show
123
mild pre-ignition heat release trends in the NTC region. Ignition delay times are relatively
insensitive to small variations in the temperature in the NTC region, though at the lowest
temperatures (~700K), ignition delay times increase with decreasing temperature.
The current data provide critical kinetic targets for the validation and refinement of jet
fuel surrogates and kinetic mechanisms. Performance studies of several current kerosene-
based kinetic mechanisms including those of Ranzi [48], Zhang et al. [19], Dagaut and
Cathonnet [8], and Lindstedt and Maurice [112] were conducted, using different
surrogate mixtures. All the mechanisms, except Ranzi, predicted longer ignition delay
times at all temperatures investigated under current experimental conditions. The
Lindstedt and Maurice activation energy prediction is different from that of the
experiments, and this mechanism also fail to show any roll-off trend below 1050 K. Both
the Dagaut and Cathonnet and the Lindstedt and Maurice surrogate mixtures and their
mechanisms may need to include higher molecular weight components in their models to
capture real jet fuel behavior. The Zhang et al. mechanism and the surrogate mixtures
studied fail to capture the high temperature trends in ignition delay times, and also fail to
predict the low temperature behavior. The Ranzi mechanism gives the closest agreement
with data under current conditions, when compared to other mechanisms. The Ranzi
mechanism captures the high-temperature trend but not the lower-temperature data;
however, it was the only mechanism to predict NTC behavior in ignition delay times.
8.1.2 Surrogate Fuels Oxidation
Ignition times and OH species concentration time-histories were measured during n-
dodecane, MCH and n-heptane oxidation over a wide range of temperatures, pressures,
stoichiometries, and fuel concentrations, using the HPST.
In the case of n-dodecane, measurements were made over temperatures of 727 to 1422
K, pressures of 15 to 34 atm, and equivalence ratios of 0.5 and 1.0. These are the first
gas-phase shock tube ignition time and OH time-history data for n-dodecane. The
measured n-dodecane ignition delay data have low scatter, and provide clear evidence of
distinct variation with equivalence ratios, evidence of pre-ignition heat release, and
evidence for NTC roll-off behavior at low temperatures. We also found that using n-
dodecane as a single-component surrogate for jet fuels may be adequate for studying
124
certain combustion characteristics of jet fuels such as ignition delay time at high
temperatures. A comparison of the measured OH concentration time-histories with four
recent mechanisms reveals that wide variations occur in the predictions of the OH
concentration time-histories and ignition times at high temperatures and current models
require refinement to better match these data. The n-dodecane data presented in this study
provide unique and critical kinetic targets to evaluate n-dodecane model performance,
and should provide benchmark information for jet fuel surrogate modeling efforts.
τign data for MCH/air (Mixture: MCH=1.96%, O2=20.60%, N2=77.44%, =1.0) in the
range T=795-1098 K and P=17.2-49.2 atm are the first gas-phase ignition delay time
measurements in a shock tube above 4atm. The current high-pressure MCH ignition
delay time measurements were characterized by low-scatter, exhibited “strong ignition”
features, showed NTC-type behavior at 45 atm for temperatures below 880 K, and are fit
by a pressure dependence of τign ~ P -0.87. In the high-temperature region above 912K, an
overall activation energy of 24.9 kcal/mol was obtained. These high-pressure
measurements complement earlier ignition time studies of MCH in a RCM by Pitz et al.
[22] and extend both the pressure and temperature range of available MCH ignition time
data. Finally, a comparison of the current MCH and jet fuel measurements indicates that
MCH may be an effective single-component surrogate for jet fuel ignition times at high
pressures over a wide temperature range that includes the NTC regime above 800K.
OH time-histories were measured during MCH and n-heptane oxidation behind
reflected shocks waves in the HPST. Reflected shock conditions included temperatures of
1121 to 1332 K, pressures near 15 atm, and initial fuel concentrations of 750 and 1000
ppm in oxygen/argon for an equivalence ratio of 0.5. These new data are the first OH
species time-history measurements in the MCH and the first high-pressure OH species
time-history measurements in the n-heptane oxidation systems, and are highly
reproducible. Current data (for MCH and n-heptane) show approximately the same
behavior, i.e., there is no significant OH formation at early times, and during ignition the
OH concentration rapidly increases, reaches a peak value and slowly falls off. The
measured peak XOH follows an approximately linear dependence on temperature and at
fixed , ign increases with decreases in either temperature or initial fuel mole
concentration. Also, for the same initial fuel concentration (at a fixed T, P), measured ign
125
values are longer in MCH than in n-heptane although the peak XOH values are nearly the
same.
Current MCH and n-heptane OH data provide critical high-pressure validation targets
for kinetic mechanisms of these fuels (and jet fuel surrogate mechanisms). Predictions of
several current kinetic mechanisms (3 for MCH, and 11 for n-heptane) were conducted,
and the results show significant differences between measured and predicted OH profiles
(also in peak XOH, ign). Sensitivity and oxidation pathway analysis show key reactions
and intermediates that influence fuel oxidation at high-pressures and high-temperatures
for MCH and n-heptane. Based on this detailed analysis, we have observed that
modifying the rates of several reactions within their uncertainty limits significantly
improves ignition time predictions, however agreement with other targets, such as peak
OH concentrations, remain either unchanged or worsens. In particular, the rate of one of
the critical reactions under current conditions, i.e. CH3+HO2=CH3O+OH, to a new value
(within the uncertainty limits) is sufficient to improve predictions of the Chaos et al. [52]
n-heptane mechanism. Using this new rate and earlier recommendations by Libby et al.
[68] (based on their OH measurements during 1,3-butadiene oxidation, a key
intermediate during MCH oxidation) in the Orme et al. [36] MCH mechanism results in
very good ign predictions.
Based on current high-pressure OH measurements in H2/O2 systems, we observe that
the latest H2 mechanisms yield predictions that are nearly the same and very close to the
experimental results. However, the OH predictions for H2 combustion by the Pitz et al.
[22] and the Ranzi [48] mechanisms, while similar to each other, predict peak XOH values
considerably lower than the experimental results (and also differ significantly from the
predictions by the three latest H2 mechanisms). Current analysis indicates that the
differences in OH shown by the Pitz et al. and the Ranzi mechanisms during MCH
oxidation may not be due to differences in the rates used for important radical reactions
of their H2 submechanisms, though those reactions most dominantly contribute to OH
ROP during MCH oxidation. In general, the ability of the Ranzi et al. JP-8 surrogate
mechanism to predict OH (for current n-heptane, MCH oxidation experiments) is
considered to be very good.
126
Our analysis show that improved understanding of stable intermediate chemistry,
especially that of alkenes, and additionally of 1,3-butadiene in the case of MCH, is
critically needed to understand overall fuel oxidation and to improve kinetic predictions.
8.1.3 Reactions of OH with Alkenes
Reactions of hydroxyl (OH) radicals with two important alkenes (ethylene and
propene) were studied behind reflected shock waves. OH+ethylene measurements were
conducted in the range of temperatures from 973-1438 K and pressures from 1.99-10.18
atm for three initial concentrations of ethylene (500ppm, 751.1ppm and 1000ppm). The
experimental range for OH+propene spanned temperatures of 890-1366 K and pressures
of 2.024-2.615 atm (average 2.3 atm) at an initial propene concentration near 300 ppm.
OH radicals were produced by shock-heating tert-butyl hydroperoxide (CH3)3- CO-OH,
and monitored by narrow-line width ring dye laser absorption of the well characterized
R1(5) line of the OH A-X (0, 0) band near 306.7 nm. OH time-histories were modeled by
using a modified natural gas oxidation mechanism from GRI 3.0 [161] with a propene
submechanism from JetSurF 1.0 [46] and rate constants for the reactions of OH with
ethylene and propene were extracted by matching modeled and measured OH
concentration time histories in the reflected shock region. Detailed error analyses yielded
an uncertainty estimate of ± 22.8% and ±16.5% for OH+ethylene (at 1201 K) and
OH+propene (at 1136 K), respectively. Current data are in excellent agreement with
previous measurements in our temperature range and extends the temperature range of
OH+propene. Current canonical TST calculations using inputs from a previous
theoretical study by Senosiain et al. [171] are in excellent agreement with data over a
wide range for OH+ethylene and resulted in an expression (600-2000 K): k1=2.23 x 104
(T)2.745 exp(-1115/T) (cm3mol-1s-1). Similarly current canonical TST calculations using
inputs from a previous theoretical study by Zádor et al. [49] are in excellent agreement
with data over a wide range for OH+propene and resulted in an expression (700-1500 K):
k2=1.94 x 106 (T)2.229 exp(-540/T) (cm3mol-1s-1). In the current study, rate measurement
for the decomposition of TBHP have been obtained in the range 745-1014 K using both
incident and reflected OH data. For the decomposition rate measurements, detailed error
analyses yielded an uncertainty estimate of ± 20% and ±15.6% for incident (at 795 K)
127
and reflected (at 890 K), respectively. For the reaction, TBHP(CH3)3CO+OH, an
expression k3=8.13 x 10-12 (T)7.827 exp(-14598/T) (s-1) is obtained combining all the
available data for this reaction in the range 400-1050 K. The current incident shock
method may prove to be an excellent way to study fuel decomposition reactions in the
negative temperature coefficient regime.
8.1.4 OH+1,3-Butadiene
The overall reaction rate coefficient of OH radicals with 1,3-butadiene was measured
in the temperature range of 1011 K to 1406 K at pressures near 2.2 atm. OH time-
histories, monitored using laser absorption, were modeled using a comprehensive 1,3-
butadiene oxidation mechanism based on the earlier work of Laskin et al. Detailed error
analyses yielded an uncertainty estimate of ±13% at 1200K for the rate coefficient of this
reaction. The current data are in good agreement with the previous estimates of Laskin et
al. [56]. Current variational transition-state theory study of the abstraction reaction agreed
with measurements above 1200 K; however, below this temperature the calculations
systematically underpredicted the overall rate coefficient. The present experimental study
are the first measurements of the reaction of OH with 1,3-butadiene above 1200K and the
present theoretical calculations are the first ones on the kinetics of the H-abstraction this
reaction.
8.1.5 Archival Publications
The work described in this thesis has resulted in seven journal publications,
J1) Vasu S. S.; Davidson D. F.; Hanson R. K., ‘OH time-histories during oxidation of
n-heptane and methycyclohexane at high-pressures and temperatures’, Combustion and
Flame 2009, 156, 736-749. Selected as Cover Feature Article title.
J2) Vasu S. S.; Davidson D. F.; Hong Z.; Vasudevan V.; Hanson R. K., ‘n-Dodecane
oxidation at high pressures: Measurements of ignition delay times and OH concentration
time histories’, Proceedings of the Combustion Institute 2009, 32, 173-180.
128
J3) Vasu S. S.; Davidson D. F.; Hong Z.; Hanson R. K., ‘A shock tube study of
methylcyclohexane ignition over a wide range of pressure and temperature’, Energy and
Fuels 2009, 23, 175-185.
J4) Vasu S. S.; Davidson D. F.; Hanson R. K.; ‘Shock tube experiments and kinetic
modeling of toluene ignition’, Journal of Propulsion and Power, 2010, 26 (4), 776-783.
J5) Vasu S. S.; Davidson D. F.; Hanson R. K., ‘Jet fuel ignition delay times: Shock
tube experiments over wide conditions’, Combustion and Flame 2008, 152, 125-143.
J6) Vasu S. S.; Zádor J., Davidson D. F.; Hanson R. K., Golden D. M., Miller J. A.,
‘High-temperature Measurements and a Theoretical Study of the Reaction of OH with
1,3-Butadiene’, J. Phys. Chem. A., in press, Aug. 2010, doi: 10.1021/jp104880u.
J7) Vasu S. S.; Hong Z.; Davidson D. F.; Hanson R. K., Golden D. M., ‘Shock
Tube/Laser Absorption Measurements of the Reaction Rates of OH with Ethylene and
Propene’, J. Phys. Chem. A., in review, Aug. 2010.
8.2 Recommendations for Future Work
8.2.1 Multi-species Measurements in Multi-
component Mixtures
Kinetic models to describe practical fuel ignition chemistry use surrogate fuel
mixtures and detailed reaction mechanisms. The current study examined shock tube
oxidation of jet fuels and single-component surrogates. Time-resolved OH laser
absorption at high-pressure was used as a stringent performance test for various kinetic
models. Several important laser-based diagnostics have been advanced recently in our lab
[194] and are provided in Table 8.1. To produce a complete picture of the chemistry of
the ignition process, several of these diagnostics can be used simultaneously in each
experiment. While formulating surrogate kinetic models by mixing different single-
component submechanisms, the “cross-term” reactions may become important and the
presence of each component can affect the oxidation chemistry of other [33,195,196].
Co-oxidation between reactants occurs quasi-exclusively via the radical pool. For
mixtures containing aromatic compounds, cross-term reactions involving resonance-
129
stabilized radicals could be of some importance, at least to explain the formation of minor
products [197]. Hence multi-species time-histories in multi-component surrogates using
shock tubes would not only aid in developing surrogate mixtures (by comparing with
similar data in real fuels), but also can serve as one of the most stringent kinetic
validation targets. Such measurements are easily achievable in the current HPST by
redesigning the end section to accommodate multiple laser measurements.
8.2.2 Measurements of Reactions in the NTC
Regime Using OH Lasers
Several important OH reactions (e.g., OH plus fuels and intermediates [197-199]) in
the NTC regime can be measured using the strategy outlined here. Such measurements
would be valuable for developing accurate kinetic mechanisms in the NTC region. The
use of TBHP as an OH precursor to measure the rates of any species with OH has been
well established in our lab (see, for example, current work). However, most of these
measurements are conducted in the range 900-1500 K, because at lower temperatures (<
900K) TBHP does not decompose immediately behind reflected shocks, and at higher
temperatures TBHP might completely decompose behind incident shock waves. In this
thesis, TBHP decomposition rate behind incident shock was successfully modeled to
extract a rate for the decomposition of TBHP (TBHPOH + (CH3)3CO). In Figure 6.8,
the OH sensitivity plot indicates strong sensitivity to the TBHP decomposition initially,
but sensitivity to the OH+ethylene reaction increases at later times. Hence, the measured
decomposition rate can be used together with a diode-laser-based diagnostic to measure
temperature (e.g., [194,200]) in order to extend the current TBHP/fuel/Ar technique (to
measure reactions of OH with fuels) to the NTC regime. Such measurements can be
performed behind incident shocks at a location 5 cm from the endwall (compared to the
current 2 cm location in LPST) so that there is sufficient test time. Mixtures need to be
optimized for the target reaction and the use of temperature diagnostic will eliminate
most of the problems (see [168-170]) associated with performing kinetic measurements
behind incident shocks at long test times.
130
8.2.3 Kinetics of Biobutanol
Biobutanol promises to be a superior renewable fuel when compared to ethanol, and
very recently there have been many attempts in the combustion community to develop a
kinetic mechanism for biobutanol or n-butanol [201-207]. There is a steady rise in
utilization of butanol in blends and stand-alone fuels as an increasing number of
organizations are researching its viability to replace ethanol [208,209]. Additionally,
recent work has shown that 1-butanol can be converted to alternative jet fuels [210,211].
However, there exists no measurement of the reaction rate of OH with 1-butanol at
combustion conditions and there is disagreement among these latest modelers (see Figure
8.1). TBHP method used in this study can be utilized to measure the reaction of OH with
1-butanol. Specifically, 20ppm TBHP and 150 ppm 1-butanol mixtures in argon can be
shock heated at 2 atm in the range 1000-1300 K. Sensitivity analysis using the Sarathy et
al. [201] mechanisms indicate (not shown here) that the OH decay in these experiments is
highly sensitive to the overall reaction of OH with 1-butanol and a rate can be extracted
using the procedure outlined in this thesis. Very recent measurements of OH+1-butanol
rate constant are now available (see Vasu et al. [212]) and measurements of OH+butanol
isomers are currently in progress.
0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.21E12
1E13
1E14
OH
+1-
But
anol
rat
e,cm
3/m
ol/s
1000/T(K)
Black et al. Moss et al. Sarathy et al.
Figure 8.1 OH+1-butanol rate used in latest mechanisms (Black et al. [205], Sarathy et al.
[201], Moss et al. [202]).
131
Additionally, 1-butanol decomposition reactions can be measured using the OH laser
absorption technique. The decomposition of 1-butanol is poorly understood and there is
lack of experimental data [201-207]. OH profiles can be measured in shock-heated
30ppm butanol and 20% oxygen in argon in the range 1300-1600 K. Sensitivity analysis
at 1350K and 2atm using the Sarathy et al. [201] mechanism (not shown here) indicates
that the OH concentration profiles are highly sensitive to the 1-butanol unimolecular
decomposition channels and the well-known H+O2=O+OH reaction. The rate of the
decomposition of 1-butanol can be extracted by matching the measured and modeled OH
traces (using a detailed mechanism, for example, Sarathy et al. [201]). Such a strategy has
been previously used by Cook et al. [167] in our laboratory to measure the decomposition
of dimethylether. Kinetics of other types of biofuels (e.g., biodiesels) will be of
immediate future interest and several important reactions in such fuel oxidation systems
can be measured using the strategy outlined here.
Table 8.1 Laser-based quantitative absorption diagnostics developed in our lab [194].
Species Wavelength (nm) Using frequency doubling
CH3 216 NO 225 HO2 225 Benzyl 266 Toluene 266 OH 306.5 NCN 329 Propargyl 335 NH 336
Using ring dye lasers CN 388 CH 431
Using frequency modulation NH2 597 HCO 614
Using infrared diode lasers Species Wavelength (μm) H2O 1.4 CO 2.3 CO2 2.7 H2O 2.5 HC fuels 3.4 CO 4.6 NO 5.2 C2H4 10.5
132
Appendix A: Modes of Ignition in High-
Pressure Fuel/Air Mixture Ignition The occurrence of ramp behavior (DP/Dt) before ignition in the case of MCH/air and
the non-occurrence of ramp behavior in the case of n-dodecane/air (as seen in Chapter 4)
may be related to the ignition regimes or modes of ignition, which are explained in this
section. To illustrate, Figure A.1 provides pressure traces from HPST [115,143] study of
toluene/air in HPST at pressures close to 20 atm. These pressure traces show the three
standard ignition modes (strong, mild, and mild-to-strong transition) as exhibited by
some fuels and is very well documented in the literature (see for example, Fieweger et al.
[129] study in iso-octane). Mild ignition occurs at low temperatures (traces A and B in
Figure A.1); there are no post-ignition pressure oscillations, only a smooth gradual rise in
pressure is seen. Emission measurements confirmed that there is mild ignition in these
cases. Strong ignition occurs at higher temperatures (trace E in Figure A.1); here, the
pressure rises rapidly due to a rapid release of energy and is accompanied with a strong
pressure peak and oscillations. No ramp occurs before ignition in this case. Ignition in the
mild-to-strong transition regime occurs between these two limits (traces C and D in
Figure A.1); in the two cases shown, mild ignition is followed by a strong ignition event.
Interestingly, another aromatic fuel (-methyl naphthalene) ignition study conducted by
Pfahl et al. [119] using 3 different shock tubes (under similar conditions to those shown
in Figure A.1) exhibit similar 3 different modes of ignition. Following the same analogy
as in [119,129], the ramp behavior seen in the case of MCH/air (Figure 4.8) falls under
mild to strong transition regime.
133
0 400 800 1200 1600 2000
0
2
4
Davidson et al. (2005)Toluene/air, =1.0
Pre
ssur
e (R
elat
ive
Sig
nal)
Time, s
A 971K, 17.2atm B 995K, 16.5atm C 1076K, 16.5atm D 1097K, 16.6atm E 1173K, 14.9atm
A
B
C
D
E
Figure A.1 Pressure time-histories from Davidson et al. [115] study near 20atm.
The shock tube ignition occurs in different modes depending on several parameters
including but not limited to the regime of temperature and pressure, mixture
concentrations, and geometric details of the apparatus. Refs. [38,119,129] observed that
ignition in n-alkanes (n-heptane, n-decane) does not show the key mild ignition features
such as ramp behavior, instead exhibits two-stage ignition due to NTC behavior, and
always exhibited strong ignition modes in the range 700-1200 K. Fuels such as hydrogen
[141], methane [213], cyclo-alkanes (current study), alkenes (ethylene [214]), aromatics
(toluene [115,143], -methyl naphthalene [119]) and branched-alkanes (iso-octane
[115,129]) exhibit mild ignition and transition modes (pressure ramp followed by strong
ignition). In jet fuels, DP/Dt as defined in the current study (Figure 4.8) occurs only near
700K and not at higher temperatures (see Chapter 3). Hence the ignition modes
classification is inherently related to chemical class of the fuels such as, n-alkanes,
branched alkanes, cyclo-alkanes, aromatics, etc.
Several theories in the literature have tried to explain the occurrence of these modes as
discussed below. In a recent work, Bartenev and Gelfand [134] stated the importance of
formation of exothermic centers as spatial non-uniformities of the induction period and
reviewed many such theories. According to [134] initial attempts to determine the criteria
of realization of various modes of ignition (in hydrogen) were performed by Soloukhin
and co-workers [215,216] as a continuation of the second explosion limit (purely
chemical), but excluded any gasdynamic influence. Oppenheim and co-workers
[138,217-221] (in hydrogen, n-heptane, iso-octane) considered coherence of exothermic
134
centers and derived a critical condition in terms of the derivative of ignition time with
temperature at constant pressure to mark boundaries between strong and mild ignition
regimes. Oran and co-workers [222,223] proposed sensitivity of a field of ignition delays
i.e., ign(x,y,z), to acoustical perturbations and showed that amplitude and frequency of
sound waves and sensitivity of chemical processes to the change in temperature are
important factors in deciding between mild or strong ignition. Building on the ignition
regime classification theory proposed by Zeldovich [137], which was based on relative
magnitudes of various speeds such as spontaneous reaction front velocity, speed of sound,
rate of normal detonation, and laminar flame speed, Walton et al. [132] successfully
explained mild and strong ignition in iso-octane ignition. In addition, Walton et al. [132]
noted the strong sensitivity of such transition processes to the mole fraction of fuel.
Penyazkov et al. [214] compared velocities of reflected shock wave and reaction front at
different locations in order to classify mild and strong ignition regimes in ethylene.
According to Bartenev and Gelfand [134], the transition from mild to strong ignition
for the majority of gas combustible mixtures occurs at reference temperatures of about
900-1200K and the critical parameters are very sensitive to changes of initial conditions,
gasdynamic effects, derivative of ignition delay with temperature, etc. Mild ignition is a
strong feedback mechanism controlled mainly by temperature and can originate from
local hotspots. But due to the very nature of NTC chemistry, where reactions may be
more vigorous in the colder gas, thus serving to smooth out any temperature difference,
feedback is suppressed and that is the reason why fuels (such as n-alkanes) exhibiting
NTC behavior do not show mild ignition at similar conditions to fuels such as toluene
and MCH. One theory alone cannot explain all the features of such ignition regimes and
their limits and experiments must performed specifically to assist in improving the
understanding these phenomena [134]. Experiments are currently in progress in our
laboratory to study further the nature of these ignition regimes.
135
Appendix B: Influence of Impurities,
Particles, and Wall Effects on Ignition Many researchers have hypothesized that impurities, particles and facility surfaces can
influence homogeneous gaseous ignition (mainly in fuels which are resistive to ignition
such as MCH) at low temperatures and thus can affect the ignition delay time
measurements. Hence a short discussion on these topics is provided here.
As observed by Wang et al. [224] in shock tubes, mild ignition in H2-air-steam
mixtures might start from localized centers at random positions along the tube. One of the
reasons hypothesized for this behavior was the presence of particles and ignition
originating at particles. The authors also noted that contamination on tube surface
reduced their ignition delay time when compared to a clean tube for T>1100K. This was
supported by Chaos and Dryer [225] using computational methods, where they showed
that surfaces could promote chemical induction by catalytic enhancements in H2 systems.
Haskell et al. [226,227] in their RCM studies (in 2-methylpentane/hydrogen/air) found
that particles (present in gases) as small as 0.01m were responsible for non-uniform
ignition and also that invisible particulate matter deposits on the wall chamber (even in
fuel-lean cases) acted as a particle reservoir, which due to turbulence and/or thermal
shock of compression interfered with the ignition process. This interaction was thought to
be due to catalytic oxidation of fuel on particle surface. However, Haskell et al. noted that
it took multiple RCM runs (after cleaning the combustion chamber) before most surface
influences are removed (they speculated that this occurred because combustion cycles
fusing the deposits to the wall) and that experiments with increasing hydrogen content
were always controlled by particle ignition. Recently, Strozzi et al. [228,229] reported
that particles promoted RCM methane/air ignition (even in lean mixtures) by producing
hot spots leading to early ignition.
In shock tubes, the majority of impurities and particles arrive from the mixing system.
Hence precautions were taken in the current study to minimize the presence and possible
transporting of such impurities in accordance with procedures recommended by [97].
Impurities typically affect ultra-lean, ultra-low concentration gas mixtures and the effect
136
is typically manifested as large scatter in experimental results [97,224,226,229]. The
effect of impurities on experiments is only critical at very high temperatures near 2000K
compared to current experiments near 1000K [97]. In the current work, even in the lowest
concentration experiments (500ppm fuel) conducted for T>1200, excellent
reproducibility was obtained for ignition times and OH concentration profiles (see
Chapter 5). Gelfand et al. [135,230] observed ignition acceleration below 1100K due to
desorption of O atoms from the surface. However, these experiments were at 100atm
using 5-15% hydrogen in pure oxygen bath gas. In other lean ignition studies (syngas),
Kalitan et al. [231] found that if impurities (such as, H atoms sticking to the wall) were
present in significant amounts, there would be a general acceleration of ignition times for
all temperatures (and not just at low temperatures). Moreover, in the current HPST
experiments using high-temperature (1300-1500K) pure oxygen shocks, we did not see
OH emission which implies that the shock tube walls have minimal H-atom impurities.
Hence the discrepancies seen in predictions and current data in the NTC region (see
Chapter 4) cannot be attributed to the presence of impurities.
Mantzaras et al. [232-235] conducted extensive hetero/homogeneous combustion
studies in hydrogen/air and hydrocarbon/air mixtures over highly reactive catalysts using
both experimental and computational methods. According to the authors, there can hardly
be any adsorption of fuel onto the stainless steel surface of the shock tube (before the
shock arrives), i.e. at temperatures as low as 340 K. Hydrocarbons, either higher or lower,
would require temperatures of at least 200 °C to react on specific catalyst surfaces
(notably noble metals) and stainless steel would be inactive to fuels used in this study
even at such temperatures. Hence there can be no plausible effect of the surface before
the shock passage. Also, in a hetero-/homogeneous systems, influence of heterogeneous
pathway on homogeneous ignition is minimal (even at atmospheric conditions) and the
impact of the surface pathway gets weaker as pressure increases (i.e., the higher the
pressure, the more important the gas-phase reactions). After the passage of the reflected
shock at 1000 K, it may be high enough to start catalytic oxidation of fuel. However, this
still would require special catalysts such as noble metals or special metal
oxides. Mantzaras et al. [232-235] also stated that the gas-phase produced
radicals can recombine on the stainless steel surface. The surface chemistry does play a
137
role on the lower (first) explosion limit because this limit refers to very low
subatmospheric pressures (~1/100 atm). This is also the reason why the first explosion
limit depends on the particular vessel of the experiment (quartz, steel, ceramic, etc)
because radical recombination is different on these materials. However, the second and
third explosion limits (pressure~1 atm or more) are hardly influenced by the vessel type
(like in the case of the current experiments). In summary, during high-pressure
experiments in shock tubes, impact of radical adsorption/desorption surface reactions and
hetero/homogeneous coupling is minimal on ignition. In fact at conditions presented in
this study, the wall can only act as inhibiting homogeneous ignition and not promoting it
[235].
There are two underlying mechanisms through which particles (isolated aluminum
diaphragm particles in HPST) in shock tubes can affect gas-phase chemistry [236-240].
The first one depends on the lifting of particles by shock waves from the wall into the
gaseous mixture. When a shock wave slides over a layer of particles sitting on a surface
(independent of the type of surface), the particles experience hydrodynamic instabilities
(Kelvin-Helmholtz) and after a certain lag (~ 300s), the particles are lifted with average
velocities (which is proportional to the velocity of the shock wave) of the order of several
mm/s [236]. A single particle (d > 50 m) will slide over the surface and is not lifted
until it collides with some obstacle (height of the obstacle being less than 20 times higher
than particle size) [237]. The implication is that particles located on a smooth surface can
eventually be made to ascend only when they are in a group of similar particles and
collide (because of Magnus effect they experience rotation after collision and changes
direction) with one another (i.e., layers of particles or a significant number of particles
must be present), which is not the case in current ignition experiments.
The second mechanism is concerned with the effect of already suspended particles in
gaseous mixture ignition. From the detailed shock tube studies conducted by Boiko and
co-workers [238-240] several conclusions can be made about ignition of Al particles in
fuel/air mixtures: 1) the ignition temperature of Al particles is typically around 2000 K
and a major heating mechanism of the particle is through thermal conduction from gases,
and oxide layers typically inhibit ignition [238]; 2) gaseous ignition delays in heptane/O2
are not affected by the presence of Al particles (T=1000-2000K) and activation energy of
138
ignition delay times are not affected [239]; 3) the kinetics of ignition in mixtures of metal
powder in air and volatiles such as methane and kerosene in the range T=1300-1700K are
primarily determined by the kinetics of the volatile compounds (and not the kinetics of
particles) and at low temperatures the ignition delay times are not affected by the
presence of particles [240]; and 4) heating of particles by inert gases plays a noticeable
role only above 2000K [240]. The fact that the influence of particles on gaseous ignition
is limited to very high temperatures was confirmed recently by our group [241] in a
shock tube. In that study, it was reported that at T<1175 K (P <10 atm) there was no
influence of Al particles (d =0.1 m) on the ignition of pure n-dodecane/21%O2/Ar
mixtures and above that temperature, the particles accelerated ignition slightly. Based on
the above mentioned facts, it can be safely assumed that particles promoting ignition are
not a concern in current HPST ignition experiments.
139
Appendix C: Shock Tube Boundary
Layer and Facility Effects on Ignition The ideal shock tube theory assumes the following: diaphragm breaks instantly
forming a constant speed shock wave; thermodynamic conditions and the flow velocity
remain constant between the shock and the contact front; and, reflection from endwall
produces uniform conditions behind reflected shock wave [242]. Actual shock tubes
slightly deviate from ideal behavior: there is a finite diaphragm breaking and shock
formation time; a boundary layer builds up due to viscous effects behind moving incident
shock waves which affects the shock velocity (this problem was originally conceived and
boundary layer models were developed by the pioneering work of Mirels and co-workers
[243-247]); interaction of the reflected shock wave with the boundary layer produces
bifurcation [248], and non uniform test conditions and affects the shock velocity; and
contact-surface/reflected-shock interaction affects the available test times [92]. The
effects of the above-mentioned deviations on test conditions in the HPST have been well
studied, and attenuation and boundary layer models have been developed [92]. Assuming
the linear attenuation of incident shocks [92] and an ideal shock tube, it is currently
estimated that at the measurement location (1 cm from endwall), the reflected shock
would see a fluid particle which traveled from a maximum distance of 3.8cm (~ 1000K)
away from endwall. Because the incident shock velocity was higher at this 3.8cm
location, it is possible that the fluid particle from this upstream location might be at a
higher temperature than those that originated at 1 cm location (when the reflected shock
arrives at 1cm location). However, the difference in temperature behind incident shock
velocity at these two locations (3.8cm and 1cm) is only 1K and hence we can rule out the
possibility of higher T5 in current fuel/air measurements at the 1 cm location due to
attenuation of the incident shock alone.
At some conditions, due to these non-ideal effects, pressure (and therefore temperature)
increases (defined as dP/dt in Chapter 3) with time at a fixed axial location, and the effect
of non-ideal gas dynamics behind reflected shock wave are more severe at lower
pressures (<10atm) and high temperatures (T>1500K) [92]. Recent temperature
140
measurements behind reflected shocks conducted in many shock tubes (including the
HPST) using laser-based and other techniques [92,103,116,194,200,249,250] indicate the
following: if pressure remains constant with time at an axial location, then temperature
does not change; and if pressure changes due to non ideal effects or due to small energy
release, then temperature change can be approximated by isentropic relations. As seen in
Figure 3.8 this rise in pressure with time usually starts from time zero (see also
[7,141,251]). It should be kept in mind that incident shock attenuation is not the only
parameter reflecting the magnitude of non ideal effects. The effect of dP/dt on measured
ignition time data was estimated in the case of jet fuel ignition (Chapter 3) and was found
to be negligible. In the case of n-dodecane/air ignition, dP/dt effect was taken into
account using CHEMSHOCK [103] (Chapter 4) and results were found to be not
significantly different from those of constant U, V modeling (see Figure 4.3). Note that
none of the MCH/air ignition data show a dP/dt, instead show ramp just before ignition
(defined as DP/Dt) as described in Chapter 4 (Figure 4.8).
Bifurcation occurs because the boundary layer is not able to negotiate the pressure rise
across the reflected shock when brought to rest relative to it, and is therefore trapped and
carried along at the foot of the shock [252,253]. Bifurcation features normally appear in
diatomic and polyatomic gases (such as current fuel/air mixtures) but not in argon diluted
mixtures and its features have been well-known through experimental visualization
utilizing color schlieren [254] and side-wall pressure measurements [255,256].
Bifurcation has been modeled in many computational studies [257-260]. Bifurcation
affects determination of time zero because of the uncertainty in determining the arrival of
the normal shock wave at the sidewall location and its effects are severe as one moves
away from the endwall and also for short ignition delay times (<100 s). However,
bifurcation should not affect the core portion of the post-shock region, which comprises
most of the flow area [255]. In the current fuel/air experiments bifurcation features are
visible in pressure measurements (see Figure 4.1), but the effect of which on the
determination of time zero is minimal since the lowest measured ignition time is greater
than 100s.
Even though the effect of nonideal effects in the current fuel/air results are thought to
be negligible, it should be noted that non-ideal (and other facility related) effects vary
141
from facility to facility in a manner not accounted for in the usual boundary layer theories
and they must be characterized for each shock tube [261,262]. For example, in the shock
tube described by Michael and Sutherland [261], the attenuation of incident shock wave
was reported to be zero, but still deceleration of the reflected shock wave occurred in
nonreactive mixtures, and the correction procedures for such effects are different from
that of the HPST [92]. As reported by Bowman and Hanson [262], multiple diagnostics at
multiple axial locations must be used to characterize shock tubes before comparing data
from different shock tubes.
142
Appendix D: TST Theory and
Calculations The failure of simple collision theory to explain certain bimolecular reactions led to
the formation of transition state theory (TST). A short description of this theory and the
main concepts are given here (adapted from [263-265]). The transition state, in classical
theory, is defined with a ‘dividing surface’ separating reactant and product regions. In
canonical TST, the TST rate constant is proportional to the total flux of classical
trajectories from reactant to product side of the dividing surface calculated with Maxwell-
Boltzmann weighting at a given temperature [265].
Consider a schematic atom + diatomic molecule reaction as shown in Figure D.1 [264].
The reaction is A + BCX≠AB + C. The transition state is the highest energy point on
the graph labeled X≠, and the reactants at this point are known as the activated complex.
TST assumes that X≠ is in equilibrium with reactants, [X≠]=K≠[A][BC]. K≠ is the
equilibrium constant for the formation of X≠ from A+BC. The overall rate of forming
products (AB and C), ρ, is k≠[X≠], where k≠ is the first-order rate coefficient for the
decomposition of X≠ into products. Since ρ=k[A][BC]= k≠[X≠], we have k=k≠K≠.
Assuming that the removal of the transition state to form products does not affect the
equilibrium between X≠ and reactants, calculating the rate coefficient of the reaction, k,
reduces to one of evaluating k≠ and K≠.
The equilibrium constant, K≠, can be calculated from parameters, such as bond lengths,
bond frequencies and molecular weights, or from partition functions using the results of
statistical mechanics. By using the Born-Oppenheimer approximation, molecular energy
can be subdivided into its component parts, translation, rotation, vibration, and electronic
energy [266]. The total partition function Q is the product of the individual partition
functions QT (translation), QR (rotation), QV (vibration), QE (electronic). Then we have
k= k≠ K≠ = k≠ (QX/QAQBC) exp(-Δεo/kBT), where Δεo= difference between the lowest
energy levels of A+BC and X≠ (i.e., activation energy at 0K) and kB is the Boltzmann
constant. Note that QX is different from QX≠ in order to take into account dissociation
(asymmetric stretching). The motion of the transition state and associated energy levels
143
are treated in the same way as those of stable molecules. However, since X≠ lies at the
maximum in the potential energy surface for motion along the reaction path (Figure D.1),
there is no restoring force during asymmetric stretch vibration. As molecule A moves into
form the A-B bond, the B-C bond breaks and C moves out. Hence this is a dissociation
mode and is treated separately.
The reaction may be simulated by the motion of a point mass over the potential energy
surface, i.e., the oscillatory motion of the point mass along the reaction path corresponds
to dissociation of the complex and the motion perpendicular to the reaction path indicates
the symmetric stretching mode. If we specify that the complex exists when the reaction
coordinate lies within a length δ, centered around X≠, the mean rate coefficient for
dissociation of the complex is: k≠=υ/δ, where υ is the one dimensional average velocity.
Assuming a Maxwell distribution, υ can be calculated and, hence, the partition function
for the dissociating mode [264]. We have QX=Qd.QX≠, and with some algebra, k = K
kBT/h (Qx≠ /QAQBC) exp(-Δεo/kBT). We have inserted the transmission coefficient K,
which allows for the possibility that not all activated complexes lead to products, since
some may be reflected back to the reactants. The MultiWell-2010.1 code (by Barker et al.
[180-183]) calculates the canonical TST rate using inputs such as, frequencies, moments
of inertia, energies and molecular symmetry information for reactants, products and the
transition state. Input files, used in the calculation of OH+ethylene and OH+propene rates
are given at the end of this section.
According to quantum mechanics, there is a finite possibility of a wavefunction
appearing on the other side of a potential barrier. This penetration into classically
forbidden regions is called tunnelling and an obvious example of such a barrier is the
potential energy barrier between reactants and products. This barrier is of finite width and
therefore there will be a non-zero probability of the particle appearing on the other side of
the barrier even if it classically does not have the energy to react. In the current work,
tunnelling is calculated assuming a 1-D asymmetric Eckart [184] barrier.
TST theory is exact if, and only if, no trajectories cross the transition state dividing
surface more than once [265], otherwise the TST overestimates the exact (equilibrium)
rate constant. Therefore, in such cases, the transition state dividing surface need to be
optimized so that the flux through it is minimum. Variational TST (VTST) takes into
144
account such an approach, and VTST always provides an equal or a better estimate of the
rate constant than simple TST. Variational effects have been found to be negligible in the
case of OH+ethylene [178] and in the case of OH+1,3-butadiene, variational effects are
shown in Chapter 7.
Figure D.1 3-D potential energy surfaces for the collinear reaction A+BCAB+C.
145
Input Files to Multiwell-2010.1 Code
OH+ethylene
146
OH+propene (abstraction from CH3)
147
OH+propene (abstraction from CH)
148
OH+propene (abstraction from CH2 cis)
149
OH+propene (abstraction from CH2 trans)
150
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