the oxidation and autoignition of jet fuels
TRANSCRIPT
The Oxidation and Autoignition of Jet Fuels
Matthew Oehlschlaeger Mechanical, Aerospace, and Nuclear Engineering Rensselaer Polytechnic Institute, Troy, NY
Sponsor: AFOSR, Dr. Chiping Li
Motivation: changing fuel supply • Combustion properties….
– are dependent on fuel composition – of low-volatility and alternative fuels are less well
characterized
• Shock tube studies as high-pressures for fuel/air mixtures (engine-like conditions)
• Quantitative targets for the validation of kinetic models • Direct comparisons of fuel reactivity
Methodology
Payoff
Methodology: heated shock tube technique
• Isolation of homogenous gas-phase kinetics
• Reactants are heated “instantaneously” – Zero-dimensional model – Temperature and pressure can be accurately determined
(typ. 1-2% uncertainties)
• High pressures achieved behind reflected shocks – Present studies ~7-50 atm
• Extended test times (10 ms) achieved using tailored
driver gases
Vacuum section
RPI shock tube facility
Heated and insulated mixing vessel
Driver
Heated and insulated driven section
Test location w/ optical access
Shock velocity detection
Diaphragm
Inner diameter = 5.7 cm; initial temperatures up to 200
C
Mixing manifold
P [atm] T [K] τ [ms] φ Accessible 1 - 200 650 + 0 - 10 Typ. for fuel/air 5 - 60 650 - 1300 0.05 - 10 0.25 - 2.0
Condition Space
Ignition delay measurement
Ignition delay
Incident Shock Velocity
Ignition Time Measurement
to DAQ
to counter-timers for velocity
Kistler transducer to DAQ
incident shock
reflected shock
PZTs
Filtered Si photodetector (OH* emission)
Shock tube test
section
Example results
Uncertainties: ±25% in ignition delay ±1.5% in temperature
Example results
Uncertainties: ±25% in ignition delay ±1.5% in temperature
High temp
Low temp
Negative- temperature- coefficient (NTC)
Example results RH
R
RO 2
QOOH
OOQOOH
ketohydroperoxide + OH
’
olefin + HO
cyclic ether + OH
OH + radical
O 2
O 2
RH
R
RO 2
QOOH
OOQOOH
ketohydroperoxide + OH
’
olefin + HO2
cyclic ether + OH
OH + radical
O 2
O 2
Beta-scission / fragmentation
Fuel decomposition
Beta-scission + R’’
• Temperature dependence mechanistically explained by models in literature • Quantitative prediction is a work in progress
Uncertainties: ±25% in ignition delay ±1.5% in temperature
High temp
Low temp Negative-
temperature- coefficient (NTC)
Experiment vs surrogate modeling pressure dependence φ dependence
Generalizations • a priori surrogate kinetic modeling deviates with real fuel ignition delay by on average a factor of two – sufficient for “engineering accuracy”?
• Kinetic sensitivities/dependencies much larger in NTC than high-T regime • In condition and fuel composition/structure parameter spaces
Jet fuel ignition variability
~ 4x
DCN = 31
DCN = 61
0
10
20
30
40
50
60
70
80
90
100
Sasol IPK JP-5 Jet A HRJ-5 S-8 Shell GTL
n-alkane
iso-alkane
cyclo-alkane
aromatic% b
y vo
lum
e
DCN: 31 42 47 59 59 61
DCN a proxy for NTC ignition delay?
The IQT-based DCN measurement made is at ~830 K and 22 atm
Ignition delay vs DCN
Ignition delay vs DCN
The entrance to the high-T regime occurs at higher T with increasing DCN
Jet fuel composition
n-alkanesiso-alkanescyclo-alkanesaromaticsother
n-alkanesiso-alkanescyclo-alkanes
Jet A S-8
methyl alkanesdimethyl alkanestrimethyl alkanes
iso-alkanes
methyl alkanesdimethyl alkanes
iso-alkanes
Conventional and alternative jet fuels contain Large quantities of lightly branched alkanes
Collaborative study of lightly-branched alkanes
• RPI: shock tube ignition delay • Nat. Univ. Ireland Galway: shock tube and
RCM ignition delay • LLNL: kinetic modeling
• Compounds studied:
C8 alkane comparison
Compound RON DCN
2,5-dimethylhexane 56
3-methylheptane 37 43
2-methylheptane 22 53
n-octane -19 64
At high-pressure fuel/air conditions Branching has: • little to no influence at high T • significant influence in NTC region
→ dependence on number and location of methyl substitutions
C8 alkane comparison: T < 1000 K
Equivalence ratio dependence: 3-methylheptane
Higher temperatures • small φ-dependence; crossover in φ-dependence at 1200 K for 6.5 atm case
Moderate to low temperatures • larger φ-dependence
Crossover in φ-dependence
• T > 1200 K: ignition controlled by H + O2 → OH + O – Increased φ, increased fuel scavenging of H atoms:
fuel + H → R + H2
• T < 1200 K: H + O2 + M → HO2 + M competes – Increased φ, increased radical production:
fuel + HO2 → R + H2O2, H2O2 + M → 2OH + M
• Additionally at T > 1200 K: inhibitive decomposition
(+ radical)
resonantly stable
Observations from C8 alkane isomer studies
• High temperatures – Little to no difference in ignition delay for C8 isomers
• Similar bond strengths, similar intermediate pool, similar reactivity
– Model captures pressure- and φ-dependence but is ~2x longer than measured ignition delay
• Low temperatures – Model in better agreement with both RCM and ST data – Reactivity dependent on location and number of
methyl substitutions
Low-T chemistry: 2-methylheptane vs n-octane 2-methylheptane n-octane
RH
R
RO2
QOOH
OOQOOH
O2
O2
Low-T chemistry: 2-methylheptane vs n-octane
2-methylheptane n-octane 2-methylheptane: slower OOQOOH isomerization; no H atom available at OOH site
Low-T chemistry: 3-methyl vs 2-methyl heptane
Inhibitive cyclic ether formation results due to slow OOQOOH isomerization
3-methylheptane 2-methylheptane RH
R
RO2
QOOH
O2
OOH OO
High-temperature deviations
At high T&P • Sensitive reactions are within C0-C2 chemistry and fuel + HO2 R + H2O2 • Over prediction of alkane ignition delay is common among literature models
Summary
• Jet Fuels – Ignition delay variability large in the NTC – Correlates with DCN
• Not yet clear how universal this observation is
• Normal and lightly-branched alkanes – Number and location of methyl substitutions has a
strong influence on NTC ignition delay – Isomer reactivity differences explained by competition
among low-T radical branching and propagation pathways
– Perhaps still some issues with high-T alkane kinetics
Acknowledgements
• RPI: Dr. Haowei Wang, Weijing Wang
• LLNL: Drs. Bill Pitz, Charlie Westbrook, Mani Sarathy, and Marco Mehl
• Princeton: Drs. Fred Dryer, Yiguang Ju, Stephen Dooley, and Sang Hee Won
• NUI Galway: Drs. Henry Curran and Darren Healy
• AFOSR (sponsor): Dr. Chiping Li
Sensitivity analysis 3-methylheptane/air, 20 atm, 1100 K
C0-C2 sensitivity: common to all alkanes at high temperatures
H2O2 formation