1-s2.0-s0010218098001345-main444

Ignition of Ethane, Propane, and Butane in Counterflow Jetsof Cold Fuel versus Hot Air Under Variable Pressures

C. G. FOTACHE, H. WANG, and C. K. LAW*Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544

This study investigates experimentally the nonpremixed ignition of ethane, propane, n-butane, and isobutane ina configuration of opposed fuel versus heated air jets. For each of these fuels we explore the effects of inertdilution, system pressure, and flow strain rate, for fuel concentrations ranging between 3100% by volume,pressures between 0.2 and 8 atm, and strain rates of 100600 s21. Qualitatively, these fuels share a number ofcharacteristics. First, flame ignition typically occurs after an interval of mild oxidation, characterized by minimalheat release, fuel conversion, and weak light emission. The temperature extent of this regime decreases withincreasing the fuel concentration, the ambient pressure, or the flow residence time. Second, the response tostrain rate, pressure, and fuel concentration is similar for all investigated fuels, in that the ignition temperaturesmonotonically decrease with increasing fuel content, decreasing flow strain, and increasing ambient pressure.The C4 alkanes, however, exhibit three distinct p-T ignition regimes, similar to the homogeneous explosionlimits. Finally, at 1 atm, 100% fuel, and a fixed flow strain rate the ignition temperature increases in the orderof ethane , propane , n-butane , i-butane. Numerical simulation was conducted for ethane ignition usingdetailed reaction kinetics and transport descriptions. The modeling results suggest that ignition for all fuelsstudied at pressures below 5 atm is initiated by fuel oxidation following the high-temperature mechanism ofradical chain branching and with little contribution by low-to-intermediate temperature chemistry. 1999 byThe Combustion Institute

INTRODUCTION

Ignition represents a key process in the opera-tion and satisfactory performance of practicalcombustors. Most of the understanding gainedto date on this phenomenon is based on studiesof homogeneous systems, in which transporteffects are not taken into consideration. Hence,the only aspects relevant to homogeneous igni-tion are radical-concentration growth due tochain reactions and/or thermal feedback as aresult of exothermic reactions. The ignitionphenomena in the limit of well-mixed, spatiallyhomogeneous systems have been extensivelystudied in closed static reactors, shock tubes,continuous flow devices, rapid compression ma-chines, and perfectly stirred reactors. Thesestudies have been well reviewed [e.g., 15].

In many realistic environments such as theDiesel engine, however, ignition occurs in thepresence of local spatial inhomogeneities, man-ifested as notable gradients of temperature,

concentration, and flow velocity. To be morespecific with the above statement, we note thatthe ignition of liquid fuel droplets in a high-temperature oxidizing environment is influ-enced by convective-diffusive mixing of evapo-rating fuel with oxygen, as well as heatconduction due to spatial temperature gradi-ents. The existence of such gradients means thatconvective-diffusive transport processes can ex-ert significant influence on the ignition eventwhen their characteristic time scales becomecomparable to the chemical time scales relevantat ignition.

Our present work is a step in a larger ongoingprogram aiming to understand the interaction atignition between chemical kinetics and the con-vective-diffusive transport associated with anonpremixed system. Extending from previousstudies [612] in which the ignition of hydrogen,CO, and methane in convective-diffusive sys-tems was thoroughly examined, the focus here ison the ignition of C2C4 alkanes, namelyethane, propane, n-butane, and i-butane, undera wide envelope of the system pressure, fuelconcentration, and flow strain rate.

The importance of the selected hydrocarbonsin combustion processes cannot be overstated.Ethane and propane are common constituents

*Corresponding author. E-mail: [email protected] address: United Technologies Research Center,East Hartford, CT 06108.Present address: Department of Mechanical Engineering,University of Delaware, Newark, DE 19716.

COMBUSTION AND FLAME 117:777794 (1999) 1999 by The Combustion Institute 0010-2180/99/$see front matterPublished by Elsevier Science Inc. PII S0010-2180(98)00134-5

of natural gas; butane is found in commercialgasolines, and is the lowest alkane for whichmore than a single structural isomer is possible.Its two isomers present different knocking char-acteristics under conditions typically found inspark ignition engines [13, 14], which is re-flected in their respective Research OctaneNumbers of 94 (n-butane) and 102 (i-butane).Propane and butane are the simplest hydrocar-bons that show the complete spectrum of higherhydrocarbon combustion: low temperaturechain-branching and cool flames, negative tem-perature coefficient (NTC), intermediate andhigh temperature oxidation [13].

Finally, the kinetic mechanisms of theseC2C4 hydrocarbons are necessary buildingblocks in the hierarchical structure of the mech-anisms of higher hydrocarbons oxidation [15],consequently their study is an essential step inunderstanding the combustion of more complexfuels. Given the importance of these fuels, it isnot surprising that an extensive amount of re-search has been directed at understanding theircombustion characteristics. Comprehensive ki-netic models have been proposed to account forhomogeneous C2C4 hydrocarbon ignition indifferent temperature regimes [e.g., 1521]. Asystematic, detailed study on the comparativeignition characteristics of ethane, propane, andbutane in nonpremixed, diffusive systems, how-ever, has not been conducted.

Our present work is a contribution in thisdirection. Specifically, using the prototypicalconfiguration of nonpremixed jets of hot airversus cold fueldiluent mixture in counterflow,we have determined experimentally the ignitiontemperatures for ethane, propane, n-butane,and i-butane, and their variation with fuel dilu-tion, system pressure, and flow strain rate. Inaddition, numerical modeling using detailed ki-netics and molecular transport descriptions wasperformed for ethane ignition, and the experi-mental results are interpreted on the basis ofthis numerical simulation. The ignition re-sponses of propane and the two isomers ofbutane are explored mechanistically, extendingfrom the knowledge gained from modelingethane ignition, the known mechanisms of C4-hydrocarbon ignition, and the comparativetrends of the present experimental results.

METHODOLOGY

Experimental Specifications

A detailed description of the experimental ap-paratus can be found in Ref. 7. Briefly, thecounterflow ignition apparatus consists of two20 mm i.d. tubular quartz burners separated bya 20 mm distance, which are housed in a vari-able pressure chamber (0.115 atm). The burn-ers direct an electrically heated air jet down-ward against an upward flow of cold (300 K)fuel/nitrogen mixture, as seen in Fig. 1a. Gasflows are monitored using mass flow controllers(Teledyne Hastings), and the heaters are oper-ated by a temperature feedback controller (J-Kem Model 250). The fluctuation of the result-ing air temperature was estimated by thethermocouple emf readings to be 612 K oncestable heater operation was achieved.

Commercially available N2 (99.9%), C2H6(99.5%), C3H8 (99.0%), n-C4H10 (99.5%), andi-C4H10 (99.5%) were used without further pu-rification for pressures below 2 atm. The high-pressure experiments were conducted usingmanufacturer-certified pressurized mixtures of6% n-butane and i-butane in N2.

Ignition is achieved in this experiment by a

Fig. 1. Schematic illustration of (a) the counterflow systemand the profiles of fuel, oxygen, and intermediate concen-trations and temperature along the centerline of the coun-terflow of cold fuel versus hot air jets, and (b) the steady-state solution for the variation in the peak intermediateconcentration as a function of air boundary temperature.The turning point at the end of the lower branch (mildoxidation) represents the ignition state.

778 C. G. FOTACHE ET AL.

slow and stepwise increase of the temperatureof the air jet until flaming condition. Thus, if wedefine a characteristic system response as thepeak concentration of a representative radicalspecies, this response would increase as the airboundary temperature Tair is increased; follow-ing the trajectory depicted in Fig. 1b. Eventu-ally, a turning point is achieved where thesystem will transition abruptly to the upperbranch which characterizes the flaming condi-tion. The ignition state is characterized by theflow state just prior to the onset of the flamingcondition. The characteristic ignition tempera-ture is measured by the Tair beyond which theflame ignition occurs. For the present convec-tive-diffusive system, this is the only unambigu-ous way of defining such a characteristic ignitiontemperature.

Data acquisition involves the measurementalong the stagnation streamline of the distribu-tions of axial velocity and temperature. The gastemperature is measured using a bare Chromel-Alumel (Omega Engineering K-type) thermo-couple with an average bead diameter of ;0.16mm. The thermocouple readings are correctedto account for radiative/convective heat trans-fer, with an uncertainty varying between 615and 625 K. This error is mostly responsible forthe extent of the absolute temperature errorbars used when comparing the experimentaldata with results of the numerical simulation.The relative errors which affect the trend of thedata, however, are substantially lower, rangingbetween 61 to 63 K for the flame ignitiontemperatures.

The axial velocity profile u 5 u( z) is deter-mined through laser Doppler velocimetry(LDV). Subsequently, the LDV data is fittedlocally with polynomials to determine the max-imum gradient k 5 2(du/dz)max, which quan-tifies the flow strain rate. To enable the com-parison of results obtained at variable pressures,we employ the pressure-weighted strain ratedefined as k 5 k( p/p0), where p is the pressureand p0 the standard pressure of 1 atm. Thisrepresents an adequate approximation to therigorous density-weighted strain rate [8].

In addition, spectroscopic experiments areconducted to analyze the light emission fromthe weakly reactive state established prior to

ignition using the setup described in Refs. 22and 23.

Numerical Procedure

The numerical simulation of ethane ignition isperformed along the stagnation streamline us-ing detailed chemical kinetics and transportdescriptions. We have also attempted to modelthe ignition of C3C4 hydrocarbons using asimplified model similar to those reported inRefs. 24 and 25, but were unable to obtaincomputational ignition under the strain rate andpressure conditions of the current study. Simu-lation of C3C4 hydrocarbons using a detailedmodel is computationally expensive and beyondthe scope of the present study.

The ethane/air oxidation mechanism em-ployed in this work was taken from Hunter et al.[26]. To understand better the mild reactionstate prior to ignition, we also investigated thevisible chemiluminescent emission above thestagnation surface. Based on our spectroscopicinvestigation, a large part of emission comesfrom the excited formaldehyde. To simulate thechemiluminescent emission, we appended themechanism of Ref. 26 with the following reac-tions, involving the formation and deactivationof excited formaldehyde CH2O* [27],

CH3O 1 OH3 CH2O* 1 H2O (R283)

CH2O* 1 M3 CH2O 1 M. (R284)

The rate parameters of these reactions aretaken from Ref. 28. The resulting model in-volves 48 species and 284 reversible reactions.

In the calculations the numerical code de-scribed in Kreutz and Law [8] was employed,which is based on the steady-state stagnationflow code of Smooke and coworkers [29, 30].For convenience we have used the potentialflow/boundary layer formulation of the govern-ing equations. It is noted that previous ignition[7] and flame [23] studies have indicated thatthe flame structure and the ignition tempera-tures do not change appreciably when usingalternative flowfield descriptions, such as theplug flow boundary condition.

The computational procedure involves ob-taining the multiple branches of a system re-sponse S-curve [31], quantified in this work by

779HYDROCARBON IGNITION IN CONVECTIVE-DIFFUSIVE SYSTEMS

the peak concentration of a representative rad-ical versus the boundary temperature of theheated air (Fig. 1b). The turning point on thelower branch of the S curve identifies theignition state.

To analyze the dominant ignition chemistrywe have employed in this study the Computa-tional Singular Perturbation (CSP) method ofLam [32, 33] and the S-curve sensitivity methoddescribed in Kreutz and Law [34]. In the CSPanalysis, we first compute the spatial distribu-tions of species and temperature using theconvective-diffusive ignition code. We then se-lect initial conditions for the CSP routine cor-responding to the composition and temperatureat a given spatial location, generally the peak ofa representative radical. The composition of theexplosive mode, together with the flux analysisof the species pointed at by its radical pointers[32], provide the information necessary to iden-tify the dominant reactions or reaction setsresponsible for the ignition behavior.

RESULTS

In most situations a state of mild reactivity isachieved with the increase in air temperatureprior to the establishment of the vigorouslyburning flame. This state is signaled by chemi-luminescent glow emitted from a narrow, disk-shaped region situated above the stagnationplane, on the air side of the flow field. Thisluminous zone is accompanied by little or nomeasurable local temperature rise. The airboundary temperature at which this luminosityis first observable in a darkened environment ishereafter referred to as the luminosity threshold(LT) temperature, which can be determinedexperimentally to an accuracy of 610 K.

When the air temperature is raised evenfurther from the LT temperature, the glowintensity increases progressively until ignition ofthe diffusion flame occurs. Because the chemi-luminescent glow is directly related to the oxi-dation chemistry prior to ignition, it is likely thatthe chemistry responsible for this glow is alsorelated to the onset of ignition. We shall reportboth the LT and ignition temperatures below.

Effects of Fuel Concentration

Figure 2 plots the LT and the flame ignitiontemperature as functions of the fuel concentra-tion in nitrogen, for ethane at 1 atm and a strainrate of 300 s21. The system response to fueldilution displays two distinct regimes. The firstregime is characterized by a high sensitivity ofthe LT and ignition temperature to fuel concen-tration, and occurs at low fuel concentrations,approximately below 20% by volume. Below;5% ethane in N2 the transition to burningregime is no longer abrupt, and the flame itselfcannot be sustained when the air temperature isreduced. These weak situations are consideredto be out of the ignitability range. The secondregime occurs for concentrations in excess of;30% in the fuel jet, and is described by arelatively constant ignition temperature. Thisappears to be in contrast to the inhibitive effectof high equivalence ratio which is observed inshock-tube [35, 36] and spark ignition (richmixtures) experiments [37, 38]. Similar fuelconcentration effects were observed, however,in our earlier studies on ignition of nonpre-mixed counterflowing methane/air [12]. Thereason for such an insensitivity to fuel concen-tration can be well understood from a previous

Fig. 2. Effects of fuel concentration on ethane ignition andluminosity threshold temperatures at p 5 1 atm and k 5300 s21. Symbols are experimental measurements, and linesrepresent calculated results. Error bars indicate the absoluteerror.


study [9] of methane ignition in a convective-diffusive system, and on the basis of the currentsimulation study which will be discussed later.

The same trends in the dependence of LTand ignition temperatures on fuel concentrationin the fuel jet are found in the present work forpropane/air, n-butane/air, and i-butane/air at 1atm, as seen in Figs. 3 and 4, respectively. In thecase of propane the transition from the concen-tration-sensitive regime to the insensitive re-gime occurs at a fuel concentration similar tothat of ethane, namely 20 to 30% of fuel in thejet. A comparison between n-butane and i-butane reveals that the ignition temperature ofi-butane is higher than that of n-butane byabout 60 K over the entire fuel concentrationrange shown in Fig. 4, although the transitionbetween the two regimes occurs at similar fuelconcentrations.

To conclude this section we comment brieflyon the nature of the preignition glow. Weexamined emission spectra taken for flow con-ditions close to the flame ignition temperature.An analysis of these spectra revealed the pres-ence of excited formaldehyde (CH2O*) bandscharacteristic of cool flame emission [39, 40]. Inaddition to these, we have also identified thecharacteristic 3064 peak emission from OHand a weaker emission from CH near 4315 ,

for all four fuels investigated. Similar spectralcontributions were identified in an earlier workon propane cool flames and preignition glows byGaydon and Moore [41], and in the study onengine knock by Smith et al. [42].

Effects of Aerodynamic Strain Rate

The ignition of paraffinic hydrocarbons involvesintermediate species [15] whose lifetimes maybe comparable to the characteristic time scalesof mass and heat transport [43]. Thus, transporteffects are expected to exert a significant influ-ence on the ignition event. To quantify thisinfluence we have determined the response ofignition temperatures to changes in the flowstrain rate. In our counterflow configuration thereciprocal of the strain rate is a direct measureof the convective residence time.

Figure 5 presents the experimentally mea-sured LT and ignition temperature as a functionof strain rate for 6% ethane in the fuel jet and1 atm pressure. It is seen that increasing theflow strain rate uniformly raises the air temper-ature required for flame ignition. Here, anincrease in the strain rate by a factor of 4elevates the ignition temperature by about 60 K.The LT temperature, however, is less affectedby the strain rate, and remains nearly constant,as seen in Fig. 5.

Fig. 3. Experimentally determined effects of fuel concen-tration on propane ignition and luminosity threshold tem-peratures at p 5 1 atm and k 5 300 s21. Error bars indicatethe relative error. Lines are fits to experimental data.

Fig. 4. Experimentally determined effects of fuel concen-tration on n-butane and i-butane ignition and luminositythreshold temperatures at p 5 1 atm and k 5 300 s21.Lines are fits to experimental data.


Figures 68 present respectively the varia-tions of the ignition temperature with respect tostrain rate for propane, n-butane, and i-butane.It is seen that these fuels exhibit similar re-sponse to flow straining as just discussed forethane. The sensitivities to strain appear to becomparable in magnitude among the four hy-

drocarbons. In general, an increase in the strainrate by a factor of 4 requires the air temperatureto rise by 4060 K to achieve flame ignition.

Moreover, the effects of strain rate and fuelconcentration on ignition do not seem to be

Fig. 5. Ethane ignition and luminosity threshold tempera-tures versus flow strain rate at p 5 1 atm, 6% C2H6 in thefuel jet. Symbols are experimental data, and lines representcalculated results. The error bars indicate the relative error.

Fig. 6. Propane ignition and luminosity threshold tempera-tures versus flow strain rate at p 5 1 atm, 6% C3H8 in thefuel jet. Symbols are experimental data, and lines are fits toexperimental data.

Fig. 7. n-Butane ignition and luminosity threshold temper-atures versus flow strain rate at p 5 1 atm, 5% n-C4H10 inthe fuel jet. Symbols are experimental data, and lines are fitsto experimental data. The error bars indicate the relativeerror.

Fig. 8. i-Butane ignition and luminosity threshold temper-atures versus flow strain rate at p 5 1 atm, 5% i-C4H10 inthe fuel jet. Symbols are experimental data, and lines are fitsto experimental data. The error bars indicate the relativeerror.


strongly coupled. As demonstrated respectivelyin Figs. 9 and 10 for n-butane and i-butaneignition at 1 atm, changing the fuel concentra-tion by a factor of 5, i.e., from 3 to 15% in thefuel jet, merely shifts the ignition temperature-versus-strain rate curves upward, but does notaffect the qualitative trend significantly. Con-

versely, performing the fuel concentration ex-periments described in Figs. 24 at higher strainrates would only shift the data upward in theignition temperatureconcentration curve,without modifying the trend.

A second similarity among the four fuels isthe relative insensitivity of the LT temperatureto strain rate variation. An exception appears tobe propane, for which it would be hard todiscount the change in LT temperatures seen inFig. 6.

Effects of Pressure

An increase in the system pressure is generallybelieved to facilitate ignition, as surmised fromthe simple thermal explosion theory of Semenov[44] for reactions with positive overall pressureexponent. In reality, however, hydrocarbon ig-nition presents more complex responses in dif-ferent pressure ranges, such as the cool flamephenomena, NTC regimes, and multiple-stagedand oscillatory ignition [13, 45]. This variabilitywith pressure implies that conclusions derivedfrom atmospheric pressure studies cannot besimply extrapolated to higher or lower pres-sures. Recognizing, therefore, that a truly sys-tematic study of ignition must encompass anextended range of the system pressures, weexpand in this section our investigation ofC2C4 nonpremixed ignition to study the effectsof pressure variation between 0.2 and 8.0 atm.

Figure 11 plots the experimentally deter-mined ignition and LT temperatures of 6%ethane in N2 as a function of pressure, for aconstant pressure-weighted strain rate of 300s21. The data in Fig. 11 shows that both the LTand the ignition temperature decrease when thesystem pressure is raised. The same overalldependence on system pressure can be observedwith the other hydrocarbons studied, as will beseen shortly. This dependence is expected, con-sidering that the rate-limiting processes for rad-ical growth are typically second-order, andtherefore depend on the square of the reactantpressure. To a first approximation, the balanc-ing diffusive losses remain constant with pres-sure variation, because of the near-constancy ofthe density-weighted diffusivities, hence theneed for a higher air temperature when pres-sure is reduced.

Fig. 9. n-Butane ignition temperatures versus flow strainrate at p 5 1 atm and different fuel concentrations in thefuel jet. Symbols are experimental data, and lines are fits toexperimental data.

Fig. 10. i-Butane ignition temperatures versus flow strainrate at p 5 1 atm and different fuel concentrations in thefuel jet. Symbols are experimental data, and lines are fits toexperimental data.


An additional observation which appears tobe valid for all four hydrocarbons may be in-ferred. As pressure is increased, the tempera-ture domain of the glow regime diminishes, asseen in Fig. 11 by the gradual closure of the LTand ignition temperature. In particular, at pres-sures higher than 5 atm, no glow is detectedexperimentally before flame ignition and the LTtemperature curve merges with that of the igni-tion temperature.

Propane has a similar ignition response topressure as that observed with ethane. This isseen in Fig. 12 in the variation of the experi-mental LT and ignition temperature with pres-sure determined for 6% propane in N2 and aconstant pressure-weighted strain rate of 300s21. A somewhat higher sensitivity to pressureincrease is observed in the range of high pres-sures.

Finally, Figs. 13 and 14 plot respectively thepressure dependence of the experimentally de-termined ignition temperature of n-C4H10 andi-C4H10, for 5% fuel in N2 and a constantpressure-weighted strain rate of 300 s21. It isseen that C4 nonpremixed ignition is character-ized by three distinct regimes, although theoverall dependence remains one of facilitatingignition upon pressure increase. Under all con-

ditions, n-butane ignites more readily than i-butane.

It is seen that regime I occurs at low tointermediate pressures, below ;5 atm, and issimilar to that encountered with ethane andpropane, namely a similar qualitative depen-dence of ignition temperature on pressure and

Fig. 11. Effects of pressure on ethane ignition and luminos-ity threshold temperatures at k 5 300 s21 and 6% C2H6 inthe fuel jet. Symbols are experimental measurements, andlines represent calculated results.

Fig. 12. Effects of pressure on propane ignition and lumi-nosity threshold temperatures at k 5 300 s21 and 6% C3H8in the fuel jet. Symbols are experimental measurements, andlines are fits to experimental data.

Fig. 13. Effects of pressure on n-butane ignition and lumi-nosity threshold temperatures at k 5 300 s21 and 5%n-C4H10 in the fuel jet. Symbols are experimental measure-ments, and lines are fits to experimental data.


flame ignition preceded by glow. Regime IIoccurs around 5 atm and is characterized by arapid decrease of the ignition temperature witheven small pressure increases. This regime,manifested as a plateau in Figs. 13 and 14,appears similar to the behavior observed inautoignition studies of n-butane/air mixtures[46], where it is attributed to a transition in thedominant chemistry to a low temperature mech-anism. Regime III is characteristic of high pres-sures, in excess of ;6 atm, and shows aninsensitivity of the ignition temperature to pres-sure variation. In this regime, the ignition tem-peratures of n-butane and i-butane are respec-tively ;900 and 1010 K, and flame ignition is nolonger preceded by glow.

If the transition of P-T behavior to regimes IIand III can be attributed to a transition in thedominant chemistry to the low-temperature orNTC regime, we expect that propane shouldalso exhibit such a p-T behavior. This is becausepropane also exhibits NTC behavior. Indeed,the p-T ignition curve of propane of Fig. 12levels off more significantly than that of ethanein Fig. 11, when the pressure is higher than 4 to5 atm. Hence, the p-T curve of propane mayenter into regime II or even regime III, if thepressure is further elevated from the upper limitof p approximately equal to 7 atm. Clearly,

more experiments are needed to resolve thisissue.

Numerical Simulation of Ethane Ignition

To understand the convective/diffusive effectsand the chemical processes responsible forflame ignition, we present the results of numer-ical modeling of ethane ignition in this section.The simulation was performed under conditionsidentical to those of the experiments describedearlier.

A typical numerical solution is shown in Fig.15, which plots the axial distributions of majorspecies, temperature, and five relevant interme-diates (H, HO2, CH2O, C2H5, and C2H5O2) fora situation just prior to flame ignition, at 6%ethane in N2, 300 s

21 strain rate, atmosphericpressure, and Tair 5 1276 K. The selection ofthe intermediate species is based on the consid-eration that while the H atom is characteristic ofhigh-temperature ignition mechanism domi-nated by

H 1 O23 O 1 OH (38)

chain branching [47], the HO2 and C2H5O2 arerelevant to low-to-intermediate temperature hy-

Fig. 14. Effects of pressure on i-butane ignition and lumi-nosity threshold temperatures at k 5 300 s21 and 5%i-C4H10 in the fuel jet. Symbols are experimental measure-ments, and lines are fits to experimental data.

Fig. 15. Axial temperature and species concentration pro-files as a function of the distance from the fuel-jet nozzleprior to flame ignition computed for p 5 1 atm, k 5 300s21, 6% C2H6 in the fuel jet.


drocarbon ignition dominated by the peroxidechemistry.

We note that the response of the peak radicalconcentration at flame ignition is similar amongall radicals, and is characterized by the turningof the response curve as a function of a systemparameter, like the air temperature. Figure 16plots two representative response curves, for thepeak concentrations of H and HO2 as a functionof air temperature. The computational flameignition is determined from such a plot as theturning point of the response curves at ;1270K, above which the steady-state solution, e.g.,the H-atom concentration, can exist only on theupper branch of the S-curve of vigorous burn-ing. We have verified previously [9] that theignition temperatures achieved by this proce-dure are nearly identical to those obtained usingcontinuation algorithms which trace the re-sponse through the turning point [48]. In addi-tion, the critical turning points are obtained atthe same air temperatures, as expected from thesteady-state nature of the analysis [9]. Indeed, itis seen that both H and HO2 curves turn at thesame temperature, as such the ignition state canbe defined by examining the response of any

radical with respect to air temperature. Here wechoose the H atom as a representative speciesfor the ignition temperature identification.

The computed ignition temperature is com-pared with the experimental data for ethane inFigs. 2, 5, and 11. The smooth curves in Fig. 2are generated through 10 computational cases.It is seen that the numerical simulation repro-duces qualitatively the dependence of the igni-tion temperature on the fuel concentration (Fig.2), the strain rate (Fig. 5), and the pressure (Fig.11). In particular, the transition in the ignitiontemperature response to fuel concentration iswell simulated by the numerical model, as seenin Fig. 2. The calculated ignition temperaturesare, however, uniformly higher than the exper-imental values by less than 100 K. This differ-ence is not substantial considering the uncer-tainties in both reaction kinetics and in thetransport coefficients.

Figure 16 also shows multiple stages of radi-cal growth at the temperature below the ignitionpoint. First, when the air temperature is gradu-ally raised up to ;1050 K, the peak concentra-tion of the H atom grows at a nearly uniformrate E 5 dln[H]max/d(21/RTair) of about 52kcal/mole. This slope is comparable to the acti-vation energy Ea of the initiation step,

C2H6 1 O23 C2H5 1 HO2, (R222)

which has Ea 5 51 kcal/mole, and is found todominate the system response in this range oftemperatures of diminished reaction activities.

Around 1100 K for the situation shown in Fig.16, an abrupt acceleration of the radical growthensues. Subsequently, the system enters a stageof mild reactivity, characterized by small heatrelease and partial fuel conversion. Some inter-mediates, such as CH2O or HO2, are formed inamounts larger than those on the vigorousflaming branch, as seen in Fig. 16b for HO2. Theexistence of large amounts of such radical spe-cies in the mild oxidation region is likely tocorrespond to the chemiluminescent glow ob-served experimentally.

It should be recognized, however, that thesignal at the very onset of mild oxidation may betoo weak to be visually detectable. Indeed, whenwe plot the computational inflection tempera-ture identified in Fig. 16a and compare it with

Fig. 16. System response curves: peak (a) H and (b) HO2concentrations computed for p 5 1 atm, k 5 300 s21, 6%C2H6 in the fuel jet. Solid line: the base case calculation;dashed lines: computations without chemical heat release.


the experimental LT temperature, we foundthat the inflection temperature is significantlylower than the experimental detection of theglow, although the response is similar tochanges in the fuel concentration (Fig. 2), strainrate (Fig. 5), and pressure (Fig. 11).

An alternate possibility of modeling the LT isto assign a threshold of detection, for example,in terms of the excited formaldehyde concentra-tion, and subsequently monitor the air temper-atures required to achieve this threshold. At 1atm, 6% ethane in N2, and a flow strain rate of300 s21, the experimental LT was found to be;1170 K. The corresponding computed CH2O*concentration is 1.5 3 10218 mol/cc. If we usethis threshold concentration as the calibrationpoint and define the air temperature required toachieve this threshold as the computational LTtemperature, we found that we can model theLT fairly well, as seen in Figs. 2 and 5 bycomparing the dashed lines with the experimen-tal data (circles).

The computed response of LT temperatureand inflection point responses to pressure areplotted in Fig. 11. It is seen that the inflectionpoint disappears at p 5 5 atm, that is, no abrupttransition occurs between the initiation-domi-nated and mild oxidation regimes. In addition,the preset threshold for CH2O* is reached nearthe ignition point, or not at all for pressures inexcess of ;6 atm, in agreement with the visualobservation of the onset of glow. For this rea-son, the computed LT and ignition temperaturecurves merge at ;6 atm.

DISCUSSION

Mechanism of Flame Ignition at Atmosphericand Subatmospheric Pressures

We begin discussion by noting that the airboundary temperature at ignition defined in theMethodology section is not necessarily the tem-perature corresponding to that experienced bychemical reactions responsible for flame igni-tion. First, the fuelair mixing region of thecounterflow system is a region where a signifi-cant temperature gradient exists (Fig. 15). Sec-ond, the heat release associated with mild oxi-dation prior to ignition often causes a small rise

in temperature above the air temperature, as isseen in Fig. 15. Nonetheless, we note that themore physically appropriate local tempera-tures corresponding to the peak of the H atom,or to the maximum temperature in the flow, areactually very close in value to the boundarytemperature (within 520 K). It follows that theignition response is virtually identical when us-ing any of these three representations. The airboundary temperature quantifies a global re-sponse, which is appropriate when comparingthe ignitability of different fuels. In addition,this selection greatly facilitates the comparisonbetween experimental and modeling results.

The consequence of spatial concentrationand temperature gradients is that the knowl-edge previously learned in homogeneous igni-tion system is valuable but may not be directlyapplicable to the current nonhomogeneous ig-nition system. In particular, the present situa-tion cannot be simply understood by consider-ing radical growth and destruction following areaction time, because the latter cannot beunambiguously defined in the present diffusivesystem.

In an earlier study on hydrogen/air ignition[8] it was found that the active radicals peakednearly at the same spatial location, which sug-gested the concept of an ignition kernel ofessentially premixed nature. This concept fur-ther led to understanding the effects of changesin external parameters such as the flow strainrate. The intermediate species profiles in Fig. 15show the existence of a spatially narrow reactionzone, encompassing both the peroxide and H-atom dominated reaction chemistries which oc-cur at different spatial locations within thenarrow reaction zone. For consistency, we shallstill call the reactive region of the counterflowsystem as the ignition kernel, which develops asa result of fuel and oxidizer mixing.

The ignition kernel extends mainly on theoxidizer side of the stagnation plane, as exem-plified by the peak concentration location ofC2H5 relative to the stagnation surface. This isreasonable considering that the general Arrhe-nius dependence of reaction rates favors ahigher temperature environment at the expenseof decreasing fuel concentration. Consequently,significant reactions occur by-and-large with aneffectively fuel-lean stoichiometry. Because the


reaction zone is on the oxidizer side of thestagnation surface, the fuel must come fromconvective/diffusive transport across the stagna-tion surface.

For ethane ignition, Fig. 15 shows that a clearseparation exists between the peaks of theC2H5O2 and H concentrations, whereas theHO2 profile is more broadly distributed acrossthe reaction zone. In general, three types ofintermediates are found: (a) the high-tempera-ture, highly reactive radicals, like the H atom,whose short lifetimes translate into narrowlypeaked concentration profiles situated close tothe oxidizer boundary; (b) the peroxide species,such as C2H5O2, which are preferentiallyformed at lower temperatures, and thus closerto the fuel side; and (c) the relatively stablespecies, such as CH2O and HO2, which situatebetween the high-temperature radicals and thelow-temperature peroxides, and exhibit broaderdistribution profiles because they are morelikely to be transported out of the reaction zoneand remain unreacted. The coexistence of allthree types of species suggests that in thepresent convective/diffusive system the fuel canbe oxidized via a complete spectrum of thehigh-, intermediate-, and low-temperature reac-tion chemistries of hydrocarbon fuels. Thesechemistries are: (a) reaction kinetics dominatedby the H atoms at the high-temperature, (b)that in the intermediate temperature range withthe HO2 as a key radical, and (c) the low-temperature chemistry of peroxide reactions, asexemplified by C2H5O2 and C2H5O2H.

Two related questions follow immediatelyfrom the above discussion. First, is flame igni-tion a consequence of the peroxide chemistry,or the H-atom dominated high-temperature ki-netics? Second, does peroxide chemistry pre-condition the high-temperature kinetics? Weshall address these questions below by means ofsensitivity analyses.

Figure 17 presents a computational sensitivitytest, where the peak H-atom concentrations areplotted as a function of the air temperature forthe case of 6% ethane in N2, 1 atm pressure, and300 s21 strain rate. These conditions are thesame as those sample calculations shown inFigs. 15 and 16. It is seen that removing therepresentative peroxide reactions

C2H5O2H3 C2H5O 1 OH (R229)

C2H5 1 O23 C2H5O2 (R214)

from the simulation does not cause any appre-ciable effects on flame ignition, i.e., the temper-ature at the turning point remains essentiallythe same. Yet removing the high-temperaturechain branching reaction

H 1 O23 OH 1 O (R38)

causes the disappearance of the turning point,or flame ignition. In addition, removing thereaction

C2H5(1M)3 C2H4 1 H(1M) (R74)

causes the disappearance of the inflection pointprior to ignition, forces the growth of radicalspecies to require higher air temperatures, andthus causes a higher ignition temperature ascompared to the base case.

These sensitivity analyses suggest that flameignition in this case is caused by the high-temperature reaction mechanism dominated bythe chain-branching reaction (R38), and thelow-temperature, peroxide chemistry is unim-portant. The low-temperature chemistry ap-pears to affect the concentration of the H-atom,but it does not affect the ignition temperature.

Fig. 17. Effect of removing key elementary steps from thekinetic model on the variation of the H-atom response curveas a function of the air temperature. The conditions are thesame as those in Fig. 15.


This is in direct contrast to the general under-standing of the homogeneous ignition system, inwhich both low- and intermediate-temperaturechemistry is known to affect the ignition behav-ior [49] by preconditioning the high-temper-ature ignition.

An important distinction between the presentdiffusive configuration and the homogeneoussystems is the following. In a homogeneoussystem, when temperature is increased at aconstant pressure the cool flames initiated bythe low temperature oxidation will be extin-guished by the inherent transition into the NTCregime. In our system the NTC behavior willmanifest itself at most in a slowdown of theincrease of reactivity with increasing air temper-ature. This is because the nonpremixed systemoffers considerably more freedom. When thetemperature becomes too high, the low-temper-ature mechanism can simply adjust its positioncloser to the cold fuel side. Consequently, we donot observe in our nonpremixed system thesequence ignitionextinctionignition whichtranslates into the well-known ignition penin-sula of the hydrocarbon explosion limits [50].

Because of the presence of mild oxidationand heat release prior to ignition, the ignitionevent may also be affected by chemical heatrelease feedback prior to ignition. To investi-gate this possibility, a numerical experiment isperformed, in which the heat release is turnedoff in the energy conservation equation. Theresults, plotted with dashed lines in Fig. 16,indicate that while the onset of mild oxidationmay be resolved accurately without heat release,the flame ignition is no longer realized withoutthermal feedback. This result indicates thatflame ignition occurs primarily as a combinationof high-temperature kinetics and heat releasefeedback, whereas the mild oxidation regime isentirely kinetically driven.

The above analyses should be applicable toethane ignition in general at pressures below 1atm where the peroxide chemistry is less impor-tant as compared to the case at 1 atm shown inFigs. 15 through 17, because the air temperatureat ignition is higher for p , 1 atm than that at1 atm (Fig. 16). The analysis may be equallyapplicable at elevated pressure, based on theconsideration that the pressure-versus-temper-ature response curve of Fig. 16 does not expe-

rience qualitative change over the range ofexperimental conditions. Based on the sameconsideration, one may further speculate thatthe high-temperature chemistry causes ignitionfor propane over the entire pressure rangeshown in Fig. 12, and for n-butane and i-butaneignition in regime I (high-temperature and low-pressure) of Figs. 13 and 14. Because of theuncertainties associated with the reaction kinet-ics of peroxides and the transport properties ofthe peroxide species, discussion of a conclusivenature regarding the mechanism of ignition forbutane in regimes II and III can only be con-ducted in future modeling studies.

It is interesting to note that the occurrence ofkinetically dominated transition followed bythermokinetic ignition has been identified forfuels previously investigated in our convective-diffusive system, namely H2, CO/H2, CH4, andCH4/H2. Moreover, the kinetically driven inflec-tion point may become an actual turning point[9, 12] within certain ranges of the strain rate,pressure, and fuel concentration, suggesting thepossibility of double-staged ignition in the senseof two abrupt transitions to higher states ofreactivity.

Further Examination of Reaction Kinetics

In this section, we further examine the domi-nant reaction kinetics of flame ignition and mildoxidation regimes. To analyze the first acceler-ation of the radical concentration growth in thesystem response, we have performed CSP anal-yses for the peaks of the ethyl peroxy C2H5O2and H radical distributions for the case of 6%ethane previously discussed, at the air boundarytemperature corresponding to the inflectionpoint (1102.5 K) of the S-curve.

The composition of the explosive mode at thepeak of the C2H5O2 radical reveals that amechanism of low and intermediate tempera-ture is responsible for the C2H5O2 radicalgrowth at the spatial location where the temper-ature is approximately 400 K below the bound-ary air temperature. The attack on fuel occursmostly through

C2H6 1 HO23 C2H5 1 H2O2 (R221)

C2H6 1 OH3 C2H5 1 H2O. (R113)


Subsequently, the ethyl radical undergoes oxi-dation through the competing routes:

C2H5 1 O23 C2H4 1 HO2 (R213)

C2H5 1 O2 C2H5O2 (R214)

The C2H5O2 radical produced in R214 may (a)internally isomerize to form C2H5O2H

C2H5O23 C2H4O2H (R236)

which then dissociates through

C2H4O2H3 C2H4O 1 OH (R237)

or (b) abstract an H atom to form ethylhy-droperoxide C2H5O2H which then providesbranching through homolytic fission of the OObond:

C2H5O2H3 C2H5O 1 OH. (R229)

The competing conjugate alkene route R213is favored at slightly higher temperatures, whenthe equilibrium in R214 shifts to the reactantsbecause of the higher activation energy of thereverse reaction. This thermal switch mecha-nism is conventionally considered to be respon-sible for the onset of the NTC regime [51, 52] inhigher hydrocarbons, although more recentlyWilk et al. [53] have proposed an alternativecriterion for n-butane transition to NTC.

The CSP analysis at the peak of the C2H5O2concentration shows the increased importanceof the HO2 chemistry through the reactions:

CH3 1 HO23 CH3O 1 OH (R119)

C2H5 1 HO23 C2H5O 1 OH (R212)

C2H5O2 1 HO23 C2H5O2H 1 O2. (R232)

Additional branching comes about from thedecomposition of the hydrogen peroxide H2O2

H2O2(1M)3 OH 1 OH, (R-85)

with H2O2 produced primarily by R221 andHO2 1 HO23 O2 1 H2O2. The reaction R-85,characteristic of the intermediate temperaturemechanism, is highly temperature-sensitive andhence peaks farther upstream, closer to the hotair boundary. The main branching process atthe peak of C2H5O2 remains reaction R229,which is representative of the low-temperaturechemistry.

At the upstream peak of the H-atom concen-tration, however, the CSP analysis indicates thatreactions characteristic of the intermediate- andhigh-temperature mechanisms become active.The explosive mode is dominated by the ther-mal decomposition of ethyl:

C2H5(1M)3 C2H4 1 H(1M) (R-74)

and the propagation route R213. Features ofthe low-to-intermediate mechanism are stillpresent through the influence of R214. How-ever, the combination of high temperature andH production through R-74 results in activatingthe branching reaction (R38), which is crucial inflame ignition as discussed previously.

Further evidence supporting the above dis-cussion is that the removal of R214, the sourceof low-temperature oxidation, from the numer-ical simulation actually promotes ignition. It isseen in Fig. 17 that the rise of the H atomconcentrations occurs at lower temperaturesand the ignition temperature is slightly lowerthan the base case calculation in which R214 isincluded. This result can only be interpreted bythe fact that R214 and thus the low-temperatureoxidation competes for the C2H5 radical, whichis critically needed to initiate flame ignition viaethyl decomposition to form the H atoms. Theoverall effect of fuel scavenging by low-temper-ature chemistry is, however, not very significant.The removal of the low-temperature pathwaysfrom the mechanism affects only minimally theignition temperature.

In addition to the CSP analysis, we alsoperformed an S-curve sensitivity analysis [34].The sensitivity coefficients normalized to theirlargest value are plotted in Fig. 18. A positivesensitivity coefficient means that the reactionpromotes flame ignition, whereas a negativesensitivity signifies that the reaction retardsignition. The sensitivity results indicate that it isthe high-temperature reactions which have themost significant influence in radical growth justprior to ignition, namely the ethyl thermal de-composition R-74, the branching reaction R38,the oxidation of CH3 by HO2 R119, and amongothers,

CO 1 OH3 CO2 1 H (R99)

HCO 1 M3 H 1 CO 1 M. (R166)


The destruction of the radicals is sensitive to thereactions

OH 1 HO23 H2O 1 O2 (R87)

HCO 1 O23 HO2 1 CO. (R167)

Thus the sensitivity analysis further supports theconclusion that ethane ignition at 1 atm pres-sure is caused mainly by the high-temperaturechain branching mechanism. Figure 18 alsoshows that the oxidation of C2H5 by O2 (R213)has a negative effect on ignition, because itcompetes against R-74. In addition, the internalisomerization of C2H5O2 to C2H4O2H alsoshows a negative sensitivity coefficient, againindicating that the low-temperature chemistryinhibits, albeit mildly, the flame ignition. Theseresults are consistent with the CSP analysis.

In summary, the following interpretation ofthe ignition response can be reached. When theair temperature is low, initiation reaction R222provides a stable, if small, pool of ethyl andhydroperoxy radicals. As the temperature israised, this pool increases sufficiently such thatethyl decomposition begins to supply sufficientH radicals to trigger the high-temperaturebranching reaction R38. This explains the firstacceleration of the system response. The low-temperature mechanism of peroxide is also ini-tiated. This mechanism contributes to the in-crease in concentration of the peroxide species,but it does not contribute significantly to theproduction of the H atom concentration, whosegrowth ultimately controls the flame ignition.

Effects of Transport Processes, FuelConcentration, and Fuel Structure

The responses of the ignition temperature tostrain rate shown in Figs. 510 point to the factthat species transport indeed affects the ignitionprocesses. The strain-rate dependence has beenattributed [8, 9] to a decrease in the availableresidence time within the ignition kernel. Theconvective residence time is inversely propor-tional to the flow strain rate in the counterflowconfiguration [54], therefore decreases uponincreasing strain. Likewise, the ignition kernelwidth varies inversely with the square root ofthe strain rate [8], hence the rate of radical lossthrough diffusive transport out of the kernelalso increases with increased flow straining. Inaddition, the fuel must diffuse across the stag-nation surface and be transported against theconvective flow of the oxidizer stream to initiatereaction. The numerical modeling results forethane further support this explanation. Whenstrain rate is increased the reaction zone widthis reduced, and the species concentration gradi-ents increase, resulting in enhanced rates ofdiffusive loss. The computed results agree qual-itatively with those obtained experimentally(Fig. 5), even though the flame ignition temper-atures remain substantially overpredicted.

The characteristic dependence of the ignitiontemperature on fuel concentration (Figs. 24)appears intuitively correct in that at low fuelconcentrations a fuel deficit would result in astate of low reactivity, and hence the need forhigher ignition temperature. At high fuel con-centrations the rate of reaction processes be-comes limited by fuel diffusion to the ignitionkernel. The diffusionally limited supply of fuelresults in a fuel-lean stoichiometry even at100% fuel in the jet [9]. The result is thatincreasing the fuel concentration does not suc-ceed in raising or reducing the required ignitiontemperature.

The relatively constant ignition temperatureattained at large fuel concentrations allows us tocompare in a systematic fashion the ignitabilitycharacteristics of the four paraffins. In order todo this we define a nonpremixed ignitiontemperature as the ignition temperature in thelimit of high fuel concentration (100%), for afixed characteristic flow strain rate and system

Fig. 18. S-curve sensitivity analysis under the same condi-tions as in Fig. 15.


pressure. Figure 19b plots the variation of thesenonpremixed ignition temperatures as a func-tion of the number of carbon atoms in the fuelmolecule. The ignition temperatures for n-bu-tane and i-butane were estimated by extrapola-tion of least-square polynomial curve fitsthrough the data in Fig. 4.

It is seen that the ignition temperature ini-tially decreases from methane to ethane, thenincreases from ethane to propane, and to bu-tane. This could be a manifestation of thedifferences in reaction kinetics, as well as theinfluence of decreasing mass diffusivity whichaccompanies the transition to larger hydrocar-bons. The lower fuel diffusivity implies a posi-tion of the reaction zone closer to the stagnationplane, thereby occurring at lower temperatures.As a result, the boundary temperature mustincrease for ignition to occur. Moreover, nu-merical simulations performed for ethane igni-tion appear to support this explanation. Specif-ically, our calculations show that the ignitiontemperature can increase by 3040 K if thediffusivity of ethane is artificially decreased tothat of butane. While this increase may not besufficient to modify the ranking in Fig. 19b, itdoes substantiate the importance of fuel diffu-sivity in effecting ignition.

We must note, however, that the trend in Fig.19b is not determined entirely by the diffusiveproperties of the fuel. Ignition is a result ofcomplex interactions between chemical kineticsand transport phenomena. An example is theignition of methane [9]. As seen in Fig. 19b,methane remains the most difficult hydrocarbonto ignite, its higher diffusivity notwithstanding.In addition, the two isomers of butane havesimilar transport properties, however, i-butaneis more difficult to ignite than its straight-chaincounterpart as seen in Fig. 19b, and throughoutthe ranges of pressure, fuel concentration, andstrain rate investigated.

We also plot in Fig. 19a the shock-tubeignition temperatures from Burcat et al. [55],and in Fig. 19c the corresponding autoignitiontemperatures (AIT) taken from the compilationof Zabetakis [56]. The shock-tube ignition tem-peratures were estimated from the experimentaldata under stoichiometric conditions of fuel/oxygen mixtures in argon diluent, at the exper-imental ignition delay times of ;0.3 ms. It isseen that while the qualitative dependence ofthe ignition temperature on fuel structure be-tween the shock-tube experiments and thepresent convective-diffusive system sharemarked resemblance, the present results do notfollow the trend of the autoignition tempera-tures of the same fuels. The autoignition exper-iments benefit from very long residence times,hundreds of seconds [56] whereas the charac-teristic residence time in our system is of theorder of milliseconds, and the ignition delaytimes in shock-tube experiments are typicallytens or hundreds of microseconds. These differ-ences further support our conclusion that withinthe parameter space adopted in our study (withthe exception of the high-pressure regimes IIand III for n-butane and i-butane seen in Fig. 13and 14), the convective/diffusive ignition is ini-tiated by fuel oxidation following a high-tem-perature reaction mechanism. For n-butane andi-butane with p greater than approximately 5atm, it is tempting to argue that ignition inregimes II and III is significantly influenced ordominated by the intermediate- and low-tem-perature fuel oxidation mechanisms, because ofthe significant differences in the p-T responsesfrom those at lower pressures. This inference,

Fig. 19. Comparison among (a) shock-tube [55], (b) coun-terflow nonpremixed ignition (this work), and (c) autoigni-tion [56] temperatures as a function of the hydrocarbonchain length.


however, needs to be validated in future kineticmodeling studies.

CONCLUSIONS

We have conducted an extensive experimentalinvestigation on the nonpremixed ignition char-acteristics of C2C4 hydrocarbons in an aerody-namically strained flow of opposed cold fuelversus hot air jets. The ignition responses ofethane, propane, n-butane, and i-butane werefound to possess a number of similarities, sum-marized below.

First, flame ignition is preceded by the onsetof chemiluminescent glow, except at high pres-sures ( p . ;5 atm). This weak light emission isdue to excited formaldehyde and OH, and isindicative of the existence of mild oxidation, assubstantiated by the numerical modeling ofethane ignition. Second, similarity exists con-cerning the effects of external parameters onignition. Thus, the ignition temperature initiallydecreases with an increase in fuel concentra-tion, but it may reach a constant value at largefuel concentrations. Increased aerodynamicstraining progressively increases the ignitiontemperature, whereas an increase in the systempressure facilitates ignition overall.

Distinct regimes of pressure sensitivityemerge for the two butane isomers, which maycorrespond to the transition between the low-to-intermediate and high-temperature oxida-tion mechanisms observed in homogeneous sys-tems. Isobutane is found to be more difficult toignite than n-butane, except at the lowest pres-sures of this study, where the two isomers igniteat similar temperatures.

Limited kinetic modeling study suggests thatignition at pressures below 5 atm is initiated byfuel oxidation following the high-temperaturemechanism of radical chain branching. The low-to-intermediate temperature fuel oxidation ki-netics is present in the system, and occurs at adifferent spatial location from the region dom-inated by the high-temperature oxidation kinet-ics. The low-to-intermediate temperature chem-istry contributes little to flame ignition.

Lastly, in our nonpremixed system the ignit-ability decreases in the order ethane . pro-pane . n-butane . i-butane at 1 atm. This

ranking is opposite to the one shown by theautoignition temperatures. Our future researchwill involve detailed modeling of the C3 andhigher hydrocarbons ignition, and a more accu-rate experimental characterization of the mildoxidation regimes and their relation to possibletwo-staged ignition phenomena.

The authors wish to thank Dr. Y. Tan and Dr.T. G. Kreutz of Princeton University for help inconnection with the experimental results, and forthe useful technical observations, respectively. Wealso wish to acknowledge the fruitful comments ofDr. C. K. Westbrook. This work was supported bythe Army Research Office under the technicalmonitoring of Dr. David Mann.

REFERENCES

1. Fish, A., in Organic Peroxides (D. Swern, Ed.,) Wiley,New York, 1970, Vol. 1, pp. 149151.

2. Pollard, R. T., in Comprehensive Chemical Kinetics,(C. H. Bamford and C. F. H. Tipper, Eds.), Elsevier,New York, 1977, Vol. 17, pp. 249367.

3. Glassman, I., Combustion, 3rd ed., Academic Press,San Diego, 1996, p. 325.

4. Griffiths, J. F., and Scott, S. K., Prog. Energy Combust.Sci. 13:161197 (1987).

5. Spadaccini, L. J., and Colket, M. B. III, Prog. EnergyCombust. Sci. 20:431460 (1994).

6. Kreutz, T. G., Nishioka, M., and Law, C. K. CombustFlame 99:758766 (1994).

7. Fotache, C. G., Kreutz, T. G., Zhu, D. L., and Law,C. K. Combust. Sci. Technol. 109:373394 (1995).

8. Kreutz, T. G., and Law, C. K. Combust. Flame 104:157175 (1996).

9. Fotache, C. G., Kreutz, T. G., and Law, C. K. Combust.Flame 108:442470 (1997).

10. Trujillo, J. Y. D., Kreutz, T. G., and Law, C. K.Combust. Sci. Technol. 127:127 (1997).

11. Fotache, C. G., Kreutz, T. G., and Law, C. K. Combust.Flame 112:522532 (1998).

12. Fotache, C. G., Sung, C. J., Sun, C. J., and Law, C. K.,Combust. Flame 112:457471 (1998).

13. Leppard, W. R. (1988). SAE Paper No. 881606.14. Leppard, W. R. (1989). SAE Paper No. 892081.15. Westbrook, C. K., and Dryer, F. L., Prog. Energy

Combust. Sci. 10:157 (1984).16. Westbrook, C. K., and Pitz, W. J., Combust. Sci.

Technol. 37:117152 (1984).17. Pitz, W. J., Westbrook, C. K., Proscia, W. M., and

Dryer, F. L., Twentieth Symposium (International) onCombustion, The Combustion Institute, Pittsburgh,1984, p. 831.

18. Warnatz, J., Combust. Sci. Technol. 34:177200 (1983).19. Pitz, W. J., and Westbrook, C. K., Combust. Flame

63:113133 (1986).


20. Wilk, R. D., Green, R. M., Pitz, W. J., Westbrook,C. K., Addagarla, S., Miller, D. L., and Cernansky,N. P., SAE Paper No. 900028, Intl. Congress andExposition, Detroit, MI, Feb. 26March 2, 1990.

21. Kojima, S., Combust. Flame 99:87136 (1994).22. Law, C. K., Sung, C. J., Yu, G., and Axelbaum, R. L.,

Combust. Flame 98:139154 (1994).23. Sung, C. J., Liu, J. B., and Law, C. K., Combust. Flame

102:481492 (1995).24. Li, H., Miller, D. L., and Cernansky, N. P., SAE Paper

No. 960498, Intl. Congress and Exposition, Detroit,MI, Feb. 2629, 1996.

25. Wang, S., Miller, D. L., and Cernansky, N. P., SAEPaper No. 961154, Intl. Spring Fuels and LubricantsMeeting, Dearborn, MI, May 68, 1996.

26. Hunter, T. B., Wang, H., Litzinger, T. A., and Fren-klach, M., Combust. Flame 104:505523 (1996).

27. Gaydon, A. G., and Wolfhard, H. G., in Flames, TheirStructure, Radiation, and Temperature, 4th ed., JohnWiley and Sons, New York 1979, p. 255.

28. Barbieri, G., Di Maio, F. P., Lignola, P. G., andLoiacono, M. L., Combust. Sci. Technol. 106:83102(1995).

29. Miller, J. A., Kee, R. J., Smooke, M. D., and Grcar,J. F., Paper No. WSS/CI 84-10, Spring TechnicalMeeting of the Western States Section of the Combus-tion Institute, Boulder, CO, 1984.

30. Smooke, M. D., Puri, I. K., and Seshadri, K., Twenty-First Symposium (International) on Combustion, TheCombustion Institute, Pittsburgh, 1986, pp. 17831792.

31. Fendell, F. E., J. Fluid Mech. 21:281303 (1965).32. Lam, S. H., Combust. Sci. Tech. 89:375404 (1993).33. Lam, S. H., and Goussis, D. A., Int. J. Chem. Kin.

26:461486 (1994).34. Kreutz, T. G., and Law, C. K., Combust. Flame.

114:436456 (1998).35. Burcat, A., Crossley, R. W., Scheller, K., and Skinner,

G. B., Combust. Flame 18:115123 (1972).36. Hidaka, Y., Gardiner, W. C., Jr., and Eubank, C. S., J.

Mol. Sci. 2:141153.37. Bond, J., Sources of Ignition, Flammability Characteris-

tics of Chemicals and Products, Butterworth Heine-mann, Oxford, 1991, pp. 1415.

38. Wang, C. S., Sibulkin, M., Combust. Sci. Technol.91:163178 (1993).

39. Kondratiev, V., Acta Phys. URSS 4:556558 (1936).40. Sheinson, R. S., and Williams, F. W., Combust. Flame

21:221230 (1973).41. Gaydon, A. G., and Moore, N. P. W., Proc. Royal Soc.

London A233:184194 (1955).42. Smith, J. R., Green, R. M., Westbrook, C. K., and Pitz,

W. J., Twentieth Symposium (International) on Com-bustion, The Combustion Institute, Pittsburgh, 1984,pp. 91100.

43. Griffiths, J. F., Prog. Energy Combust. Sci. 21:25107(1995).

44. Semenov, N. N., Chemical Kinetics and Chain Reac-tions, Clarendon Press, Oxford, 1935.

45. Griffiths, J. F., Combust. Flame. 93:202206 (1993).46. Chandraratna, M. R., and Griffiths, J. F., Combust.

Flame 99:626634 (1994).47. Westbrook, C. K., and Dryer, F. L., Eighteenth Sympo-

sium (International) on Combustion, The CombustionInstitute, Pittsburgh, 1981, p. 749.

48. Nishioka, M., Law, C. K., and Takeno, T., Combust.Flame 104:157175 (1996).

49. Westbrook, C. K., and Pitz, W. J., in Numerical Ap-proaches to Combustion Modeling (E. S. Oran, Ed.),Prog. Astro. Aero. 135:57 (1991).

50. Kane, G. P., Chamberlain, E. A. C., and Townend,D. T. A., J. Chem. Soc. 436443 (1937).

51. Benson, S. W., Prog. Energy Combust. Sci. 7:125 (1981).52. Lignola, P. G., and Reverchon, E., Prog. Energy Com-

bust. Sci. 13:7596 (1987).53. Wilk, R. D., Cohen, R. S., and Cernansky, N. P., Ind.

Eng. Chem. Res. 34:22852297 (1995).54. Schlichting, H., Boundary Layer Theory, McGraw-Hill,

New York, 1979, p. 95.55. Burcat, A., Scheller, K., and Lifshitz, A., Combust.

Flame 16:2933 (1971).56. Zabetakis, M., in Flammability Characteristics of Com-

bustible Gases and Vapors, Bulletin 627, U.S. Bureau ofMines, 1965.

Received 19 May 1998; accepted 9 September 1998


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