Fuel 97 (2012) 695–702

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Fuel

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Laminar flame speeds and extinction stretch rates of selectedaromatic hydrocarbons

Xin Hui a, Apurba K. Das a, Kamal Kumar b, Chih-Jen Sung b,⇑, Stephen Dooley c, Frederick L. Dryer c

a Department of Mechanical and Aerospace Engineering, Case Western Reserve University, Cleveland, OH 44106, USAb Department of Mechanical Engineering, University of Connecticut, Storrs, CT 06269, USAc Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, USA

a r t i c l e i n f o

Article history:Received 16 November 2011Received in revised form 19 February 2012Accepted 20 February 2012Available online 7 March 2012

Keywords:AromaticsLaminar flame speedExtinction limitPremixed combustion

0016-2361/$ - see front matter � 2012 Elsevier Ltd. Adoi:10.1016/j.fuel.2012.02.045

⇑ Corresponding author. Address: Department of Mversity of Connecticut, 191 Auditorium Road, Storrs,860 486 3679; fax: +1 860 486 5088.

E-mail address: cjsung@engr.uconn.edu (C.-J. Sung

a b s t r a c t

The laminar flame speeds and premixed extinction limits of n-propylbenzene, 1,2,4-trimethylbenzene,1,3,5-trimethylbenzene, and toluene have been studied experimentally to assess the effects of differentalkyl substitutions to the benzene ring on flame propagation and extinction. The experiments were car-ried out in a twin-flame counterflow setup under atmospheric pressure. The laminar flame speeds of fuel/air mixtures at two unburned mixture temperatures of 400 K and 470 K were determined over an equiv-alence ratio range of / = 0.7–1.4. Additionally, the extinction stretch rates of fuel/O2/N2 mixtures at anunburned mixture temperature of 400 K were measured over an equivalence ratio range of / = 0.8–1.6,with an oxidizer composition of 16% O2 and 84% N2 by mole. The experimental laminar flame speedsand extinction stretch rate values were compared to simulated results, for each fuel, using detailedkinetic models available in the literature. The simulation results were found to be in reasonable agreementwith the current experimental data, except for 1,2,4-trimethylbenzene, where the model under-predictsthe extinction limits significantly. Sensitivity and flux analyses were conducted to identify reactionsand species to which the computed results were most sensitive.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Gaining a basic understanding of combustion characteristics ofaromatic hydrocarbon fuel components is important to understandreal transportation fuel combustion characteristics, since thesefuels contain substantial amounts of aromatics [1]. Aromatic com-pounds increase the energy density of transportation fuels and areof significance especially in automotive fuels to improve antiknockperformance as a result of their high octane ratings [2]. It has alsobeen demonstrated that the presence of aromatic components inthe fuel blends can have a significant impact on the global combus-tion response, such as laminar flame speed and extinction limit(e.g., [3–5]). In other venues, the formations of polyaromatichydrocarbons from aromatic combustion are important as precur-sors to soot formation [6]. Though aromatics can be entirelyremoved from future air transportation fuels, the hardware legacyin this field appears to require a minimum content of aromatic spe-cies to maintain elastomer sealing qualities [7]. Therefore, it is offundamental and practical significance to refine the current under-standing of aromatic combustion characteristics and their coupling

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echanical Engineering, Uni-CT 06269-3139, USA. Tel.: +1

).

performances with other fuel structures present in the transporta-tion fuels.

Extensive studies (e.g., [8–10]) have focused on constructingreliable surrogates for transportation fuels, including gasoline, die-sel, and kerosene jet fuels. The objective is to determine a specificmixture composition to best emulate desired characteristics of aspecific real fuel of interest. The simplest aromatic, benzene, is lim-ited as a component in fuels as a result of its carcinogenic charac-teristics. On the other hand, toluene is a major component found ingasoline, and the reactions describing its pyrolysis and oxidationprovide much of the submodel components for describing morecomplex aromatic combustion chemistry [11].

A recent study by Dooley et al. [12] proposed a jet fuel surrogatecomposed of n-decane/iso-octane/toluene. This surrogate wasshown to emulate very well a specific target jet fuel in terms ofpredicting homogeneous auto-ignition characteristics, species evo-lution, and diffusion flame properties. Matching of surrogate fuelmixtures with real fuel combustion property targets of hydrocar-bon/carbon (H/C) ratio, derived cetane number (DCN), thresholdsoot index (TSI), and average molecular weight were utilized toemulate the real fuel behavior. However, larger molecular weightcomponents are required to match the typical TSI and averagemolecular weight of jet fuels [13]. Thus in the follow-on researchDooley et al. [14] have pursued surrogate mixtures containing lar-ger molecular weight alkyl aromatics, specifically n-propylbenzene

Table 1Molecular structures of the aromatic components investigated in this study.

Toluene n-Propylbenzene 1,2,4-Trimethylbenzene 1,3,5-Trimethylbenzene

696 X. Hui et al. / Fuel 97 (2012) 695–702

and 1,3,5-trimethylbenzene. Other trimethyl benzenes have alsobeen used by other investigators in constructing surrogate mix-tures [15]. For all of the above practical drivers, it is essential toimprove understanding of the combustion chemical kinetics ofneat toluene and the C9 aromatics. Without doubt, the oxidationof these aromatics is much less well characterized than the oxida-tion of normal and iso-alkane components found in real fuels.

A brief overview of combustion studies on the C9 alkyl aromat-ics appearing in the literature is provided as follows. Brezinsky [16]studied the oxidation of n-propylbenzene in the plug-flow reactor.Dagaut et al. [17] studied the oxidation of n-propylbenzene in thejet-stirred reactor under atmospheric pressure and over thetemperature range from 900 to 1250 K and developed a detailedchemical kinetic model. Farrell et al. [18] reported the laminarburning velocities of various aromatics, including n-propylbenzene,1,2,4-trimethylbenzene, 1,3,5-trimethylbenzene, etc., at the preheattemperature of 450 K and pressure of 3 atm. However, the burningvelocity data of [18] were not stretch-corrected. Honnet et al. [15]developed a kinetic model for a kerosene surrogate composed ofn-decane and 1,2,4-trimethylbenzene. Recently, a semi-detailedn-propylbenzene kinetic model has been proposed by Won et al.[19] to investigate diffusive extinction. Despite these previous ef-forts, a deficiency of the experimental data essential to developingcomprehensive reaction models that are predictive over a broadrange of experimental venues and combustion continues to exist.This is especially true for stretch-corrected laminar flame speeddata for these aromatic fuel constituents.

To this end, the objective of this work is to provide fundamentalcombustion properties, namely laminar flame speeds and extinc-tion stretch rates of three C9 aromatics, n-propylbenzene (n-PB),1,2,4-trimethylbenzene (1,2,4-TMB), and 1,3,5-trimethylbenzene(1,3,5-TMB). The three C9H12 isomers are expected to have differ-ent combustion characteristics primarily as a result of their differ-ent alkyl substitutions, as their adiabatic flame temperatures andmass diffusive properties are similar. In addition to the above threearomatic hydrocarbons, this work also includes additional study oftoluene, the simplest alkyl aromatic. The molecular structures ofthe aromatic components studied herein are shown in Table 1.The comparison of these isomers along with toluene allows forinvestigating molecular structure effects on flame propagationand extinction of premixed stretched flames. Further, by compar-ing the existing kinetic model predictions against the presentexperimental data, we aim to provide more insight into the con-trolling reactions within these models that most strongly affectflame propagation and extinction.

2. Experimental specifications

2.1. Experimental setup

The experimental determinations of laminar flame speed andextinction stretch rate of the specific fuel/oxidizer mixture wereconducted in the counterflow twin-flame configuration. The sche-matic of the flow control system and counterflow setup is shown in

Fig. 1a. The counterflow burner was composed of two opposed,convergent nozzles with an exit diameter of 14 mm. The separa-tion distance between the two nozzles was kept close to one diam-eter. The two burners were supplied with the same premixedcombustible mixture, and two identical stretched flames as shownin Fig. 1b were then established between the nozzles after ignition.

The combustible mixture was prepared as described below. Byinjecting the liquid fuel and heated nitrogen into a prevaporizerat a given rate, the fuel was first atomized into fine droplets andthen vaporized in the heated prevaporizer chamber. Subsequently,the vaporized fuel was mixed with other gases (e.g., O2 and addi-tional N2) to form a combustible mixture of desired composition.Gas flow rates were regulated by using calibrated sonic nozzles.A bypass valve before the burner was used to vary the flow rateof the combustible mixture through the burner. This arrangementallowed for the variation of the stretch rate without changing themixture composition and seeding density. Two shrouding nitrogencoflows were provided to both nozzles to prevent any interactionwith the ambient air and stabilize the flames. The entire flow cir-cuit was heated and maintained at an appropriate temperatureto prevent condensation of fuel vapor. A nebulizer was employedto generate silicone fluid seeding particles to enable digital particleimage velocimetry (DPIV) measurements of the flow field. The de-tails regarding the mixture preparation, seeding, and DPIV tech-nique can be found in Refs. [3,5,20]. Based on our previous study[3], the full-scale accuracy in velocity measurement is estimatedto be 0.83% and 1.25% in axial and radial directions, respectively,for the present setup.

2.2. Laminar flame speed determination

The images obtained by DPIV were processed to extract velocitymaps which were further analyzed to determine the stretch-af-fected reference flame speed and the associated stretch rate. Thereference flame speed Su,ref is defined as the minimum axial veloc-ity upstream of the flame. At the location of Su,ref, the radial velocitygradient is obtained. This radial velocity gradient a is related to theaxial velocity gradient K, conventionally used to denote the stretchrate, by K = 2a It should be noted that the linear radial velocity pro-file enables an unambiguous experimental determination of thestretch rate [3].

Based on the variation of Su,ref with K, laminar flame speed S0u

can be determined by either linear or nonlinear extrapolation tozero stretch rate. It has been demonstrated that linear extrapola-tion overestimates the laminar flame speed compared to nonlinearextrapolation [21]. Here, the reported laminar flame speed datawere determined by using the nonlinear extrapolation methodbased on the theoretical analysis of Tien and Matalon [22]. Fig. 2shows an example of nonlinear extrapolation of reference flamespeeds in n-PB/air flames at unburned mixture temperature ofTu = 400 K and / = 0.7, 1.0, and 1.4. The standard error value asso-ciated with the extrapolation procedure to deduce the laminarflame speed is shown as an error bar when plotting the reported data.Further note that the variation in slopes of the Su,ref �K dependence

Fig. 1. (a) Schematic of the flow control system and counterflow setup and (b) picture of counterflow twin flames.

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Ref

eren

ce F

lam

e Sp

eed

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Stretch Rate (s-1)

φ=0.7

φ=1.0

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Fig. 2. Experimental data (symbols) showing reference flame speed versus stretchrate for n-PB/air mixtures at an unburned mixture temperature of Tu = 400 K andequivalence ratios of / = 0.7, 1.0, and 1.4, along with the demonstration of nonlinearextrapolation (lines).

X. Hui et al. / Fuel 97 (2012) 695–702 697

at different equivalence ratios is due to the non-unity Lewis num-ber effect [23]. The raw data to which the nonlinear extrapolationmethod was applied are available as Supplementary material.

2.3. Extinction stretch rate measurement

The extinction stretch rate was also measured in the counter-flow setup. Instead of using the oxygen content in normal air, theoxidizer for extinction experiments was composed of 84% N2 and16% O2 by mole. The higher N2 concentration allows for the flamesto extinguish at moderate values of stretch rate, while still underlaminar flow conditions. In the experiments, extinction wasbrought about by gradually increasing the flow rate through theburners until an abrupt flame blow-off was observed. Similar toflame speed measurement, the extinction stretch rate is based ontwice the radial velocity gradient at the reference flame speed loca-tion. Typically, a sequence of 32 image pairs was captured justbefore the extinction. The image pairs were then processed todetermine the corresponding extinction stretch rate. Therefore,the extinction stretch rate value reported herein is the averagefrom 32 or more velocity maps obtained from repeated runs. Thestandard deviation is also presented as an error bar of the reportedextinction stretch rate value.

3. Computational specifications

3.1. Kinetic models

The kinetic models used to simulate toluene flame speeds weretaken from the work of Metcalfe et al. [11]. The detailed chemicalkinetic model is composed of 328 species and 1888 elementary

reactions. In the same paper, a reduced model was derived fromthe detailed one, consisting of 139 species and 525 reactions. Thereduced model of Ref. [11] was reported to be valid over the tem-perature range of 1300–2000 K. As shown in due course, thisreduced model was found to well reproduce computed laminarflame speeds of toluene/air mixtures using the detailed model.Thus, for ease of computation, the reduced model was employedin generating the flame response curves for determining theextinction limits.

The toluene model also served as the base model for the n-PBmodel, recently published by Won et al. [19]. The n-PB submodel(that was added to the toluene model of Metcalfe et al. [11]) iscomposed of 8 species and 50 reactions that describe the pathwaysof fuel decomposition, hydrogen abstraction, and alkyl radical andother intermediate consumption. As described by Won et al. [19],the n-PB submodel was constructed by an assumed direct analogyto toluene oxidation kinetics for the reactions occurring from thebenzyl-type position and by analogy to those of propane for thereactions occurring at the alkyl positions. The so derived n-PBmodel was tested against diffusion flame extinction results in[19] and some shock tube ignition delay data in [24]. However,no comparisons have been made previously against premixedflame data.

For 1,2,4-TMB flames, the kinetic model was taken as thatdeveloped for representing the Aachen kerosene surrogate [15].This model, which includes semi-detailed descriptions of n-decaneand 1,2,4-TMB chemistry, is composed of 118 species and 527reactions.

3.2. Numerical approaches

Laminar flame speed and extinction stretch rate were simulatedusing the PREMIX code [25] and the opposed-flow code [26],respectively, in conjunction with the CHEMKIN [27] and TRANS-PORT [28] packages. The opposed-flow code was modified by usingthe one-point temperature controlling method of Nishioka et al.[29] to generate the flame response curve. The turning point ofthe flame response curve defines the extinction limit. At the turn-ing point, the computed maximum axial velocity gradient ahead ofthe flame was used to determine the extinction stretch rate. Con-sistent with the experimental determination, in the opposed flamemodeling the local stretch rate based on the axial velocity gradientis equal to twice the radial velocity gradient. Fig. 3 shows the typ-ical flame response curves by plotting maximum flame tempera-ture to stretch rate variation. Note again, the vertical tangencypoint of the curve is the turning point and defines the extinctionlimit.

Both the PREMIX and opposed-flow codes allow for the use of themixture-averaged or multi-component formulation of moleculartransport processes. The multi-component formulation leads to

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Fig. 3. Computed extinction curves of 1,2,4-TMB/oxidizer mixtures at an unburnedmixture temperature of Tu = 400 K and equivalence ratios of / = 1.0, 1.4, and 1.6.

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Fig. 4. Differences between mixture-averaged formulation and multi-componentformulation in laminar flame speed and extinction limit simulations for 1,2,4-TMBflames at Tu = 400 K.

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(a) Comparison of Laminar Flame Speeds, Tu=400K

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(b) Comparison of Laminar Flame Speeds, Tu=470 K

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Fig. 5. Comparison of laminar flame speeds of toluene/air, n-PB/air, 1,2,4-TMB/air,and 1,3,5-TMB/air mixtures at unburned mixture temperatures of (a) Tu = 400 K and(b) Tu = 470 K.

698 X. Hui et al. / Fuel 97 (2012) 695–702

more accurate transport properties and is preferable for extinctionstretch rate calculation. However, it is significantly more computa-tionally expensive and results of premixed hydrocarbon flames byusing multi-component formulation are expected to be within 5%of those by using mixture-averaged formulation [30]. Fig. 4 showsthe differences between these two transport formulations in lami-nar flame speed and extinction stretch rate calculations for 1,2,4-TMB flames. It can be seen from Fig. 4 that the differences aremostly less than 3% in both laminar flame speed and extinctionstretch rate simulations. Thus all the simulated results reportedin the following were performed by using the mixture-averagedformulation.

4. Results and discussion

Fig. 5 compares the atmospheric laminar flame speeds of n-PB/air, toluene/air, 1,2,4-TMB/air, and 1,3,5-TMB/air mixtures atTu = 400 K and 470 K. It can be seen that all the laminar flamespeeds increase by about 30% with Tu increased from 400 K to470 K. Among the aromatics, n-PB has the highest laminar flamespeed, followed by toluene and the two TMBs. Farrell et al. [18] re-ported the same flame speed ranking for these four aromatic com-ponents under experimental condition at Tu = 450 K and p = 3 atm.Their results also showed that the laminar burning velocities(without stretch correction) of 1,2,4-TMB are slightly higher thanthose of 1,3,5-TMB. However, within the experimental uncertaintythere was no discernible difference observed in laminar flamespeed results of these two TMBs in the present study.

It is noted that for a given equivalence ratio the adiabatic flametemperature ranking is in the order of toluene/air > n-PB/air > TMB/air. However, the difference in adiabatic flame tempera-ture is not very significant, especially for fuel lean conditions.Although the adiabatic flame temperatures of toluene/air mixturesare somewhat higher than those of n-PB/air mixtures, toluene/airlaminar flame speeds are lower. Also, the diffusive properties ofthe four aromatic hydrocarbons are quite similar. Therefore, thedifference in laminar flame speeds of the aromatic components islargely caused by the different oxidation kinetics of each fuel asa result of the different alkyl substitution structures.

Comparing the two mono alkyl substituted benzenes, n-PB andtoluene, a significant difference between their oxidation mecha-nisms is the pathway of fuel molecule disintegration. As also sug-gested by Dagaut et al. [17], the high reactivity of n-PB is due to thespecies produced from reactions of the longer side-chain of n-PB.As for toluene and TMBs, their structure difference lies in the de-gree of the methyl substitution. It is reasonable to expect radicalregeneration processes in TMB flames to be slower than in tolueneflames as the TMBs have more numerous opportunities to producethe uniquely stable benzyl type radicals that dominate the reactiv-ity of both systems [31]. Thus, in addition to the effect of adiabaticflame temperature, the chemical kinetic effect due to their struc-ture difference also contributes to the difference in laminar flamespeeds of toluene and TMBs. Other studies [32] on xylenes alsoshowed that the m-xylene (1,3-dimethylbenzene) has a lowerflame speed than toluene. These results suggest that the degreeof the substitution can have a strong effect on fuel reactivity. More-over, the position of substitution can also play an important role inaffecting the fuel reactivity. It has been reported in previous flamestudies [18,19,32] that o-xylene (1,2-dimethylbenzene) is more

Fig. 6. Pictures of near-extinction flames of n-PB/oxidizer mixtures: (a) lean flameof / = 0.9 (Le = 2.97) and (b) rich flame of / = 1.2 (Le = 0.95).

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Comparison of Extinction Limits, Tu=400 K

Toluene1,3,5 -TMB1,2,4 -TMBn-PBE

xtin

ctio

n St

retc

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ate

(s-1

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[N2]/([N2]+[O2])=0.84

Fig. 7. Comparison of extinction stretch rates of toluene/oxidizer, n-PB/oxidizer,1,2,4-TMB/oxidizer, and 1,3,5-TMB/oxidizer mixtures at an unburned mixturetemperature of Tu = 400 K.

X. Hui et al. / Fuel 97 (2012) 695–702 699

reactive than p-xylene (1,4-dimethylbenzene) and m-xylene. How-ever, to quantify and isolate the influence of the degree and posi-tion of substitution, more experimental data are still required fora systematic comparison of the aromatic components.

In the flame extinction experiments, two different modes of flameextinction have been observed. In the first mode, the lean flames wereextinguished with a finite separation distance between each other, asshown in Fig. 6a. In the second mode, the rich flames merged into eachother before extinction occurred as shown in Fig. 6b. These differentmodes of flame extinction result from the coupled effects of positivestretch rate and non-unity Lewis number (Le) [23,33]. For the aromatichydrocarbons studied herein, the effective Le of lean (rich) mixtures aregreater (smaller) than unity. When the positive stretch rate is imposedon the lean flame (Le > 1), flame temperature decreases because ther-mal energy loss from the reaction zone exceeds chemical energy gaindue to mass diffusion of the deficient reactant [23]. Flame extinctionoccurs when the flame temperature drops to the extent that the flamecannot sustain itself. Vice-versa when the mixture is fuel rich (Le < 1),the combined effect of positive stretch and disparate diffusion of massand energy leads to an increase in flame temperature and enhancedcombustion until the twin flames move towards the stagnation planeand merge into each other. Further increasing the stretch rate re-duces flow residence time thus decreasing the flame temperature.The flame extinction eventually occurs due to the incompletecombustion.

Fig. 7 depicts the experimentally determined extinction stretchrates of n-PB/oxidizer, toluene/oxidizer, 1,2,4-TMB/oxidizer, and1,3,5-TMB/oxidizer mixtures as a function of equivalence ratio atTu = 400 K. It can be seen that all the extinction stretch rates peakon the fuel rich side at / � 1.4 which is much richer than the valuewhere the respective laminar flame speeds peak. Among all thearomatics, n-PB has the highest resistance to extinction, followedby toluene and TMBs in descending order. Comparing the extinc-tion limits of n-PB and toluene, n-PB is consistently higher than tol-uene and the difference becomes larger when the fuel/oxidizermixtures become rich. Based on the laminar flame speed results,it is seen that n-PB flame is more reactive, resulting in shorter char-acteristic reaction time than toluene flame, thus it can sustainhigher stretch rate. The extinction limits of the TMBs are substan-tially lower than those of n-PB and toluene, which is consistentwith the laminar flame speed results that TMBs are less reactive

than n-PB and toluene. Between the two TMBs, even though thelaminar flame speed results show no discernible difference, theextinction limit results, while within the experimental uncertainty,nevertheless show that extinction stretch rates of 1,2,4-TMB areslightly higher than those of 1,3,5-TMB. Same trend has been re-ported in the diffusion flame extinction experiments [19]. 1,3,5-TMB has a more symmetrical structure compared to 1,2,4-TMB,which could be the reason that 1,3,5-TMB is more stable. In addi-tion, it is conjectured that for 1,2,4-TMB some interaction betweenthe radicals formed at the 1 and 2 positions can b-scission to pro-duce a quantity of radical pools, while the isolation of the methylconfigurations in 1,3,5-TMB would not allow for such an interac-tion and hence would rely more on bi-molecular reactions to prop-agate the radical chains. Though the detailed chemistry for the twoTMBs is still not clear, the experimental results suggest that the po-sition of the substitution can have an impact on the flame extinc-tion response of the fuel isomers.

Fig. 8 compares the measured laminar flame speeds with com-puted ones of toluene/air, n-PB/air, and 1,2,4-TMB/air mixtures atTu = 400 K and 470 K. It can be seen that the computed values byeach model are all in fairly good agreement with experimentaldata. However, all the models under-predict the laminar flamespeeds on the lean side. It is further noted that the use of the de-tailed and reduced toluene models of Metcalfe et al. [11] yields clo-sely-matched laminar flame speeds for both toluene and n-PBsimulations. This close agreement also justifies the use of the re-duced toluene model in extinction calculations.

Fig. 9 compares the measured extinction stretch rates withcomputations of toluene/oxidizer, n-PB/oxidizer, and 1,2,4-TMB/oxidizer at Tu = 400 K. Note again the oxidizer in this extinctionstretch rate experiments is composed of 84% N2 and 16% O2 (bymole). It can be seen that the n-PB model, the combination of thereduced toluene model of [11] and the n-PB submodel of [19],over-predicts the extinction stretch rates with a maximum devia-tion less than 20%. A similar over-prediction was also reported inthe diffusion flame extinction study by Won et al. [19]. Thoughthe reduced toluene model predicts the peak at / = 1.3, leaner thanthe experimental value of / = 1.4, the overall prediction is in rea-sonable agreement with the experimental data. The 1,2,4-TMBmodel [15] substantially under-predicts the extinction stretchrates with a maximum deviation of 40%. Similar trends were foundin diffusion flame extinction study by Won et al. [19]. Their resultssuggested that the under-prediction of 1,2,4-TMB model is mainlydue to the deficiency of its toluene submodel at atmosphericpressure. Despite the deficiency of these models, a reasonable pre-diction of the present experimental data has been shown in Figs. 8and 9. Further evaluations of these models against mechanistically

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Equivalence Ratio, φ

Fig. 8. Comparison of experimental and computed laminar flame speeds for (a)toluene/air mixtures, (b) n-PB/air mixtures, and (c) 1,2,4-TMB/air mixtures.

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Fig. 9. Comparison of experimental and computed extinction stretch rates for (a)toluene/oxidizer mixtures, (b) n-PB/oxidizer mixtures, and (c) 1,2,4-TMB/oxidizermixtures.

-0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6

HCO+O2<=>CO+HO2

C6H5O+H(+M)<=>C6H5OH(+M)

H+OH+M<=>H2O+M

HO2+OH<=>H2O+O2

C6H5CH3(+M)<=>C6H5CH2+H(+M)

HO2+H<=>2OH

C6H5O+O<=>o-C6H4O2+H

HCO+M<=>H+CO+M

CO+OH<=>CO2+H

H+O2<=>O+OH

Mass Burning FluxExtinction limit

Normalized Sensitivity Coefficient

Sensitivity Analysis for n-PB (φ=1.0, Tu=400 Κ)

Fig. 10. Normalized sensitivity coefficients of mass burning flux and extinctionlimit in the n-PB flame at Tu = 400 K and / = 1.0.

700 X. Hui et al. / Fuel 97 (2012) 695–702

revealing experimental data would be enlightening in confirmingthe veracity of each model. Such further analyses can be used toprovide more definitive kinetic analyses of these flames.

Fig. 10 shows the normalized sensitivity coefficients of massburning flux and extinction limit for n-PB flames at stoichiometriccondition and Tu = 400 K. Not surprisingly, the main chain-branch-ing reaction H + O2 = O + OH is the most sensitive reaction in bothlaminar flame speed and extinction limit simulations. The reactionof CO + OH = CO2 + H, which is responsible for most of the heat re-lease, also shows a great positive sensitivity. It can also be seenthat extinction sensitivities are much larger than laminar flamespeed sensitivities, emphasizing the importance of kinetics in sim-ulating the extinction limit. Similar results are also obtained fromsensitivity analysis of toluene and 1,2,4-TMB flames.

To provide more insight, a reaction path analysis was performedon each of the present models to identify the key species that areresponsible for the predicted reactivity of each aromatic. The maindifference in the present kinetic models is from the consumptionpathways available to the fuel structure due to the different alkylsubstitutions. Recognizing this and that each model has a similar-ity after the fuel molecule breaks down to smaller aromatic

Fig. 11. Reaction path analysis for the n-PB freely-propagating and near-extinction flames at Tu = 400 K and / = 1.0.

0

1x10-3

2x10-3

3x10-3

4x10-3

5x10-3

0 0.02 0.04 0.06 0.08

Mas

s Fr

actio

n of

(C

2H4+

C2H

2)

Spatial Coordinate (cm)

n-PB Flame

TolueneFlame 1,2,4 -TMB Flame

(a) C2 Species in Aromatic Flames

0

2x10-3

4x10-3

6x10-3

8x10-3

1x10-2

Mas

s Fr

actio

n of

Ben

zylic

Rad

ical

s

n-PB Flame

TolueneFlame

1,2,4 -TMB Flame

(b) Benzylic Radicals in Aromatic Flames

0 0.02 0.04 0.06 0.08

Spatial Coordinate (cm)

Fig. 12. Spatially-resolved mass fraction profiles of (a) combined C2H2 and C2H4

and (b) benzylic radicals in the freely propagating toluene/air, n-PB/air, and 1,2,4-TMB/air flames at Tu = 400 K and / = 1.0.

X. Hui et al. / Fuel 97 (2012) 695–702 701

fragments, the path analysis is only performed for the initial fewsteps of the oxidation process.

Fig. 11 plots the propyl substitution breaking pathway of n-PBpremixed flames at / = 1.0 and Tu = 400 K. Both freely-propagatingand near-extinction flames were analyzed and compared. The ini-tial fuel breakdown is dominated by the hydrogen abstractionreactions, the top three channels in Fig. 11, which account for73% of total fuel consumption in freely propagating flame and87% of total fuel consumption in near-extinction flame. The restfuel consumption is through the unimolecular decomposition reac-tions, with the forth channel in Fig. 11 being the major one. This isexpected in premixed flames as hydrogen abstraction reactions aremore important than unimolecular decomposition reactions in theinitial fuel breakdown. The abstraction can occur from primary,secondary, and the benzylic-type CAH bonds, yielding three phen-ylpropyl (C6H5C3H6) isomers. It is noted that the n-PB submodel of[19] was constructed by an assumed direct analogy to the equiva-lent processes of propane. As such, by b-scission these phenylpro-pyl radicals are described to decompose unimolecularly to formeither benzyl (C6H5CH2) and ethylene (C2H4), phenyl (C6H5) andpropene (C3H6), or styrene (C6H5C2H3) and methyl (CH3) radicals.The styrene submodel is then described to ultimately result in phe-nyl and acetylene (C2H2). Recognizing the importance of styrene asan intermediate and the simplicity of the present model in describ-ing styrene oxidation chemistry, further attention to the mecha-nism of styrene oxidation is warranted.

It is expected that the production of these C2 species and theradicals that accompany their formations are responsible for n-PB’s relatively higher reactivity shown in the laminar flame speedand extinction limit in premixed combustion. Although not shownin the figure, the same flux analysis of toluene and 1,2,4-TMBflames also shows that most of the fuel breaking pathways arethrough the hydrogen abstraction reactions. However, due to theirmethyl substituted structures, there are no opportunities for beta-bond scission reactions and thus no C2 species that can be formed

702 X. Hui et al. / Fuel 97 (2012) 695–702

in the early fuel consumption. Moreover, after the initial radical at-tack on toluene or TMB, oxidation process leads to the formation ofbenzylic radicals. These benzylic-type radicals are resonantly sta-bilized and do not have the opportunity to propagate the radicalchain by beta-bond scission reactions.

To further quantify the presence of the C2 and benzylic radicalsin flames, Fig. 12a compares the computed profile of the combinedmass fractions of C2H2 and C2H4 in the freely propagating flames at/ = 1.0 and Tu = 400 K. It can be seen that the total mass fraction ofthe key C2 species in the n-PB flame is much higher than those intoluene and 1,2,4-TMB flames. The excess of C2 species in the n-PB flame is found to be produced during the initial propyl substitu-tion breakdown. Fig. 12b shows spatially-resolved benzylic radi-cals, C6H5CH2 in the n-PB and toluene flames as well as C6H5C3H6

in the 1,2,4-TMB flame, under the same conditions. It is seen thatthe benzylic radical in the 1,2,4-TMB flame is notably higher thanthose in the n-PB and toluene flames. The high concentration ofbenzylic radicals in the 1,2,4-TMB flame could explain its lowerreactivity observed in the experiments.

5. Conclusions

Laminar flame speeds and extinction stretch rates have beenexperimentally and numerically determined in toluene, n-PB,1,2,4-TMB, and 1,3,5-TMB premixed flames under atmosphericpressure. The experimental results of laminar flame speed andextinction limit showed that n-PB has the highest reactivity fol-lowed by toluene and TMBs. The simulation results were shownto be in reasonable agreement with the present experimental data,except that the 1,2,4-TMB model significantly under-predicts theextinction stretch rates. Sensitivity analysis of the present modelsdemonstrated that both flame phenomena are mostly sensitive tochain-branching and heat release reactions. Further flux analysisrevealed that the present models produce high reactivity of n-PBas a result of C2 species and the accompany radicals produced dur-ing the disintegration of propyl side chain, while the high concen-tration of resonantly-stabled benzylic radicals is responsible forthe low reactivity of TMB. Continuing efforts are underway to re-fine the n-PB model employed herein and develop 1,3,5-TMB mod-el so as to encompass a larger range of experimental venues. Wealso note that mechanistically revealing experiments will be par-ticularly useful in developing higher fidelity models for these alkylaromatics.

Acknowledgements

This work has been supported by the Air Force Office of Scien-tific Research Multi University Research Initiative under GrantNo. FA9550-07-1-0515.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.fuel.2012.02.045.

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