Proceedings

Proceedings of the Combustion Institute 31 (2007) 1215–1222

www.elsevier.com/locate/proci

of the

CombustionInstitute

High temperature ignition and combustionenhancement by dimethyl ether addition

to methane–air mixtures q

Zheng Chen, Xiao Qin, Yiguang Ju *, Zhenwei Zhao,Marcos Chaos, Frederick L. Dryer

Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, USA

Abstract

The effects of dimethyl ether (DME) addition on the high temperature ignition and burning propertiesof methane–air mixtures were studied experimentally and numerically. The results showed that for a homo-geneous system, a small amount of DME addition to methane resulted in a significant reduction in the hightemperature ignition delay. The ignition enhancement effect by DME addition was found to exceed thatpossible with equivalent amounts of hydrogen addition, and it was investigated by using radical poolgrowth and computational singular perturbation analysis. For a non-premixed methane–air system, itwas found that two different ignition enhancement regimes exist: a kinetic limited regime and a transportlimited regime. In contrast to the dramatic ignition enhancement in the kinetic limited regime, the ignitionenhancement in the transport limited regime was significantly less effective. Furthermore, laminar flamespeeds as well as Markstein lengths were experimentally measured for methane–air flames with DME addi-tion. The results showed that the flame speed increases almost linearly with DME addition. However, theMarkstein length and the Lewis number of the binary fuel change dramatically at small DME concentra-tions. Moreover, comparison between experiments and numerical simulations showed that only the mostrecent DME mechanism well reproduced the flame speeds of both DME–air and CH4–air flames.� 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Keywords: Dimethyl ether; Ignition enhancement; Laminar flame speed; Markstein length

1. Introduction

Environmental regulations and energy diversi-ty produce an urgent need to develop new cleanfuels. Dimethyl ether (DME), which has low sootemission, no air or ground-water pollution effects,

1540-7489/$ - see front matter � 2006 The Combustion Institdoi:10.1016/j.proci.2006.07.177

qSupplementary data for this article can be accessed

online. See Appendix A.* Corresponding author. Fax: +1 609 258 6233.

E-mail address: yju@princeton.edu (Y. Ju).

and can be mass produced from natural gas, coal,or biomass [1–5], is emerging as a substitute forliquefied petroleum gas (LPG), diesel fuels, andliquid natural gas (LNG). In addition, DME canalso be used as an ignition enhancer in propulsionsystems and internal combustion engines [6].Recently, the study of DME combustion hasreceived significant attention [1–6]. In kineticresearch, several detailed chemical mechanismsfor low and high temperature DME oxidation[7–11] have been developed and validated againstburner stabilized flames [12,13], non-premixed

ute. Published by Elsevier Inc. All rights reserved.

1216 Z. Chen et al. / Proceedings of the Combustion Institute 31 (2007) 1215–1222

counterflow flame ignition [14], and laminar flamespeeds [15–17]. More recently, DME jet diffusionflames were also studied [18]. It was shown thatdue to the existence of oxygen atoms in DME,lift-off characteristics are distinctly different fromother hydrocarbon fuels.

DME has shown promise as an additive and/orfuel extender. Recently, Yao and Qin [19] haveundertaken studies on DME addition to methanefor homogeneous charge compression ignition(HCCI) engines. In such cases, the coupling ofDME kinetics with those of methane involvesthe low temperature kinetics of DME. On theother hand, DME/methane utilization in burnersand in gas turbine applications is expected toinvolve principally high temperature kinetic cou-pling effects. For example, the effects of DMEon the high temperature ignition of methane havebeen experimentally studied by Amano and Dryer[6], who showed that DME was an effective pro-moter of high temperature methane ignition.However, the underlying kinetic coupling betweenDME and CH4 responsible for the observed igni-tion enhancement was not explored in any detail.In addition, kinetic coupling effects on flameproperties and auto-ignition in non-premixed sys-tems have not been studied. Moreover, the exist-ing kinetic mechanisms have not been validatedagainst the flame speeds of DME/CH4 mixtures.

It is well known that flame properties such asburning rate and flame stability depend on theoverall activation energy and the Lewis number(Le) [20]. Kinetic coupling may result in a dramat-ic change in the overall activation energy with asmall amount of DME addition to methane.DME has a molecular weight larger than air sothat the Le is larger than unity for lean DME/air mixtures in comparison to methane/air mix-tures. On the other hand, DME is expected toreact more quickly in the preheating zone decom-posing to form lighter molecules. Therefore, it isof interest to investigate how the effective mixtureLewis number depends on the DME content inbinary fuels with disparate molecular weights,such as DME/CH4 mixtures.

The objective of the present study was toinvestigate kinetic coupling effects of DME addi-tion on the high temperature ignition and burn-ing properties of methane–air mixtures.Experimentally measured laminar flame speedsof DME–air and CH4–air mixtures were com-pared with predictions by existing DME mecha-nisms including a recently developed model byZhao et al. [21]. The latter mechanism is thenused to study the effect of DME addition onthe ignition enhancement in both homogeneousand non-homogeneous systems. Finally, theflame speeds of DME/CH4–air mixtures weremeasured by using outwardly propagatingspherical flames. The results were comparedwith model predictions and the effect of kinetic

coupling on the Markstein length and Lewisnumber was studied.

2. Experimental/computational methods and kineticmodel selection

The laminar flame speed and Markstein lengthof DME/CH4–air premixed flames were measuredby using outwardly propagating spherical flamesin a dual-chambered, pressure-release-type, andhigh pressure combustion facility [17]. Pre-mix-tures were prepared by using the partial pressuremethod from pure methane and DME com-pressed gas sources. The purities of the DMEand CH4 were 99.8% and 99.9%, respectively.Experiments were conducted for DME/CH4–airmixtures: {aCH3OCH3 + (1 � a)CH4} + air, withvalues of the volume fraction, a, ranging fromzero to one. The combustible mixture was spark-ignited at the center of the chamber with the min-imum ignition energy so as to preclude significantignition disturbances. The flame propagationsequence was imaged by using Schlieren photog-raphy. A high-speed digital video camera operat-ing at 8000 frames per second was used torecord the propagating flame images. To avoidpossible effects caused by the initial spark distur-bance and wall interference, data reduction wasperformed only for flame radii between 1.5 and2.5 cm. The pressure rise in the combustion cham-ber was monitored using a pressure sensor. At thesmall flame radii studied in this work, the pressurerise is about 2%, resulting in a nearly constantpressure flame propagation condition.

The stretched flame speed was first obtainedfrom the flame history and then was linearlyextrapolated to zero stretch rate to obtain theunstretched flame speed [17,22]. The results pre-sented here are the averaged value of at leasttwo tests at each experimental condition. The esti-mated experimental error for flame speed determi-nations is approximately 5%. All experimentswere performed at an initial temperature of298 ± 3 K and at atmospheric pressure. In orderto examine the available kinetic mechanisms, themeasured flame speeds were compared with thenumerical results obtained using PREMIX [23].The Markstein length and the effective Lewisnumber of the binary fuel mixtures were thenextracted from the measured flame speed data.

The effect of adding DME on ignition enhance-ment of methane was investigated numerically intwo different systems, a homogeneous flame con-figuration to examine the kinetic ignition enhance-ment, and a non-premixed counterflowconfiguration to examine the effect of transport.The ignition time of homogeneous mixtures atconstant pressure and enthalpy was calculatedby using SENKIN [24]. For the non-premixedsimulations, the quasi-steady temperature and

Fig. 2. Laminar flame speeds of CH4–air mixtures as afunction of equivalence ratio at 298 K, atmosphericpressure.

Z. Chen et al. / Proceedings of the Combustion Institute 31 (2007) 1215–1222 1217

species distributions of counterflowing DME/CH4

(298 K at the boundary) and hot air jets (1400 Kat the boundary) were determined under a frozenflow constraint. At time zero, chemical reactionswere allowed in the pre-calculated frozen flowfield. Ignition time was recorded when the firstincrease in the temperature field exceeded 400 K,indicating thermal runaway. Simulations wereconducted using an unsteady potential counter-flow flame code described by Ju et al. [25,26]. Tofurther examine the effect of flow residence timeon ignition enhancement, the stretch rate in thefrozen flow configuration was varied from lowstretch to that near the ignition limit.

In order to properly model and interpret thepresent work, the chemical kinetic model used inthe calculations must be capable of predictingthe pure fuel–air laminar flame speed and hightemperature shock tube ignition properties. Onemight expect that comprehensively developeddetailed mechanisms for DME oxidation wouldalso be capable of predicting high temperaturekinetic properties for methane oxidation. Themeasured laminar flame speeds of pure DME–air and CH4–air mixtures at atmospheric pressureand room temperature are shown in Figs. 1 and 2,respectively, along with the predictions utilizing anumber of different DME mechanisms.

It is seen that the earlier DME mechanismspublished by Curran and co-workers [8–10,14](2000-Mech and 2003-Mech) are not able to wellreproduce measured flame speeds for both leanand rich mixtures. Although a more recentlyupdated version (2005-Mech [11]) predicts flamespeeds much better than the previous ones, thereis still a large discrepancy for lean DME–air andCH4–air flames. Recent theoretical studies of theunimolecular decomposition of DME using theRRKM/master equation approach [21,27] yieldsignificantly different parameters from those used

Fig. 1. Laminar flame speeds of DME–air mixtures as afunction of equivalence ratio at 298 K, atmosphericpressure.

in the above models. Because decomposition andabstraction pathways are coupled during bothpyrolysis and oxidation, considerable re-assess-ment, and updating of other parameters impor-tant to high-temperature oxidation are needed toincorporate this change. A new model thatincludes the revised decomposition parameters,updates in the sub-model and thermochemistryfor the hydrogen oxidation [28], and a revisedC1–C2 species sub-model developed in recentexperimental and modeling studies on ethanolpyrolysis and oxidation [29–31], has been devel-oped [21,27]. The mechanism was constructedand tested hierarchically against a large volumeof experimental data for hydrogen, carbon mon-oxide, formaldehyde, and methanol oxidation.The new DME reaction mechanism (denoted hereas 2006-Mech) consists of 290 reversible elementa-ry reactions and 55 species, and its predictionscompare well with high-temperature flow reactordata for DME pyrolysis and oxidation [9], for oxi-dation at high temperatures in a jet-stirred reactor[7], for high-temperature shock tube ignition[2,32], for species profiles from burner-stabilizedflames [12,13], and for laminar flame speeds ofDME-air flame (Fig. 1). The model also resultsin excellent prediction of CH4–air flame speeddata (Fig. 2). On the basis of the ability to repro-duce these same reference conditions, we utilizethe 2006-Mech in the remainder of comparisonsreported in this paper.

3. Results and discussion

3.1. Ignition enhancement by DME addition

Figure 3 shows the effects of DME addition onthe ignition time of homogeneous and non-homo-geneous DME/CH4–air mixtures. The results of

0 20 40 60 80 100-4

-3

-2

-1

Homogeneous Ignition

Nonpremixed Ignition a=300 s-1

a=200 s-1

a=100 s-1

a=20 s-1

))s( yaled

noitin

gi( g

oL

Volume percentage of DME in DME+CH4

Fig. 3. Effects of DME addition on ignition delay ofmethane at atmospheric pressure (the initial temperatureis 1400 K and / = 1.0 in the homogeneous ignitionsimulation; the temperature of fuel is 298 K and that ofhot air is 1400 K in the non-premixed ignitionsimulation).

CH3OCH3+H=>CH3OCH2+H2

CH3OCH3+HO2=>CH3OCH2+H2O2

CH4+HO2=>CH3+H2O20% DME2% DME10% DME100% DME

1218 Z. Chen et al. / Proceedings of the Combustion Institute 31 (2007) 1215–1222

homogeneous ignition at initial temperature1400 K show that the addition of DME to CH4–air has a dramatic enhancement in CH4–airignition, particularly, at small amounts of DMEaddition. It is seen that for 10% of DME addition,the ignition time can be reduced by at least anorder of magnitude. As the DME addition levelreaches 40%, a further increase in DME blendinghas little effect on reducing ignition time. Further-more, the ignition enhancement will also be afunction of initial reaction temperature.

In order to understand the factors involved inthe enhancement of methane ignition by DMEaddition, radical path, and computational singu-lar perturbation (CSP) analyses [33] were made.Figure 4 shows the radical pool development(sum of CH3, H, O, OH, HO2, and C2H5) duringignition of DME/CH4–air mixtures for various

0.0001 0.001 0.01 0.1Time (s)

1.0E-006

1.0E-005

1.0E-004

1.0E-003

Mol

e Fr

actio

n

0% DME

2% DME

5% DME

100% DME

10% DME

20% DME

50% DME

Fig. 4. Radical pool (H, O, OH, HO2, CH3, C2H5)growth during homogeneous ignition of DME/CH4–airmixtures (P = 1 atm, / = 1.0, T = 1200 K).

levels of DME addition. Addition of a smallamount of DME (i.e., 2%) drastically increasesthe radical pool concentration growth. As DMEconcentration further increases, however, thiseffect is lessened which gives rise to the nonlinearignition enhancement observed in Fig. 3. CSPanalyses were performed during the initial radicalbuild-up stages to gain further insight into thereactions participating in this process (Fig. 5).The CSP methodology was implemented includ-ing temperature as one of the state variables[34], permitting thermokinetic feedback to betreated in the analyses. CSP has an advantage incomparison to typical sensitivity analysis methodsin that the entire thermokinetically coupled sys-tem is treated directly and perturbation is appliedto the complete set of differential equationsdescribing the kinetic system.

For CH4–air mixtures without DME the initialradical production is governed by the reactionCH4 + O2 fi CH3 + HO2, which has no signifi-cant contribution to radical pool growth later inthe induction period. The methyl radicals formedreact with O2 to yield CH2O and OH, or CH3Oand O, with CH3O decomposing to form CH2Oand H. Through abstraction reactions, CH2Oforms HCO which subsequently yields HO2 andCO through oxidation. The recombination reac-tion 2CH3(+M) fi C2H6(+M) is the main channelopposing the initial radical pool growth. Whenboth CH3 and HO2 are available in sufficient con-centrations, CH3 + HO2 fi CH3O + OH becomesan important radical source. The above processesall depend on developing a significant pool of CH3

0 0.2 0.4Participation Index

CH4+O2=>CH3+HO2

CH3OCH3=>CH3+CH3O

CH3+O2=>CH2O+OH

2CH3(+M)=>C2H6(+M)

CH3+O2=>CH3O+O

H+O2=>O+OH

CH4+H=>CH3+H2

CH3OCH3+CH3=>CH3OCH2+CH4

CH3O+O2=>CH2O+HO2

CH3O+M=>CH2O+H+M

CH3+HO2=>CH3O+OH

CH3OCH3+O2=>CH3OCH2+HO2

CH3+HO2=>CH4+O2

Fig. 5. Dominat reactions during initial radical build-up(DME/CH4–air; P = 1 atm, / = 1.0, T = 1200 K)obtained from CSP analysis.

10-5 10-4 10-3 10-20.0

1.0x10-5

2.0x10-5

3.0x10-5

4.0x10-5

5.0x10-5

6.0x10-5

7.0x10-5

2% DME

200s-1

10% DME

a=20s-1

HC f

o noitcarf ssa

m m

umixa

M3

Time (s)

200s-1

20s-1

Fig. 6. Evolution of the maximum mass fraction of CH3

in non-premixed ignition at different stretch rates andDME addition levels.

Z. Chen et al. / Proceedings of the Combustion Institute 31 (2007) 1215–1222 1219

and HO2 before radical pool growth of OH, O,and H can occur.

Since, the governing reaction CH4 + O2 fiCH3 + HO2 (Fig. 5) is slow relative to the similarreaction and/or thermal decomposition rates ofhigher alkanes, the ignition time for pure CH4–air mixtures is, in comparison, relatively longer.Once a small amount of DME is present, the sys-tem is strongly driven by the unimolecular decom-position of DME (Fig. 5). This reaction is themajor initial source of radicals and continues tocontribute to radical production thereafter. WithDME addition to methane, the radical poolgrowth occurs much more rapidly (Fig. 4), as itis not limited by the rate at which methyl radicalsalone can produce more reactive species. In thecase of DME, unimolecular decomposition yieldsCH3O and CH3 at the above temperatures, andsubsequent abstraction reactions of CH3 and rad-icals generated from CH3O produce CH3OCH2

which in turn yields additional radical growththrough decomposition to CH3 and CH2O. Thesereaction sequences lead to a relatively large con-centration of HO2, which in turn provides analternative mechanism for CH3 to yield radicals(CH3 + HO2 vs. CH3 + O2). CH2O reacts withOH, H, or CH3 through CH2O + X fi HCO + X(X = OH, O, H, and CH3). Formyl radicals fur-ther oxidize to produce CO and HO2. CH3Odecomposes through CH3O + M fi -CH2O + H + M, or reacts with O2 to obtainCH2O and HO2. Moreover, the large concentra-tions of HO2 also produce H2O2 and subsequentproduction of OH through H2O2 + M fi O-H + OH + M. Thus, by DME addition to meth-ane not only is CH3 more easily generated, othersources and channels are also available for gener-ating radicals, as a result it enhances ignition.

The effect of DME addition on ignition time innon-premixed counterflow systems is also shownin Fig. 3. It is seen that, unlike the homogenouscase, the ignition enhancement strongly dependson the stretch rate. At large stretch rates, the igni-tion is kinetically limited (the characteristic trans-port time is shorter than that of ignition) so that asmall amount of DME addition causes a rapidreduction of ignition time. For example, at a highstretch rate (a = 300 s�1), the short flow residencetime prevents the slow radical process from quick-ly building up the radical pool, so that the ignitiontime of pure methane in non-premixed counter-flow is considerably long. It is a kinetic limitedprocess. However, at low stretch rates, the igni-tion time is limited by the characteristic transporttime (transport limited). In this regime, the igni-tion time only slightly decreases with increasingDME concentration. Furthermore, it is noted thatfor both low and high stretch rates, the minimumignition times for large amounts of DME additionare of the same order, indicating the limiting bytransport as the kinetic ignition time is shortened.

Therefore, it can be concluded that for non-pre-mixed ignition, there are two different regimes.In the kinetic limited regime, DME addition willsignificantly reduce the ignition time. However,in the transport limited regime, the ignitionenhancement by DME addition is much lesssignificant.

Besides, Fig. 3 shows that, for very low DMEpercentages, the homogeneous ignition time islarger than that of non-premixed ignition at lowstrain rates. This is caused by the presence oftransport in nonpremixed ignition. For nonpre-mixed ignition, thermal, and mass transports havetwo effects: (1) to bring heat to the ignition kerneland preheat the fuel by the hot air, and (2) tomove away the radicals produced in the ignitionkernel, slowing down the radical pool growth. Incases of low strain rates of large ignition delaytimes, the first effect becomes important becausethe endothermic decomposition reactions ofDME are enhanced by the convective heat trans-fer and the radical transported away from theignition kernel is low at small radical gradients.

To further demonstrate the stretch and DMEaddition effects on methane ignition in non-pre-mixed configuration, the evolution of the maxi-mum mass fraction of CH3, the major radicalspecies controlling the ignition time, as discussedpreviously, is shown in Fig. 6. It is seen that withincreasing DME addition (from 2% to 10%), sim-ilar to the homogeneous case, the ignition time isreduced, however, its effect greatly depends on thestretch rate. For example, at low stretch rate(20 s�1), the ignition is only slightly enhancedwhen the DME addition changes from 2% to10%, although the initial CH3 concentration isheavily affected. On the other hand, at high stretchrate (200 s�1), ignition time is significantly short-ened by increasing the DME addition from 2%to 10%. This result further confirms the existence

0.7 0.8 0.9 1.00.0

0.1

0.2

0.3

0.4

0.5Rozenchan et al. (α=0)

α=0.00α

α=0.05α=0.10α=0.20α=0.50α=1.00

L ,ht

gnel

nietskraM

denr

ub

nU

U)

mm(

Equivalence Ratio, φ

Fig. 8. Unburned Markstein lengths of DME/CH4–airflames as a function of equivalence ratio at differentamount of DME blending at 298 K, atmosphericpressure.

1220 Z. Chen et al. / Proceedings of the Combustion Institute 31 (2007) 1215–1222

of two different regimes for non-premixedignition.

3.2. Laminar flame speeds of DME/CH4 binary fuelmixtures

Figure 7 shows the dependences of the mea-sured and predicted laminar flame speeds ofDME blended methane–air flames on the DMEaddition level at different equivalence ratios. It isseen that with the increasing amounts of DME,the laminar flame speeds of DME/CH4–air flamesincrease almost linearly with the addition,although the rate of increase is slightly larger atsmall DME addition levels. In addition, thenumerical results calculated from the 2006-mecha-nism agree reasonably well with the experimentaldata. This result indicates that although DMEaddition can increase the initial radical productionfor the acceleration of ignition, the fuel oxidationrate which is dominated by chain-propagationand termination reactions does not change signif-icantly. As a result, the burning speed of a binaryfuel mixture can be approximated as a linear func-tion of the mixture fraction of the blended fuel.

3.3. Extraction of markstein length and global lewisnumber

Figure 8 shows the unburned Marksteinlengths Lu of DME blended methane–air flamesat different equivalence ratios and DME concen-trations. The unburned Markstein length wasobtained directly from the linear flame relationSu ¼ S0

u � LuK, where K is the flame stretch [20].It is seen that for pure methane–air flames(a = 0), the unburned Markstein length increaseswith the equivalence ratio. The present data agreewell with those measured by Rozenchan et al. [35].It is well-known that the Lewis number of lean

0 20 40 60 80 10010

20

30

40

50

60

φ = 0.7

φ = 0.8

φ = 1.0)s/mc(

uS ,

deeps e

malF

Volume percentage of DME in DME+CH4

Symbols: experimentLines: prediction

Fig. 7. Variation of laminar flame speeds of DME/CH4–air mixtures at different equivalence ratios andDME blending at 298 K, atmospheric pressure.

methane–air mixtures is slightly less than unity.This positive dependence on equivalence ratio isdue to an increase in the effective Lewis numberas the mixture approaches stoichiometric condi-tions [36]. When DME is added to CH4, it is inter-esting to see that the unburned Markstein lengthincreases significantly. In contrast to the lineardependence of flame speed on DME additiondescribed above, the Markstein length increasesmore than 50% with only 10% DME addition.In addition, the Markstein length starts to havea negative dependence on the equivalence ratiofor DME levels larger than 10%. This nonlineardependence on DME addition to the binary fuelhas not been reported in the literature.

The rapid increase of unburned Marksteinlength by small amounts of DME addition impliesthat the global Lewis number or the overall Zel’-dovich number (b) of the mixture is very sensitiveto DME addition. The Zel’dovich number can becomputed from the sensitivity of flame speed tothe adiabatic flame temperature. The global Lewisnumber of the mixture can be extracted from themeasured unburned Markstein length accordingto the following relation [37]:

Lu

d¼ r

1� rbðLe� 1Þ

2�Z ð1�1=rÞ

0

lnð1þ xÞx

dx� lnðrÞ� �

;

ð1Þ

where d is the flame thickness, r the ratio ofburned and unburned gas densities, and b theZel’dovich number.

Figure 9 shows the dependences of the globalLewis number, Zel’dovich number, and unburnedMarkstein length on the percentage of DME addi-tion at / = 0.8. It is seen that the activation ener-gy decreases quickly with the DME addition. Thisis because the DME addition promotes the radical

0 20 40 60 80 1000.0

0.2

0.4

0.6

0.8

LU

Le -1.0β /20

,1- eL

βL ,02/

U)

mm(

Volume percentage of DME in DME+CH4

2.5

2.6

2.7

2.8

2.9

β (Le -1)

β )1- e

L(

Fig. 9. Effects of DME addition on the global Lewisnumber, Zel’dovich number and unburned Marksteinlengths of DME/CH4–air flames at 298 K, atmosphericpressure, and / = 0.8.

Z. Chen et al. / Proceedings of the Combustion Institute 31 (2007) 1215–1222 1221

pool growth shown in Fig. 4. However, it alsocauses rapid increase of the overall Lewis number.As a result, the reduced Lewis number, b(Le � 1),increases significantly with a small addition ofDME. Therefore the Markstein length nonlinearlydepends on DME addition level.

4. Conclusions

In the present work, the effects of DME addi-tion to methane–air mixtures on ignition, flamespeeds, and Markstein length were studied exper-imentally and computationally. New experimentaldata were obtained for the study of kinetic cou-pling between DME and methane and for the val-idation of existing kinetic mechanisms. Thefollowing conclusions can be drawn from the pres-ent work:

1. In homogeneous ignition, small amounts ofDME addition to methane lead to a significantdecrease in ignition time. The effect is evenmore profound than that of hydrogen addi-tion. This significant ignition enhancement iscaused by the rapid build-up of CH3 andHO2 radicals when DME addition is presentin the system. The resulting chain propagationreaction via CH3 and HO2 replaces the slowreactions via CH3 and O2 in the pure methanecase and thus accelerates the ignition.

2. In non-homogeneous ignition, it is found thatthe ignition enhancement is strongly affectedby the stretch rate. There exist two ignitionregimes: a kinetic limited regime and a transportlimited regime. In the kinetic limited regime,small amounts of DME addition cause a dra-matic decrease of ignition time. However, inthe transport limited regime, ignition enhance-ment by DME addition is much less effective.

3. In contrast to the nonlinear behavior in ignitionenhancement, the flame speeds of DME/CH4–air mixture are linearly proportional to theDME fraction. However, it is shown that smallamount of DME addition leads to a significantchange in Markstein length, effective Lewisnumber, and the overall activation energy.

4. The comparison between the experimentaldata with model predictions showed that the2000-Mech, 2003-Mech, and 2005-Mech donot well reproduce the flame speed data forboth DME and methane–air flames, although2005-Mech performs much better than its pre-vious versions. The results also showed that therecently developed mechanism (2006-Mech) isable to well reproduce the speeds of bothDME–air and methane–air flames, and thosefor DME addition to methane.

Acknowledgments

The authors thank Professor H.J. Curran atthe University of Ireland for providing hischemical mechanism. This work was partiallysupported by PRF#39162-AC9 and Air ForceResearch F49620-04-1-0038 and the ChemicalSciences, Geosciences and Biosciences Division,Office of Basic Energy Sciences, US Depart-ment of Energy under Grant No. DE-FG02-86ER13503.

Appendix A. Supplementary data

Supplementary data associated with this articlecan be found in the online version at doi:10.1016/j.proci.2006.07.177.

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