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Effects of C2-Chernistry on the Structure of PartiallyPremixed Methane-Air FlamesZHUANG SHU a , VISWANATH R. KATTA a , ISHWAR K. PURI a & SURESH K. AGGARWAL aa Department of Mechanical Engineering , University of Illinois at Chicago M/C 251 , 842 W.Taylor St, Chicago, IL, 60607-7022Published online: 27 Apr 2007.

To cite this article: ZHUANG SHU , VISWANATH R. KATTA , ISHWAR K. PURI & SURESH K. AGGARWAL (2000) Effects of C2-Chernistry on the Structure of Partially Premixed Methane-Air Flames, Combustion Science and Technology, 157:1, 185-211

To link to this article: http://dx.doi.org/10.1080/00102200008947316

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Combust. Sci. and Tech., 2000. Vol. 157. pp. 185-211

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Effects of C2-Chemistry on the Structureof Partially Premixed Methane-AirFlames

ZHUANG SHU, VISWANATH R. KAnA', ISHWAR K. PURl andSURESH K. AGGARWALt

Department of Mechanical Engineering, University of Illinois at Chicago MIC 251,842 W. Taylor St., Chicago, IL 60607-7022

(Received March 03, 1999; In final form January 31,2000)

Partially-premixed flames (PPF) can contain multiple reaction zones, e.g., one or two with apremixed-like structure and one being a nonpremixed reaction zone. An intrinsic feature of partiallypremixed flames pertains to the synergistic interactions between these two types of reaction zonesthat are characterized by heat and mass transfer between them. Since these interactions are stronglydependent on the distribution of the radical and stable species' concentrations, an accurate representa­tion of the flame chemistry involving these species is critical for simulating their behavior. The roleof Crchemistry in determining the structure of partially premixed methane-air flames is investigatedherein by employing two relatively detailed chemical mechanisms. The first involves only Cj-con­taining species and consists of 52 reactions involving 17 species, while the second mechanism repre­sents both C J- and Crchemistry and consists of 81 reactions that involve 24 species. A planartwo-dimensional partial1ypremixed flame established on a rectangular slot burner is simulated. Thesimulation is based on the numerical solution of the time-dependent conservation equations for masscontinuity, momentum, species, and energy. The computations are validated by comparison with theexperimentally-obtained chemiluminescent emission from excited-Cj" free radical species, as well aswith velocity measurements using particle image velocimetry, A numerical study is then conducted toexamine the applicability of C. and C2 mechanisms for predicting the structure of partially premixedflames for different levels of partial premixing and reactant velocity. Results indicate that both themechanisms reproduce the global structure of PPF over a wide range of reactant velocity and stoichi­ometry. Since the C 1 mechanism is known to be inadequate for fuel-rich premixed flames, its rela­tively good performance can be attributed to the interactions between the two reaction zones thatcharacterize the PPF structure. There are, however, imponant quantitative differences between thepredictions of the two mechanisms. The Cz mechanism is overall superior compared to the C. mech­anism in that its predictions are in closer agreement with our experimental results. The rich premixedreaction zone height obtained with the Cz mechanism is more sensitive to variations in the equiva­lence ratio as compared with predictions that are obtained using the C,-mechanism. In addition, forhigh levels of partial premixing, the methane consumption in the inner reaction zone is significantlyincreased when the Crmechanism is employed, compared to when the C ,-mechanism is used. Con-

• Innovative ScientificSolutions, Inc. Dayton,OH 45430.t Corresponding author. email: ska@uic.edu

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sequently, theamount of methane that leaks from the richpremixed to nonpremixed reaction zone issignificantly lower when the Crmechanism is used. The interactions between the inner and outerreaction zones arestronger when theCrmechanism is employed. Finally, the maximum temperaturepredicted by the Cz-mechanism is slightly lower as compared to that obtained using the Cj-chemistryalone. These differences are attributed to the presence of the Crchain in the 8 l-step mechanism,which strongly affectsthe innerpremixed reaction zone.

INTRODUCTION

Partially premixed flames (PPF) occur in many practical systems that includeBunsen burners, industrial furnaces, and gas-fired domestic burners. In ultra-leanpremixed combustors, a promising new concept to significantly reduce pollutantemissions, partially premixed combustion is an important phenomenon due to theinherent unmixedness of fuel and oxidizer. Partial premixing is an importantprocess in lifted flames since the reactants can mix prior to ignition. Partialpremixing also occurs in turbulent combustion and in spray combustion systems.The partial premixing of laminar diffusion flamelets has been considered essen­tial for the modeling and prediction of turbulent flame structure. Consequently, afundamental understanding of the structure of partially premixed flames isimportant from both practical and scientific considerations.

Previous investigations of partially premixed flames have employed bothcounterflow (Yamaoka and Tsuji 1978; Hamins et al. 1985; Seshadri et al. 1985;Law et al. 1989; Li et al. 1997) and coflow (Gore and Zhan 1996; Shu et al.1997a and I997b; Shu et al. 1998) configurations. A counterflow partiallypremixed flame can be established by flowing two opposing jets, one containinga rich fuel-air mixture and the other containing air. Similarly, a partiallypremixed flame can be stabilized in a coflow configuration, with the inner flowcontaining a rich fuel-air mixture and the outer flow containing air. A generalfinding in these studies is that for certain range of equivalence ratios, partiallypremixed flames are characterized by the existence of two distinct reactionzones; an inner rich premixed zone which is synergistically coupled to an outernonpremixed zone. Since an important part of these interactions involves theexchange of key radical species, it is essential that a detailed reaction mechanismbe employed for an accurate prediction of the double-flame structure. Li andWilliams (1998) examined the double-flame structure in a counterflow configu­ration by employing a detailed CH4-air chemistry involving C ,-, C2- , andC)"species. Shu et al. (l997a, 1997b, 1998) employed a 52-step reaction mecha­nism, involving Cj-species, to simulate steady two-dimensional par­tially-premixed methane-air flames.

The present study extends our previous analysis to include an 8 I-step mecha­nism, which contains both C.- and C2"species, for the simulation of two-dimen-

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PARTIALLY PREMIXEDMETHANE-AIRFLAMES 187

sional PPFs stabilized on a Wolfhard-Parker slot burner. The major objective isto examine the applicability of CI- and Crmechanisms for predicting the struc­ture of PPFs for different levels of partial premixing and reactant velocity. Sincethe C2-chemistry provides an important pathway for fuel consumption throughthe CH3 + CH3~ C2H6 reaction, it is known to playa significant role in predict­ing the characteristics of fuel-rich premixed combustion; for example, the lami­nar premixed flame speed as a function of equivalence ratio for rich methane-airmixtures. It is, therefore, important to examine as to why the CI-mechanism canadequately reproduce the measured structure of PPFs, while it fails to accuratelypredict the laminar flame speed under fuel-rich conditions. Another considera­tion for including the C2-mechanism is that C2-species represent a major sourceof CH2 and CH radicals that are largely responsible for the "prompt" mechanismof NOx production. The C2-containing species also playa major role in soot for­mation.

THE COMPUTATIONAL MODEL

The simulation considers a planar 2-D partially premixed flame established on arectangular Wolfhard-Parker slot burner that is schematically depicted inFig. I(a), and described elsewhere (Shu et al. 1998). A fuel-rich mixture is intro­duced from the inner slot, and air from either side of it. Identical two-dimen­sional flames are established on either side of the centerline. Numericalsimulations are conducted on one side of the symmetry plane (plane I). Theother three planes bounding the domain are the free surface (plane 2), the inflowboundary (plane 3), and the outflow boundary (plane 4). Figure I(b) contains aschematic illustration of the computational domain.

The flame simulation is based on a numerical solution of the time-dependentgoverning equations for a two-dimensional reacting flow. The governing equa­tions and the numerical model have been previously described (Shu et al. 1997aand 1998). We have also previously presented a detailed experimental-computa­tional study of the double-flame structure for different velocities and levels ofpartial premixing (Shu et al. 1998). The focus of this investigation is to examinethe effects of Crchemistry on the double-flame structure of partially premixedmethane-air flames. Two detailed mechanisms for CH4-air chemistry, as com­piled by Peters (1993), are employed. The first mechanism involves Cj-chemls­try alone, comprising of 52 reactions that involve 17 species, while the lattermechanism represents both C I - and Crchemistry and consists of 81 reactionsand 24 species.

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188 ZHUANG SHU .1 al.

Stagnant Air

~1~1 1 5 .5

~~Vair V eact air

42 /;;r - - ......---,""'......""'.--71/

-f~~-IIStagnantI AirI1105IIIII

_i. __ .

( a)

#3

( b)

150

Free Surface

J=U

#2

I=LILI=121, U=61

#4

Symmetry t

\1I

J=I I#1 I

IIIVreact Wall Vair

it,GI t

100

FIGURE 1 Schematic diagram of the 100 mm x 150 mm (or 61 x 121 gridline) computationaldomain. The symbols I and J, respectively, represent the axial and transverse gridlines. The boundaryconditions are also specified. The simulated burner consists of an inner 7.5 mm slot with two15.5 mm outer slots on eitherside of it. The wall thicknessseparating the slots is 1 mm

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PARTIALLY PREMIXED METHANE-AIR FLAMES 189

The thermodynamic and transport properties, such as the viscosity, thermalconductivity, and binary diffusion coefficients that are required in the computa­tions are considered to be temperature- and species-dependent. The methodologyto calculate these properties has been described in previous publications (Shu etat. 1997a; Kalla et at. 1994). The enthalpy h and specific heat for each speciesare calculated using the polynomial curve fits complied by Kee et aJ. (1983). Animplicit algorithm is employed to solve the time-dependent governing equations,which are integrated by using a "finite control volume" approach with a stag­gered, nonuniform grid system. Further details about the numerical procedureand the treatment of boundary conditions are discussed by Katta et al. (1994).

RESULTS

Figure 2 contains a comparison between the experimental and predicted (usingthe C I-mechanism *) images of partially premixed flames for three differentcases. The experimental images are represented in terms of the excitedC2*-chemiluminescence signal, while the computed flames are depicted in termsof the heat release rate contours. The C2*-chemiluminescence images (Shu et al.1998) were obtained using a 513x480 pixel intensified and gated solid-state cam­era (ITT F4577). As discussed by Shu et aI. (1998), the C2*-emission signal hasshown to be a marker of the heat release rate in partially premixed flames. Boththe C2*-signal and the predicted heat release clearly depict two reaction zones,one an inner premixed reaction zone and the other an outer nonpremixed reactionzone. As we have previously discussed (Shu et al. 1997a and 1997 b; Shu et at.1998). the two distinguishing features of a partially premixed flame are the exist­ence of spatially-separated reaction zones, and the synergistic interactionsbetween these regions. For the three cases depicted in Fig. 2, the simulation andexperiment show good agreement with respect to the spatial location of the reac­tion zones, implying that the C I-mechanism is capable of reproducing the PPFstructure that is observed in laboratory experiments. The region near the apex ofthe nonpremixed reaction zone has a weaker chemiluminescent intensity than thepredictions indicate. This is due to the fact that C2* radicals are relatively weakerin this region

Figure 3 compares reaction rate contours of the major fuel decomposition reac­tions, namely (a) CH4 + H <=> CH3+ H2, (b) CH4 + OH <=> CH3+ H20 , and(c) the net fuel consumption rate predicted by two mechanisms. The flame condi-

* In the following discussion. the term Cz-mechanism refers to the 8] -step mechanism that repre­sents both the C,- and C2-chemistry, while Cj-mechanisrn refers to the 52-step mechanism that onlyinvolves the Cj-chemistry.

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190 ZHUANG SHU et of.

+=2.5 C1Vreat=30trnls

+= 2.0Vreat=30trnls

\ \I I

" \ '

"'! \

<,' \ \

.~·\\1 .....·;', I I

"il), l'

+=2.0Vreat=40trnls

(mm)50

40

30

20

10

oFIGURE 2 Comparison of the predicted heat release rates (obtained by using the Cj-mechanism)with the experimentally-obtained Crchemiluminescence images of partially premixed flames forthree different cases: (a) <I> =2.0, Vreact =30 em s-'; (b) <I> =2.0, Vreact =40 em s-t; and (c) <I> =2.5,Vreact=30 em s-I. The outer slot velocity in all cases Vair:;;: 30 em s-I (See Color Plate XI at theback of this issue)

tions correspond to a mixture equivalence ratio ep = 2.5 in the inner slot, and reac­tant and air velocities, Vreac and Vair both equal to 30cm s-I. The reaction ratecontours clearly indicate that for C2-mechanism, methane is completely con­sumed in the inner premixed region with very little CH4 escaping into the outernonpremixed region. In contrast, when the Cj-mechanism is employed, methane

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PARTIALLY PREMIXED METHANE-AIR FLAMES 191

is only partly consumed in the inner premixed reaction zone, with the remainingmethane being consumed in the outer nonpremixed reaction region. Further evi­dence of this is provided by the methane concentration contours depicted inFig. 4. The higher fuel consumption in the inner reaction zone for the C2-mecha­nism is attributable to the Cz-chemistry, which provides an additional pathwayfor methyl consumption through the reaction CH3 + CH3 ~ CZH6. Further dis­cussion on this aspect is provided in a later section.

246810x.mm

2 4 6 8 10x,mm

246810x. mm'

~1-4'JC2-Mechanism

R~3-50100 100 100 \\0014

1l.<1I'·(14 II.OE-04 l·l,OF..ose.os-os 6.0&05 ·b4fi.{)5

80 H1E-llj 80 3.6E-05 80 -1.2E-04

r r r-s60I.)E-{)S

s60·Z.JE-O-I

7.7E.Q6 S60 7.7E-06 -2.8E-O-1S H"'" S 4,6&06 S .3AB4S

>-40 HE~ >'40 UIl::-06 >'40 -3.96-04l.7E-06 1.71l.()6 -4.:m-o-tI.OE-06 101::-06 -S.OE-lM

20 20 20

2 4 6 8 10x,mm

CH4, rnoleslcm3/s

20

80

100CI-Mechanism

20

80

246810x,mm

CH4 + OH <=>CH3+ H20

IllO

FIGURE 3 Comparison of the reaction-rate contours of major fuel decomposition reactions:(a) CH. + H <=> CH J + H2, (b) CH4 + OH <=> CHJ + H20 . and (c) the net consumption rate of meth­ane as predicted by the C 1- and Cz-mechanisms. 4> =2.5. Vreact =Vair :;: 30 em s-I (See ColorPlate XII at the back of this issue)

The reaction rate contours presented in Fig. 3 also indicate that the two mecha­nisms differ with respect to the major methane consumption reaction in the innerpremixed reaction zone. For the Cz-mechanism, methane consumption by Hatoms dominates that by OH radicals, while for the Cj-mechanlsm, the corre­sponding reaction with hydroxyl radicals is more important reaction. This can beattributed to the higher concentration of H atoms for the Cz-mechanism com­pared with that predicted by the Cj-mechanism. The concentrations of Hand OHpredicted by the two mechanisms are presented in Fig. 4. Another factor contrib­uting to the difference in the fuel-consumption chemistry is that the interactions

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192 ZHUANG SHU et al.

1?:'..oo4.6E-OI

, ,2.2E-OI

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2.2&-112

I JIE-l12.4.oE-OJ2.2E.(I)1,0&01

246810x. mm OH

20

100

H

15.OE-OI

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: I.JI::.Q1

1",,><>23,2E.oo

1,6U-OZ

7.9E-m

4.0E.{H

z.on-ox1.flI;.m

C2 Mechanism

80

20

100

e60s>'40

CII4

12.OU+<l ]

IAE:+fll

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I.OIl-+Ol

8 10 2 4 6 8 10xmm

CI Mechanism100 100

80 80

g60 g60';40 >'40

20 20

2 4 6 8 10 2 8 10x. mm CH4 H

FIGURE 4 Contours of the CH4, H-atom, and OH concentrations predicted by the two mechanismsfor the case discussed in context of Fig. 3 (See Color Plate XIII at the back of this issue)

between the inner and outer reaction zones are stronger when the Cz-mechanismis employed.

The heat release rate and CHO-concentration contours for the conditions corre­sponding to Fig. 3 are presented in Fig. 5. The heat release rate contours clearlyindicate that interactions between the inner and outer reaction zones are strongerwhen the C2-mechanism is used than that with the Cj-mechanism. (This is repre­sented at the base of the rich premixed and outer nonpremixed reaction zonesthrough the existence of a broadened and connected heat release zone.) Thesesynergistic interactions involve the exchange of energy and species (whichinclude radical species such as Hand OH, and stable intermediate species suchas H2and CO) between the two reaction zones. The degree of interaction can beillustrated by considering the separation distance between the two reaction zones.Use of the C2-mechanism results in a smaller separation distance compared tothat obtained after employing the Cj-mechanism. The inner and outer reactionzones heights can be based on the heat release rate contours. The premixed zoneheight can also be deduced from the CHO concentration contours that are pre­sented in Fig. 5, and from the methane-consumption-rate contours contained inFig. 3. These contours indicate that the premixed zone height predicted by the

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PARTIALLY PREMIXED METHANE-AIR FLAMES 193

246810xrnrn

CHO, moles/cm3/s

C2-Mechanism100 Q 100 WCHO

II.OE+03 1.0E-08

6.0E+02 6.OE-09

80 3.6E+02 80 3.6E-09

2.2E+02 2.2E-09

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E 60 I.3E-09

7.7E+OI E 7.7E-1O

4.6E+OI 4.6E-IO

>;40 2.8E+OI >;40 2.8E-1O

1.7E+OI 1.7E-IO

1.0E+OI 20 1.0E-IO20

00 2 8 10 2 4 6 8 10x, mm

100 CI-Mechanism 100

80 80

E 60 §60E>;40 >;40

20

FIGURE 5 Contours of heat release rate and CHO concentration predicted by the two mechanismsfor the case discussed in context of Fig. 3 (See Color Plate XIV at the back of this issue)

Cz-mechanism is higher than that simulated with the C I -mechanism. However,the (outer) nonpremixed zone heights are essentially similar when either mecha­nism is used, although the outer zone is somewhat broader upon application ofthe C2-mechanism. Additional discussion on the effects of C2-chemistry on theinner and outer zone heights is provided later.

Another important observation pertains to the correlation between the CHOconcentration and the heat release rate contours. As discussed by Najm et a!.

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246810x,mm H2co

C2-Mechanism100100 CO H2

I6.0E-tOO7.0E-tOO

4.6E-tOO 5.4E-tOO

80 80 4.2E-tOO3.6E-tOO

2.8E-tOO 3.2E-tOO

E 60 2.2E+OO 2.5E-tOO

1.7E+OO J.9E-tOOE J.5E-tOO

>'401.3E+OO

J.OE-tOO J.2E-tOO

7.7E-OI 9.0E.QI

6.0E.QI 7.0E.Q1

8 10 2 4 6 8 10x,mm

CI-Mechanism 100100

80

FIGURE 6 Contours of the CO and H2 mole fractions predicted by the two mechanisms for the casediscussed in context of Fig. 3 (See Color Plate XV at the back of this issue)

(1998), CHO is an excellent marker of heat release rate in premixed methane-airflames. In the present case, which focuses on partially premixed flames, CHO isfound to be an excellent marker of heat release in the inner premixed region.However, it seems to be a relatively poorer indicator of heat release in the outernonpremixed reaction zone, since its concentration is negligible in that regionwhere the dominant heat release reactions involve the H02 chemistry (Lee et al.1996). It is also interesting to note that the CHO concentration in the outer regionis significantly higher when the Cj-mechantsm is used due to the consumption ofthe "leaked" methane there.

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R5-61.0E-04

7.0&05

4.9E-05

3.4E-05

2.4E-05

I.7E-05

I.2E-05

8.2E-06

5.7&06

4.0E-06

00246810x,mm

H2 + OH <=>H20 + H

00246810x. mm

CO +OH <=>C02+H

PARTIALLY PREMIXED METHANE-AIR FLAMES

C2-Mechanism100 R25-26 100

1.0&04

7.2E-0580 5.IE-05 80

3.7E-05

E 60 2.6E-05E60

E 1.9E-05E

:>'401.4E-05

:>'409.7&06

7.0E-06

20 5.0&0620

2 4 6 8 10 2 4 6 8 10x. mm x. mm

100 C1-Mechanism100

80 80

§j60 E 60E

:>'40 :>'40

20 20

FIGURE 7 Comparison of the rate contours of the reactions CO + OH <=> CO2 + Hand H2 + OH <=>H20 + H predicted by the two mechanisms for the case discussed in context of Fig. 3 (See ColorPlate XVI at the back of this issue)

Figure 6 contains contours of the CO and H2 concentrations predicted by thetwo mechanisms for the flame discussed in the context of Fig. 3. Note that COand Hz are the "product" species that are formed in the inner premixed reactionzone and are also the reactant or "fuel" species that are provided to the outer non­premixed reaction zone. The Crmechanism predicts significantly higher concen­trations of CO and Hz compared with the Cj-mechanism. This is attributable tothe fact that the amount of methane consumed in the inner reaction region is sig-

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196 ZHUANG SHU et al.

nificantly higher due to the Cz-chemistry. The reaction rate contours of the CO­and Hz-oxidation reactions, CO + OH <=) COz+ H and Hz + OH <=) HzO + H,are presented in Fig. 7. For both mechanisms, the CO- and Hz-oxidation contoursthrough these reactions strongly correlate with the heat release rate contours con­tained in Fig. 5. While the first reaction appears to be similar for both mecha­nisms, the second reaction is relatively stronger in both the inner and outerreaction zones when the Cz-mechanism is used. This can be attributed to the sig­nificantly higher concentration of Hz in the inner flame and that of OH in theouter flame for this mechanism. The stronger interactions between the inner andouter flames for Cz-mechanism is also clearly illustrated in these plots.

t H (4.0)Qi(4.2}

~CHl

......,..1.6) 10 (3.0)CHI ,q (2.5)

I H CH20

... 1 H (1.9)

CH~ '" (3.01(0.2) CHO.M (5.0)

co+0i(2.2)

co,

CH,

t H (0...)~(1.11

~CHI

......-ro." I 0 It.')CHI ,(\(1.2)

I H CH20

+ 1 H (0.55)CH~ ,0« ....,)

(0.21 CHO.M (1.2)

co+QiIO.41

co,

FIGURE 8 Schematic illustration of the dominant reaction pathways involved with the two mecha­nisms for a flame established with an inner flow equivalence ratios ~::: 3.0, and withVreecr f Vair :::30 em s-I. The numbers in parentheses indicate the relative magnitude of the reactionrate. The thickness of the arrows is also used to indicate the relative magnitudes of the species con­sumption rates

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PARTIALLY PREMIXED METHANE-AIR FLAMES 197

Based on the rate distributions of the various reactions, the important pathwaysleading to the formation of CO and Hz in the inner reaction zone can bedescribed as follows. (I) Methane is converted to methyl (CH3) radicals mainlythrough its reactions with Hand OH. As indicated in Fig. 3, these reactions arerelatively stronger in case of the Cz-mechanism than when the Cj-mechanism isused, due to the faster consumption of methyl radicals through the CH 3 + CH3<:=> CzH6 channel. (2) The conversion of CH3 to CO follows three separate routesin Cz-mechanism, and two separate paths in the C ,-mechanism. These pathwaysare schematically illustrated in Fig. 8. The first two paths are common to bothmechanisms, while the third route involving Cz-species is obviously relevant inthe Cz-mechanism alone. The first pathway involves the decomposition of CH 3into CHz, which is converted to CH, that then reacts with 0z to form CHO whichis subsequently converted to CO (the dominant reaction in the conversion ofCHO to CO is CHO + M <:=> CO + H + M). The second route dominates the firstpath, and involves the oxidation of CH3 with °and 0z to form CHzO, whichthen reacts with Hand OH to form CHO, that forms CO in the manner describedabove. (3) The third route involves Cz-containing species, and begins with theformation of CZH 6. The ethane forms CzHs, mainly through the reactionCZH 6 + H <:=> CzHs + Hz, and the CzHs radicals decompose into CZH 4, that thenforms CzH3 (predominantly through the reaction CZH4+ H <:=> CzH3 + Hz). TheCZH3 radicals decompose to form CzHz, which produces CzHO and, subse­quently, CO. The chemical paths are schematically illustrated in Fig. 8.

In Fig. 9, we present reaction rate profiles of the two major chain-branching

reactions, (a) H + 0z <:=> OH +°and (b) Hz + °{=> OH + H, and of the reaction0z + H + M {=> HOz+ M. Clearly, these three reactions are important in both theinner and outer reaction zones. While the reaction rate distributions are qualita­tively similar for the two mechanisms, the first two chain branching reactions arestronger in the inner zone when the Cz-mechanism is used. This is partly respon­sible for the relatively complete consumption of methane in the inner flame incase of the Cz-mechanism. In addition, the rate contours clearly indicate strongerinteractions between the two reaction zones when the Cz-mechanism isemployed compared to those obtained with the Cj-mechanism.

Figure 10 contains a comparison between the measured Cz'-chemilumines­cence and the predicted heat release rate profiles (obtained both by using the C ,­and Cz-mechanisms) for various partially premixed flames. While the predictedand measured reaction zone topography shows excellent qualitative agreementover a wide range of equivalence ratios, there are quantitative differences withrespect to the spatial location of the inner premixed reaction zone. At higher lev­els of air addition to the inner flow (i.e., l1> :::; 1.5), the inner reaction zone heightpredicted by both the C,- and Cz-mechanisms is larger than that obtained from

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246810x. mm

246810x,mm

ZHUANG SHU et al.

e2 MechanismRI.2 100

I~~100

I ~~~II.O'E-D36.0&04 o.OE.{)5 60~

, 3.6E4t 80 3,oUl5 80 " J"E..05

r r r]JE-().t§60

IJE-{l5§607.7F.-OS 7.7lUl6 7.7&1JI'o

4.(,E-o~

;>;404.6lUl6

>-4046E-06

2,8E-OS HI::-OlI 2.SE-061.7E-oS 17EUl6 1.7r:-06

r.ce.os 20 IOE-& 20I.OE-Q6

198

100

80

§60

>-40

20

CI Mechanism

2 4 6 8 10x.mrn

H2+ 0 <=>OH +H

100

80

5 605>-40

20

246810x, mrn

02+ H +M<=>H02+M

FIGURE 9 Comparison of contours of the reactions (a) H + O2 <=> OH + O. (b) Hz + 0 <=> OH + H.and (c) O2 + H + M (::) H02 + M predicted by the two mechanisms for the case discussed in contextof Fig. 3 (See Color Plate XVII at the back of this issue)

measurements. As $ is increased, the inner reaction zone heights predicted by theC I-mechanism exhibits a relatively weaker dependence on variations in theequivalence ratio compared with those obtained using the Cz-mechanism or withthe measured results. As discussed in the following section, this can be attributedto the absence of Cz-chemistry in the Cj-mechanism. As a consequence, at lowlevels of air premixing ($ ~ 3.0), predictions using the C ,-mechanism yieldlower inner reaction zone heights compared with measurements or thoseobtained using the Cz-mechanism.

Figure JI shows the inner and outer reaction zone topographies in terms of theheat release rate contours predicted by both mechanisms for different levels ofpartial premixing. The global partially premixed flame structure regarding theexistence of two reaction zones is predicted in a qualitatively similar manner bythe two mechanisms. With both mechanisms the heights of the two reactionzones increase as the level of air premixing decreases (or as $ increases), sincethe chemical time required for the global chemistry to occur increases with largervalues of $. In addition, the inner reaction zone is observed to playa more impor-

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PARTIALLY PREMIXED METHANE-AIR FLAMES

.=3.0V........30<mI.

C1

199

+=%.0v.....<=4O<mI.

(illial!Ill

10

.0

o

CI

+-1.0V....._ml.

FIGURE 10 Comparison of the predicted heat release rates, obtained by using the Cj- and Cz-mecha­nisms, with the experimentally-obtained Cz-chemiluminescence images of partially premixed flamesfor five different cases. The outer slot velocity in all cases Vair =30 em 5- 1 (See Color Plate XVIII atthe back of this issue)

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200 ZHUANG SHU et al.

tant role at higher levels of air premixing (</> :52.0), while the outer reaction zoneis relatively stronger at lower level of air premixing (</> > 2.5).

120

80

40

C \ C2 C \ C2 C I C2 C 1 C2 C 1 C2

Phi",1.3 Vr=3Ocmfs Phi=1.5 Vr=3Ocm's Phi=2.0 Vr=3Ocmls Phi=2.S Vr=3Ocrnls Phi=3.0 V~Ocmls

FIGURE 11 Predicted heatreleaserate conWUTS for the two mechanisms for different levels of partialpremixing. For all cases, Vreact:::: Vair :::: 30 em 5-

1(See Color Plate XIX at the back ofthis issue)

It is apparent from Fig. I I that there are quantitative differences in the flamestructure predicted by the two mechanisms. These differences can be attributedto the C2-path in the C2-mechanism, which has a relatively stronger influence onthe inner reaction zone. In particular, the inner reaction zone height exhibits a farstronger sensitivity to variations in the equivalence ratio when the CTmechanismis used compared with the Cj-mechanism. Consequently, at high levels of airpremixing (</>::; 1.5), the CTmechanism yields a lower inner reaction zone height,while at lower levels of premixing (</> ~ 2.0), it yields a higher inner zone heightcompared the predictions obtained with the Cj-mechanism. The outer (non­premixed) reaction zone height is relatively unaffected by the mechanism that isused, although it is slightly lower in case of the CTmechanism. However, due tothe synergistic interactions between the two reaction zones, the spatial topogra­phy of the outer reaction zone is noticeably affected by the choice of reactionmechanism.

The global characteristics of partially premixed flames predicted by the twomechanisms are depicted in Fig. I2, which shows the temperature and the mole

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PARTIALLY PREMIXED METHANE-AIR FLAMES 201

2400

2000~

1600iEe 1200&E 800"E-

400

00 10 20 30 40 50 60

Y,mm

(a)

25

'6---e--- c

1' q>=1.5

20 ---e--- Cl, 4>=2.0

><---.r--- C

1, 4>=3.0

" '>." 15 - e- o C,_ <1>=1.5U -," - e- - C

2' $=2.0~

u. -," 10 -, "" - C,_ <1>=30'0:>::r:~ 5u

00 20 30 40 50 60

Y,mm

(b)20

'6>< 15

"-"ue 10u.

"'0:>: 5ON

00 10 20 30 40 50 60

Y,mm(c)

FIGURE 12 Temperature and mole fraction distributions of the reactants (CH4 and02) along the cen-terline as predictedby using the C1- and Crmechanisms for three differentequivalence ratios, andVreact =Vair=30 em s-l

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202 ZHUANG SHU et al.

fraction of CH4 and O2 along the centerline for three different cases. Consistentwith the results discussed earlier, the flames predicted by using the C I-mecha­nism exhibit less sensitivity to the equivalence ratio. In addition, for high levelsof partial premixing, the methane consumption in the inner premixed zone is sig­nificantly higher for the CTmechanism compared to that when Cj-mechanism isused. Consequently, for high levels of partial premixing, the amount of methanethat leaks from the rich premixed to nonpremixed reaction zone is significantlylower when the C2-mechanism is used. The maximum temperature obtained withthe C2-mechanism is slightly lower compared to that obtained using theC I-mechanism. This is due to the endothermic effects (Lee et al. 1996)caused bythe C2-pathway.

In general, quantitative differences in the predictions of the two mechanismsare relatively small at high level of partial premixing (~=1.5), i.e., when interac­tions between the two reaction zones of partially premixed flames are strong.Since the C ,-mechanism is known to be inadequate for fuel-rich premixedflames (Xue and Aggawal, 2000), its relatively good performance in the contextof PPFs can be attributed to the interactions between the two reaction zones. Asthe level of partial premixing is reduced, e.g. ~ is increased, these interactionsbecome weaker, and quantitative differences between the two mechanismsbecome more significant. This is clearly depicted in the plots of temperature andspecies mole fractions for ~=3.0 shown in Fig. 12. In this context, it is importantto note that while the rich premixed flames are chemistry dominated, the corre­sponding PPFs are characterized by both chemistry and transport. Consequently,any conclusions regarding the applicability of reduced or detailed reactionsmechanisms, that are based on premixed combustion phenomena, may not beextended to partially premixed combustion.

REACTION PATHWAYS IN C1" AND C2-MECHANISMS

As illustrated in Fig. 8, the Cj-pathway involves two routes, i.e.,

(PI)

(PIl)

These formyl radicals subsequently produce CO; which is further oxidized toform CO2, The first of these two paths is faster, since formyl production throughthe reactions

(RI)

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PARTIALLY PREMIXED METHANE-AIR FLAMES

is more significanl than its formation through the reaction

CH + O2 -t CHO + O.

203

(RIl)

(Rill)

The C2-pathway involves an additional route, i.e.,

CH3 -t C2H6 -t C2Hs -t C2H4 -t C2H3 -t C2H2 -t C2HO -t CO.(PIII)

Although the path PIlI is Iypically slower than the two Cj-channels PI and PII,it is nonetheless significant since (I) it introduces endothermic effects (Lee et at.1996), and (2) because the rate of CO formation through the reaction

C2HO + 0 -t CO + CO + H (RIV) .

has an equal magnitude to the rate of the reaction

CHO + M -t CO + H + M. (RV)

The C2-pathway PIlI also affects the C [-channel PII by increasing H-atom for­mation through reaction RIV and the reaction

(RVI)

As illustrated in Table I, this reaction results in a net production of H-atoms atlower equivalence ratios (cj>::; 1.5), but a net consumption of these atoms at higher

equivalence ratios (cj>;:: 3.0).

Therefore, the major effect of the C2-pathway PIlI at lower equivalence ratios

(cj> ::; 1.5) is to increase the overall reaction rates by increasing H-atom formation

and, thereby, augmenting the effect of the chain-branching reactions by raisingthe net availability of radical species. This is illustrated by the data presented in

Table I for some key reaction rates for flames established at cj>= 1.5 and 3.0, andcomputed by using both the C]- and C2-mechanisms. Reaction rates are pre-

sented in an integrated form, i. e., (1/A) !wi dA where Wi is the rate of pro-A

duction (consumption) of species i, and the area A is selected to include asignificant part of the inner or outer flames respectively.

The increase in H-atom formation is largely due to the effect of reactions R1Vand RVL Reaction R1V is also responsible for an increase in CO production, theoxidation of which through the reaction

CO + OH ¢} CO 2 + H (RVIl)

is also responsible for H-atom formation. The increase in H-atom formation hasa feedback effect, since, as illustrated in Table I, it synergistically raises the ratesof the initiation reactions

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204 ZHUANG SHU et al.

02 + H --t OH + 0

(RVIII)

(RIX)

This in turn increases the rates of the other initiation reactions (cf. Table I),

namely,

H2 + 0 <=> OH + H,and (RX)

(RXI)

and, thus, enhances the rate of H-atom formation. In summary, the C2-chemistryis responsible for an overall increase in the reaction rates by increasing H-atomformation and, thereby, the radical pool. Consequently, for flames established at$ ::; 1.5, as shown in Fig. II, the inner reaction zone is located upstream when itis modeled using the C2-chemistry as compared to when the Cj-chemisrry isemployed in the simulations.

The data contained in Table I also illustrates that the impact of C2-chemistry islarger on the inner premixed reaction zone than its influence on the outer non­premixed reaction zone. Since the CTchemistry is "faster", methane leakage tothe outer reaction zone is negligible when it is employed. The increase in the rad­ical pool in the inner reaction zone due to the CTchemistry synergistically influ­ences the outer reaction region, eventually resulting in a higher product (C02 andH20) formation rate in that zone.

At higher equivalence ratios, the overall reaction rate is lower than at higherlevels of air premixing (i.e., lower $), since the flame is influenced relativelymore by transport and less by the premixed methane-air chemistry. In addition,the fuel flux through the inner slot also rises as the equivalence ratio increases.Consequently, the flame occupies a larger overall spatial volume. At $ = 3, theeffect of the CTchemistry again occurs due to reaction RIV that producesH-atoms, and reaction RVI that consumes H-atoms. Once again, the rate of theinitiation reactions RVIII-RXI increases. However, as illustrated in Fig. 8 andTable I, the role of reaction RIV in augmenting H-atom formation is relativelydiminished due to the paucity of the O-atom pool at higher $. In addition, thesmaller increase in the H-atom pool at $ =3 (in comparison with $ =1.5) is off­set by H-atom consumption through the reactions RVI and

(RXIl)

(RXIII)

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PARTIALLY PREMIXED METHANE-AIR FLAMES 205

TABLEI Integrated species consumption (or production) rates fordominant reaction in the inner andouter flames at <1>=1.5 and 3.0 computed by using the C,- and Crmechanisms

l1J;/.5 l1J;3.0

Reactions Inner Outer Inner Outer

CJ C2 C/ C2 CJ C2 CJ C2

CH4

I. CH4 + H <=> CH3 + H2 -4.0 -13 -0.06 a -0.4 -1.0 -0.2 a2. CH4 + OH <=> CH3 + H2O -4.2 -4.3 -0.02 a -1.1 -0.7 -0.15 aO2

1.°2+H<=>OH+O -6.0 -14.7 -0.5 -0.15 -0.5 -0.9 -1.0 -0.6

2. O2 + H + M <=> H02 + M a -0.7 -0.4 -0.13 a a -0.45 -0.4

3. H02 + OH -> H20 + 02 a 0.12 0.3 a a a 0.35 0.3

4. CH3 + 02 -> CH20 + OH -2.5 -1.0 a a -1.2 -0.4 -0.25 acoI. CO + OH <=> CO2 + H -2.2 -3.9 -0.6 -0.16 -0.4 -0.1 -0.85 -0.7

2. CHO + M -> CO + H + M 5.0 4.6 0.1 a 1.2 0.4 0.4 a3. CH2 + O2 -> CO + OH + H 0.5 1.0 0.03 a a 0.09 a a4. C2HO + H <=> CH2 + CO / -0.7 / a / 0.06 a5. C2HO + ° -> 2CO + H / 4.4 a 0.18 a6. C2H2 + °-> CH2 + CO / 2.0 a 0.2 aHz

I. H2 + ° <=> OH + H -0.8 -3 -0.15 a -0.05 -0.1 -0.2 -0.2

2. H2 + OH <=> H20 + H -4.2 -13.4 -0.35 -0.12 -0.3 -0.8 -0.55 -0.3

3. CH20 + H -> CHO + H2 1.9 2.1 a a 0.55 0.2 a a4. CH3 + H <=> CHz + H2 1.6 1.6 a a 0.10 -0.08 0.15 a5. CH4 + H <=> CH3 + Hz 4.0 13 0.06 a 0.4 1.0 0.2 a6. C2H4 + H -> C2H3 + H2 2.4 / a 0.3 a7. C2H6 + H -> C2H, + H2 3.3 a 0.6 aCOz

I. CO + OH <=> CO2 + H 2.2 3.9 0.6 1.5 0.4 0.1 0.85 0.7

2. CH2 + O2 -> CO2 + 2H 0.7 a a 0.05 0.09 a aHzO

I. H2 + OH <=> H20 + H 4.2 13.4 0.35 0.12 0.3 0.8 0.55 0.3

2. H20 + ° <=> 20H -0.7 -1.5 -0.30 a a -0.07 -0.4 -0.3

3. H02 + H -> H20 + ° a 0.1 0.3 a a a 0.35 a4. CH20 + OH -> CHO + H2O 3 1.9 a a 0.6 0.2 0.4 a

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206 ZHUANG SHU et al.

¢>=}.5 ¢>=3.0

Reactions Inner Outer Inner Outer

CJ Cz CJ Cz C/ Cz CJ Cz

5. CH4 + OH ." CH3 + HzO 4.2 4.3 0.02 0 1.1 0.7 0.15 0

6. CZH4 + OH .... CZH3 + HzO 1.4 0 0.1 0

7. CZH6 + OH .... CzHs + HzO 0.8 0 0.1 0

eH3

I. CH 3 + H ." CHz + Hz -1.6 -1.6 0 0 -0.10 0.07 -0.15 0

2. CH) + 0 .... CH20 + H -3.0 -3.2 0 0 -1.1 -0.04 0 0

3. CH3 + O2 .... CHzO + OH -2.5 -0.5 0 0 -1.2 -0.4 -0.25 0

4. CH4 + H ." CH) + H2 4.0 13 0.06 0 0.4 1.0 0.2 0

5. CH4 + OH ." CH) + HzO 4.2 4.3 0.02 0 1.1 0.7 0.15 0

6. 2CH3 .... C2H6 / -I / 0 -0.16 0

CHO

I. CH + 02 .... CHO + 0 0.2 0.5 0 0 0 0 0 0

2. COz + CH .... CHO + CO 0.1 0.3 0 0 0 0 0 0

3. CHO + M .... CO + H + M -5.0 -4.6 -0.1 0 -1.2 -0.4 -0.4 0

4. CH20 + H .... CHO + H2 1.9 2.1 0 0 0.55 0.2 0 0

5. CH20 +OH ." CHO + H2O 3 1.9 0 0 0.6 0.2 0.4 0

6. CHO + H .... CO + H2 0 -0.1 0 0 0 0 0 0

H

I. Oz + H ." OH + ° -6.0 -14.7 -0.5 -0.15 -0.5 -0.9 -1.0 -0.6

2. H2 + 0 ." OH + H 0.8 3 0.15 0 0.05 0.1 0.2 0.2

3. H2 + OH ." H20 + H 4.2 13.4 0.35 0.12 0.3 0.8 0.55 0.3

4. 0z + H + M ." H02 + M 0 -0.7 -0.4 -0.13 0 0 -0.45 -0.4

5. COt OH ."COz + H 2.2 3.9 0.6 0.16 0.4 0.1 0.85 0.7

6. CHO + M .... CO+ H+ M 5.0 4.6 0.1 0 1.2 0.4 0.4 0

7. CH3 + 0 .... CH20 + H 3.0 3.2 0 0 1.1 0.04 0 0

8. CH4 + H ." CH3 + H2 -4.0 -13 -0.06 0 -0.4 -1.0 -0.2 0

9. C2HO + 0 .... 2CO + H / 2.2 / 0 / 0.09 0

10. CZH4 + H .... C2H3 + H2 / -2.4 0 / -0.3 0

II. C2H6 + H .... C2Hs + Hz / -3.3 0 -0.6 0

OH

1.02+H"'OH+0 6.0 14.7 0.5 0.15 0.5 0.9 1.0 0.6

2. Hz + 0 ." OH + H 0.8 3 0.15 0 0.05 0.\ 0.2 0.2

3. Hz + OH ee H20 + H -4.2 -13.4 -0.35 -0.12 -0.3 -0.8 -0.55 -0.3

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PARTIALLY PREMIXED METHANE-AIR FLAMES 207

4>=1.5 4>=3.0

Reactions Inner Outer Inner Outer

CJ C2 CJ C2 C/ C2 CJ C2

4. CO + OH <0> CO2 + H -2.2 -3.9 -{J.6 -{J.16 -{J.4 -o.i -{J.85 -{J.7

5. CH4 + OH <0>CH 3 + H2O -4.2 -4.3 -{J.02 0 -1.1 -{J.7 -{J.15 0

6. C2H4 + OH -. C2H3 + H2O -1.4 0 -{J.I 0

7. C2H6 + OH -. C2Hs + H2O -{J.8 0 -{J.I 0

0

I. O2 + H <0> OH + 0 6.0 14.7 0.5 0.15 0.5 0.9 1.0 1.0

2. H2 + 0 <0>OH + H -{J.8 -3 -0.15 0 -{J.05 -{J.l -{J.2 -{J.25

3. CH 3 + 0 -. CH20 + H -3.0 -3.2 0 0 -1.1 -{J.04 0 0

4. C2HO + 0 -. 2CO + H -2.2 I 0 -{J.09 0

Note:I. The unit of thespecies consumption (or production) rate is 10-5 moles m-3 s-I.2. The"0" indicates a smallvalue,and..I" indicates that thereaction is not included in theCJ­

mechanism.

The two Cj-pathways PI and PII and the CTpath PIlI are distinguished by acompetition between the following reactions

(RXIV)

(RXV)

(RXVI)

(RXVII)

The reactions RXIV and RXV occur along path PI, RXVI along path PII, andRXVII along PIlI. When CTchemistry is employed, reaction RXIV dominatesRXV at lower values of «I> due to the relative abundance of 0 atoms.

There is a competition for O-atom consumption between reactions RXIV andRX. Reaction RVIII results in the formation of H2 and, since, H-atoms are ingreater abundance when C2-chemistry is employed, so is molecular hydrogen.However, at the higher equivalence ratio (<<I> = 3), the O-atom pool is not suffi­ciently enhanced in the inner reaction zone due to the relative paucity of molecu­lar oxygen. Therefore, the two reactions RXIV and RX compete with each otherto consume O-atoms and, consequently, the path PI diminishes in importance,rendering the path PIlI of comparable magnitude. This decreases the overallreaction rates for the CTmechanism, since path PIlI is slower compared to path

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Z08 ZHUANG SHU et at.

PI. This also leads to a reduction in CO formation through reaction RV that is notoffset by either reaction RIV or the reaction

(RXVIII)

both of which occur along the Cz-pathway PIlI.

In addition, the lower concentration of Oz at higher <p reduces the formation ofhydroxyl radicals through the reaction RIX. This leads to a reduction in the con­sumption rates of methane and CO (through reactions RXIX and RVII respec­tively) for both the C,- and Cz-mechanisms. However, this effect is relativelymore significant in case of the latter mechanism due to the competing reactions

CH 4 + OH -t CH 4 + H20, and

C2H4 + OH -t C2H 3 + H20.

(RXIX)

(RXX)

In summary, the use of C2-chemistry enhances the overall reaction rate in theinner flame at lower equivalence ratios by increasing the radical pool in compar­ison with the Cj-mechanism. However, due to a paucity in the O-atom pool athigher equivalence ratios, this effect is significantly diminished in richer flames,such that the slower path PIlI becomes comparable to the path PI. In addition, asindicated in Table I, reaction RVI produces H-atoms at lower <p, but consumesthem at higher <p. The net effect is that the methane consumption rate is reducedwhen the C2-mechanism is used. In addition, the use of C2-chemistry furtherretards the CO-formation rate in richer flames due to a reduction in the OH-radi­cal pool. As a result, the inner reaction zone in the <p = 3.0 flame exists at higher(more downstream) locations when the Cz-mechanism is employed, but at lowerlocations when the simpler mechanism is used.

CONCLUSIONS

In the present study, the role of Cz-chemistry in determining the structure of par­tially premixed methane-air flames has been investigated by employing twodetailed mechanisms. The first mechanism involves only C ,-chemistry and com­prises 52 reactions that involve 17 species, while the second mechanism containsboth C ,- and C2-chemistry and Cz-chemistry and consists of 81 reactions and 24species. The simulation model has been validated by comparing the predictedand measured planar partially premixed flames (PPFs) established on a rectangu­lar Wolfhard-Parker slot burner. A detailed numerical study has been conductedto examine the effects of Cz-chemistry on the structure of partially premixedflames for different levels of partial premixing and reactant velocities. Theimportant observations from this investigation are as follows:

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PARTIALLY PREMIXEDMETHANE-AIR FLAMES 209

l. Both the C,- and C2-mechanisms are capable of reproducing the global fea­tures of PPFs that are observed in laboratory experiments. These include:(I) the existence of an inner premixed reaction zone and an outer non­premixed reaction zone; (2) synergistic interactions between these two reac­tion zones, whereby the inner zone provides CO and H2(intermediate fuels)to the outer zone, which in tum provides radical species (H and OH) to theinner zone; (3) variation of the inner and outer reaction zone heights withchanges in the equivalence ratio and reactant velocity; and (4) the relativedominance of the inner zone chemistry at higher levels of air premixing (i.e.,lower $) and of the outer zone at higher values of $. Since the Cj-mechanismhas been shown to be inadequate for fuel-rich premixed flames, its relativelygood performance in the context of PPFs can be attributed to the synergisticinteractions between the two reaction zones.

2. There are important quantitative differences in the PPF structures predictedby the two reaction mechanisms. First, the inner reaction zone height pre­dicted by the Cj-mechanism is less sensitive to variations in the equivalenceratio as compared with the Crmechanism. At high levels of air premixing($ ~ 1.5), the CTmechanism yields a lower inner reaction zone height, whileat lower levels of air premixing ($ ~ 2.0), it yields a higher inner zone heightcompared with predictions obtained by using the CI-mechanism. The predic­tions using the Crmechanism are more consistent with the experimentalresults. Second, methane consumption in the inner reaction zone is signifi­cantly higher when the C2-mechanism is used as compared to resultsobtained with Cj-mechanism. Consequently, for high levels of partialpremixing, the amount of methane that leaks from the rich premixed to non­premixed zone is significantly lower when the Crmechanism is used.. Third,a smaller separation distance between the inner and outer reaction zones ispredicted by the C2-mechanism, compared to the Cj-mechanism results. Thisindicates stronger interactions between the inner and outer reaction zoneswhen the C2-mechanism is used. Finally, the maximum temperature predictedby the C2-mechanism is slightly lower as compared to that obtained using theC,-mechanism. The various differences are due to the presence of Crchainin the 81-step mechanism, which strongly affects the structure of the innerpremixed reaction zone.

3. The CI-pathway, involved in the conversion of methane to CO and H2, con­sists of two channels. The first channel involves CH3 ~ CH20 ~ CHO,while the second channel involves CH3 ~ CH2 ~ CH ~ CHO. The formylradicals subsequently form CO, which is further oxidized to produce CO2,

while H2 is oxidized to form H20 . The CTroute in the Crmechanisminvolves an additional path CH3~ C2H6 ~ C2H5 ~ C2H4 ~ C2H3~ C2H2

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~ CZHO~ CO. At lower equivalence ratios (<P::; 1.5), the major effect of theCz-pathway is to increase the overall reaction rates by increasing H-atom for­mation and, thereby, augmenting the effect of the chain-branching reactionsby raising the net availability of radical species. Consequently, the inner reac­tion zone is located further upstream when the Cz-mechanism used, and fur­ther downstream when Cj-mechanism is employed. However. at higherequivalence ratios this effect is significantly diminished due to the paucity ofO-atoms, so that the slower Cz-path becomes comparable to the Cj-path. Inaddition, the reaction CzHO + H <=> CHz + CO produces H-atoms at lower <p,but consumes them at higher <p. The Cz-path also retards the CO-formationrate in richer flames due to a reduction in the OH-radical pool. The net effectis that at higher equivalence ratios, the methane consumption rate is reduced,and the inner reaction zone exists at higher axial locations when Cz-mecha­nism is employed.

4. The Cz-pathway has no direct effect on the outer nonpremixed reaction zone.However, the spatial location of this zone is indirectly affected, as it islocated closer to the inner reaction zone, by the Cz-pathway.

Acknowledgements

This research was supported by the National Science Foundation Combustionand Plasma Systems Program through Grant No. CTS-9707000 for which Dr.Farley Fisher is the Program Director. Simulations were performed on the SGIworkstations at NCSA at Urbana-Champaign.

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