Quantitative CH determinations in low-pressure flames

959

Twenty-Sixth Symposium (International) on Combustion/The Combustion Institute, 1996/pp. 959–966

QUANTITATIVE CH DETERMINATIONS IN LOW-PRESSURE FLAMES

JORGE LUQUE, GREGORY P. SMITH and DAVID R. CROSLEYMolecular Physics Laboratory

SRI InternationalMenlo Park, CA 94025, USA

Quantitative linear laser-induced fluorescence (LIF) has been applied to measure CH concentration inlow-pressure propane/air and methane/air flames. Temperature measurements at the peak of the CHdistribution give 1600 and 1700 K for those flames, respectively, using CH B-X, A-X, and OH A-X excitationLIF scans. Quenching was taken into account by measuring the temporally resolved decay of the signal.The CH absolute number density determination was carried out in two different electronic bands and theLIF signal was calibrated with two independent methods: Rayleigh scattering of N2 and Raman scatteringof H2. The results show agreement to better than 10% between electronic bands and 30% between cali-bration methods. Overall, the technique is well suited to measure concentrations in the ppm and sub-ppmrange with an average error of 25%. This degree of accuracy is adequate to test flame kinetic models. CHprofiles and peak amounts of 4–11 ppm for the propane and methane flames agree well with modelpredictions based on the GRI-Mech 2.11 mechanism. Flame CH, temperature, and NO measurementswere used to deduce rate constants for the CH ` N2 reaction near 1600 K.

Introduction

Prompt NO Production and CH Radicals

The CH radical is the key reactant in the forma-tion of nitrogen oxide emissions through the promptNO mechanism [1]. It undergoes reaction with N2from combustion air, breaking the strong N–N bondto form N atoms and HCN. These N atoms imme-diately react with O2 to produce NO; the HCN un-dergoes a further oxidation sequence involving CN,NCO, and NH radicals to eventually produce an-other N atom that also reacts with O2 to make NO.The CH radical, which initiates this mechanism, isformed in most hydrocarbon flames largely frommethyl radicals through sequential hydrogen atomabstraction and is lost largely through oxidation withO2 and water.

The prompt mechanism is one of several meansof NOx formation in combustion systems [2,3], al-though it dominates for many liquid and gaseous hy-drocarbon fuels, such as natural gas, at lower com-bustion temperatures. The other primarymechanism, thermal or Zeldovich NO, occurslargely in hot burned gases above ;1800 K; itschemical kinetics is well understood, and the maincontrol strategy is a reduction of residence time atthe final hot temperature. Prompt NO is best con-trolled by limiting the amount of CH present atmoderate combustion temperatures, near or below1600 K. For example, in a test at the Burner Engi-neering Research Laboratory at Sandia [4], a regionof NO formation was identified by chemilumines-

cent flame imaging of the CH precursor, and sub-sequent burner modifications eliminating that regionshould lower NOx emissions by 50%.

A full understanding of the role played by CH innatural gas flames, including means of formation anddestruction for various hydrocarbon fuels and fuel–air ratios, is important in developing such controlstrategies. Models of combustion chemistry are ofparticular importance in developing such an under-standing. These must, however, be capable of robustprediction of CH radical concentrations at the sev-eral part per million concentration level in order tomake useful prediction of NOx pollutant formation.Our experience with flame chemistry models hasshown that the correct prediction of absolute CHconcentration and position in a one-dimensionallaminar flame forms a key test of the relevant partof the chemical mechanism. Sensitivity studies [5]for low-pressure methane-air flames show that theCH ` N2 reaction is indeed the most important.This complicated, spin-forbidden multipath reactionproceeds through a bound HCNN complex at lowtemperature but an abstraction process at highertemperature [6]. Its rate coefficients are thus depen-dent on both temperature and pressure and are dif-ficult to determine at combustion temperatures. Acohesive theoretical picture of this reaction [7], in-corporating the pertinent experimental data and po-tential energy surfaces, based on an extrapolation ofhigher temperature shock-tube measurements [8],can provide predictive capability only within a factorof 2 at flame temperatures. The situation for other

960 LAMINAR PREMIXED FLAMES

CH reactions, including those governing its produc-tion and loss, is no better.

Because of the direct correspondence betweenNO and the total rate of the CH ` N2 reaction,accurate knowledge of CH is essential to accuratelypredict NO from models. However, the large uncer-tainty in the relevant CH reaction rate coefficientsat combustion temperatures renders this difficultfrom kinetic considerations alone. Furthermore, op-timizing or adjusting the model using experimentallydetermined NO emission in a flame as the sole cri-terion cannot distinguish between error in the CH` N2 rate coefficient or errors in the other CH re-actions. Even though a good match may be obtainedfor a specific set of conditions, it may be faulty forother cases.

In this paper, we describe the measurement of ab-solute CH concentrations in low-pressure flames,#40 torr. Low-pressure flames serve this purpose byproviding suitably high spatial resolution for preciseand detailed comparison with predictions of flamechemistry models. This presents a considerable ex-perimental challenge, because CH is present at con-centrations too low for normal absorption spectros-copy, its concentration is present at thermalequilibrium in the burned gases at far too small anamount to measure by any means, and there is nosuitable calibration source. Previous absolute mea-surements of CH have often been made in acetyleneflames, where the concentration is much higher,;200 ppm, using direct absorption at atmosphericpressure [9] and, at low pressure, saturated fluores-cence [10] and frequency modulated absorption[11]. For our measurements, we chose unsaturatedlaser-induced fluorescence (LIF). This method ishighly selective and sensitive and generally yields ex-cellent signal-to-noise ratios for relative CH profilesin premixed flames. However, operation on an ab-solute basis demands careful attention to many ex-perimental details and an optical calibration scheme.In the next section, we describe the experimentalapproach. That is followed by a description of theflame chemistry model, including comments onmodel sensitivity and optimization using predictedvs. measured CH and NO.

Experimental

Absolute LIF Measurement of CH Approach

The LIF signal, emitted by an electronically ex-cited state created by absorption of a laser photon,is given by

S 4 BI Cs Nf U F (X/4p)egV (1)F L L B F fl

This equation is valid under one important condi-tion, which holds in our experiment: the excitation

rate BIL is small compared to the inverse of the laserpulse length, , so that only a small fraction of the11sLpopulation in the absorbing ground state level is ex-cited. (IL is the laser spectral power density per unitfrequency interval in J cm12 s11 (cm11)11 and B isthe Einstein coefficient divided by the speed oflight.) The fluorescence quantum yield UF 4 A/[A` Q], where A is the Einstein emission coefficientand Q is the quench rate, RikQiNi, taken over allcontributing flame species. As described below, wedirectly measure the total quenching rate in theseexperiments. Ffl is the fraction of fluorescent emis-sion falling within the detection bandpass; fB, thefraction of total CH in the absorbing level, may becalculated knowing the local temperature T. Theline-width integral C will be discussed below.

Many of the quantities appearing in Eq. (1) andnecessary to measure the species number density Nare determined during the experiment: SF, IL, sL, T,and Q. The Einstein A and B coefficients are ob-tained from previous spectroscopic work. Transitionprobabilities in the A-X [12] and B-X [13] band sys-tems of CH have recently been reinvestigated, op-timizing an electronic transition moment to vibra-tional band intensity data for relative values andusing experimental radiative lifetimes for absolutevalues. Geometric and detection efficiency terms arecollected in the quantity (X/4p)egV. These terms(solid angle, phototube efficiency, optical transmis-sion, and probe volume) could be determined indi-vidually, but this is cumbersome and leads to a largecompounded error. Instead, this is calibrated as asingle term via Raman and Rayleigh scattering mea-surements.

We describe measurements made in 40 torr, pro-pane/air flames. These one-dimensional, laminarflames were burned on a 6-cm-diameter McKennaburner in a low-pressure chamber. Two differentstoichiometries, f 4 1.00 and 1.15 at a mass flowrate of 2.0 mg/cm2/s, were used. Measurementswere also made on a 25 torr, f 4 1.07 methaneflame (0.519 std. L/min. (SLM) CH4, 0.970 SLM O2,2.263 SLM N2 flows). A frequency-tripled Nd:YAGlaser pumped a dye laser to produce light to exciteCH near 431 nm (A-X system) or 390 nm (B-X sys-tem). The laser pulse duration sL 4 7 ns, and thelaser bandwidth is 0.22 cm11. Optical collection wasat f/3 and matched into an f/7 monochromator witha 30-nm-wide trapezoidal bandpass; detection useda Hamamatsu 1P28 photomultiplier. The signal wasprocessed by a boxcar integrator or transient digi-tizer. Further details are found in Refs. 14, 18, and19.

Raman and Rayleigh Scattering

The signal for either Raman [15] or Rayleigh [16]scattering is given by

S 4 N P (]r/]X)XegV/hv (2)R tot L

QUANTITATIVE CH IN FLAMES 961

Fig. 1. Dependence of the hydrogen Raman signal onthe product of pressure and laser energy (laser wavelength404 nm, collection wavelength 485 nm).

Ntot is the total number density of scatterers; N2 wasused for the Rayleigh signals and H2 for the Ramancalibration. One may rewrite Eq. (2) as SR 4 DPE,where E is the laser energy per pulse (PL, the pulseenergy density, times the beam cross-sectional areaA), P is the pressure of the scatterer, and D is anexperimental constant. The desired calibrationquantity Xeg(V/A) is then directly determined fromthe scattering measurements. The example in Fig. 1is a Raman calibration in H2. Each point is a com-bination of some pressure P and laser energy E; thesignal is linear in the product. In this case, pressuresbetween 10 and 90 mtorr of H2 and laser energiesbetween 0.2 and 1 mJ per pulse were used. Furtherdetails, including a discussion of the Rayleigh cali-bration, are found in Ref. 14.

Temperature

Temperature is needed for the Boltzmann popu-lation fraction fB and also to relate the measuredabsolute number density to fractional concentrationfor comparison with model output. Moreover, an ac-curate temperature profile is needed as input to theone-dimensional flame code used to describe theselow-pressure flames with heat loss [17]. Tempera-tures at the point of CH measurement were deter-mined by rotational excitation scans in CH; full tem-perature profiles had previously been determined[18] by excitation scans in OH. Fits of the Boltzmanntemperature plots in the f 4 1.15 propane flame,using the recently determined transition probabili-ties, yield temperatures of 1600 5 60 K for a mea-surement using the A-X system and 1570 5 50 Kwith B-X. OH furnishes a temperature of 1620 5

40 K at the same point. There is excellent agreementamong these three independent LIF temperaturedeterminations.

Collisional Quenching

The excited CH molecules can be removed non-radiatively, and knowledge of the fraction that emitlight, UF, is needed for any quantitative LIF mea-surements. We address the problem by direct mea-surement of the fluorescence decay time, 4 Q11sfl` A, for both the A2D and B2R1 excited states atthe CH peak. From independent measurements ofA for each state, Q (and thus UF) is determined. Inour rich propane flame, the A state decays with alifetime of 66 5 4 ns, and B has s 4 54 5 3 ns,leading to quenching rates of 0.33 5 0.03 and 0.405 0.04 ls11 torr11, respectively. This is about 20%higher than calculated values using previously mea-sured or estimated quenching rate coefficients kQi(Ref. 19), which represents good agreement.

Absorption Line Overlap

The line-shape integral C, representing the over-lap of the laser line and the CH absorption line, wascalculated assuming a Gaussian line shape for thelaser spectral distribution, with a full width at halfmaximum of 0.22 5 0.02 cm11, as determined byan external etalon. The CH line shapes are domi-nated by Doppler broadening in the low-pressureflames, with widths of 0.18 cm11 for A-X and 0.20cm11 for B-X. These yield overlap integrals CA-X 40.73 and CB-X 4 0.70; these factors cannot be ne-glected in accurate measurements.

Absolute CH Determinations

The 40 torr propane/air flames were chosen forthese measurements because they have previouslybeen well characterized [18]. First, relative concen-tration profiles were obtained using a 10-ns detec-tion gate to avoid effects of quenching variations.Placing the relative profiles on an absolute basis re-quires measurement at only one point; this was doneat the maximum for optimum signal-to-noise ratio.

Figure 2 shows a plot of SF vs. laser intensity IL.Note that very low laser energies, ,150 nJ per pulse,were used in order to ensure linearity, as demandedfor the application of Eq. (1). The slope for the B-Xsystem is larger than for A-X due to its nearly twofoldhigher absorption coefficient.

The slopes from these lines, combined with thescattering calibrations, are used to obtain absoluteconcentrations at the peak. Because two band sys-tems were used, with two different calibration meth-ods, we have four independent determinations of ab-solute CH. The results for the f 4 1.15 propane/air flame are given in Table 1.


Fig. 2. CH LIF signal vs. laser energy at the peak con-centration in the rich 40-torr propane flame. Two elec-tronic state excitations are illustrated, along with linear fitslopes.

TABLE 1CH concentration measurements (ppm) using different

LIF and calibration methods for the richpropane/air flame

Calibration Method

Electronic Transition

A-X B-X Average

Rayleigh 5.3 5 2.1 4.8 5 2.0 5.1 5 1.6Raman 6.5 5 3.2 6.7 5 3.2 6.6 5 2.3

Average 5.9 5 2.2 5.7 5 1.9 5.8 5 1.5

Fig. 3. Absolute CH concentration vs. height above theburner for the U 4 1.15 40-torr C3H8-air flame. Thedashed line is the model result using GRI-Mech 2.11.

Fig. 4. Absolute CH concentration vs. height above theburner for the U 4 1.00 40-torr C3H8-air flame. Thedashed line is the model result using GRI-Mech 2.11.

This measurement involves independent knowl-edge of many quantities, so error assessment is amajor concern. Most of the uncertainty arises fromthe calibration, due to scatter for replicate measure-ments and uncertainty in scattering cross sections.These contribute 10% for Rayleigh and 25% for Ra-man calibration to the error; scatter in LIF signalfrom day to day was as much as 10%. A more de-tailed discussion is found in Ref. 14. We assess theerror bars shown in Table 1 and average the resultsof the four measurements to obtain an overall errorof 25–30%. Final profiles, placed on an absolute ba-sis, are shown in Figs. 3–5.

Modeling of the CH Flame Chemistry

Model calculations of CH in these low-pressureflames were performed for comparison to these ab-

solute measurements and profiles, to test our currentunderstanding of prompt NO flame kinetics and todetermine the key reactions involved through theuse of sensitivity analysis. Flames were modeled us-ing the Sandia laminar flame code Premix [20], withmeasured temperature values and flow rates used asinputs, assuming no radial expansion of the flamesand recombination of hydrogen atoms at the burnersurface. The C-1, C-2, and N chemistry and ther-modynamics are from the optimized GRI-Mech 2.11mechanism [21], which includes four pathways toNO production (thermal, prompt, N2O, and NNH).


Fig. 5. Absolute CH concentration vs. height above theburner for the U 4 1.07 25-torr CH4–O2–N2 flame. Thedashed line is the model result using GRI-Mech 2.11.

TABLE 2CH sensitivity coefficients, d[CH]/[CH] 3 dk/k

ReactionCH4 flameU 4 1.07

C3H8 flameU 4 1.15

C3H8 flameU 4 1.00

C3H8 ratio(1.15/1.00)

O ` H2 4 H ` OH `0.136 `0.089 `0.088O ` CH2 4 H ` HCO 10.052 10.053 10.066O ` CH3 4 H ` CH2O 10.439 10.251 10.313 `0.062O ` CH4 4 OH ` CH3 `0.105 `0.008 `0.009O ` C2H2 4 CO ` CH2 `0.016 `0.137 `0.122O ` C2H4 4 CH3 ` HCO `0.003 `0.005 `0.045 10.040H ` O2 4 O ` OH 10.205 10.160 10.182H ` CH 4 C ` H2 10.116 10.124 10.097 10.027H ` CH4 4 CH3 ` H2 `0.105 `0.015 `0.011C2H3 4 H ` C2H2 `0.013 `0.120 `0.127H ` C2H3 4 H2 ` C2H2 `0.005 `0.065 `0.064OH ` CH3 4 CH2 ` H2O `0.056 `0.015 `0.021OH ` CH3 4 CH2(S) ` H2O `0.210 `0.052 `0.124 10.072OH ` CH4 4 CH3 ` H2O `0.065 `0.008 `0.010HO2 ` CH3 4 OH ` CH3O 10.015 10.077 10.082CH ` O2 4 O ` HCO 10.239 10.194 10.293 `0.099H ` CH2 4 CH ` H2 `0.331 `0.303 `0.416 10.113CH ` H2O 4 H ` CH2O 10.473 10.558 10.494 10.064CH2 ` O2 4 OH ` HCO 10.209 10.192 10.288 `0.096CH2(S) ` N2 4 CH2 ` N2 `0.125 `0.077 `0.099CH2(S) ` O2 4 H ` OH ` CO 10.077 10.041 10.061CH2(S) ` H2O 4 CH2 ` H2O `0.109 `0.039 `0.045HCO ` H2O ` H ` CO ` H2O `0.041 10.003 `0.030 10.034C2H3 ` O2 4 HCO ` CH2O 10.017 10.181 10.188H ` C3H5 4 C3H6 `0.000 `0.050 `0.036

To model the propane flames, we added 47 reactionsinvolving six C-3 species (propane, n-propyl, i-pro-pyl, propene, allyl, and allene), using rate constantslargely adopted from compilations of Tsang [22].These steps mostly serve to cleave the fuel into C-1and C-2 fragments, and the CH profiles are insen-sitive to these rate constants.

Model results for CH are shown by the dashedlines in Figs. 3–5. Excellent agreement, within ex-perimental error, is obtained for absolute CHamounts in all three flames. Profile peak positionsand widths are also accurately computed. Any minordifferences are comparable to experimental uncer-tainty, since the spatial resolution of the laser beamis 0.05 cm and the precision of spatial profiles judgedfrom repeated measurements is 0.02 cm.

Sensitivity analysis can determine how stringent atest of the model chemistry these observations formand the specific rate processes that are being tested.Sensitivity coefficients for peak CH amounts in thethree flames are shown in Table 2. The control ofthe key CH production and destruction reactions isapparent, as well as several destruction steps involv-ing CH precursors methyl, methylene, and vinyl. Wenote the additional path to CH through acetylene in


TABLE 3NO sensitivity coefficients, d[NO]/[NO] 3 dk/k

ReactionCH4 flameU 4 1.07

C3H8 flameU 4 1.15

C3H8 flameU 4 1.00

CH ` N2 4 N ` HCN `0.817 `0.678 `0.447C ` N2 4 N ` CN `0.014 `0.014 `0.003O ` N2 4 N ` NO `0.088 `0.115 `0.095O ` NNH 4 NH ` NO `0.075 `0.179 `0.436O ` N2 ` M 4 N2O ` M `0.004 `0.009 `0.017O ` CH3 4 H ` CH2O 10.459 10.147 10.107H ` O2 4 O ` OH 10.427 10.405 10.365OH ` CH3 4 CH2(S) ` H2O `0.146 `0.014 10.059CH ` O2 4 O ` HCO 10.189 10.125 10.126H ` CH2 4 CH ` H2 `0.260 `0.194 `0.182CH ` H2O 4 H ` CH2O 10.411 10.387 10.220CH2 ` O2 4 OH ` HCO 10.168 10.124 10.128CH2(S) ` N2 4 CH2 ` N2 `0.104 `0.048 `0.038HCO ` H2O 4 H ` CO ` H2O 10.144 10.067 10.009C2H3 ` O2 4 HCO ` CH2O 10.017 10.125 10.093

the propane flames, as some C-2 steps become im-portant in the CH sensitivity spectrum. However,only one C-3 reaction appears above the 5% sensi-tivity threshold of the tabulated values. Since the CH` N2 reaction is a very minor path for CH loss, itis not prominent in the CH sensitivity.

The ratio of CH between the rich and stoichio-metric propane flames also tests select parts of CHxreaction chemistry, as sensitivities in the last columnshow. The model peak concentration ratio (rich/stoi-chiometric) of 1.39 is in excellent agreement withthe measured 1.35 5 0.06.

The magnitude of the sensitivity coefficients in-dicates that these CH observations form a good testof the model kinetics controlling the prompt NOprecursor. The peak position and width are less sen-sitive to variation in the kinetics, as shown in pastwork [5,21,23], being driven largely by the imposedtemperature profile. Thus, the CH measurementswill provide excellent experimental targets for pro-ducing optimized kinetics mechanisms relevant toNO formation. The current GRI-Mech 2.11 mech-anism was optimized for NO formation and reburn-ing kinetics by substantially increasing the rate con-stant for CH ` H2O in order to provide betterpredictive results for one measurement of promptNO in a low-pressure flame [5] and one determi-nation of CH in a CH4-O2-Ar-NO flame [24]. Thepresent experimental targets are being developed forthe next round of the optimized mechanism devel-opment effort and agree with this action. If mecha-nism rate constants prior to this optimization, fromGRI-Mech 1.2, are used instead, the CH in the threeflames is overpredicted by 35–70%. (See, for exam-ple, the mismatch for the propane flames in Ref. 14.)

We can also derive information on the critical CH` N2 rate constant, given these absolute CH deter-minations and measurements of absolute NO levelsin these same flames. Details of the NO measure-ments will be presented elsewhere [18,19]. NO LIFmeasurements were calibrated by comparing signalswith known amounts of NO seeded into the flames.NO sensitivity coefficients in Table 3 show that theprompt NO mechanism is the chief component. Infact, the contributions of the four NO productionmechanisms are mirrored in the key rate-constantsensitivity coefficients, and the reactions to whichCH is sensitive also contribute NO sensitivities bythe appropriately reduced amounts. Hence, itshould be possible to effectively derive values for theCH ` N2 rate constant k[T(x)] using the measuredabsolute CH density [CH], temperature, and NOprofiles. Assuming all N and HCN from the CH `N2 reaction forms NO, and taking the nitrogen molefraction (N2) and flow velocity v(x) from the calcu-lation, the final prompt NO mole fraction (NOp) iscomputed by

(NO ) 4 2k[T(x)] [CH] (N ) dx/v(x) (3)p 2#Table 4 takes the observed NO for each flame,

subtracts off the nonprompt NO from the model cal-culation, and compares various prompt NO modelpredictions. Using Eq. (3) for each flame, we com-puted (a) the contribution of prompt NO from GRI-Mech 2.11 by using the model CH distribution; (b)the predicted prompt NO from the experimentalCH concentrations using the CH ` N2 rate constantfrom GRI-Mech 2.11; (c) the predicted prompt NOfrom the experimental CH concentrations using theCH ` N2 rate constant from the evaluation of Ref.


TABLE 4NO concentrations (molecule/cm3) at 2 cm in the flames according to various schemes

CH4 flameU 4 1.07

C3H8 flameU 4 1.15

C3H8 flameU 4 1.00

Observed NO 1.8 5 0.6 E12 4.0 5 1.0 E12 2.9 5 0.9 E12Model NO GRIMech 2.11 1.0 E12 1.7 E12 1.1 E12(a) Prompt NO GRIMech 2.11 0.8 E12 1.5 E12 0.7 E12

Nonprompt model NO 0.16 E12 0.22 E12 0.38 E12“Observed” prompt NO 1.64 E12 3.78 E12 2.52 E12

(b) Prompt NO from CH (exp.) [2.11] 0.7 5 0.2 E12 2.3 5 0.6 E12 1.0 5 0.3 E12(c) Prompt NO from CH (exp.) [Ref. 7] 1.0 E12 3.2 E12 1.4 E12(d) 2.11 k multiplier (see text) 2.3 5 0.9 1.6 5 0.6 2.5 5 1.0

7 (1.4 times GRI-Mech 2.11); and (d) the multipli-cative factor by which the GRI-Mech 2.11 rate con-stant would have to be raised to match the observedNO given the experimental CH values.

These results couple the ongoing NO measure-ments with the present absolute CH determinationsand suggest a significantly higher rate constant forCH ` N2 than that produced by the GRI-Mech2.11 optimization. This value is even higher than theunoptimized base rate constants obtained from thetheoretical analysis of available data in Ref. 7. (Theabove suggests a rate constant of 4.0 2 109 cm3

mol11 s11 at the “average” CH temperature of 1640K, compared to the GRI-Mech 2.11 value of 1.9 2109, and 3.1 2 109 in Ref. 7.) It should be notedthat the optimization value was the result of a singleobservational target of NO production in a low-pres-sure flame from Ref. 5. Work is underway to resolvethis conflict, by measuring absolute CH and NO invarious low-pressure flames.

Conclusions

Absolute CH concentrations were determined inthree low-pressure hydrocarbon flames using LIFcalibrated by Rayleigh and Raman scattering. Mod-eling calculations employing sensitivity analysis showthat the controlling reactions to this prompt NO in-itiator involve CH production from methylene andthe destruction reactions of CH and its precursors.These measurements form a good test and optimi-zation target for the prompt NO mechanism kinet-ics, and the agreement with predictions based onGRI-Mech 2.11 is excellent. The model alsoestablishes a direct link between absolute CH andNO measurements in these flames via the CH ` N2reaction, and the results suggest an increased valuefor this rate constant.

Acknowledgments

We thank U. Westblom, F. Fernandez-Alonso, C. R. Ma-hon, M. Tamura, and J. E. Harrington, for their assistance

in many of the flame experiments that contributed to thisanalysis, and J. B. Jeffries, for useful discussions. This workwas supported by the Basic Research Group of the GasResearch Institute.

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Quantitative CH determinations in low-pressure flames

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