combined cavity ringdown absorption and laser-induced fluorescence imaging measurements of cn(b-x)...

Combined Cavity Ringdown Absorption and Laser-InducedFluorescence Imaging Measurements of CN(B-X) and

CH(B-X) in Low-Pressure CH4-O2-N2 andCH4-NO-O2-N2 Flames

J. LUQUE,† J. B. JEFFRIES,‡ G. P. SMITH,* and D. R. CROSLEYMolecular Physics Laboratory, SRI International, 333 Ravenswood Ave., Menlo Park, CA 94025, USA

and

J. J. SCHERERLos Gatos Research, 67 East Evelyn Ave., Mountain View, CA 94041, USA

A combined cavity ringdown absorption spectroscopy and laser-induced fluorescence imaging method is usedto study CN and CH absolute concentration profiles in low pressure premixed flames featuring prompt NO andreburn chemistry. For methane-air flames with and without seeded NO, the absolute concentrations and theshapes and peak positions of CN and CH above the burner compare favorably to model predictions and validatethe chemical mechanism. Cavity ringdown absorption of CN provides part-per-billion detection sensitivity. TheCH results agree with previous laser-induced fluorescence measurements calibrated with Rayleigh scattering,after correcting cavity ringdown for laser linewidth effects and accounting for the spatial inhomogeneities of theCH distribution in the flame. © 2001 by The Combustion Institute

INTRODUCTION

Premixed, laminar, low-pressure flames havebeen examined with spatially resolved laser-induced fluorescence (LIF) diagnostics to gaininformation about the intermediates involved inthe combustion chemistry. Such methods wereused previously to study the mechanisms ofprompt NO formation and the reburn strategyfor NO reduction, with LIF determinations ofabsolute CH and NO concentrations [1–3].Well-resolved, quenching-corrected, spatialprofiles of species distributions above theburner (height profiles) are obtained that mayeasily be compared to one-dimensional flamemodels. However, calibration methods are re-quired to obtain absolute concentrations, andour earlier determinations of this importantresult relied on a Rayleigh or Raman scatteringcalibration of the optical system. This paperreports measurements of the CN structure andabsolute concentration in such flames, tostrengthen our test of the flame chemistry, and

applies the sensitive technique of cavity ring-down absorption spectroscopy (CRD) to obtainabsolute concentrations of CN and CH.

The CRD technique [4] is highly sensitivebecause of the very long effective path length oflight trapped in an optical cavity. A recentreview of this method describes combustion andother applications, including a bibliographictabulation of species detected by CRD [5]. Thesensitivity of CRD offers an alternative,quenching-independent, calibration-free mea-sure of absolute CH concentration to compareto our earlier CH LIF results [1–3], and pro-vides the sensitivity needed to measure thenative CN. As an absorption method CRD isaccumulated over a line of sight; thus, thespatial distribution of the measured speciesmust be known along the path for a properquantitative interpretation. In addition, becausethe light makes many passes in the opticalcavity, the spatial resolution of CRD can bepoorly defined [6, 7]. Therefore, we measureline images of the CH LIF signal to determinethe CRD spatial resolution along the heightdimension. These LIF line images also providethe CH and reburn CN radial distributions.Then, at each height above the burner werecord the CRD absorption and a separate

* Corresponding author. E-mail: [email protected]† Current address: Lam Research Corp., Fremont, CA.‡ Current address: Department of Mechanical Engineering,Stanford University, Stanford, CA.

COMBUSTION AND FLAME 126:1725–1735 (2001)© 2001 by The Combustion Institute 0010-2180/01/$–see front matterPublished by Elsevier Science Inc. S0010-2180(01)00286-3

linear LIF image. This combination of CRDand LIF produces a quantitative two-dimen-sional flame image, which is compared to ourprevious quantitative LIF on the centerline ofthe burner, and is also used to test the one-dimensional flow assumptions for our low-pres-sure flames. The details of CRD calibration ofLIF and LIF determination of the CRD absorp-tion path length and spatial resolution are pub-lished elsewhere [7]. Depending on the initialdata set considered, one may describe eitherLIF corrections to the CRD results, or CRDcalibration of the LIF distributions or images.

This paper describes the application of thiscombined CRD-PLIF diagnostic to theCH(B2S2-X2P) and CN(B2S1-X2S1) systemsnear 388 nm in a slightly rich 25-torr methane-oxygen-nitrogen flame. In addition, replace-ment of the nitrogen feedstock with 1% NO innitrogen permits a study of the kinetics ofreburn, in a second flame. In these flames, CHis a key initiator for both the prompt NO andreburn processes, via the reactions

CH 1 N23 N 1 HCN

CH 1 NO3 O 1 HCN, H 1 NCO,

or N 1 HCO

CN, easily formed from HCN, is an intermedi-ate species in these mechanisms, before the finalsteps of:

N 1 O23 O 1 NO

or

N 1 NO3 O 1 N2

Thus, our absolute CN measurements will pro-vide tests of the NO production and loss mech-anisms, when compared to model results. CNhas been measured previously by absorption orLIF in low-pressure flames by using nitrogenoxides as oxidizer, or in flames seeded with NOor HCN [8–14]. Recently, Shin et al. [15] mea-sured LIF profiles of native CN in an atmo-spheric pressure natural gas-boiler flame.

In addition to its important role in initiatingprompt NO formation and high-temperaturereburning mechanisms, CH is the likely precur-sor to the intense OH(A-X) flame emission, viathe chemiluminescent reaction CH 1 O2 3

OH(A) 1 CO. Because this visual marker of theflame front is easily observed and imaged, it is agood potentially quantitative diagnostic for ap-plications where lasers are impractical, such assensor development or space combustion exper-iments. To interpret microgravity diffusionflame experiments by using OH(A) signals [16–18], the above rate constant can be measured inthe low-pressure flame, provided the CH con-centration is well determined.

This paper reports the measurement of nativeCN in a methane-air premixed low-pressureflame at levels of 10 ppb, illustrating the ex-tremely high sensitivity of the CRD technique.This technique, which has been used to detectOH, CH, HCO, 1CH2, and CH3 in flames [6,19–23] is quantitative as long as any complica-tions with spatial resolution, path length, orspectral bandwidth are properly accounted for.CN was also measured during the reburn of NOseeded into the premixed gas. Results are com-pared to model predictions, to test the combus-tion chemistry of the GRI-Mech 3.0 mechanism[24]. The CH(B-X) results also provide animportant independent measurement of CHto compare to recent CRD [7] and LIF [1]work in the same flame by using the CH(A-X)system.

EXPERIMENTAL

The flame examined was our standard laminarpremixed 25.2-torr CH4/O2(30%)/N2(70%)flame [1–3] of stoichiometric ratio 1.07 and flowrate 3.22 slm supported on a 6-cm-diametersintered brass McKenna burner (McKennaProducts, Livermore, CA) with a concentricargon shroud co-flow of 0.5 slm. The CH B-X(0,0) and CN B-X(0,0) bands were excitednear 388 nm by a pulsed dye laser (LambdaPhysik LPD 3000, Acton, MA, USA) temporalwidth ;6 to 7 NS, vertically polarized, Quinolon390 dye (Lambda Physik, Acton, MA), pumpedby a 355-nm Nd:YAG laser (Spectra PhysicsGCR-4; Mountain View, CA). The laser spec-tral bandwidth measured with a monitor etalonwas 0.13 6 0.01 cm21. Laser energies between 1mJ and 1 mJ were measured with a microjoule-meter (Rj-7200, Laser Precision, Yorkville, NY,USA). The laser beam was spatially filtered with

1726 J. LUQUE ET AL.

a telescope (3:1) and pinhole (100-mm diame-ter) for better mode matching in the ringdowncavity. The burner was equipped with opticalaccess arms. At both ends, mirrors with reflec-tivity better than 99.99% and a 6-m radius ofcurvature (Los Gatos Research) were set inkinematic mounts, forming a cavity of 0.92-mlength. If this cavity were confocal, a TEM00mode of 700-mm diameter (1/e FWHM) wouldresult; although this forms an approximation forthe best-case spatial resolution, we obtainednearly the same values in our detuned cavity [7].Such a resolution is adequate for measurementsin low-pressure flames where spatial distribu-tions of chemical intermediates are several mil-limeters wide. The temporally short multimodelaser pulses have limited coherence lengths, andthus interference effects were avoided.

The burner also had optical access perpendic-ular to the arms, and an intensified CCD cam-era (Princeton Instruments ICCD-576G/RBT,Princeton, NJ, USA; with 14 bits dynamicrange) was aligned to detect the fluorescenceproduced during the ringdown absorption. Thecavity alignment was optimized with respect totwo factors: adjusting the mirrors for the longestoff-resonance CRD decay time, and simulta-neously observing and minimizing the width ofthe spatial distribution of the LIF with the laserresonant on the R1 [8] CH B-X(0,0) line. CRDbeam widths were typically 0.8-mm FWHM,narrow enough to resolve low-pressure flamefeatures. Details on the use of simultaneous LIFto optimize the CRD spatial resolution havebeen presented elsewhere [7].

The burner was mounted on a motorizedstage for vertical scans. The accuracy of thevertical scale was estimated to be 0.25 mm. TheCRD light at the other side of the exit mirrorwas collected with a 9558QB EMI photomulti-plier and digitized with a 500-MHz Tektronix520C oscilloscope (Wilsonville, OR, USA). ALabview program handled data acquisition andlaser synchronization. The transients, averagedover 10 to 20 laser shots, were fit to a singleexponential decay between 90% and 10% of thesignal intensity.

The laser energy hitting the front mirrorbefore injection into the cavity was less than 10mJ and the laser energy inside was less than 10nJ, thus ensuring that the absorption is in the

linear, unsaturated regime even for strong tran-sitions like CN(B-X) and CH(B-X). CCD im-ages of light scattering from the ringdown laserin the chamber were used to calibrate thedistance above the burner. The spatial uncer-tainty was approximately 60.25 mm, as judgedby the reproducibility of profiles and the abilityto determine the zero of the scale. The CRDstudy of the CH(A-X) system in low-pressureflames [7] gives additional experimental details,including optical alignment procedures, the ef-fects of laser linewidth, and the use of LIF todetermine spatial resolution and the distribu-tion of the absorber.

Single-pass laser-induced fluorescence mea-surements were also taken, by replacing thefront and end mirrors with quartz windows andby using the CCD camera as a one-dimensionalarray detector. This alignment was used toassemble all the two-dimensional images shownlater, as sums of one-dimensional results atvarious burner heights. The laser beam diame-ter was 1 mm and images were composed ofhorizontal slices averaged over 0.6-mm height(sums of four camera pixels) taken every 0.35mm above the burner up to a height of 16 mm.The intensifier gate was slightly delayed fromthe laser pulse to avoid light scattering, and theintegration time is 20 ns to minimize the effectof fluorescence yield variations from quenchingchanges at different locations in the flame.

Analysis Method

In a cavity ringdown experiment, the monochro-matic light pulse coupled into the cavity de-creases in intensity with time due to mirror,scattering, and absorption losses. The light in-tensity collected beyond the exit mirror on everyother pass decays exponentially with a ringdownlifetime t given by:

t~n! 5 ~L/c)/[T 1 L1 s(n)Nd] (1)

where L is the cavity length, c is the speed oflight, T 1 L are mirror and scattering losses, sis the cross-section for the absorber, N is thenumber density of the absorber, and d is theeffective distance in the path for an absorberdistributed homogeneously. The ringdown timet is measured as the excitation wavelength is

1727CN AND CH CRD-LIF IN CH4 FLAMES

varied to produce a laser absorption spectrum.The spectra are analyzed in terms of total lossper pass 5 L/ct(n). Within this description,mirror and scattering losses appear as a back-ground in the spectrum, and absorption peaksdepend on number densities and absorptioncross-sections. For a pair of mirrors with high-reflectivity R each, T can be approximated by2lnR. At 388 nm, optimal flame values of T 1L are 0.00015, expressed as a cavity loss rate of150 ppm/pass and observed as a 20-ms ringdowndecay time. Laser intensity variations do notcontribute to the uncertainty, as they would intypical absorption measurements, because theabsorption is determined as a time decay.

To extract number densities, the cross-sectionwith the laser tuned to the peak of the spectralfeature is calculated from:

s~n! 5 B fB h n [G(n)/Dn] (2)

where B is the Einstein absorption coefficientgiven by the simulation/database program LIF-BASE [25–27], fB is the Boltzmann factor, i.e.,the fraction of molecules in the quantum statesampled by the laser, h is the Planck constant, nis the transition frequency and g(v) 5 G(n)/Dn isthe overlap integral of the laser and absorptionlinewidths (Dn is the laser bandwidth).

Densities derived from CRD measurementsare particularly subject to error from the over-lap integral G(n)/Dn, the effective absorber pathlength d, and the failure to account for anon-uniform distribution of absorbers along thepath length. The CRD transient is a singleexponential only in the monochromatic limit ofconstant s(n). With our multimode laser band-width comparable to the flame Doppler widths,weaker absorption in the wings rings down at aslower decay rate than at line center. Thisproduces a multi-exponential transient and cangive inaccurate quantitative results if analyzedas a single exponential. In the CH(A-X) CRDstudy [7], using an etalon to narrow the laserbandwidth from 0.13 to 0.07 cm21 produced a15% increase in the apparent number density.The computed overlap integral provides a line-averaged cross-section and gives only a first-order, single-exponential correction. A simplecorrection factor for multiple exponential ef-fects on the decay constant can be computed

from the absorbance and linewidth ratio, with-out repeated computed convolutions [28]. Thecorrection for the CH(B-X) R-branch workreported here is smaller than the (A-X) Q-branch values [7], which increased the density10% to 15%. The size of the correction dependson the ratios of laser bandwidth to spectralabsorption bandwidth, and of absorption ring-down time to background ringdown time. Thisfactor affects both absolute number density andheight profiles, because spatial distributions aremore likely to be distorted near their maxima(stronger absorption) and thus appear widerthan expected. For large absorptions, one willalso see deviations from exponential decay atthe longer times, an effect reflecting differentialabsorption in the wings of the lines. Recentwork by Mercier et al. [23] examined the influ-ence of finite laser bandwidth for pulsed dyelaser CRD measurement of CH(B-X) absorp-tion in low-pressure methane-air flames.

Spatial resolution and uniformity are a sec-ond issue for CRD measurements. Curvature ofthe flame and gradients at its edges degrade thespatial resolution, add uncertainty to the pathlength d, and introduce non-uniformities intemperature and species distributions. The si-multaneous LIF imaging addresses these prob-lems. The CRD absorption vertical profiles canbe converted into species distributions vs. heightat the flame center by multiplying by the ratio ofcenter LIF to the integrated line-of-sight LIFextracted from the images. Optimizing the ver-tical CRD spatial resolution in the flame de-pends on several factors: initial laser beamdiameter, divergence, beam walk inside thecavity, and the beam waist. The two-dimen-sional LIF image recorded by the CCD arraymeasures this resolution and assists in its opti-mization. Our CRD beam widths were typically0.8 mm. Background and mirror losses for our390-nm unseeded flame setup were usually be-tween 140 to 200 ppm/pass, and could be as lowas 100 ppm without the flame.

CRD Results

Figure 1 shows the CRD transient decay for thestrong CH B-X(0,0) R1 [8] line at its maximumabsorption in the flame, compared to the tran-sient ringdown taken off resonance. The ring-


down is easily observable over four exponentialdecay constants and linear over three. BecauseCH is present at only 10 ppm, the high sensitiv-ity is evident. For the very high CH absorptionsand fast ringdown times measured by Thomanand McIlroy [6] in a rich 31-torr methane flameusing A-X(0,0) CRD, the onset of non-expo-nential behavior is observed earlier in the decay.

The top panel of Fig. 2 shows a cavity-ringdown spectrum in the methane/oxygen/ni-trogen flame at 0.6 cm above the burner. TheCH B-X(0,0) R branch is easily identified. At388.34 nm there is a much weaker absorptionsystem—the weak fuzzy feature is not noise.The blown-up inset image of that section revealsthe P branch of CN B-X(0,0) transition. WhenNO is added to the flame (1% in the N2), theCN features are revealed in detail and are twoorders of magnitude larger and comparable toCH, as shown in the lower panel. The back-ground absorption (L) also increases ;50 ppm,

Fig. 1. Decay of the cavity ringdown transient signal plottedon an exponential scale vs. time, at the R1 [8] CH B-X(0,0)absorption, recorded at the height of maximum CH in the25-torr F 5 1.07 CH4-O2-N2 flame, and averaged over 250laser shots. The top trace is at an off-resonance wavelengthat the same distance above the burner.

Fig. 2. Top: Cavity ringdown absorption spectrum near 388 nm at a height of 0.65 cm in the 25-torr F 5 1.07 CH4-O2-N2

flame, showing features of the CH B-X(0,0) band. The R [8] lines are marked by an asterisk. Inset: magnification of the CNB-X(0,0) CRD absorption feature in this spectrum. Bottom: similar CRD absorption spectrum for the 25-torr CH4-NO-O2-N2

flame, now dominated by the CN lines. The P [12] lines excited are marked by an asterisk.


possibly due to continuum absorption by NO2formed in the circulating exhaust gases in theburner chamber. Using the cross-section of Liuet al. [29] for NO2 absorption at 390.13 nm, wecalculate that a concentration of , 2 ppm in theexhaust gas in the cavity would produce thiseffect.

An analysis of the rotational level populationsfrom the CH(B-X) R-branch CRD absorptionspectrum shown in Fig. 2 can be used to deter-mine temperature. The absorption coefficientsfrom Luque and Crosley [25, 27] used here havebeen shown to give accurate temperatures inLIF flame experiments [30]. The temperatureobtained from a Boltzmann plot, by using inte-grated absorption areas, is 1708 6 30 K at adistance of 0.65 cm above the burner. This is inexcellent agreement with CRD measurementsusing the CH A-X(0,0) Q branch and with LIFvalues [7].

Number density profiles were taken withCRD for the unseeded flame by using the CHB-X(0,0) R2 [8] line and the CN B-X(0,0)P-branch bandhead. These two spectral featureshave nearly temperature independent cross-sec-tions between 1000 K and 2000 K, and thus theirabsorbances provide good representations ofthe species spatial distributions. In the case ofCN, the signal-to-noise ratio is improved by afactor of ;3 by using the bandhead instead ofone of the resolved rotational lines (which weuse for absolute concentration determinations).The final CRD absorption concentration pro-files vs. height are the result of the differencebetween a burner height scan taken with theexcitation laser on the absorption line and onewith the laser tuned off resonance. This proce-dure is important for CN(B-X), because theabsorption losses are very small and changes inthe background ringdown time at different lo-cations in the flame can alter the apparentspatial distributions; the L term in Eq. (1) variessomewhat with height.

The CRD absorption results are shown inFig. 3, with no corrections made for the laserlinewidth overlap multi-exponential, and assum-ing a completely flat flame of 6-cm diameter.The lines give the predictions of flame modelcalculations using the Sandia Premix code [31]and the GRI-Mech 3.0 kinetic mechanism [24].Compared to LIF results for CH in this flame

from current and previous studies (and to themodel) [1–3], the raw CRD absorption CHconcentration (d 5 6 cm and no lineshapecorrection) is ;30% lower and peaks 1 mmhigher above the burner. Roughly one third ofthis discrepancy will be removed by the line-shape factor correction. Spatial factors exam-ined below will account for most of the remain-ing difference.

The uncorrected CRD absorption native CNpeak concentration of 1.1 3 109 cm23 (8 ppb) at0.65 cm with a 0.45-cm FWHM agrees well withthe model prediction of 1.04 3 109 cm23 at 0.61cm with a 0.42-cm FWHM. This agreementwithin an approximate 30% error bound con-firms the model expectations. The laser band-width correction for the native CN CRD mea-surement is negligible for two reasons. Whenthe background ringdown time and the absorp-tion ringdown time are very similar, non-lineari-ties are greatly reduced and the two decays arewell described by single exponentials, as wasobserved and discussed by Thoman and McIlroyfor 13CH in flames [6]. In addition, we excite ata bandhead where overlapping transitionspresent a broader absorption feature. The de-tection limit for CN is at least 10 times moresensitive than CH, due to larger absorptioncoefficients. This increased sensitivity facilitatesthe detection of trace amounts of CN. Theestimated detection limit is 2 3 108 cm23 CN at1700 K in our system, at the ppb level, and

Fig. 3. Uncorrected CRD absorption absolute CH (F) andCN (E) profiles vs. distance above the burner for the 25-torrF 5 1.07 CH4-O2-N2 flame. Lines are the predictions of aone-dimensional premixed flame model using the GRI-Mech 3.0 mechanism. CN results are multiplied by 1000.


represents only 5 3 106 per quantum state. Thismight translate to a limit of 50 parts per trillionCN at atmospheric pressure, depending onbackground absorbers and path length.

Spatial Distributions

Two-dimensional spatial distributions of CHand CN were examined with planar LIF. Theflame front displays a non-negligible curvature,which has an effect on line-of-sight measure-ments like CRD. Results for the 1% NO in N2flame are shown in Fig. 4, calibrated to absoluteamounts by using the CRD absorption measure-ments at 0.65 cm above the burner, and includ-ing laser bandwidth corrections. CH was excitedwith the R2 [8] line. CN profiles were measuredusing excitation at the P bandhead. The CNCRD absorption absolute measurement usedthe P1(12) and P2(12) lines, which are separatedby 0.1 cm21 and have 0.15 cm21 Doppler line-widths at the 1700 6 200 K flame temperatureswhere CN is present. From a full simulation of

the convolution of the laser bandwidth with theoverlapped lines, we determine an overlap inte-gral value of G(n)/Dn 5 4.0 6 0.3 cm.

The results in Fig. 4 confirm the nominal6-cm flame diameter, but clearly show a declineat the edge of the flame as well as the evidenceof curvature. Some inhomogeneity is evidentnear the peak amounts, largely from noise onthe edge of the color scale transition. Thedistributions demonstrate the necessity of con-sidering spatial corrections to interpret theCRD results.

From this combined PLIF-CRD data, onecan derive vertical absolute concentration pro-files at the center of the burner. Horizontalintegration of the LIF signals produces verticalline-of-sight LIF profiles of CH and CN, whichcan be used to examine the spatial CRD correc-tion factors associated with the lack of a perfect6-cm diameter one-dimensional flame. Figure 5compares profiles from the CRD absorptiondata, from horizontal integrals of the PLIFsignals (labeled line-of-sight LIF), and from

Fig. 4. Planar images of absolute CH (top) and CN (bottom) concentrations above the burner for the 25-torr F 5 1.07CH4-NO-O2-N2 flame, derived from linear images of the LIF and the CRD absorption data taken as a function of heightabove the burner. Nominal 6-cm burner diameter is shown, and corrections for laser bandwidth effects have been applied tothe CRD data.


point LIF profiles taken in the usual fashion atthe center of the flame over a 0.5-cm length(labeled point-center LIF). The line-of-sightintegrated LIF and CRD absorption profilesagree as expected. Figure 5 also shows thecenterline LIF data, calibrated by the PLIFprofile and line-of-sight CRD absorption con-centration measurement. Flame curvature shiftsthe line-of-sight CH structure outward by 0.7mm from the flame center peak, and widens itfrom 0.28-cm to 0.34-cm FWHM. An accuratespatial accounting and correction also increasesthe CH and CN peak concentrations by about20% above the integrated CRD absorption val-ues measured. CN has twice the CH width, andthus these spatial effects are less important;corrections to peak positions and widths areonly 10%. The near identity of the line-of-sightLIF and CRD profiles in Fig. 5 implies thateither can be used, when calibrated or correctedby the appropriate CRD or LIF measurements,to determine flame center concentrations vs.distance above the burner.

Figure 6 shows the absolute vertical profilesof CH and CN in the center of the seeded flameafter all the LIF-CRD factors are considered,fully corrected for spatial distributions and line-width factors. The CH results show excellentagreement with the predictions of the GRI-

Mech 3.0 model [24], and are also consistentwith earlier determinations by using Rayleighand Raman calibrated LIF [1–3]. Determina-tions performed by using CRD absorption onthe CH A-X(0,0) band gave similar agreementwith the earlier calibrated LIF data [7]. Thepeak CH amount is reduced 8 6 2% uponsubstitution of the 1% NO in N2, which agrees

Fig. 5. Comparison of three CH (left) and CN (right) profiles vs. distance above the burner for the 25-torr F 5 1.07CH4-NO-O2-N2 flame: LIF from the center of the flame (point center, dashed lines), LIF of Fig. 4 summed along the laserbeam (line of sight LIF, solid lines), and CRD absorption (dots).

Fig. 6. Final absolute CH (F) and CN (E) profiles vs.distance above the burner at the center of the 25-torr F 51.07 CH4-NO-O2-N2 flame, obtained by normalizing theLIF signals with the lineshape corrected and spatially cor-rected CRD absorption values. The CN values have beenmultiplied by a factor of 10. The lines show the modelpredictions using GRI-Mech 3.0.


with the model calculations (also an 8% reduc-tion), and is consistent with the observations ofWilliams and Fleming [11] for a similar seededflame. The CN model results for this reburnsystem predict the peak concentration very well,within the error limits. The model CN peakposition and profile decay are each about 10%earlier or faster than observed. The results arenot consistent with the finding by Juchmann etal. [14] that models significantly overpredicttheir measured CN in a 10-torr CH4-O2 flameseeded with NO. Their CH observations arealso overpredicted, although to a lesser extent.

Figure 7 shows the absolute CH and CNvertical profiles at flame center for the un-seeded flame, obtained from the CRD andPLIF data fully corrected for spatial distributionand linewidth factors. For these low native CNamounts, we did not obtain PLIF images due tolow signal levels. Instead we used the line-of-sight and flame center LIF distributions of CNin the seeded flame, along with the unseededCRD absorption, to determine the unseededflame absolute CN concentrations. This shouldintroduce minimal additional uncertainty, dueto the similarity of the two observed CN CRDspatial distributions. Model results using GRI-Mech 3.0 are also shown. Excellent agreementof the analyzed data is seen for CH, both peak

and profile, in accord with previous absoluteCH determinations for this flame. The CNmodel result matches the measured 9 ppb peakamount and also fits the observed FWHM andbroad peak position. This constitutes agreementat the estimated uncertainty limits. Models us-ing GRI-Mech 3.0 [24] also fit CN profilesexcellently in rich and lean 25-torr H2/O2/Arflames seeded with HCN [10], and predict CNprofiles well for 10-torr CH4/O2/Ar flamesseeded with NO or N2O [10]. Predicted de-creases late in the flames are slightly too fast[24].

In order to examine the chemistry being testedby the CN observations, sensitivity analysis calcu-lations [31] were also undertaken for the GRI-Mech 3.0 models [24] of the two flames. Thesensitivity coefficient Si 5 ln[CN]MAX/lnki

gives the fractional change in maximum CNconcentration in response to a relative changein a particular rate coefficient ki. Results for thepeak CN concentrations are shown in Fig. 8.The chief difference between the normal andreburn (NO-seeded) flames lies in the initiationsteps, CH 1 N2 vs. C or CH 1 NO. Similarsensitivities are recorded to other reaction setscontrolling CN chemistry, CH chemistry, andflame front location, respectively, as one movesdown the figure. Note however that reburn isaided by the H 1 CH 3 C 1 H2 reactionbecause C can initiate reburn but not promptNO. CH sensitivities were considered previ-

Fig. 7. Final absolute CH (F) and CN (E) profiles vs.distance above the burner at the center of the 25-torr F 51.07 CH4-O2-N2 flame, obtained by correcting CRD profilesfor laser lineshape effects and for spatial distributions usingthe LIF images. The CN correction assumes the samespatial distribution as in the NO seeded flame (see text).The CN values have been multiplied by a factor of 1000. Thelines show the model predictions using GRI-Mech 3.0.

Fig. 8. The largest sensitivity coefficients for CN at itsmaxima in the 25-torr F 5 1.07 CH4 flames, without(native) and with (reburn) 1% of the N2 replaced by NO.


ously, [1] and feature the reactions of Fig. 8 thatdo not involve CN.

From the good comparison of the CH CRDprofiles with previous Rayleigh calibrated LIF,we can conclude that the CRD method is capa-ble of providing low pressure flame concentra-tions of equal accuracy. Correction factors thatmust be considered differ—spatial distributionsand linewidth terms for CRD, vs. simplequenching lifetimes for calibrated LIF. For mo-lecular transitions with short fluorescence life-times, such as predissociating levels of CH(B)or the very strong CN(B-X) with its radiativelifetime of only 65 ns [25], it is hard to accu-rately measure quenching rates or use a shortgate to negate its influence. The CRD method isadvantageous for such situations.

CONCLUSIONS

In summary, CRD absorptions of CH(B-X) andCN(B-X) in low-pressure flames were measurednear 390 nm in low-pressure methane flames tostudy prompt NO formation and reburn. TheCN radical is detected by CRD at a peak densityof only 1.1 3 109 cm23, the ppb trace levelspresent in normal air flames associated withprompt NO. The agreement between CN con-centrations measured in unseeded and NOseeded flames and GRI-Mech 3.0 model calcu-lations is better than 20%, which further vali-dates the prompt NO and reburn mechanisms.

CH number densities from B-X(0,0) CRDare statistically equivalent to the CRD valuesobtained with the CH A-X(0,0) transition, andalso agree with those from LIF calibrated byRayleigh and Raman scattering. Analysis forabsolute concentrations by using CRD requirescorrections for laser bandwidth and consider-ation of the width and spatial inhomogeneity ofthe radical distribution. This latter factor ismeasured by using LIF imaging. Absolute two-dimensional concentration images are con-structed from the CRD calibrated LIF. TheCRD-LIF concentrations are as accurate asthose from the LIF-Rayleigh method, and CRDis particularly well suited for species with shortfluorescence lifetimes.

This work was supported by the Basic ResearchGroup of the Gas Research Institute, and by theNASA Microgravity Combustion Program Con-tract NAS3-99143.

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Received 26 January 2001; revised 28 May 2001; accepted25 June 2001


combined cavity ringdown absorption and laser-induced fluorescence imaging measurements of cn(b-x)...

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