AIAA-95-3114 Temperature and Mixture Fraction Profiles in Counterflow Diffusion Flames Using Linewise Raman Imaging

J. A. Wehrmeyer, S. Yeralan, and K. S. Tecu University of Missouri-Columbia Mechanical and Aerospace Engineering Dept. Columbia, MO 65211

31 st AIANASMUSAUASEE Joint Propulsion Conference and Exhibit

July 10-12,1995/San Diego, CA For permladon to copy or republish. contad the Amsrtcan instltuta of Aeronautic. and Aatronautlca 370 L'Enfant Promenade, S.W., Waehlngton, D.C. 20024

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d TEMPERATURE AND MIXTURE FRACTION PROFILES IN COUNTERFLOW

DIFFUSION FLAMES USING LINEWISE RAMAN IMAGING

Joseph A. Wehrmeyer: Serdar Yeralanf Kirk Tecut University of Missouri - Columbia, Columbia Missouri 65211

Abstract numerical efforts.

L

Temperature and major species concentration profiles in physical space are obtained in Hz/Nz ver- sus air opposed flow diffusion flames of various fuel dilutions. Three fuel jet compositions are examined: mole fractions of 21% Hz and 79% Nz, an equimolar mixture of Hz and Nz, and undiluted Hz. Mixture fraction and scalar dissipation profiles are derived from the major species concentration and tempera- ture profiles and clearly show effects of differential diffusion and strain rate. Measured peak tempera- tures agree well with numerical results of others, ex- cept for low strain rate, undiluted fuel flames. To obtain the measurements, a Raman imaging system is used, capable of providing time averaged linewise measurementsof high accuracy (3%) and high spatial resolution (160 am). A collinear alignment of laser source and opposed jet burner axis is used to provide 32 separate measurement locations along a 2.56 mm portion of the flame’s centerline.

Introduction

Diffusion flames generally involve interactions between chemical reaction, diffusion, and convec- tion, with these interactions influenced by the flame’s characteristic strain rate. Several calculational ef- forts have been directed at understanding the in- fluence of strain rate upon diffusion flames. These efforts have modeled as the fuel CH41-3, and CO/Hza Though comparisons with experiments exist for CH4 flame^^*^ there exists little experimental H2 flame data with which to compare to numerical results.’.lO~ll Hydrogen has applications as a fuel for high speed combustion because of its high reaction rate, and as a fuel for clean combustion technology. Thus it is of interest to experimentally investigate la- minar Hz-air diffusion flames to complement previous

*Assistant Professor, Member AIAA tPh.D Student. Student Member AIAA

This paper describes experimental tempera- ture and major species mole fraction measurements, using Raman spectroscopy, for a range of different Hz-air diffusion flames. These flames are created in an opposed jet burner configuration where the fuel jet has one of three different dilutions of H2 with N2. These three are: X H ~ = 0.21, XN. = 0.79; X H ~ = 0.5, X N ~ = 0.5 (equimolar); and X H ~ = 1, XN. 7 0 (undiluted); where Xi is the mole fraction of specles i. Both inlet boundary temperatures are approxima- tely 300K. The three fuel jet conditions are chosen because they have been numerically investigated by others?-6 Strain rate information is experimentally obtained using laser Doppler velocimetry (LDV), prD- viding centerline axial velocity profiles. These velo- city profiles yield characteristic strain rate informa- tion using the method of Chelliah et al.’ Mixture fraction and scalar dissipation rate values are calcu- lated at various locations in the flame using only the major species information provided by the Raman system. Thus relationships between reactive scalars and mixture fraction are established experimentally.

Experimental CounterfIow Burner

In order to provide laminar flames at the high strain rates needed to see strain-induced effects in HZ flames, high jet velocities are necessary. To lower the Reynolds number and maintain laminar flow, small diameter nozzles are used. At high strain rates, Ba- lakrishan et a1.I0 have attributed deviations between their measured and numerically predicted flame ex- tinction data to turbulence caused by high Reynolds number conditions produced in their burner. Thus 5 mm dia. contoured nozzles are used in this work. Luna” has shown that as long as the nondimensional separation (separation distance/nozzle dia.) between the nozzles is less than 1.5 a stable flowfield results. This work uses a seDaration distance of 5 mm for a

tPh.D Student, Student Member AIAA *Copyright 0 by J . A. Wehrmeyer. Published by the Ame-

..-” ria” Institute of Aeronautia and Astronautics, In=. with

nondimensional separation of 1. A schematic of the burner configuration is

shown in Fig. 1. Coflow shrouds (20 mm dia.) of permission.

1 American Institute of Aeronautics and Astronautics .

intensified CCD is apparent from Fig. 2, where peak signals are at 3500 counts and the background noise level is only at the 10 count level.

The flame of Fig. 2 is produced by the folk- wing flowrates, Q,, of species i: Q H ~ = 6.8 elmin, Q N ~ = 6.8 e/min, and Qa,r = 9.8 P/min. This corre- sponds to an experimentally measured strain rate, a, of 3830 s-', determined from an experimental prc+ cedure described in a later section. Flowrates and experimentally measured strain rates for all the fla- mes examined in this work are listed in Table 1.

Calibration

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Major species number densities are obtained through their direct relationships to Raman signals. Temperature is obtained from the resulting total number density, using the perfect gas law, and 88-

suming a constant pressure flowfield. To transform the raw data of Fig. 2 into tem-

perature and major species concentration informa- tion, the following calibration is performed, similar to that used in reference 14. The o p p d jet nozz- les are separted to enable a Hencken multi-element burner to be inserted between them and flames of several stoichiometries are established sequentially using the burner. The composition of the post flame zone is predicted by assuming chemical equilibrium and measuring the flowrates of Nz, 0 2 , and Hz to the burner. Flame temperatures are measured using a series of uncoated 50 pm diameter Pt3O%Rh vs. Pt6%Rh thermocouples. Various bead sizes are used to allow radiation correction using the method of Nich~lls.'~ Catalytic effects should be negligible in the post flame region used for calibration.

Figure 3 shows the calibration procedure re- sults, plotted according to the elemental hydrogen mixture fraction, &, defined as:

u

low velocity Nz dilute the excess fuel to prevent, or at least hinder, its combustion. A horizomtal confi- guration is used to allow the combustion products to exit around the shrouds, lessening their heating, and to provide a more efficient optical system. Buoyancy effects should be negligible in the momentum domi- nated flow near the burner centerline.

Raman Diagnostic System

The Raman diagnostic system, shown schema- tically in Fig. 1, is described in detail el~ewhere.'~ Some important attributes of the system that make it amenable to this particular application include using a pulsed, frequency doubled Nd-YAG laser of rela- tively low pulse energy (300 mJ/pulse). This allows tighter focusing of the beam, producing better spa- tial resolution. To account for the resulting low signal levels, multipulse averaging is performed. This is ac- complished directly on charge-coupled device (CCD) by using an unintensified, cryogenically cooled model. The absence of the intensifier's amplification allows a more efficient use of the wide dynamic range provi- ded by the 14 bit A/D circuitry of the CCD's control- ler. Cryogenic cooling is necessary to eliminate dark noise as a significant noise contributor when multi- pulse averaging is accomplished on the CCD itself. Signal from 200 laser pulses is accumulated on the CCD with this number somewhat arbitrary, being a compromise between maximizing signal levels (7000 counts for 300K pure Hz) and minizing data collec- tion time (20 sec using the 10 Hz laser).

The Raman system is aligned so that the laser beam passes through a window and into one nozzle, down its centerline, through the flame, and into the other nozzle, finally passing out through another win- dow. In this way complete information is obtained for the flame without sequential measurements. This configuration also provides higher spatial resolution, since a laser intercepting the flame transversly to the flame axis can only resolve flame structure down to the the beam waist dimension, whereas a collinear alignment of beam and flame axis allows resolution down to smaller dimensions. This is described in a later section.

Raman Flame Spectra

An example of the raw data obtained from the Raman system is shown in Fig. 2. Here it is seen that the imaged length of the laser beam is divided into 32 separate measurement locations, each corre- sponding to an imaged laser beam length of 80 pm. The usefulness of the wide dynamic range of the un-

where ZH is the mass fraction of elemental hydrogen, ZH,O is ZH in the oxidizer stream, and ZH,F is ZH in the fuel stream. For the display of calibration results ZH,O is assumed 0 and ZH,F is assumed 1. Since minor species are not measured <H is only estimated, though to a good approximation, by the equation:

Examination of Fig. 3 shows good agreement W

between measured and calibration conditions for all

2 American Institute of Aeronautics and Astronautics

major species and temperature. Error bars are shown for both calibration and measurement data: Calibra- tion uncertainties are due to flowmeter uncertainties (1% of flowmeter full scale values). Measured values are subject to shot noise, which results in typical un- certainties of 3% in temperature and mole fraction measurements.

SDatial Resolution

4 These reduced data are shown in Fig. 5. To aid in describing the qualitative changes in the profiles as strain rate changes, reduced data are also shown for another equimolar flame where a = 1250 s-'. By comparing the two sets of data it can be seen that as strain rate increases there is an increased spatial gradient for the 0 2 and Hz profiles. The resulting re- actant breakthrough and incomplete combustion cau- ses the depression in the peak temperature and peak HzO mole fraction values. Thus qualitatively Fig. 5 shows the expected trends with increased strain rate: the thinning of the flame and its reduction in tempe-

One critical issue with proper measurement of thin diffusion flames is the spatial resolution of the measuring system. The laser beam is focused to a

rature beam waist of - 500 Nm and thus provides a cylin- drical sample volume whose radius-is 250 pm. -The axial spatial resolution is determined by the binning of the CCD, which provides 80 pm of resolution, but is also affected hy the focusing characteristics of the W a n detection system and the source depth. The source depth in this case is 250 pm in either direc- tion, which when coupled with the F/2 colllection optics adds an additional 60 pm to the axial spatial resolution.

The degree of imperfect focusing of the optics is determined by inserting a 15 pm dia. Sic fiber into the opposed jet flame while the laser is off. Figure 4 shows the image of the minimum and maximum imaged wavelengths and at six different axial locati- ons for the fiber (the figure is for a composite image of six individual images). With the CCD comple- tely unhinned, the image of the glowing fiber has a linewidth corresponding to an additional 40 pm of spatial resolution. Convoluting all of these contribu- tions t6gether results in an overall spatial resolution in the Raman measurements of 160 pm. Figure 4 also shows that the linewidth remains essentially constant at the six flame locations, which indicates that image blurring and apparent location shifting due to index of refraction gradients does not degrade system per- formance at any location in the flame, as has been reported for Raman measurements where the laser is transverse to the burner axis.I6 Thus the spatial re- solution is 160 pm for any flame location and there is a linear relationship between CCD pixel location and flame location.

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Results and Discussions

Mixture Fraction Profiles in Phvsical SDace

Using Eq. 2, the mole fraction information di- played in Fig. 5 can be transformed into t" profiles. Figure 6 (for the 3830 s-l case) shows that the prc- file of & versus axial location behaves monotonically through the flame.

Scalar dissipation rate, x, information can also be derived from the major species data. The defining equation for ,y is:

(3)

where x is the axial coordinate and D is an appro- priate diffusion coefficient. For this work D is amole- weighted average using all four major species diffu- sion coefficients." Diffusion rate data for each spe- cies is obtained by assuming it diffuses into N2.l' The plot of x versus axial location, also in Fig. 6, peaks at - 1000 s-l near the location for the stoichiometric value of (H assuming equal diffusivities (&= 0.304).

In order to reduce noise in the x profile, a 5 point central differencing technique is used to deter- mine the gradient of &. Therefore the spatial resc- lution in measurements of x is greater than for the major species, temperature, and (H measurements, but this does not greatly alter the peak value of ,y since it changes slowly across the flame.

Mixture fractions can be based on the other two elemental species as well as elemental hydrogen. To see the effect of differential diffusion, profiles of mixture fraction based on elemental nitrogen, &, and on elemental oxygen, €0, are obtained by using Eq. 1 applied to the appropriate element. Again a5 for

Reactive Scalar Profiles in Phvsical Srmce &I, Eq. 1 is approximated in a good fashion by using only the major species information.

By using the calibration shown in Fig. 3, the raw Raman data of Fig.2 are transformed into tempe- rature and major species concentration data that are functions of axial location in the opposed jet flame.

Figure 6 shows these two mixture fraction pro- files, both of which differ from the EH profile, indica- ting differential diffusion plays a role in the structure of this atmospheric pressure flame. On the air side

-Ci

3 American Institute of Aeronautics and Astronautics

of the flame both of the new mixture fraction profi- les drop to slight negative values due to differential diffusion of HzO with respect to both Nz and 0 2 .

On the fuel side of the peak temperature location (- 1.2 mm) the three ( profiles intersect and the faster diffusion of Hz over either Nz or HzO depresses the (H profile below the other two. In fact the IN profile achieves values above one. These “overshoots” of the <N and (0 profiles at the physical ends of this equi- molar flame are expected to occur based on numerical predictions for a Tsuji-type counterflow flame.4

The numerical work of reference 4 also pre- dicts that diluting the fuel jet to XH. = 0.21 will cause the (H profile to no longer be monotonic in physical space, but instead have a characteristic S shape. This is indeed seen experimentally by the data shown in Fig. 7, which is for a flame with a fuel jet of X H 2 = 0.21, X N ~ = 0.79, and astrain rate of 1050 s-l. The (H profile is non-monotonic and dips downward on the rich side of the axial location (- 1.3 mm) for the stoichiometric value of [H (0.61) for equal diffusivities. The peak value of x occurs near the stoichiometric location for equal diffusivities but drops down to insignificantly low values (below the noise level) where the (H profile becomes non- monotonic, and then rises again on the fuel jet side of the non-monotonic region. Overshoots in the EN and (0 profiles, though smaller than for the previous flame, are predicted to occur for this flame4 but they cannot be discerned in the profiles of Fig. 7, probably because of the noise level of the experimental data. The probable reason for the (H profile not ending at one on the rich side is because the Hz flowmeter may have been providing a higher flowrate than indicated, though still within its error specification. Reference to Table 1 shows the low Hz flowrates used in these XH> = 0.21 flames.

Profiles (H and x for an opposed jet flame fue- led by an undiluted fuel jet are shown in Fig. 8. Here the important differences between the ( profiles oc- cur at low values. The stoichiometric value of (H for equal diffusivities is 0.0283 and is located in physical space at x w 0.9 mm. Profiles of ( exhibit deviati- ons due to differential diffusion on the lean side of stoichiometric. For example, (N drops below (H from about 0.5 mm to 1.0 mm. In addition the [o profile at first drops below the other two profiles from 0 to 0.3 mm but rises above from 0.4 to 1.0 mm. All of these traits between the three profiles have been nu- merically predicted by others, though for a flame of significantly less strain rate!

Experimental discrepancies between the three ( profiles of Fig. 8 at high values of ( are due to

- a systematic error in the calibration of the Raman system. The Hz S(11) rotational Raman line, l e cated at 609 nm, interferes with the H2 Stokes Q- branch at 607 nm, producing a crosssensitivity that results in a slightly higher measure of Nz mole frac- tion when there is a signicant amount of Hz present. Even though the increase in Nz mole fraction is only 2%, it is still enough to discernably reduce the values of (H and EN. For the other fuel jet exit conditions this error results in insignificant changes in the ( pro- files.

Velocitv Profiles and Strain Rates

In order to arrive at a good estimate of the strain rate for the flames examined in this work, ex- perimental velocity profiles are obtained using a one- component laser Doppler velocimetry (LDV) system. The LDV system was operated in the forward scatte- ring mode and was aligned perpendicular to the bur- ner axis. Seeding was provided by l pm nominal dia. A t 0 2 particles. Strain rates imposed on the flames are determined from the slope of the experimental data just on the air side of the flame.’

Figures 9 and 10 show the axial and radial ve- locity profiles obtained in the two flames of Fig. 5. Radial mean and radial rms velocities are generally below 0.1 m/s everywhere along the burner center- line. Axial rms velocity values were also below 0.1 m/s except near the stagnation plane, due to low data rates there.

Figures 11 and 12 show axial velocity profiles for the other two fuel stream conditions examined in this work. Figure 11 shows the axial velocity profile along the centerline of a flame with a fuel jet of 21% Hz and 79% Nz. The peak temperature of this flame is considerably less than for the equimolar case, re- sulting in the reduction in the velocity profile’s flame zone dip compared to those in Figs. 9 and 10. Just the opposite trend occurs for the profile of Fig. 12, which is for a flame with a fuel jet of undiluted Hz. In this figure the flame zone is markedly broader and the profile is greatly affected by the significantly hig- her temperature of this pure fuel flame.

The horizontal axes of Figs. 9 thru 12 do not correspond to any specific reference, such as nozzle location or stagnation plane. In fact, they cannot not be compared to one another in an absolute fashion for the translation stage was reset to a different zero value for each velocity profile traverse.

In addition to these four flames, four other fla- mes were examined using LDV. An effort was made for these eight flames to represent the low, middle, and high values for the strain rates experienced by

4 American Institute of Aeronautics and Astronautics

the three different types of fuel jet exit conditions. A high strain rate measurement €or the XH. = 0.21 flame was not obtained due to problems with flame extinction occuring when this flame was seeded.

The relationhip between experimentally mea- s u r d strain rate and air jet flowrate was determined to be approximately a direct one when all 8 flames are considered together. To arrive at estimated strain

in physical space. With the increase in strain rate there is an increase in peak XM, rising from - 200 SKI to - 500 s-l as the strain rate increases from 380 s-' to 1550 s-'. These peak values occur near tM = 0.41, on the lean side of (M for stoichiometric, equidiffusive conditions.

Eauimolar H,/N-, Fuel

L./

rates for all other flames, linear interpolation or ex- trapolation is used. All strain rates are displayed in Table 1, where the experimentally measured strain rates are indicated with an asterisk.

Reactive Scalars in Mixture Fkaction Space

Figures 18 thru 22 show data for flames with equimolar ( X H ~ = 0.5, X N ~ =, 0.5) &/Nz fuel jets. For these figures & is used since it displays a mc+ notonic rise with increasing axial location, as shown in Fig. 6. Figure 18 shows four temperature profiles

d

Rather than display the temperature and ma- jor species information as functions of axial location, it is more beneficial from a comparison standpoint to plot them aa functions of (. Fuel with XH- = 0.21. XN. = 0.79

Recalling from Fig. 7 the non-monotonic be- haviour of (H, it is desirable to choose some other definition of ( for the independent variable. Dixon- Lewis and Missaghi4 found an appropriate variable by an arbitrary weighting of the three (i to produce a mean mixture fraction, (M, given by

(4)

which is the independent variable used for Figs. 13 thru 17. Figure 13 is a plot of temperature versus (M for dilute fuel jet exit conditions ( X H ~ = 0.21, XN> = 0.79). The four curves of Fig. 13 represent the lowest and highest strain rates examined, along with two in- termediate values of strain rate. Recalling that the stoichiometric value of 5 at equal diffusivities is 0.61 for this flame, it can be seen that temperature peaks very near this value of (M and that the most signi- ficant differences between the four curves occur near (M = 0.61. Figure 14 shows Xo, versus (M. The four curves drop more or less linearly down to a value of (m higher than 0.61. There is much more of a strain rate influence shown in the HzO profiles of Fig. 15 than for the 0 2 profiles. This is also observed for the Hz mole fraction profiles of Fig. 16, Here the reac- tant breakthrough can be observed by the increased Hz mole fraction value with increased strain rate for low values of &, Figure 17, showing scalar dissipa- tion rate information, plots XM versus <M, where XM is defined as in Eq. 3. except that the spatial gra- dient of <M is used. This value of scalar dissipation rate does not fluctuate to the extent of the one plot- ted previously in Fig. 7 since now <M is monotonic

American Institute of Ae 5

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for this type of flame, again two representing the low and high limits of the strain rates examined. Here the peak temperatures are higher than for the tem- peratures displayed in Fig. 13 since the fuel is less diluted. The decreased fuel dilution also results in a lower stoichiometric <H for equidiffusive conditions (&=0.304). The Xo, versus profiles of Fig. 19 show that again, 89 for the Xfi2 = 0.21 case, the pro- files approach zero at a value of & (- 0.40) that is greater than for stoichiometric, equidiffusive condi- tions, The effects of differential diffusion and strain rate can be seen in these profiles at very low values of EH with the almost vertical drop (due to differen- tial diffusion) of the low strain rate profiles becoming less significant with increasing strain rate. Also the increased reactant breakthrough arsociated with in- creased strain rate manifests itself in these profiles around <H = 0.4 by increasing the value of (H where XO, becomes 0, going from 0.35 for a = 390 s-' to 0.42 for a = 3830 s-'. The XH.O and X H ~ ver- sus t" profiles are shown in Figs. 20 and 21 and both display the influence of differential diffusion at low strain rate (with the peak XH.O values occuring on the rich side of <H = 0.304), and increased reac- tant breakthrough (with increased X H ~ values at (H c 0.304). Scalar dissipation rates, given in Fig. 22 for these equimolar flames, are generally lower than for their more dilute counterparts, for a given strain rate. For example, the equimolar flame for a = 390 s-' has a peak x of 100 s-' while the X H ~ = 0.21 flame for a = 380 s-'has a peak X M of over 200 S- ' .

Undiluted H7 Fuel

Figures 23 thru 27 show results for four fla- mes with undiluted ( X H ~ = 1, x~~ = 0) fuel jets, with strain rates ranging from 490 s-' to 1240s-'. The highest strain rate achievable was limited by the maximum flowrate (10 elmin) that could be accu- rately measured by the Hz mass flowmeter. While

mautics and Astronautics

this upper limit is far from the calculated extinction strain rate for these flames (> 8000 s-') it is still significantly high enough to produce strain-induced perturbations upon the results.

The temperature profiles in Fig.22 show a peak temperature of 2350 K for the lowest strain rate case (490 s-') with the peak temperature de- creasing to 1950 for a = 1240 s-'. These tempera- tures are significantly higher by about 100 to 300 I< when compared to the numerical predictions of Gut- heil and william^^*'^, and are still higher but by le ser amounts when compared to Dixon-Lewis et aL3t4

Figures 24 thru 26 show the three reactive ma- jor species mole fractions as functions of &. The insets in each show the influence of increased strain rate, an influence that only shows around the stoi- chiometric, equal diffusivity mixture fraction (CH = 0.0283). Reactant breakthrough can be seen in these undiluted flames with increased values of XO, for (H > 0.0283 and in increased values of XH. for (H < 0.0283 for increased strain rate. The peak value for X H ~ O drops from - 0.29 for a = 490 s-' down to 0.275 for a = 1240 s-'.

The profiles of x in Fig. 27 show an interesting feature around & = 0.0283 where there is a relative minimain the profiles for the lower strain rate flames. This dip in the scalar dissipation rate profile near the stoichiometric value of < has been predicted nu- merically for undiluted Hlair diffusion flames5*"7'9 and for CO/Hz/Nrair diffusion flames.8 The unusual feature is that this dip is experimentally measured in flames at relatively high strain rates. Numerical prediction^'^.'^ show the dip to he absent in ,y profi- les for strain rates above 400 s-' and yet it appears in the two experimental x profiles for 490 and 810 s-'.

Summarv and Conclusions

Temperature and major species concentration profiles in physical space were obtained in H?/Nz/air diffusion flames of various fuel jet dilutions by using a Raman imaging system capable of providing time

I/ which have been numerically predicted to O C C U I , ~ are seen in the experimental profiles.

Four flames for each of the three fuel jet con- ditions have been described in the previous sections. A total of 32 flames (13 for X H ~ = 0.21, 12 for X H ~ = 0.5, and 7 for X H ~ = 1.0) were examined. Rat- her than display all the data for this entire set, a summary of peak temperatures and peak XH.O va- lues measured in all these flames is presented (see Fig. 28).

The three peak temperature curves compare well to the numerical work of ~ t h e r s ~ - ~ except for the undiluted fuel jet flames at low strain rates, where the measured peak temperature is from 100 to 300 K hig- her than any of numerical predictions. Another dis- crepancy between the undiluted fuel jet results and numerical predictions is in the profiles of x . The ex- perimental profiles show a relative minima near (H = 0.0283 that is not predicted to occur at the high strain rates experimentally measured5~17~1s More in- vestigation of these undiluted fuel jet flames is nee- ded to determine if these discrepancies are associated with systematic experimental errors in determining characteristic strain rate and/or if modifications to numerical models are warranted.

Acknowledzement W This research is supported by the National Sci-

ence Foundation (grant CTS-9210988) and the Engi- neering Foundation (grant RI-A-91-06). We wish to acknowledge Greg Holscher and James Seaba for the help in obtaining the velocity data, Praveen Kumar for help in producing the figures, and Gerald Pellett for providing the nozzles for the opposed jet burner.

References

[l] Chelliah, H. K., C. K. Law, T. Ueda, M. D. Smooke, and F. A. Williams. 1990. 23rd Sym- posium (International) on Combustion (Combu- stion Institute, Pittsburgh) pp.503-511. - - - -

[2] Sick, V., A. Arnold, E. Diepel, T. Dreier, W. Ketterle, B. Lange, J. Wolfrum, K. U. Thiele, F. Behrendt, and J. Warnatz. 1990. 23rd Sympo- sium (International) on Combustion (The Com- bustion Institute. Pittsbureh) DD. 495-501. and

averaged linewise measurements of high accuracy (3%) and high spatial resolution (160 pm). To pro- vide the one-dimensional images, a cryogenically coo- led CCD was used, because of its higher detection efficiency and dynamic range over intensified CCD's .- - , ..

Flame 65: 137. that are typically used for Raman imaging." By transforming the mole fraction profiles in

physical space into three elemental species < Profiles, the effect of differential diffusion upon flame struc- ture is clearly seen. The characteristic S shape of the

in the and [H profiles for equimolar flames, all of

[3] Dixon-Lewis, G., T. David, and P. H. Gaskell. 1986. Axhiuum Combustionis 6: 3-20.

(H profile for the X H ~ = 0.21 flameand the overshoots [4] Dixon-Lewis, G., and M. Missaghi. 1988. 22nd Symposium (International) on Combustion (The

W

6 American Institute of Aeronautics and Astronautics

Combustion Institute, Pittsburgh) pp. 1461- d 1470.

[5] Gutheil, E. and F. A. Williams. 23d Sympo- sium (Internationol) on Combustion (The Com- bustion Institute, Pittsburgh) pp. 513-521.

[6] Darabiha, N., and S. Candel. Combustion Sci- ence and Technology 86: 67-85.

[7] Brown, T. M., S. Nandula, P. A. Skaggs, R. W. Pitz, G. L. Pellett, W. Roberts, L. G. Wilson, and K. M. Isaac.1994. AIAA paper 94-00226 presented at the 32nd Aerospace Sciences Me+ ting, Reno, NV, Jan. 10-13.

[8] Drake, M. C., and R. .I. Blint. 1988. Combustion Science and Technology GI: 187-224.

[9] Norton, T. S., K. C. Smyth, J. Houston Miller, and M. D. Smooke. 1993. Combustion Science and Technology 90: 1-34.

[lo] Balakrishnan, G., D. Trees, and F. A. Williams. 1994. Combustion and Flome 98: 123-126.

[ll] Trees, D., T. M. Brown, I<. Seshadri, M. D. Smooke, G. Balakrishnan, R. W. Pitz, V. Giovangigli, and S. P. Nandula. 1995. Com- bustion Science and Technology (in press).

[12] Luna, R. E. 1965. Ph.D. Thesis, Princeton Uni-

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Optics Letters 20: 934-936. 1131 Wehrmeyer, J. A., S. Yeralan, and K. S. Tecu.

[14] Cheng, T. S., J. A. Wehrmeyer, and R. W. Pitz. 1992. Combustion and Flame 91: 323-345.

[15] Nicholls, E. L. 1900. Physical Reuiew 10: 234-

[16] Law, C. K., C. 3. Sung, G. Yu, and R. L. Axelbaum. 1994. Combustion and Flome 98: 139-154.

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[17] Nandula, S. P., T. M. Brown, and R. W. Pitz. 1994. Combustion and Flame 99: 775-783.

[18] Bilger, R. W. 1980. AIAA Journal20 962-970.

[19] Gutheil, E., and F. A. Williams. 1989. Western States Section of the Cornbustion Institute, Pa- per 89-109.

Table 1. Volumetric flowrates of air, Ha, Na and strain rates for flames examined.

Qai,, elmin QH, , elmin Q N ~ , elmin a, 6-1

0.85 0.21 0.79 380* 1.06 0.26 0.97 470 1.27 0.31 1.15 560 1.47 0.36 1.35 650 1.73 0.42 1.58 760 1.95 0.47 1.77 860 2.11 0.52 1.91 930 2.40 0.59 2.15 1050* 2.63 0.64 2.37 1160 2.84 0.70 2.57 1250 3.03 0.75 2.73 1330 3.21 0.79 2.90 1410 3.54 0.86 3.20 1550

XH. = 0.21 XN. = 0.79

X H ~ = 0.5 X N ~ = 0.5 0.81 0.59 0.58 390 1.04 0.73 0.73 500 1.70 1.20 1.20 800 2.65 1.88 1.86 1250* 3.47 2.42 2.41 1620 4.27 2.98 2.99 1970 5.15 3.62 3.66 2350. 6.75 4.74 4.66 2930 6.85 4.75 4.73 2960 7.67 5.40 5.31 3230 9.13 6.43 6.43 3650 9.81 6.81 6.76 3830*

XH1 = 1 XN1 = 0 1.17 2.26 0.00 490* 1.34 3.14 0.00 550 1.85 4.00 0.00 740 2.04 5.79 0.00 810* 2.12 7.71 0.00 860 2.36 8.54 0.00 1000 2.73 9.86 0.00 1240'

7 American Institute of Aeronautics and .Astronautics

Doubled NdYng h e r

CL"

Fig. 1. Experimental Raman system.

"¶ A Xlll

2.50

W A W - E " m

Fig. 2. Opposed jet flame Raman spectra: X H ~ = 0 . 5 X ~ 2 = 0.5. a = 3830 s-l.

. 2500 ' i " ' fi TEMPERATURE

1.0 0

0 0.02 0.04 0.06 6.06 0.1 0.12 0.14 EWENTAL mDROGEN M W R E FRACTION

Fig. 3. Raman system calibration data Filled symbols indicate Raman data, open circles indicate calibration COllditiOns.

-

1 12000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Fig. 4. Intensity vs. position at the two extreme imaged wavelengths for six images of a heated 5 p n dia. Sic fib%.

2000

;;I s 1000 2

p

1500

E x

500

0

1 0.0 0.50 1.0 1.5 2.0 2.5 AXIAL LOCATION. rnm

Fig. 5. Reactive scalars in physical space for two equimolar H2/N2 vs. air flames at diffaent strain rates.

Fig. 6. €, &d H2A9 flame of Figs. - 2 and 5, a = 3830 s-l.

in physical space for the equimolar

W

8 American Institute of Aeronautics and Astronautics

. '

0 AXULVELOCIN RADIAL VELOCITY n

2 E 1 -

F t 0

0 .

W ' >

D

5 0 . **e-... .. ........... P ....... 0

0 -I 2 -1- dqoo~>oo" 0 4 '

0 SLOPE = 1250s

-2 '

4 0 AXULVELOCIN

RADIAL VELOCITY n -0

1 0'

yl

10' ! ZI

9 k? yl

* 10' 5

s P

100 -2 *.

lo" 0.0 0.50 1.0 1.5 2.0 2.5

AXIAL POSITION. mm

I

Fig. 7. 5 and X in physical space: x H 2 = 0.21, XNZ = 0.79, a = 1050 s-1.

Y F 0 t 0

0 .

W ' >

D

5 0 . **.-... .. ........... P ....... 0

0 -I 2 -1- dqoo~>oo" 0 4 '

. 0 - SLOPE = 1250s

Fig. 8.5 and x in physical space: X w = 1. X N ~ = 0, a = 1240 s-1.

Y

-2 I I

a - E .

c- 2 , ci -2-

> . W

0

0

0 0 AXlALVELOCllY x RADIAL VELOCrPl U

0 ' .................... 0

- 1 SLOPE - 38308

0

0 I . . . . . . -1.2 -0.7 -0.2 0.3 -6 . ' . . L . . . . . - I .7 AXIAL LOCATION, mm

Fig. 9. Axial and radial velocity measurements along centerline for equimolar flame of Figs. 2.5. and 6: a = 3830 -

0 -

-1

2 -2 2 6

2, . . . . .

.

-

AXIAL LOCATION. mm Fig. 10. Axial and radial veiocity measurements along centerline for equimolar flame of Fig. 5 a = 1250 s-1.

-1.5- -0.7 -0.2 0.3 0.8 -, .2

AXIAL LOCATION, mm

Fig. 11. Axial and radial velocity measurements along centerline for flame of Fig. 7 X H ~ = 0.21. X N ~ = 0.79. a = 1050 s-l.

e E

1 L . - - . . I

f 9 6 W >

0 - 1

SLOPE c 1240s 0

-3' . ' . " ' . . . . . . . . . ' . . . . -1.0 -0 .5 0.0 0.5 1.0 1.5 2.0

AXIAL LOCATION. mm

Fig. 12. Axial and radial velocity measutements along centerline for flame of Fig. 8: X H ~ = 1. XNZ = 0. a = 1240 s-1.

9 American Institute of Aeronautics and Astronautics

1400

1200

Y

;loo0 p: = k- 3 800

--0-650s-1 s 600

400

0 0.2 0.4 0.6 0.8 1

0.20

$ 0.16

< E 0.12

Y _I

0 2 0 . 0 8

I”

0.04

0.00

0 0.2 0.4 0.6 0.8 1

W

5, Fig. 16. HZ mole fraction vs. mean mixture fraction for four flames at different s w n rates: x H 2 = 021. xN2 = 0.79.

5, fig. 13. Taperatun VS. mean mixture fmxion for flames at different strain rates: Xu2 = 0.21. xN2 = 0.79.

d

d

0.20

z 0 0.1 5 L 2 y 0.10

0"

Y

0 x

0.05

0.00

0.0 0.20 0.40 0.60 0.80 1.0 ELEMENTAL HYDROGEN MIXTURE FRACTION

Fig. 19. 0 2 mole fraction vs. SH for four flames at different strain ratex X m = 0.5. XNZ = 0.5.

0.25

G 2 0.15

Y -I 0 5 0.10 0, 8

0.05

0.00 0.0 0.20

ELEMENTAL Fig. 20. H 2 0 mole fraction vs. tu for four flames at

0.5

0.4 z I- V 0

0.3 Y

Y -I : 0.2 1

0.1

0.0 0.0 0.20 0.40 0.60 0.80 1.0

ELEMENTAL HYDROGEN MIXTURE FRACTION Fig. 21. H2 mole fraction vs. €,H for four flames at different S ! X l h rates: xH2 = 0.5, xN2 = 0.5.

-1250 i' -2350 s- '

0.0 0.20 0.40 0.60 0.80 1.0 ELEMENTAL HYDROGEN MIXTURE FRACTION

Fig. 22. r vs. t H for four flames at different strain rates: xH2 = 0.5.-&2 = 0.5.

2000

Y

J I- ~ 1 5 0 0

s % yo00

W

500

0.0 0.20 0.40 0.60 0.80 1 .o ELEMENTAL HYDROGEN MIXTURE FRACTION

Fig. 23. Temperature vs. 5~ for four flames at different F$wiamteS: xH2 = 1.0. xN2 = 0.0.- __ _.-.

0.20

E 0.15 d lj 0.10

0"

y.

0 z

0.05 - 8 1 0 s ' ' -1oooi' -1 240s' '

0.00 0.0 0.20 0.40 0.60 0.80 1 .o

ELEMENTAL HYDROGEN MIXTURE FRACTION

Fig. 24. 0 2 mole fraction vs. €.H for four flames at different Sh;iin xH2 = 1.0, xN2 0.0.

11 American Inshituk of Aeronautics and Astronautics

0.25

0

v

L*

2 0

F 0.20

0.15

, ~ O . l O 5

0.05

0.00 0.0 0.20 0.40 0.60 0.80 1.0

ELEMENTAL HYDROGEN MIXTURE FRACTION

2200. Y

u' 2000

$1800

2 1600 I

1400 Y 3 1200

I "

1 .o 0.0 0.20 0.40 0.60 0.80 ELEMENTAL HYDROGEN MIXTURE FRACTION

~

~

~

1 ~

Fig. 25. H20 mole fraction vs. SH for four flames at different strain rates: Xm = 1.0. xN2 = 0.0. Fig. 27. x vs. SH for four flames at efferent strain

xH2 = 1.0, xN2 = 0.0.

0.0 0.20 0.40 0.60 0.80 1.0 ELEMENTAL HYDROGEN MIXTURE FRACTION

Fig. 26. H2 mole fraction vs. 5~ for four flames at different rateS: xH2 = 1.0, xN2 = 0.0.

2400r . 0.35

0.30 E "8

0.25

1000

800 10.10

0 1000 2000 3000 4000 STRAIN RATE, i'

Fig. 28. Experimentally determined peak temperatures and peak X H ~ O values vs. strain rate for three fuel jet casesexamined

c

12 American Institute of Aeronautics and Astronautics