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Comparison of UV raman scattering measurements in a turbulent diffusion flame with reynolds-stress model predictions
Twenty-Fourth Symposium (International) on Combustion/The Combustion Institute, 1992/pp. 287-294
COMPARISON OF UV RAMAN SCATTERING TURBULENT DIFFUSION FLAME WITH MODEL PREDICTIONS
MEASUREMENTS REYNOLDS-STRESS
IN A
F. LIPP, J. HARTICK, E. P. HASSEI, AND J. JAN1CKA Technicsche tlochschule Darmstadt Fachgebiet Energie- und Kraftwerkstechnik Petersenstr. 30 D-6100 Darmstadt Germany
Simnltaneous space- and time-resolved measurements of CO~, O2, Nz, CH4 and H~O have been made using spontaneous Raman scattering in a turbulent CHJNo diffusion flame. A narrowband XeCl-excimer-laser working at 308 nm and an intensified multichannel camera were used to give flail information about all major species and spectral background. Knowledge of the background is very important for measurement accuracy. This paper presents new experimental results containing concentrations, temperatures, mixture-fractions f, f-plots and rms of fluctuations of f in comparison to numerical predicted results obtained with a Reynolds-stress flame model. The agreement is very good in the rich zones and poor in the flame zones. Strong fluorescence background from OH radicals was found in the high temperature regions of the flame resulting in smaller numbers of evaluatable spectra.
Introduction Most practical combustion devices are turbulent, nonpremixed flames. The development of appropriate computer models is very important with respect to lower development costs, higher fuel efl~ciencies, and lower emissions. Such models should be based upon a fundamental, physical understanding of turbnlence and chemical reactions. Empirical constants used in the models depend on comparisons with experiments in nonreacting flows and flames. Measurements of as many as possible key model variables are required to test assumptions for modelling turbulence-chemistry interactions. Until now few comprehensive measurements, even in laboratory-scale nonpremixed flames, have been attempted because of the diMculties in obtaining these data with sufficient spatial resolution, without significantly perturbing the flame under study. Following the previous work of [1] in the last years there were some groups which intensively improved the Raman-scattering-technique for flame measurements and who applied it mainly for H2 flames and CH4 flames near extinction. 2'3'4 For further references see these and [5]. A Raman apparatus was built based mainly on a narrowband UV XeCl-excimer-laser, a spectrometer with high resolution and an intensified diode array camera. This gives full information about Raman Qbranchs of all major molecules and vely important
information about the spectral structure of the background. A single shot spectrum is evaluated using 700 spectral data from the camera by a computer fit with the aid of a temperature dependent molecular library of the spectral shapes of the Qbranchs. The experimental apparatus, data reduction scheme, computer program, and typical spectra were presented and discussed in detail in [6]. New experimental results are presented characterizing a CH4/Nz diffusion flame and containing concentrations, temperatures, mixture-fractions .f, fplots, and rms of fluctuations of f (= (f,,z)~/~) in comparison to numerical predicted results obtained with a Reynolds-stress flame model.
Experimental Arrangement The optical setup is shown in Fig. 1. A narrowband tuneable XeCl-excimer-laser (Lambda Physics) which produces UV light (308 nm, bandwidth 0.003 nm, 120 mJ, 20 ns) has been used. Measured pulse to pulse energy fluctuation is + / - 1 . 7 % of the mean value, thus no energy referencing was made. By adjusting the distance between and the tilt of the two focussing lenses LI and L2 the focus diameter and shape can be changed thus preventing optical break down. The resulting focus has an approximately circular cross-section. Raman photons are collected by two quartz lenses (I,3, IA) with di-
287
288
TURBULENT COM BUSTION--NONPREMIXED
EXCIHERLASER )*=308nm ~SCILLATOR
DAD. L : H : P ! SV :
y
DIOOE ARRAY DETECTO~ LENS HIRROR POLYCHROHATOR SAHPI E VOLUHE
Fie,. 1. Raman scattering experimental setup in an isometric projection.
ametcrs of 80 mm and with focal lengths of 150 mm and 360 mm respectively, the pesition of the lenses is adjusted for optimal signal by minimizing lens aberrations. An aluminum backscattering mirror rereflects Raman photons scattered away from the spectrometer (Spex, f = 500 ram, single grating 600 lines/mm, f# = 4.6). The spectrometer was mounted such that the entrance slit (0.3 mm * 2.7 mm) was parallel to the sample volume. The Raman-photons were received and converted by an intensified diode array detector (Spectroscopy Instruments, 700 intensified channels, each 0.025 m m * 2.5 mm, gate time 50 us). The overall spatial resolution was 3.0 mm * 0.15 mm * 0.15 mm. The spectral range registered by the detector in one picture is 56.0 nm which is sufficient to measure the vibrational Qbranches of all major molecules in a hydrocarbon flame from CO2 to H2. The burner of the free turbulent CH4/N~ diffusion flame consists of a tube with a diameter D = 6.0 mm and a co-flowing airstream with 0.5 m / s. The fuel stream exit parameters were: mass-fraction of CH4 = 0.3, measured temperature = 300 K, average velocity = 18.7 m/s, Re-number = 6950. The measured downstream levels normalized with the nozzle diameter were: x / D = 0.83, 5, 10, 15, 20,30, ..., 80.
Raman Data Reduction The data reduction scheme is based on linear Raman theory. 7 Resonant Raman effects were thought
to be small compared to other measurement inaccuracies. There is one exception, the relative Raman scattering cross section of oxygen measured at room temperature is enhanced by a factor of approximately 1.9. At constant total pressure the number of measured Raman photons Ni is connected to the relative number densities r of molecular species i with [7, 3, 6], N~ = AI'~r
- exp(-hcvtkT)))
(1)
were Ni [ - ] is the number of detected photons, A [K] an experimental constant, Fi [ - ] is the relative Raman scattering cross-section, r [ - ] the relative number density, T [K] the temperature, h [Js] the Planck's constant, c [m/s] the speed of light, k [J/ K] the Boltzmann's constant, vi [l/m] the Raman shift wave number and i the molecular species i. If all major molecules are measured the sum of the relative number densities must be equal one. r = I
(2)
i
This yields an implicit fl~rmula for the temperature. Once temperature is known determination of concentrations is done using formula (1). The Fi factors were measured at room temperature or at boiling temperature for water. They include chromatical and other transmission effects of the optics. The data reduction formalism consists of the following steps: 1.) A correlation routine ensures the wavelength position of the experimental spectrum
XeCl EXCIMER LASER RAMAN SCATI'ERING MEASUREMENTS always to be the same. 2.) The background is calculated by a simple function which is fitted to the spectrum. This is a very important step in the program since all results crucially depend on the background fit. 3.) The chosen background function is subtracted from the experinaental spectrum. 4.) Determination of integrated counts of the peaks of different molecules with the help of a previous established temperature dependent library of spectra for each molecular species, including correction of the interference from CH 4 on Oz. 5.) From all counts and all /'i factors, temperature and concentrations are calculated using Eqs. (1) and (2). 6.) Since the measurement of the entire flame consists of 35000 spectra it is not possible to look for those spectra which are misinterpreted by the computer code. To get a measure about the quality of the analysis of a spectnnn, a theoretical spectrum is composed from the resulting concentration-, temperature- and background data using the spectrum library and compared to the experimental spectrum by a least square sum. If this error sum is higher than a certain limit, the spectrum is ignored for filrther evaluation. This has to be considered seriously to avoid biasing of statistics. At each flame location 100 spectra were taken. Some of these spectra show very strong OH-fluorescence which is induced by the laser (OH-LIF) although the laser was very carefully locked to a wavelength which gives minimum fluorescence. Up to now these spectra could not be evaluated and were neglected for statistical evalnation. As the OHconcentration is correlated to temperature the average temperature is expected to be too low in flame regions containing high concentrations of OH. For comparison purposes single-shot data were averaged applying Favre formalism.
Theoretical Calculations The mathematical model consists of submodels for turbnlence (characterization of turbulent flow fields), comlmstion (description of momentary chemical reaction) and coupling (interaction of combustion and turbulent flow).
The Turbulence Model: The Reynolds-stress-model for Favre-averaged quantities consists of equations for continuity, mean veh)city, Reynolds-stress-tensor and dissipation. In [8] a detailed description of the closure of the nonclosed pressure-rate-of-strain correlations, the third order moments in the Reynolds-stress-equation, and the source term of the dissipation equation is presented. The constants used here are taken from model 1 in [81. The main difference between this model and standard Reynolds-stress-models~ is the
289
invariant- and turbulent Reynolds-number-dependent formulation of the retunl-coefllcient cm and the function ~0 in the source term of the dissipation equation. This should yield to a greater universality of the turbulence model.
The Combustion Model: Turbulent Damk6hler-numbers greater than one typically present in diffusion flames suggest the assumption of an infinitely fast one- step reaction. Damkfhler-numbers of the investigated flame are estimated to be 0.8 near the nozzle to 7.8 at x/D = 50 by [10]. If the radiation is neglected the momentary composition of the mixture and its temperature can be calculated by chemical equilibrium under adiabatic conditions. This results in a functional relation between momentary fuel-oxidator mixture and the momentary composition and temperature of the mixture. Because of low strain rate, interest mainly focussed on positions x/D = 30 to 80 and low observed CO-concentrations (see Numerical Solution) an equilibrium model is more appropriate than a strained laminar flame model.
The Coupling Model: Using the Svab-Zeldovich-formalism, linear combination of the pde's for the chemical components results in the mixture-fraction equation [11]. To derive the statistical moments of the mixture-fraction dependent variables, the mixture-fraction's probability density fimction (pdf) must be known. Here the B-function c_alculated with the Favre-averaged mixture-fraction f and variance of the mixture-fraction f,,z is used to approximate the real pdf. 12 Integration of the product of the functional relations provided by the combustion model and the mixture-fraction pdf results in the Favre-averaged mean temperature and mean mass-fractions of the chemical components. The partial differential equations for f and f,,2 contain velocity mixture-fraction correlations u'[f". For a consequent second moment closure pde's for these terms are integrated in the mathematical model. The complete Reynolds-stress-model contains pde's for: continuity, mean velocity tii, Reynolds-stressir
Fr
tensor ui ui mean mixture-fraction
mixture-frac-
tion variance f,2 and velocity mixture-fraction correlations u}'f".
Numerical Solution: The numerical solution was carried out with an improved version of the Patankar and Spalding program. 13 The boundary conditions are taken
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FIG. 2. Comparison of experimental and theoretical axial decay of centerline mixture fraction fc, jet spreading rate as ros/(lOD) = radius(fc/2)/(lOD), centerline temperature Tc and fraction of evaluated spectra r versus normalized flame axis x/D. ~ is valid only for each axial point and particularly not for the entire level. Lines: prediction. as follows: mean velocity is estimated with the assumption of a developed turbulent pipe flow, mean mixture-fractions were set to equal one except at the last gridpoint, which was set approximately to zero, Reynolds-stress-components were approximated by measurements of Laufer in [14] in developed turbulent pipe flows, mixture-fraction variances were set to be 0.2 * 10 -4, velocity mixturefraction correlations u"f' were set proportional to U tt l) tt. Chemical equilibrium is calculated by minimizing of the chemical potential yielding temperature, composition, density and viscosity as functions of mixturefraction. The components N~, Oa, HaO, O, H, OH, COa and CH4 were taken into account. Neglection of Ha and CO seems to be justified because the Raman measurements determined mole-fractions less than 0.8% of these components and the calculations of Ha and CO in adiabatic equilibrium yields much larger concentrations of Ha and CO in fuel rich regions.
u"u" and v"]~' proportional to
Results and Discussion In Fig. 2 the correspondence of Jfc decay is very good. Differences in the spreading rate mainly at x/D = 50 and x/D = 60 levels can be explained by interferences from OH-LIF. The predicted temperature is equal to the measured at lower levels up to x/D = 30 and is approximately 250 K higher above. The temperature differences may be explained by high intensity of OH-LIF, spatial resolution and the slight overprediction due to neglection of radiation.
FRACTION
f
FIG. 3. f-plots of concentrations in form of mass fractions of the species versus mixture fraction f determined from the C atom ratio. The concentrations are added to a constant to get all curves in one figure. Each dot is the result of a single shot Raman spectrum but for clearity of the picture the values of only every tenth spectra out of all spectra from the entire flame (x/D = 0 to 80) are plotted. Lines: prediction. In Fig. 3 and 4 f-plots for all x/D are to be seen. The mixture-fraction was determined from C-atombalance, because OH-LIF interferes with the H20peak and so O- and H-atom based balances2 yields poorer results. Agreement is very good for lean (f = 0) and rich (f = 1) mixtures. Closer to the stoichiometric point scattering is a lot bigger and the number of evaluatable spectra is remarkably lower. There is small scattering for CH4 due to the relative high Raman scattering cross section. Oa-concentrations in the lean region were scattered around the predicted values whereas in the rich flame zones about two percent mass-fraction were measured al-
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FIG. 4. f-plot of temperatures versus mixture fraction f as described in fig. 3. Lines: prediction.
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N O R M A L I Z E D RADIUS ~7
FIG. 6. Favre averaged mass fractions of CO2, O~, n20, CI-I4 versus 7/at x/D = 30. Lines: prediction.
diction.
though they have to be zero. It was found that there were problems with the computer algorithm to separate the O-2/CO2 peaks especially if background is not linear. So measured O2-concentrations in the inner flame zones are made up artificially by the computer code and are not found in the spectra. For CO2 and for f = 0 to f-stoichiometric nearly all points are located at the theoretical line because the mixture-fraction is calculated from the C-atombalance. The differences between the data and equilibrium curves are due to OH-LIF and the strain-rate which is particularly more critical at upstream locations. Also at the rich mixture areas from x/D = 0 to 10 the turbulent Damk0hler-number is less than one and the assumption of chemical equilibrium is more critical. 15 The f-plot of H20 is disturbed by OH-LIF. All experimental temperatures (Fig. 4) are approximately 300 K lower than predicted, due to OH-LIF biasing, see discussion of Fig. 2. Figures 5 to 9 give results of temperatures, mixture-fraction f/fc, rms of fluctuations of mixturefraction f" /fc (~z)l/Z /fc and mass-concentrations X~ versus normalized radius "q at the beginning of the similarity region (x/D = 30) and where the flame is exhausted (x/D = 80). The differences at x/D = 30 in Fig. 5 and 6 are due to spatial averaging effects, OH-LIF and strain-rate. The Kolmogoroff length scale is about 0.8 mm at x/D = 30 and 3.5 mm at x/D = 80. As expected at x/D = 80 position measured values are in better agreement because of low OH-concentrations due to complete flame burnout and larger turbulent length scales, see Fig. 5, 8, 9. In Fig. 6 the concentrations on the axis are in good agreement except that O2 mass-fraction has to =
be zero, which was explained in discussion of Fig. 2. In Fig. 7 and 9 agreement of the mixture-fraction curves is quite good, although the number of evalutable spectra is small in the reaction zone (Fig. 7). At this location_ (7/ = 1), a biasing of statistic by magnifying off"/fc is expected. The slight increase may be caused by this effect. However, the overall continuous curve of f"/fc shows no strong influence. The experimental f"/fc in Fig. 9 lies in the same range of magnitude as the predicted. At the higher levels some spectra (approximately 30%) with fluorescence from soot or PAH were found. Most of them don't interfere with the evaluation as they show clear peaks which are separated in the spectra from the peaks of other species. Only very few spectra (1%) show a very strong continuum which masks all other peaks. Hence, soot 1.0 ~
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TURBULENT COMBUSTION--NONPREMIXED
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investigations seem to show that it is not possible to prevent the induction of OH-fluorescence with an well-locked narrowband and tunable XeCl-laser, because of the inevitable spectral background of the laser (ASE).6 It is difficult to correct the spectra for the OH-LIF because the OH-LIF is not equal to OH-emission from thermal equilibrium and it is very intense.
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Fie,. 8. Favre averaged mass fractions of CO2, O~, H20, CH~ versus r/at x/D = 80. Lines: prediction. or PAH-fluorescence is a minor complication in this diluted flame with our equipment (particularly the multi-channel-detector) as the peaks can be separated. As a rough estimation the error i~ single shot temperatures is 5%, in concentrations 3% at low temperatures and 10% at high temperatures, each of maximum value. This holds for spectra without and with low OH-LIF. If there is strong OH-LIF the spectra can not be evaluated, yielding a biasing in the pds of temperature and concentration conditioned by low OH-concentrations. The precision of f is dependent on many factors mostly on the OH-LIF interference and is estimated to be maximum 10% for the spectra with the largest amount of OH-LIF which were evaluated. The viability of this measurement technique will depend on the possibility to handle the OH-LIF problem, see [6], where this point is discussed in great detail. Our 1.0
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Acknowledgement This work was supported by Deutsche Forschungsgemeinschaft, contract number llD5-Ja544/ 1-1.
-
9
The experimental results presented in this paper demonstrate that XeCl-UV-laser excited spontaneous Raman scattering is a sensitive analytical tool for measurement of CO2, O2, N2, CH4, H20 concentrations and temperature. Comparison with equilibrium assumption calculations show satisfactory agreement between the calculations and the data concerning mixture-fraction averages and standard deviations. Because of the small strain rate, equilibrium assumption is sufficient. The main problem at present state of research is OH-LIF in the reaction zone. This leads to rejection of a great number of spectra resulting in serious biasing of the statistics, because the measurements are conditional on low OH-concentration. It may be beaten by changing the laser wavelength. The information obtained with these measurements is very important for a deeper physical understanding of flame phenomena and therefore very suitable for improving the combustion and coupling models. Such a complete description of thermodynamic state of a turbulent flame is at the moment not possible with any technique other than Raman scattering.
x/D
REFERENCES
0.6-
1. WIDHOPF, G. F. AND LEDERMAN,S.: A1AA-J. 9, 2, 1971. 2. MASRI, A. R., BILGER, R. W. AND D[BBt,E, R. W.: Combust. Flame 74 267, 1988. 3. PITZ, R. W., WEnBMEYEB, J. A., BOWLINe,,, J. M. AND CHENG, T. S.: App Opt 29, 15 2325
0.2-
,o,~;~,
0.o
0
~ NORMALIZED
:~
" RADIUS
:3
4 p~
FIC. 9. f/f~ and the rm,~ of fluctuations of the mixture fraction f"/J'c = (f,"~)~/2/]- versus "0 at x/D = 80. Lines: prediction.
(1990). 4.
5.
M . , D R A K E , M. C., P E N N E Y , C . M . A N D PITZ, R. W.: J. Quant. Spectrosc. Radiat. Transfer, 40, 3, 363 (1988). E C K B B E T H , A . C . : Laser Diagnostics for Combustion Temperature and Species (Abacus, Cambridge, MA 1988). LAPP,
XeC1 EXCIMER LASER RAMAN SCATTERING MEASUREMENTS 6. HASSEL, E. P.: UV Raman Scattering Measurements in F/ames Using a Narrowband XeCI Excimer Laser, Applied Optics, (submitted May 1991). 7. LONG, D. A. : Raman Spectroscopy, McGraw Hill (1977). 8. JnNICI
293
11. LOCKWOOD, F. C. AND NAGUIB, A. S.: Cornbust. Flame, 24, 109 (1975). 12. JANICKA, J. AND KOLLMANN, W.: AGARD-Conference Proceedings 275, (1975). 13. PATANKAN,S. V. AND SPALDING, D. B.: Heat and Mass Transfer in Boundary Layers, London Intertext books, (1970). 14. ROTrA, J. C.: Turbulente Str6mungen, Stuttgart, Teubner, (1971). 15. DRAKE, M. C., Prrz, R. W. AND LAPP, M.: Laser Measurements in Nonpremixed Hydrogen-Air Flames for Assessment of Turbulent Combustion Models, AIAA-paper NO. AIAA-84-0544, (1984).
COMMENTS G. M. Dobbs, United Technologies Research Center, USA. How well injection-locked was your laser? While you were careful to tune between OH absorption lines to minimize fluorescence, could the broadband background you recorded have arisen from the pedestal part of the laser output distribution? Some of the additional pro~esses which can lead to a broad background signal in UV Raman Spectra are laser modulated soot incandescence, fuel and PAH fluorescence, and optical breakdown. Could you please provide more information on how you were able to rule these out and attribute all of the background you observed to OH fluorescence?
Author's Reply. We acknowledge the importance of the question to assess the viability of the 308 nm Raman system. Therefore the basis of the question is an important part of the contents of Ref. 6. However, for the sake of completeness we answer very comprehensively. Laserlocking was greater than 98%, as demonstrated in Ref. 6, Fig. 7. With this laser it is nearly impossible to tune between OH absorption lines due to ASE laser background and the existence of many lines of many bands. Broadband background was identified as a superposition of a weak linear part from the laser output, from very strong OH LIF and from rare PAH soot LIF as shown in Fig. 6 in Ref. 6 .
Robert Barlow, Sandia National Laboratories, USA. (similar comment submitted by Donald W. Sweeney, Sandia National Laboratories', USA). 1. The authors conclude that they have achieved good S/N with this technique. However, scatter
plots show variations in major species concentrations that are much larger than expected for a fully connected flame (no extinction). Scatter is particularly large for CO2. Much of this due to the poor optical collection efficiency of the system. Is there also a particular problem with background subtraction and fitting for the CO2 spectrum. 2. Is 308 nm really a viable wavelength for Raman measurements if there is always a broadband component that produces significant OH fluorescence interference? 3. The scatter plots indicate that hydrocarbon fluorescence interference is not a problem in these nitrogen diluted methane flames. What is the explanation? Does nitrogen dilution eliminate interference or is interference present in the spectra and successfully handled by the data reduction procedure?
Author's Reply. 1. The scattering of the CO~ data is caused by the low efficiency of the receiving lenses and by the fact that the COz peaks are located at the same wavelength position as the 2-2 band of O H (see Ref. 6). 2. 308 nm is suitable for Raman spectroscopy in flames if it is possible to subtract the OH LIF background in an adequate way. After the submission of the presented paper a data reduction computer routine was developed and applied with success for the evaluation of Raman spectra from a H J N 2 diffusion flame. Experiments described therein show a maximum of 28% of not evaluatable spectra at one position. Changing the laser wave-length may be disadvantageous due to the possibility of inducing PAH and soot fluorescence or the difficulty of the description of resonant Raman effects. 3. Both suggested answers are correct. Firstly di-
294
TURBULENT COM B U S T I O N - - N O N PRE MIXE D
lution with N2 eliminates most of the hydrocarbon interferences occuring in the spectra from the higher x/D levels of an undiluted flame. Many of these spectra show complicated fluorescence structures especially for higher hydrocarbon flames. Secondly as mentioned in the paper approx. 30% of all spectra from the higher x/D levels of the investigated flame show hydrocarbon fluorescence. These spectra are well handled by the data routing thus demonstrating the advantage of us-
ing 700 informations from the multichannel system.
REFERENCE 1. LIPP, F., HARTICK, J., HASSEL, E. P. AND JANICKA, J.: Raman Measurements in a Turbulent H2/Nz Diffusion Flame; Applied Physics B; submitted 6/92.
COMPARISON OF UV RAMAN SCATTERING TURBULENT DIFFUSION FLAME WITH MODEL PREDICTIONS
MEASUREMENTS REYNOLDS-STRESS
IN A
F. LIPP, J. HARTICK, E. P. HASSEI, AND J. JAN1CKA Technicsche tlochschule Darmstadt Fachgebiet Energie- und Kraftwerkstechnik Petersenstr. 30 D-6100 Darmstadt Germany
Simnltaneous space- and time-resolved measurements of CO~, O2, Nz, CH4 and H~O have been made using spontaneous Raman scattering in a turbulent CHJNo diffusion flame. A narrowband XeCl-excimer-laser working at 308 nm and an intensified multichannel camera were used to give flail information about all major species and spectral background. Knowledge of the background is very important for measurement accuracy. This paper presents new experimental results containing concentrations, temperatures, mixture-fractions f, f-plots and rms of fluctuations of f in comparison to numerical predicted results obtained with a Reynolds-stress flame model. The agreement is very good in the rich zones and poor in the flame zones. Strong fluorescence background from OH radicals was found in the high temperature regions of the flame resulting in smaller numbers of evaluatable spectra.
Introduction Most practical combustion devices are turbulent, nonpremixed flames. The development of appropriate computer models is very important with respect to lower development costs, higher fuel efl~ciencies, and lower emissions. Such models should be based upon a fundamental, physical understanding of turbnlence and chemical reactions. Empirical constants used in the models depend on comparisons with experiments in nonreacting flows and flames. Measurements of as many as possible key model variables are required to test assumptions for modelling turbulence-chemistry interactions. Until now few comprehensive measurements, even in laboratory-scale nonpremixed flames, have been attempted because of the diMculties in obtaining these data with sufficient spatial resolution, without significantly perturbing the flame under study. Following the previous work of [1] in the last years there were some groups which intensively improved the Raman-scattering-technique for flame measurements and who applied it mainly for H2 flames and CH4 flames near extinction. 2'3'4 For further references see these and [5]. A Raman apparatus was built based mainly on a narrowband UV XeCl-excimer-laser, a spectrometer with high resolution and an intensified diode array camera. This gives full information about Raman Qbranchs of all major molecules and vely important
information about the spectral structure of the background. A single shot spectrum is evaluated using 700 spectral data from the camera by a computer fit with the aid of a temperature dependent molecular library of the spectral shapes of the Qbranchs. The experimental apparatus, data reduction scheme, computer program, and typical spectra were presented and discussed in detail in [6]. New experimental results are presented characterizing a CH4/Nz diffusion flame and containing concentrations, temperatures, mixture-fractions .f, fplots, and rms of fluctuations of f (= (f,,z)~/~) in comparison to numerical predicted results obtained with a Reynolds-stress flame model.
Experimental Arrangement The optical setup is shown in Fig. 1. A narrowband tuneable XeCl-excimer-laser (Lambda Physics) which produces UV light (308 nm, bandwidth 0.003 nm, 120 mJ, 20 ns) has been used. Measured pulse to pulse energy fluctuation is + / - 1 . 7 % of the mean value, thus no energy referencing was made. By adjusting the distance between and the tilt of the two focussing lenses LI and L2 the focus diameter and shape can be changed thus preventing optical break down. The resulting focus has an approximately circular cross-section. Raman photons are collected by two quartz lenses (I,3, IA) with di-
287
288
TURBULENT COM BUSTION--NONPREMIXED
EXCIHERLASER )*=308nm ~SCILLATOR
DAD. L : H : P ! SV :
y
DIOOE ARRAY DETECTO~ LENS HIRROR POLYCHROHATOR SAHPI E VOLUHE
Fie,. 1. Raman scattering experimental setup in an isometric projection.
ametcrs of 80 mm and with focal lengths of 150 mm and 360 mm respectively, the pesition of the lenses is adjusted for optimal signal by minimizing lens aberrations. An aluminum backscattering mirror rereflects Raman photons scattered away from the spectrometer (Spex, f = 500 ram, single grating 600 lines/mm, f# = 4.6). The spectrometer was mounted such that the entrance slit (0.3 mm * 2.7 mm) was parallel to the sample volume. The Raman-photons were received and converted by an intensified diode array detector (Spectroscopy Instruments, 700 intensified channels, each 0.025 m m * 2.5 mm, gate time 50 us). The overall spatial resolution was 3.0 mm * 0.15 mm * 0.15 mm. The spectral range registered by the detector in one picture is 56.0 nm which is sufficient to measure the vibrational Qbranches of all major molecules in a hydrocarbon flame from CO2 to H2. The burner of the free turbulent CH4/N~ diffusion flame consists of a tube with a diameter D = 6.0 mm and a co-flowing airstream with 0.5 m / s. The fuel stream exit parameters were: mass-fraction of CH4 = 0.3, measured temperature = 300 K, average velocity = 18.7 m/s, Re-number = 6950. The measured downstream levels normalized with the nozzle diameter were: x / D = 0.83, 5, 10, 15, 20,30, ..., 80.
Raman Data Reduction The data reduction scheme is based on linear Raman theory. 7 Resonant Raman effects were thought
to be small compared to other measurement inaccuracies. There is one exception, the relative Raman scattering cross section of oxygen measured at room temperature is enhanced by a factor of approximately 1.9. At constant total pressure the number of measured Raman photons Ni is connected to the relative number densities r of molecular species i with [7, 3, 6], N~ = AI'~r
- exp(-hcvtkT)))
(1)
were Ni [ - ] is the number of detected photons, A [K] an experimental constant, Fi [ - ] is the relative Raman scattering cross-section, r [ - ] the relative number density, T [K] the temperature, h [Js] the Planck's constant, c [m/s] the speed of light, k [J/ K] the Boltzmann's constant, vi [l/m] the Raman shift wave number and i the molecular species i. If all major molecules are measured the sum of the relative number densities must be equal one. r = I
(2)
i
This yields an implicit fl~rmula for the temperature. Once temperature is known determination of concentrations is done using formula (1). The Fi factors were measured at room temperature or at boiling temperature for water. They include chromatical and other transmission effects of the optics. The data reduction formalism consists of the following steps: 1.) A correlation routine ensures the wavelength position of the experimental spectrum
XeCl EXCIMER LASER RAMAN SCATI'ERING MEASUREMENTS always to be the same. 2.) The background is calculated by a simple function which is fitted to the spectrum. This is a very important step in the program since all results crucially depend on the background fit. 3.) The chosen background function is subtracted from the experinaental spectrum. 4.) Determination of integrated counts of the peaks of different molecules with the help of a previous established temperature dependent library of spectra for each molecular species, including correction of the interference from CH 4 on Oz. 5.) From all counts and all /'i factors, temperature and concentrations are calculated using Eqs. (1) and (2). 6.) Since the measurement of the entire flame consists of 35000 spectra it is not possible to look for those spectra which are misinterpreted by the computer code. To get a measure about the quality of the analysis of a spectnnn, a theoretical spectrum is composed from the resulting concentration-, temperature- and background data using the spectrum library and compared to the experimental spectrum by a least square sum. If this error sum is higher than a certain limit, the spectrum is ignored for filrther evaluation. This has to be considered seriously to avoid biasing of statistics. At each flame location 100 spectra were taken. Some of these spectra show very strong OH-fluorescence which is induced by the laser (OH-LIF) although the laser was very carefully locked to a wavelength which gives minimum fluorescence. Up to now these spectra could not be evaluated and were neglected for statistical evalnation. As the OHconcentration is correlated to temperature the average temperature is expected to be too low in flame regions containing high concentrations of OH. For comparison purposes single-shot data were averaged applying Favre formalism.
Theoretical Calculations The mathematical model consists of submodels for turbnlence (characterization of turbulent flow fields), comlmstion (description of momentary chemical reaction) and coupling (interaction of combustion and turbulent flow).
The Turbulence Model: The Reynolds-stress-model for Favre-averaged quantities consists of equations for continuity, mean veh)city, Reynolds-stress-tensor and dissipation. In [8] a detailed description of the closure of the nonclosed pressure-rate-of-strain correlations, the third order moments in the Reynolds-stress-equation, and the source term of the dissipation equation is presented. The constants used here are taken from model 1 in [81. The main difference between this model and standard Reynolds-stress-models~ is the
289
invariant- and turbulent Reynolds-number-dependent formulation of the retunl-coefllcient cm and the function ~0 in the source term of the dissipation equation. This should yield to a greater universality of the turbulence model.
The Combustion Model: Turbulent Damk6hler-numbers greater than one typically present in diffusion flames suggest the assumption of an infinitely fast one- step reaction. Damkfhler-numbers of the investigated flame are estimated to be 0.8 near the nozzle to 7.8 at x/D = 50 by [10]. If the radiation is neglected the momentary composition of the mixture and its temperature can be calculated by chemical equilibrium under adiabatic conditions. This results in a functional relation between momentary fuel-oxidator mixture and the momentary composition and temperature of the mixture. Because of low strain rate, interest mainly focussed on positions x/D = 30 to 80 and low observed CO-concentrations (see Numerical Solution) an equilibrium model is more appropriate than a strained laminar flame model.
The Coupling Model: Using the Svab-Zeldovich-formalism, linear combination of the pde's for the chemical components results in the mixture-fraction equation [11]. To derive the statistical moments of the mixture-fraction dependent variables, the mixture-fraction's probability density fimction (pdf) must be known. Here the B-function c_alculated with the Favre-averaged mixture-fraction f and variance of the mixture-fraction f,,z is used to approximate the real pdf. 12 Integration of the product of the functional relations provided by the combustion model and the mixture-fraction pdf results in the Favre-averaged mean temperature and mean mass-fractions of the chemical components. The partial differential equations for f and f,,2 contain velocity mixture-fraction correlations u'[f". For a consequent second moment closure pde's for these terms are integrated in the mathematical model. The complete Reynolds-stress-model contains pde's for: continuity, mean velocity tii, Reynolds-stressir
Fr
tensor ui ui mean mixture-fraction
mixture-frac-
tion variance f,2 and velocity mixture-fraction correlations u}'f".
Numerical Solution: The numerical solution was carried out with an improved version of the Patankar and Spalding program. 13 The boundary conditions are taken
TURBULENT COMBUSTION--NONPREMIXED
290 1.0
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FIG. 2. Comparison of experimental and theoretical axial decay of centerline mixture fraction fc, jet spreading rate as ros/(lOD) = radius(fc/2)/(lOD), centerline temperature Tc and fraction of evaluated spectra r versus normalized flame axis x/D. ~ is valid only for each axial point and particularly not for the entire level. Lines: prediction. as follows: mean velocity is estimated with the assumption of a developed turbulent pipe flow, mean mixture-fractions were set to equal one except at the last gridpoint, which was set approximately to zero, Reynolds-stress-components were approximated by measurements of Laufer in [14] in developed turbulent pipe flows, mixture-fraction variances were set to be 0.2 * 10 -4, velocity mixturefraction correlations u"f' were set proportional to U tt l) tt. Chemical equilibrium is calculated by minimizing of the chemical potential yielding temperature, composition, density and viscosity as functions of mixturefraction. The components N~, Oa, HaO, O, H, OH, COa and CH4 were taken into account. Neglection of Ha and CO seems to be justified because the Raman measurements determined mole-fractions less than 0.8% of these components and the calculations of Ha and CO in adiabatic equilibrium yields much larger concentrations of Ha and CO in fuel rich regions.
u"u" and v"]~' proportional to
Results and Discussion In Fig. 2 the correspondence of Jfc decay is very good. Differences in the spreading rate mainly at x/D = 50 and x/D = 60 levels can be explained by interferences from OH-LIF. The predicted temperature is equal to the measured at lower levels up to x/D = 30 and is approximately 250 K higher above. The temperature differences may be explained by high intensity of OH-LIF, spatial resolution and the slight overprediction due to neglection of radiation.
FRACTION
f
FIG. 3. f-plots of concentrations in form of mass fractions of the species versus mixture fraction f determined from the C atom ratio. The concentrations are added to a constant to get all curves in one figure. Each dot is the result of a single shot Raman spectrum but for clearity of the picture the values of only every tenth spectra out of all spectra from the entire flame (x/D = 0 to 80) are plotted. Lines: prediction. In Fig. 3 and 4 f-plots for all x/D are to be seen. The mixture-fraction was determined from C-atombalance, because OH-LIF interferes with the H20peak and so O- and H-atom based balances2 yields poorer results. Agreement is very good for lean (f = 0) and rich (f = 1) mixtures. Closer to the stoichiometric point scattering is a lot bigger and the number of evaluatable spectra is remarkably lower. There is small scattering for CH4 due to the relative high Raman scattering cross section. Oa-concentrations in the lean region were scattered around the predicted values whereas in the rich flame zones about two percent mass-fraction were measured al-
2500
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1.0
f
FIG. 4. f-plot of temperatures versus mixture fraction f as described in fig. 3. Lines: prediction.
XeCI EXCIMER LASER RAMAN SCATs 1800 1
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MEASUREMENTS
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FIG. 5. Favre averaged temperature versus ~7 at
x/D = 30 and x/D = 80. 77 is the normalized radius ~ = radius/ro.5, ro5 = radius(fJ2). Lines: pre-
2
3
4
N O R M A L I Z E D RADIUS ~7
FIG. 6. Favre averaged mass fractions of CO2, O~, n20, CI-I4 versus 7/at x/D = 30. Lines: prediction.
diction.
though they have to be zero. It was found that there were problems with the computer algorithm to separate the O-2/CO2 peaks especially if background is not linear. So measured O2-concentrations in the inner flame zones are made up artificially by the computer code and are not found in the spectra. For CO2 and for f = 0 to f-stoichiometric nearly all points are located at the theoretical line because the mixture-fraction is calculated from the C-atombalance. The differences between the data and equilibrium curves are due to OH-LIF and the strain-rate which is particularly more critical at upstream locations. Also at the rich mixture areas from x/D = 0 to 10 the turbulent Damk0hler-number is less than one and the assumption of chemical equilibrium is more critical. 15 The f-plot of H20 is disturbed by OH-LIF. All experimental temperatures (Fig. 4) are approximately 300 K lower than predicted, due to OH-LIF biasing, see discussion of Fig. 2. Figures 5 to 9 give results of temperatures, mixture-fraction f/fc, rms of fluctuations of mixturefraction f" /fc (~z)l/Z /fc and mass-concentrations X~ versus normalized radius "q at the beginning of the similarity region (x/D = 30) and where the flame is exhausted (x/D = 80). The differences at x/D = 30 in Fig. 5 and 6 are due to spatial averaging effects, OH-LIF and strain-rate. The Kolmogoroff length scale is about 0.8 mm at x/D = 30 and 3.5 mm at x/D = 80. As expected at x/D = 80 position measured values are in better agreement because of low OH-concentrations due to complete flame burnout and larger turbulent length scales, see Fig. 5, 8, 9. In Fig. 6 the concentrations on the axis are in good agreement except that O2 mass-fraction has to =
be zero, which was explained in discussion of Fig. 2. In Fig. 7 and 9 agreement of the mixture-fraction curves is quite good, although the number of evalutable spectra is small in the reaction zone (Fig. 7). At this location_ (7/ = 1), a biasing of statistic by magnifying off"/fc is expected. The slight increase may be caused by this effect. However, the overall continuous curve of f"/fc shows no strong influence. The experimental f"/fc in Fig. 9 lies in the same range of magnitude as the predicted. At the higher levels some spectra (approximately 30%) with fluorescence from soot or PAH were found. Most of them don't interfere with the evaluation as they show clear peaks which are separated in the spectra from the peaks of other species. Only very few spectra (1%) show a very strong continuum which masks all other peaks. Hence, soot 1.0 ~
,
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?/?c and
the rms of fluctuations of the
mixture fraction f,,/]c = (~'~)1/~/)cc versus ~7 at x/D = 30. Lines: prediction.
TURBULENT COMBUSTION--NONPREMIXED
292 0.3
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9
CO 2 02
9
H2 0
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~
investigations seem to show that it is not possible to prevent the induction of OH-fluorescence with an well-locked narrowband and tunable XeCl-laser, because of the inevitable spectral background of the laser (ASE).6 It is difficult to correct the spectra for the OH-LIF because the OH-LIF is not equal to OH-emission from thermal equilibrium and it is very intense.
i
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Fie,. 8. Favre averaged mass fractions of CO2, O~, H20, CH~ versus r/at x/D = 80. Lines: prediction. or PAH-fluorescence is a minor complication in this diluted flame with our equipment (particularly the multi-channel-detector) as the peaks can be separated. As a rough estimation the error i~ single shot temperatures is 5%, in concentrations 3% at low temperatures and 10% at high temperatures, each of maximum value. This holds for spectra without and with low OH-LIF. If there is strong OH-LIF the spectra can not be evaluated, yielding a biasing in the pds of temperature and concentration conditioned by low OH-concentrations. The precision of f is dependent on many factors mostly on the OH-LIF interference and is estimated to be maximum 10% for the spectra with the largest amount of OH-LIF which were evaluated. The viability of this measurement technique will depend on the possibility to handle the OH-LIF problem, see [6], where this point is discussed in great detail. Our 1.0
-
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i
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*~_
'.
i
f/fc
E
r'/t e = 80
Acknowledgement This work was supported by Deutsche Forschungsgemeinschaft, contract number llD5-Ja544/ 1-1.
-
9
The experimental results presented in this paper demonstrate that XeCl-UV-laser excited spontaneous Raman scattering is a sensitive analytical tool for measurement of CO2, O2, N2, CH4, H20 concentrations and temperature. Comparison with equilibrium assumption calculations show satisfactory agreement between the calculations and the data concerning mixture-fraction averages and standard deviations. Because of the small strain rate, equilibrium assumption is sufficient. The main problem at present state of research is OH-LIF in the reaction zone. This leads to rejection of a great number of spectra resulting in serious biasing of the statistics, because the measurements are conditional on low OH-concentration. It may be beaten by changing the laser wavelength. The information obtained with these measurements is very important for a deeper physical understanding of flame phenomena and therefore very suitable for improving the combustion and coupling models. Such a complete description of thermodynamic state of a turbulent flame is at the moment not possible with any technique other than Raman scattering.
x/D
REFERENCES
0.6-
1. WIDHOPF, G. F. AND LEDERMAN,S.: A1AA-J. 9, 2, 1971. 2. MASRI, A. R., BILGER, R. W. AND D[BBt,E, R. W.: Combust. Flame 74 267, 1988. 3. PITZ, R. W., WEnBMEYEB, J. A., BOWLINe,,, J. M. AND CHENG, T. S.: App Opt 29, 15 2325
0.2-
,o,~;~,
0.o
0
~ NORMALIZED
:~
" RADIUS
:3
4 p~
FIC. 9. f/f~ and the rm,~ of fluctuations of the mixture fraction f"/J'c = (f,"~)~/2/]- versus "0 at x/D = 80. Lines: prediction.
(1990). 4.
5.
M . , D R A K E , M. C., P E N N E Y , C . M . A N D PITZ, R. W.: J. Quant. Spectrosc. Radiat. Transfer, 40, 3, 363 (1988). E C K B B E T H , A . C . : Laser Diagnostics for Combustion Temperature and Species (Abacus, Cambridge, MA 1988). LAPP,
XeC1 EXCIMER LASER RAMAN SCATTERING MEASUREMENTS 6. HASSEL, E. P.: UV Raman Scattering Measurements in F/ames Using a Narrowband XeCI Excimer Laser, Applied Optics, (submitted May 1991). 7. LONG, D. A. : Raman Spectroscopy, McGraw Hill (1977). 8. JnNICI
293
11. LOCKWOOD, F. C. AND NAGUIB, A. S.: Cornbust. Flame, 24, 109 (1975). 12. JANICKA, J. AND KOLLMANN, W.: AGARD-Conference Proceedings 275, (1975). 13. PATANKAN,S. V. AND SPALDING, D. B.: Heat and Mass Transfer in Boundary Layers, London Intertext books, (1970). 14. ROTrA, J. C.: Turbulente Str6mungen, Stuttgart, Teubner, (1971). 15. DRAKE, M. C., Prrz, R. W. AND LAPP, M.: Laser Measurements in Nonpremixed Hydrogen-Air Flames for Assessment of Turbulent Combustion Models, AIAA-paper NO. AIAA-84-0544, (1984).
COMMENTS G. M. Dobbs, United Technologies Research Center, USA. How well injection-locked was your laser? While you were careful to tune between OH absorption lines to minimize fluorescence, could the broadband background you recorded have arisen from the pedestal part of the laser output distribution? Some of the additional pro~esses which can lead to a broad background signal in UV Raman Spectra are laser modulated soot incandescence, fuel and PAH fluorescence, and optical breakdown. Could you please provide more information on how you were able to rule these out and attribute all of the background you observed to OH fluorescence?
Author's Reply. We acknowledge the importance of the question to assess the viability of the 308 nm Raman system. Therefore the basis of the question is an important part of the contents of Ref. 6. However, for the sake of completeness we answer very comprehensively. Laserlocking was greater than 98%, as demonstrated in Ref. 6, Fig. 7. With this laser it is nearly impossible to tune between OH absorption lines due to ASE laser background and the existence of many lines of many bands. Broadband background was identified as a superposition of a weak linear part from the laser output, from very strong OH LIF and from rare PAH soot LIF as shown in Fig. 6 in Ref. 6 .
Robert Barlow, Sandia National Laboratories, USA. (similar comment submitted by Donald W. Sweeney, Sandia National Laboratories', USA). 1. The authors conclude that they have achieved good S/N with this technique. However, scatter
plots show variations in major species concentrations that are much larger than expected for a fully connected flame (no extinction). Scatter is particularly large for CO2. Much of this due to the poor optical collection efficiency of the system. Is there also a particular problem with background subtraction and fitting for the CO2 spectrum. 2. Is 308 nm really a viable wavelength for Raman measurements if there is always a broadband component that produces significant OH fluorescence interference? 3. The scatter plots indicate that hydrocarbon fluorescence interference is not a problem in these nitrogen diluted methane flames. What is the explanation? Does nitrogen dilution eliminate interference or is interference present in the spectra and successfully handled by the data reduction procedure?
Author's Reply. 1. The scattering of the CO~ data is caused by the low efficiency of the receiving lenses and by the fact that the COz peaks are located at the same wavelength position as the 2-2 band of O H (see Ref. 6). 2. 308 nm is suitable for Raman spectroscopy in flames if it is possible to subtract the OH LIF background in an adequate way. After the submission of the presented paper a data reduction computer routine was developed and applied with success for the evaluation of Raman spectra from a H J N 2 diffusion flame. Experiments described therein show a maximum of 28% of not evaluatable spectra at one position. Changing the laser wave-length may be disadvantageous due to the possibility of inducing PAH and soot fluorescence or the difficulty of the description of resonant Raman effects. 3. Both suggested answers are correct. Firstly di-
294
TURBULENT COM B U S T I O N - - N O N PRE MIXE D
lution with N2 eliminates most of the hydrocarbon interferences occuring in the spectra from the higher x/D levels of an undiluted flame. Many of these spectra show complicated fluorescence structures especially for higher hydrocarbon flames. Secondly as mentioned in the paper approx. 30% of all spectra from the higher x/D levels of the investigated flame show hydrocarbon fluorescence. These spectra are well handled by the data routing thus demonstrating the advantage of us-
ing 700 informations from the multichannel system.
REFERENCE 1. LIPP, F., HARTICK, J., HASSEL, E. P. AND JANICKA, J.: Raman Measurements in a Turbulent H2/Nz Diffusion Flame; Applied Physics B; submitted 6/92.