Dual-stokes cars system for simulataneous measurement of temperature and multiple species in turbulent flames

Dual-stokes cars system for simulataneous measurement of temperature and multiple species in turbulent flames

Twenty-Third Symposium (International) on Combustion/The Combustion Institute, 1990/pp. 1893-1899 D U A L - S T O K E S CARS SYSTEM F O R S I M U L T...

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Twenty-Third Symposium (International) on Combustion/The Combustion Institute, 1990/pp. 1893-1899

D U A L - S T O K E S CARS SYSTEM F O R S I M U L T A N E O U S M E A S U R E M E N T OF T E M P E R A T U R E A N D M U L T I P L E S P E C I E S IN T U R B U L E N T F L A M E S KEVIN W. BOYACKI" AND PAUL O. HEDMAN Department of Chemical Engineering and Advanced Combustion Engineering Research Center Brigham Young University, Provo, Utah 84602

A dual-Stokes coherent anti-Stokes Raman scattering (CARS) instrument has been used to make simultaneous time- and space-resolved measurements of temperature and the mass fractions of N2, CO, O~, and COz. Calculation of the mixture fraction, lj, a conserved scalar, is possible for each data point, making the technique useful in turbulent combustion environments. The viability of this instrumental approach has been demonstrated by calibrations in laminar CO/N2 fiat flames of many different stoichiometries. Maximum single-shots rms values due to instrument fluctuations are attained in near stoiehiometric mixtures and are -+45 K for temperature, ---0.042for YN2 and Yco2, -+0.015 for Yo2 and Yco, and -+0.036 for mixture fraction. Measurements have been made using this instrument in turbulent nonpremixed jet flames of CO/N2 with small amounts of H2. These measurements demonstrated that for turbulent systems, limitations are imposed on the CARS technique due to insufficient dynamic range and image persistence problems with the intensified photodiode array (IPDA) detector. These limitations are minimized with proper experimental parameters and data correction methods. Introduction A detailed look at fundamental combustion processes has been undertaken over the past several years with the advent of laser diagnostic techniques capable of non-intrusive measurement of instantaneous flame properties at high spatial resolution. Of the various diagnostic techniques, spontaneous Raman scattering has become the method of choice in probing these fundamental systems as it is capable of simultaneously measuring 8-10 species. 1 Mixture fraction, defined as the local ratio of mass originating from the primary stream to total mass, is a commonly used conserved scalar, and can be measured directly when all major species of a reacting system are measured simultaneously. Coherent anti-Stokes Raman spectroscopy (CARS) is another non-intrusive diagnostic technique which has gained acceptance for use in combustion environments. With spatial resolution similar to that of spontaneous Raman scattering, CARS has the potential to be used for the instantaneous measurement of multiple flame properties in turbulent reacting flows. One advantage of CARS is the high

tpresent address: Sandia National Laboratories, Division 6517, P.O. Box 5800, Albuquerque, NM 87185.

repetition rates possible (20-30 Hz.) with current instruments. However, most CARS research has been limited to the study of a single molecular constituent. Very few studies of multiple species have been undertaken. Several multi-color CARS approaches have been proposed and demonstrated, 2-4 yet to date there have been few investigations in practical combustion environments using multi-color techniques.5-s While it is unlikely that CARS will supplant spontaneous Raman scattering as the preferred tool in clean flames, its signal strength and directionality enable it to be used in many practical combustion systems were Raman experiences dit~eulties due to the luminous or dirty nature of the environment. As the future role of CARS is most likely to probe these hostile environments, it is imperative that the technique be developed such that multi-species CARS measurements become common. Measurement of multiple species in a clean gas flame is a logical step in the development of this capability. Thus, the objective of this study was to demonstrate simultaneous single-shot measurements of temperature, multiple species, and a conserved scalar, in a turbulent gas flame using a twocolor CARS technique. The combustion system to be studied has been selected with the above objective in mind. The fuel used was CO/N2 with




small amounts (<3%) of Hz. The four major species in the CO/air system (Nz, CO, 02, COz) constitute between 97-100% of the total composition. Measurement of these four species allows direct determination of mixture fraction to goocl approximation. The CARS instrument has been configured to measure these four species. The accuracy of and problems associated with this approach and their bearing on the use of this technique in turbulent environments have also been investigated.



The fundamentals of CARS have been treated extensively in the literature9'1~ and will only be briefly reviewed here. CARS is a coherent, nonlinear wavemixing technique in which laser beams of frequency COl and to2 are focussed together. When the frequency difference o~l-to2 is close to the Raman frequency of a particular molecule, the molecule will experience an oscillating polarization, resulting in a resonantly enhanced Raman signal at frequency toa = 2aol - to2. Varying the frequency difference oJi-toz allows different molecular constituents to be selectively probed. When proper phase matching of input frequencies is conserved, the antiStokes signal builds up constructively and coherently and the entire signal can be collected over a very small solid angle with a minimization of interferences. Use of a broadband dye laser for to2 allows single-shot measurements consistent with the laser pulse duration (10 ns). The CARS instrument at Brigham Young University is shown schematically in Fig. 1. H A frequency-doubled Nd:YAG laser (Quanta-Ray DCR2A) at 10 |lz. supplies the power for the entire system. Its primary output at 532 nm is split to form two pump beams, and to pump two broadband dye lasers. One dye laser used Rhodamine 640 dye and is centered at 2210 cm -l to probe N2 and CO, while the other employs a combination of Rhodamine 575 and 610 dyes and is centered at 1500 cm-i to probe O2 and CO2. An optical delay of 1.35 cm is introduced between pump beams to ensure that they are uncorrelated. The four beams are brought together into a dual folded-BOXCARS configuration where the folding angle of each (tOl, tab toz) combination is approximately 70~ rather than the commonly used 9005 .' 250 mm lenses are used to focus and collimate the beams, and the resulting CARS beams are separated from the residual input beams and focussed into a 50 p,m diameter fiber optic feeding the spectrometer. In turbulent flames it is critical that the same volume be probed by all simultaneously operating diagnostic systems. 12 The diagnostic volume profiles for the two CARS processes have been measured using a thin glass cover slide and found to




. . . . . (F~T PI P.R,~ GP , ~ C M

BS~. ~





.... ',





FIG. 1. Schematic of the BYU dual-Stokes CARS instrument. Legend: CM, compound mirror; D, frequency doubler; DC, dye cell; F, optical fiat; FO, fiber optic; GP, glan polarizer; (,T, Galilean telescope; tIM, half mirror; IPDA, intensified photodiode array; L, lens; M, mirror; P, prism; PMT, photomultiplier; PR, partial reflector; S, beamsplitter; T, beam trap; (),, quarter-wave plate.

be coincident. 11 .90% of each CARS signal is generated within a volume 1.8 mm in length and 200 p,m in diameter. This volume is certainly larger than the Kolmogorov scales, and some spatial averaging would be expected to occur in measurements in turbulent flames. 13 The beam crossing and beam waists are separated by 2 mm to reduce the energy density at the crossing. Thus each beam waists separately, allowing higher laser energies to be used without incurring saturation or ionization of the medium. Low temperature spectra taken at operating conditions were examined for stimulated Raman effects and showed no signs of saturation. Detection of the four molecular signatures is accomplished in the 0.75 meter, ]'/3.5 spectrometer in which mirrors and baffles have been placed such that the 1360-1570 cm - l and 2080-2350 cm - l portions of the Raman spectrum are positioned next to each other on the detector. Overlap of signal from the two spectral portions occurs over a very narrow region of the detector (~50 pixels) and can be neglected. Up to 1000 consecutive single-shot spectra can be recorded using the PARC 1421B detector. The detector timing sequence consists of two read scans followed by four cleansing scans. The spectra are sent to a MicroVAX II computer for storage and analysis. Figure 2 shows a single-shot dual-Stokes CARS spectrum taken in a turbulent CO/Nz jet diffusion flame. All four molecular signatures are clearly distinguishable. The discontinuity in frequency is shown to fall between the 02 and CO signatures. Temperature and concentration information are obtained by using the nonlinear leastsquares interpolation package FTCARS 14 to fit experimental



1 13vo = Yco = Yco - 7 Yoz. aFO

280 240




Scalar 13c is for the carbon atom balance, and 13ro is for the fuel-oxygen atom balance assuming the H2 mass fraction approaches zero. Svo is the stoichiometric mass ratio of oxygen to fuel and is based on the chemical equation



160 (,.) N


CO 2

0 "40300

1 / 2 ( n + 1 ) O2

I~ I 4' 00

1500 '


2200 '

2300 '

CO2 + n H20. 2400

Raman Shift (cm "1) Fie. 2. Single-shot spectrum taken with the dualStokes CARS instrument in a turbulent nonpremixed jet flame. The moleeular signatures of Nz, CO, O2, and CO2, (1388 cm -1) are all clearly visible in the spectrum. spectra to libraries of theoretical spectra. Two-parameter libraries for each molecular species were calculated using the Sandia National Laboratories CARSFT code 14 to allow simultaneous fitting of temperature and • (susceptibility/mole fraction). 15 The instrument function used was an approximate Voigt function, and was determined from spectra taken in room temperature air for each day's data. Before fitting, all spectra were corrected for detector nonlinearity.16 Temperature and susceptibility were fit simultaneously for the N2 and CO2 signatures of each spectrum. Due to their relatively small signal strengths, the CO and O2 signatures were fit for susceptibility while holding temperature fixed at the value obtained for the N2 spectrum. Although two separate values of temperature were obtained for each measurement, N2 temperatures are reported exclusively as Nz thermometry is much more advanced than that of COz. Measurement of • for all major species allows in-situ determination of the total nonresonant susceptibility of the medium, X,r, which is calculated from the individual susceptibilities (Xnr)--I = X ,=1



Individual mole fractions are given by


X, = Xor/(•

Upon conversion of mole fractions to mass fractions, the mixture fraction, ~, is calculated from mass fractions for each data point from two conserved scalars [3 defined as 12.01


I~c = 2s.0----iYco + ~




In the limit of no H2, SFO is equal to 0.571. Mixture fraction, ~, is calculated from scalar 13, as

~,= • i -



where subscripts f and a signify the fuel and air streams, respectively. An overall mixture fraction is reported which is the average of the carbon and fuel-oxygen mixture fractions.

Measurements in Laminar Premixed Flat Flames To investigate the viability and accuracy of the dual-Stokes CARS system, measurements were made over fiat flames of many different stoiehiometries were the local composition could be assumed to reach equilibrium. The fiat flame burner consists of a series of capillary tubes interspersed with open passages (ratio 1:3) in a ceramic honeycomb matrix. The burner is 2 inches in diameter, with a shroud which extends the flow area to a square 3 inches on a side. A square pyrex tube extends from the top of the burner to further isolate the flow and provides fiat surfaces for optical access. The fuel used in these tests was 50% CO/50% Nz with enough H2 to stabilize the flame. A set of 500 single-shot data points was taken for each calibration value of mixture fraction at a position 5 mm above and directly over the center of the burner. These calibrations were performed primarily to determine the accuracy with which composition and mixture fraction measurements could be made. As the burner does not operate adiabatically, temperatures were not used as a calibration, but were rather assumed to be within the commonly accepted N2 CARS accuracy of 30--50 K for averaged measurements. Previous measurements taken in a tube furnace have shown agreement between m e a n N2 CARS temperatures and thermocouple measurements to within -+20 K at 1100 K. u Mean values of mass fraction calculated from uncorrected fit values of X,r/Xi for each calibration data set are shown as open symbols in Fig. 3. These



i' I


ir!i'! ~ i !

I ! '!" i









~ ' ~!i






i i i i i i



i I' ]

0.5 0.4

0.2 0.1 0s



0.3 0.4 0.5 0.6 0.7 Fuel-lean I--Fuel-rich


Mixture Fraction FIG. 3. Mean composition and mixture fraction values from single-shot CARS data sets taken over laminar fiat flames of known composition. Uncorrected (1"3, 9 l~) and corrected (11, O, A) data are compared to calculated equilibrium composition values (solid lines) and the known values of mixture fraction (dashed lines). measured values are compared to calculated equilibrium mass fractions at the calibration values of mixture fraction, which are represented by the solid and vertical dashed lines, respectively. The data should fall at the intersection points of these lines. Errors in both mass fraction and mixture fraction are indicated by the distance of the data from the intersection points. Examination of the data shows that the measured values of Yc% are too high in the fuel-lean region, while the measured values of Yco are too low in the fuel-rich region. These biases are reflected in the calculated mixture fraction values as well. These errors appear to be systematic and can be corrected if the values of • and xnJXco given by spectral fits are adjusted properly. Linear correlations have therefore been derived and applied to the single-shot data. n Since the COz bias seems confined to fuel-lean mixtures, the first correlation corrects Xnr/Xco2 only when O2 is present. However, simulations have shown that this bias is not due to interactions between the O2 and CO2 signatures. The second correlation corrects X,r/XCo, which is too large by a nearly constant factor of 1.25. This correction is also necessary in measurements of CO/air mixtures at room temperature, showing that it is independent of temperature and can be applied to data over a large temperature range. Simulations have shown that this error is not due to interactions between N2 and CO. The sources of these biases are presently undetermined and are the subject of further investigation. Figure 3 also shows mean values of mass fraction

from the same data corrected using the two correlations described above (filled symbols). Overall agreement with equilibrium values is much better than before the correction was made. Although perfect agreement with theory is not obtained, the agreement is acceptable and provides sufficient accuracy to justify use of the CARS system for multispecies measurements in turbulent CO/N2 flames. Representative single-shot temperature and mass fraction values from selected calibration data sets are shown in Fig. 4. The scatter in each data set shows uncertainty which is directly attributable to shot to shot variations in the CARS instrument in a constant property environment. Instrument induced rms of fluctuation values of flame properties have been calculated for these data. Maximum rms values for single-shot data points are found near the stoichiometric point (~stoic = 0.447) and are -+0.042 for YN2 and Yco2, +0.015 and Yo2 and Yco, and -+0.036 for mixture fraction. These rms values decrease considerably in the fuel-lean and fuel-rich flame regions. Rms values for temperature fall between -+35-45 K for all data sets. These fluctuation values are highly dependent upon the signal strengths encountered. For lower signal strengths, fluctuation values would increase due to decreased S/N ratio in the spectra.

Measurements in Turbulent Nonpremixed Jet Flames The piloted nonpremixed jet burner used in this study is described elsewhere in detail. 11 Briefly, the burner consists of a primary jet 4.9 mm in diameter centered radially in a stainless steel pipe 146 mm in diameter. Windows placed 180~ apart allow optical access for the laser diagnostic system. Coflowing air is provided by fans in the exhaust line. Sets of 700 single-shot data points were taken in the jet burner in flames consisting of 70% CO/30% Nz with various amounts of Hz. The cold jet Reynolds number of these flames was 7900. All measurements in each flame were taken at a location 25 jet diameters downstream of the jet exit and 1.4 jet diameters from the centerline. Figure 5 contains scatterplots which show simultaneous temperature and mass fractions Yco, Yo2, and Yco2, versus mixture fraction for a flame whose fuel contained 2.8% H2. This data set exhibits a wide range of mixture fraction values (0-0.7) with temperatures and mass fractions which approach equilibrium values over the range of mixture fractions measured, and shows the applicability of CARS to simultaneous measurements of temperature and multiple species in turbulent flames. In flames where the majority of data points give very hot temperatures and CARS signal intensities are fairly constant from shot to shot, no major problems are encountered experimentally.










2200 1800 ~ 1400 ~




11300 [~

0.6 o











0.2 0.1 0.0 0.1

0.2 4

0.3 0.4 0.5 0.6 Fuel-lean I--Fuel-rich


0.8 I~

Mixture Fraction FIG. 4. Single-shot temperature, composition, and mixture fraction values from laminar fiat flames. The first 75 of 500 points in 6 of the calibration data sets are shown. The solid and dashed lines are as in Fig. 3. However, in flames where much larger temperature excursions occur, problems associated with the use of intensified photodiode arrays (IPDA) can severely limit the applicability of CARS as a diagnostic.

The most significant of these problems is the limited dynamic range of IPDA detectors. One proven solution is the use of an optical splitter in the signal beam. 17As This method consumes detector space and precludes simultaneous detection of multiple species. In a second method, measurements are taken at two laser powers and the data sets are combined to form one good set.l~ This requires extensive experimentation to determine the correct 'cutoff' criteria. A third method is to take measurements at the highest laser power which can be used without saturating the detector on any laser shots. CARS measurements were taken at two different laser powers in a flame containing 0.9% H2. Temperature data from these measurements are shown in conditional pdfs (by mixture fraction ranges) in Fig. 6. The data in Fig. 6(a) were taken at low laser power and show clearly the bimodal nature of the temperature distribution. The data in Fig. 6(b) were taken at a higher laser power. Comparison of the high and low power data sets shows that cold points are not well represented when high laser powers are used. Fit temperatures are too high and show a broader temperature range than expected. At the low laser power, cold spectra distribute as expected at all mixture fraction values. Hot spectra from both data sets have similar distributions, generally increasing in temperature with increasing mixture fraction. As accuracy in fitting cold spectra is crit-







. _-

800 [ "

" 400



x 1200 ] 1600

, 2000


0.3 L~


.003 r



, A ~ , .


0.0 ,




-,~r. . . . . . . . . . 0.3 0.4 0.5 0.6 0.7

Fuel l e a n - -



0.8 ,

' 003[


Mixture Fraction

t/~176 .001

FIG. 5. Scatterplot of simultaneous single-shot temperature and mass fractions Yco, Yo2, and Ycov versus mixture fraction from a turbulent nonpremixed jet flame of CO/N2 (2.8% H2) of Reynolds number 7900. These data were taken at x/D = 25, y/D = 1.4. The solid lines show calculated adiabatic equilibrium limits while the dashed lines indicate mixing without reaction for Yco and Yov

0g 0



/ ~ , ' ,rv



_ ~r - i ~

800 1200 1600 Temperature


~ 0 j____ - 0.11 ,



Fro. 6. Conditional pdfs of temperature at different values of mixture fraction taken in the same flame with pump laser powers of (a) 24 mJ/pulse, and (b) 44 mJ/pulse, edfs for ~ = 0.0-0.1, 0.1-0.2, 0.2-0.3, and 0.3-0.4 are given.



TABLE 1 Effect of image persistence correction on CARS derived flame properties (rrm~ values in parenthesis)

Property Ts2 (K) Y~2 Yco Y,~ Yc(,2 (~c - ~)/~

Without correction 855 0.647 O.111 0.173 0.067 0.207 0.094

With correction

(551) (0.064) (0.046) (0.050) (0.071) (0.074) (0.259)

879 0.660 O. 109 0.157 0.073 0.220 0.029

ical, the third alternative above is suggested for multi-species measurements in turbulent flames. However, this method may work only due to the decreased dynamic range needs resulting from addition of N~ to the fuel. Image persistence is another problem associated with P-20 based IPDA detectors and is critical in turbulent combustion environments where low intensity signals can directly follow very high intensity signals. 2~ An attempt has been made to quantify image persistence for our detector and to correct single-shot turbulent flame data accordingly. H Our results agree qualitatively with results of others,16 and show that persistence is a function of signal intensity. For the detector and data acquisition sequence used, persistence is given by the function Persistence (% of signal) = 3.01 Counts -o.194.


This correction is applied to each spectrum by subtracting calculated persistence from the previous spectrum. Equation 7 has been applied to the single-shot data set taken at low laser power from Fig. 6(a). These data were chosen to illustrate the correction because they contain very severe temperature excursions. Statistics for the data set with and without correction for image persistence are given in Table I. The primary differences are a modest increase in temperature, a significant decrease in Yo2, and a significant increase in Yco2. Perhaps the most significant difference is in (~c - ~)/~, which shows that uncertainty between the two calculated values of mixture fraction (Eqs. 3 and 4) is decreased greatly when the correction is applied. These differences lead to the conclusion that reasonable measurements of multiple species can be made even in severe conditions where image persistence has large effects. A promising solution to the problems associated with IPDA detectors may be found with charge-coupled devices21 and a new generation of

Ratio of means with/without

(584) (0.048) (0.048) (0.051) (0.080) (0.080) (0.170)

1.028 1.020 0.982 0.908 1.090 1.063 3.241

IPDA detectors 16 which have improved dynamic range and persistence characteristics. Conclusion CARS has been successfully applied to the simultaneous measurement of temperature and multiple species. This is the first study we know of in which a CARS instrument has been used to measure single-shot temperature and four species concentrations simultaneously. The viability of this approach was demonstrated by calibrations in CO/N2 fiat flames of different stoichiometries. With the application of calibration factors, agreement between data and theory is acceptable and provides sumcient accuracy to justify use of the CARS system for multi-species measurements in turbulent C O / N~ flames. Further development is necessary to allow full application of CARS to hydrocarbon fuel systems, although this approach could be used fi)r relative concentrations. Measurements taken using this instrument in turbulent nonpremixed jet flames pointed out the problems of limited dynamic range and image persistence which are associated with the use of IPDA detectors. Dynamic range limitations were circumvented by taking measurements at a low laser power. Image persistence was found to give rise to significant errors, particularly in composition. Its effect can be lessened by subtracting the portion of a spectrum which results from persistence as part of the data fitting procedure.

Acknowledgments Original funding for the CARS instrument was provided by the U.S. Department of Energy. This work was sponsored by the Advanced Combustion Engineering Research Center, which is funded by the National Science Foundation, the State of Utah, 25 industrial participants, and the U.S. Depart-

DUAL-STOKES CARS SYSTEM ment of Energy. Special thanks goes to Alan Eckbreth for his continual help and Richard Palmer for developing and making available the CO2 CARS model.




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