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AIAA-99-0643

Molecular Filtered Rayleigh Scattering Applied to Combustion and Turbulence

G.S. Elliott, N. Glumac Department of Mechanical and Aerospace Engineering Rutgers University, Piscataway, NJ 08854

C.D. Carter Innovative Scientific Solutions Iricorporated, Dayton, OH 45440

37th AIAA Aerospace Sciences Meeting and Exhibit

January 1 l-14,1999 / Reno, NV

For permission to copy or republish, contact the American Institute of Aeronautics and Astronautics 1801 Alexander Bell Drive, Suite 500, Reston, VA 20191


AIAA-99-0643 Molecular Filtered Rayleigh Scattering Applied

to Combustion and Turbulence

Gregory S. Elliott+, Nick Glumac’ Rutgers University

Piscataway, NJ OS855

and

Campbell D. Carter* Innovative Scientific Solutions Inc.

Dayton, OH 45433

ABSTRACT

Molecular Filtered Rayleigh Scattering (FRS) has been demonstrated to measure the instantaneous and average temperature field in combustion environments. FRS employs an optical cell placed in front of an intensified CCD camera which records the Rayleigh scattered signal from the flow field illuminated by a sheet of laser light from a Nd:YAG pulsed laser. The laser is tuned to an absorption line of iodine vapor which is contained in the optical cell. This causes background scattering from solid surfaces and particles to be strongly absorbed, while much of the Doppler broadened Rayleigh scattering is transmitted through the filter. The gas temperature can then be deduced from the measured transmission of the molecular Rayleigh scattering. Two different premixed flames were investigated, a hydrogen-air flame created using a Hencken burner and a methane-air flame. The accuracy of the FRS measurements was investigated by comparing FRS- derived temperatures with calculated values and temperatures recorded with coherent anti-Stokes Raman spectroscopy. For the hydrogen-air flames, the FRS method gave temperatures within 2% of the expected value. Methane-air flames were investigated to show the effectiveness of FRS to obtain instantaneous two-dimensional temperature information in a buoyantly driven flame. The FRS thermometry system was then utilized to investigate a stagnation-flow methane/air flame and compare the results with a 1-D model. To enhance the capabilities of FRS, the feasibility of simultaneously measuring the instantaneous velocity field using Particle Image Velocimetry was also demonstrated.

BACKGROUND Among current nonintrusive laser diagnostics there

are few techniques suitable for temporally resolved (resolution < Ips), two- dimensional temperature measurements in reacting flows. Two techniques that have been demonstrated for this purpose are planar laser- induced fluorescence (PLIF) and Rayleigh scattering (unfiltered). Fluorescence techniques can be classified as

-. + Assistant Professor, Mechanical and Aerospace Engineering, Rutgers University, Piscataway, NJ 08855, Member of AI,&4 * Innovative Scientific Solutions Incorporated, Dayton, OH 45440

Copyright 0 1998 by Greg Elliott. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission.

either single or multiple frequency methods. Single frequency techniques, such as variants of two-line molecular fluorescence (TLMF) (Cattolica et al., 1983) and absolute fluorescence (ABF) (Seitzmann et al., 1985), have been demonstrated in flames previously. The TLMF technique using overlapped transitions from two vibrational bands is best suited for high temperatures where significant population of the v = 1 vibrational state is present. The ABS technique typically requires seeding of the reactants with a fluorescing species so that the seed species mole fraction remains constant. The required use of a nonreacting seed species is a significant limitation in practical flames. Multiple frequency methods in which two lasers are used simultaneously to pump different transitions using NO (McMillin et al., 1993) and OH (Seitzman et al., 1993) have also been demonstrated. These methods are much more versatile and can

1


potentially produce accurate planar temperature fields. However, the two-laser techniques are more complicated and expensive to implement.

Rayleigh scattering represents a simpler approach that requires only a single laser at a fixed frequency. Fourguette et al. (1986) and Dibble et al. (1995) have used variations on this approach to obtain instantaneous temperature fields with accurac’les better than 4% over the range of 300 to 2000 K. However, in both cases, the fuel and air were carefully filtered to insure minimal interferences from particle scattering (which can saturate the Rayleigh scattering signal), and measurements are difficult near surfaces where the information may be needed.

Recently, techniques employing molecular filters have been introduced in a variety of flow diagnostic techniques offering the possibility of measuring single or multiple properties simultaneously. The molecular filter is simply a cylindrical optical cell that contains a molecule which has absorption transitions within the frequency tuning range of the laser. The molecular filter is placed in front of the receiving optics to modify the frequency spectrum of the scattering. Miles et al. (1992) first demonstrated filtered Rayleigh scattering (FRS) employing molecular iodine filters in conjunction with an injection seeded, frequency-doubled, Nd:YAG laser (h = 532 m-n). With injection seeding the linewidth is narrow, and the laser frequency can be tuned to match the transitions of iodine. Using the FRS technique for background suppression, the laser is tuned so that unwanted scattering from walls and windows is absorbed while the Doppler shifted Rayleigh (or Mie) scattering from molecules or particles in the flow field is shifted outside the absorption well. In addition to qualitative flow visualizations, Miles et al. (1992) showed that average properties of the flow at each point in the illuminated plane can be obtained with molecular scattering, when the laser frequency is tuned through the width of the absorption well of the iodine. The resulting intensity profile is then compared to a theoretical profile and the average velocity, density, temperature, and pressure are thus determined. Using a similar FRS point measurement system Elliott and Samimy (1996) were able to measure these properties instantaneously. Recently, Hoffman et al. (1995) demonstrated that FRS could be used to obtain temperature measurements in lightly sooting flames, but there is still a need for more comprehensive treatment of the uncertainty of the measurement technique and its application to flow fields of research interest.

In the present paper, FRS will be demonstrated and evaluated for measuring temperatures in combustion

environments. The technique will first be compared to adiabatic flame temperature calculations and measurements made using coherent anti-Stokes Raman spectroscopy (CARS). The usefulness of the technique for measurements, in particle laden flames, and in large two-dimensional regions of the flame wiil be demonstrated. Also, the FRS technique will be used to measure the axial and radial temperature variation in a stagnation-flow flame and compared to a 1-D computer model at the same conditions. In order to better describe the characteristks of the stagnation-flow flame preliminary measurements were made to demonstrate the feasibility of incorporating FRS thermometry simultaneously with Particle Image Velocimetry (PIV) to measure the instantaneous temperature and velocity field together.

EXPERIMENTAL ARRANGEMENT

Figure 1 gives a schematic of the burner and optical arrangement for the measurements conducted at the Gas Dynamics and Laser Diagnostics Research Laboratory at Rutgers University. Three different burners were used to study the feasibility of the technique and stagnation-flow flames with flow conditions given in Table 1. Initially the feasibility of FRS was studied using a hydrogen-air flame created in a Hencken burner with a 25mm square combustion region. The temperatures in this flame have been measured over a wide range of conditions using CARS (at the Air Force Research Laboratory), and the flat temperature profile across the burner, makes it ideal for testing the FRS technique. The second burner was a copper burner with an array of 64, 1 mm diameter, holes within an area of I69 mm’. Premixed methane, nitrogen, and oxygen were independently controlled and fed into the copper burner at various mixtures and flow rates (Table 1). This burner was used to study the ability of FRS to make instantaneous two dimensional temperature measurements, as well as, studying the feasibility of making simultaneous FRS and PIV measurements. The third burner was used exclusively in the stagnation-flow methane/air flame measurements. This water cooled copper burner had 241, 0.76 mm diameter, holes distributed over an effective diameter of 3.2 cm. The burner was operated at three flow rates at an approximately constant equivalence ratio.

The interrogation laser beam for the FRS system was formed into a thin collimated sheet with a ~ combination of cylindrical and spherical lenses. The beam was from a frequency doubled Spectra Physics GCR-230 laser capable of 650 m.J per pulse. The

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ND:YAG laser has an injection seeder to provide a narrow line width (- 100 MHZ) needed and the laser frequency can be tuned through absorption lines of iodine around 532 run. The FRS laser used in these experiments has a pulse duration of approximately 10 ns with a repetition rate of IO Hz. Photodiodes were connected to a Stanford Research System boxcar integrator to monitor the laser energy and laser frequency fluctuations using a second iodine filter and calibrate the camera filter. The SRS computer interface allowed the laser to be tuned in frequency during the experiments and when measuring the iodine absorption spectra. The Rayleigh scattering signal is collected using a Princeton Instruments lbbit intensified CCD camera. Images are stored on a Pentium 100 MHZ personal computer providing camera control, laser synchronization, and laser frequency control.

The PIV laser system incorporates a Spectra Physics frequency doubled (h = 532 nm) PIV-200 double pulse Nd:YAG laser. The laser is capable of producing two 200 mJ 10 ns pulses with a repetition rate of 10 Hz and a variable delay between pulses from 1 ns to 0.1 s. The beam was aligned with the FRS beam using a high power beam splitter and therefore used the same sheet forming optics. The PIV images were collected using an g-bit Kodak Megaplus camera and digitized using a Pentium 400 MHZ personal computer. The camera is capable of taking a pair of images with delay times as short as 1 p.s. Two Stanford Research System DG535 delay generators are used to control the timing of the PIV laser and camera. The PIV Nd:YAG laser is synchronized to each FRS laser pulse so that it produces two pulses with the initial one delayed by lus with respect to the Nd:YAG FRS laser. The PIV camera is synched to the FRS camera so that the image pair is taken with each FRS image. Particles used in the PIV measurements were 2.4 ym diameter ceramic spheres.

A major component of the FRS system is the iodine filter as shown in Figure 2. The iodine filter is simply a glass cylinder 9 cm in diameter and 24 cm in length with flat optical windows on both ends. Iodine vapor is formed in the cell by inserting a small amount of iodine crystals and evacuating the cell. The cell temperature (T,,,,) is raised above the ambient temperature with electrical heat tape so that no iodine crystallizes on the windows. The coldest point in the cell is set in the side arm (T,), which is housed in a water jacket and maintained at a constant temperature by a circulation water bath. The temperature of the side arm controls the vapor pressure (number density) of the iodine in the absorption cell. Once the side arm temperature is set the valve is closed so that the number density of the iodine is fixed (this is termed a starved cell arrangement by

Mosedale et al., 1998). Figure 3 presents the absorption spectra with the optically thick absorption lines used in the present experiments located at 18789.28 cm“. The profile was taken with the cell operated at Tee,, = 358 K and T, = 318 K.

Figure 4 gives a schematic of the stagnation-flow flame studied using FRS and PIV. The substrate was water cooled to 300 K. The distance between the substrate and burner (L) was varied from 1.25 to 2.5 cm in this study. Three different substrates were investigated with diameters of 1.9 cm, 3.2 cm, and 5.1 cm. The burner was operated at three different flow conditions as given in Table 1. For the copper array burner used to

study the feasibility of simultaneous FRS and PIV measurements, two independent sources of air were used with one passing through the fluidized bed particle seeding vessel. This allowed the flow rates to be regulated to create the desired flame conditions, while maintaining the particle density for the PIV measurements. The PIV and FRS camera systems were located on opposite sides of the stagnation-flow flame as shown in Figure 1.

THEORETICAL DESCRIPTION

When interrogating a flow field with a laser and collecting the Rayleigh scattering signal from molecules within the flow field, there are several important parameters governing the scattered intensity (i.e. intensity of the illuminating light, polarization, frequency of the illuminated light, etc.). In the current use of the FRS technique in combustion environments, the intensity and spectral profile of the scattering are needed to deduce the temperature. Since the flames studied here are at atmospheric pressure, Brillouin scattering effects should be considered. Several different models exist for calculating the Rayleigh scattering spectral profile (for example see Yip and Nelkin, 1964 and Tenti et al., 1972), and these have been confirmed by experimental measurements (Lao et al., 1976). The shape of the scattered spectrum is typically parameterized by the dimensionless frequency X and the y parameter. The frequency is nondimensionalized by the thermal broadening and is given by by

(1)

where ;1 is the wavelength of the incident light, (v -v,) is

3


the frequency difference from line center, 8 is the angle between the incident and scattered wave vectors, k is the Boltzmann constant, T is the temperature of the gas, and M is the molecular mass. The y parameter, which is the ratio of the collisional frequency to the acoustic spatial frequency, characterizes the shape of the Rayleigh scattering spectrum and is given by

ap J

M ’ = 4psin(O/2) 2kT

(2)

Here, P is the pressure and u is the viscosity. For y of order of unity or greater (the kinetic regime), Brillouin components become important and kinetic models must be used. For y <Al (low pressures or high temperatures) the scattering spectrum is Gaussian and the Brillouin components can be neglected (Pitz et al., 1976). The total spectral Intensity (Pitz et al., 1976) is given by the intensity of the scattering of the ith gas species weighted with its mole fraction x, and Rayleigh cross section qi and is given by

I(v) = CI,Nx, Xi OR, ri(T.P,Mi,B7V) (3)

where C is the optics calibration constant, I, is the incident laser light intensity, N is the total number density, and r, is the scattering distribution of the ith gas species determined by the kinetic model (i.e. S6 model used by Tenti et al., 1972). The intensity collected by the camera is a convolution of the total spectral intensity (I(v )) and the transmission profile (A(v)), as shown schematically in Figure 5, and is given by

S(v) = I(v’)A(v - v’)dv’ s (4)

The scattering from particles and surfaces has a narrow linewidth and is attenuated by the iodine filter. Since the Rayleigh scattering from molecules is thermally broadened, part of the scattered intensity (16% to 42%) passes outside the absorption profile (as illustrated by the shaded region of Figure 5). This is an important characteristic of FRS, since molecular Rayleigh scattering is generally weak relative to surface and particle scattering. In order to eliminate the optical calibration constant on the incident laser it-radiance distribution, flame images were nondimensionalized by

scattering collected from the air at ambient conditions. For the present experiments only premixed flames

are considered and the dominant species is nitrogen, therefore the scattering will be assumed to be mostly from nitrogen. This is a common assumption in Rayleigh scattering temperature measurements in combustion and has been evaluated in the uncertainty analysis shown in detail elsewhere. With these assumptions, and since the

_flames are all at atmospheric pressure, the intensity is a function only of temperature for a given optical arrangement; thus the normalized intensity is

The local gas temperature influences the collected intensity through the gas number density (N) and by changing the thermal broadening, which increases the portion of the scattering transmitted by the filter. For the stagnation-flow flame experiments the convolution of the Rayleigh scattering with the iodine line used (18787.8 cm-‘) results the temperature versus normalized intensity shown in Figure 6. Thus, if the normalized intensity ratio (S(v)/&,.(v)) is known the temperature can be determined from the profiles shown in Figure 6. It should be noted that different absorption lines were used in the initial study of the Hencken burner, but the trends in the calibration curve are similar (see Elliott et al., 1997 for additional details).

It should be noted that the profile can be shifted in frequency relative to the illuminating laser due to the Doppler shift which is given by

where k, and k, are the observed and incident unit light wave vectors, respectively, and 1 is the flow velocity vector. In the present experiments, however, the flow rates are relatively low and the scattering is collected normal to the primary velocity component, resulting in a negligible Doppler shift effect.

TEMPERATURE MEASUREMENTS

,Feasibilitv of FRS Thermometrv In order demonstrate the feasibility of using FRS to

obtain accurate temperature measurements a hydrogen air

4


flame was created using a Henken burner operated at various equivalence ratios. Figure 7 presents the two- dimensional temperature field (x-y view) for a hydrogen- air flame (equivalence ratio 4 = 0.25) 2 cm above the Henken burner with the temperature profile in the center of the appropriate image given below. It should be noted that no particular care was taken to physically eliminate particles in the flame or in the surroundings. Scattering from particles generally will dominate the Rayleigh scattering; however, in the present experiment particle scattering is strongly absorbed smce it is not thermally broadened. The adiabatic flame temperature in this case is 1065 K. Figure 7a shows a single-shot instantaneous temperature field with the average temperature away from the edges of the flame being 1062 K. The instantaneous profile has large temperature variations in what should be a relatively flat profile, as shown by CARS measurements. Most likely this temperature variation is due to the photon shot-noise statistics. Thus the signal- to- noise ratio can be improved by either increasing the number of photons collected (more laser energy per pulse), low pass-filtering, or binning multiple pixels together. Figure 7b shows the effect of binning 3 adjacent pixels together (Fig. 7~). As expected, the random temperature fluctuations decrease, but at the cost of decreased spatial resolution (0.1 mm’ to 1 mk~ ). Figure 7c is the two-dimensional temperature field made by averaging 40 instantaneous images together. As expected this shows the least amount of temperature fluctuation, but of course the instantaneous information is lost.

Figure Sa and 8b present the average temperature and standard deviation in the flat portion of the flame 2 cm above the burner surface. The temperatures measured with FRS show excellent agreement with the adiabatic flame temperatures and the CARS measurements. The maximum deviations are at the highest temperatures with the FRS measurements always slightly lower, as expected. It should be noted that the CARS measurements use the highest temperature as a calibration and may actually have even better agreement with the FRS measurements than is shown here. The standard deviation of the temperature across the flat portion of the flame is given in Figure 8b for the instantaneous, bin-by- three, and average temperature fields for various equivalence ratios. Binning the pixels of the instantaneous image by three decreases the signal to noise ratio by approximately three for all cases which would be the case if the fluctuations were a result of photon statistics. The percent of temperature fluctuation for the average, instantaneous, and instantaneous bin-by-three temperature fields were found to be approximately 2%,

8%, and 3% respectively. A detailed uncertainty analysis [Elliott et al., 19971 gave an uncertainty of approximately 8% for these instantaneous measurements.

One of the greatest advantages of F’RS for two- dimensional temperature measurements is in its ability to capture instantaneous fluctuations of unsteady flames. Figure 9 shows the average (Figure 9a) and instantaneous temperature fields (Figs. 9b-d) of the premixed methane- nitrogen-oxygen flame with an equivalence ratio of 1.21 and an adiabatic flame temperature of 2510 K. Using FRS the flame temperature was measured at 2265 K. As expected, the measured temperatures are below the adiabatic flame temperatures due to the averaging process, heat losses to the burner surface, and mixing with ambient air. The temperature in the premixed region would be expected to be at a temperature of approximately 300 K, but was measured to be lower due to the presence of the methane which has a much higher Rayleigh scattering cross section. Almost all the instantaneous images show buoyantly driven vortices rolling up on the edge of the flame and the associated temperature variation within them. A detailed uncertainty analysis has be conducted previously indicating an uncertainty of approximately 3% in the reaction region of the flame [Elliott et al., 19971

FRS Thermometrv in Stapnation-Flow Flames The investigation of stagnation-flow flames as

illustrated in Figure 4 is primarily motivated by applications in material processing. These applications include atmospheric and low pressure stagnation-flow flames used to synthesize polycrystalline diamond films and nanopowders. One of the essential processing parameters which should be controlled is the temperature gradient in the flame and across the cooled substrate. The axial temperature profile has been measured using CARS by Bertagnolli and Lucht [1996] and laser induced fluorescence by Glumac [ 19961. Both investigators compared their measurements with 1-D combustion models and reported temperatures above the adiabatic flame temperature for acetylene/oxygen flames under some conditions. Currently, there are few nonintrusive experiments which have been conducted to measure the two dimensional temperature field in these axisymmetric stagnation-flow flame geometries. In particular one would like to know the accuracy of 1-D models used, as well as, when radial temperature variations become important for varying experimental parameters.

Figure 10 gives the average temperature field measured using FRS (calculated using 300 instantaneous images) for varying flame conditions (given in Table I), burner substrate separation distances (L), and substrate

5


diameters (D,). The ability of the IRS thermometry to measure the temperature close to surfaces was particularly important so that the temperature field near the substrate could be quantified. Comparing the temperature field across the various diameter substrates, the largest substrate diameter (Fig. 10~ and d), has a flatter radial temperature profile next to the substrate over a greater percentage of the substrate. It is also observed that the flat temperature profile terminates radially into a low temperature region of the flame for the largest substrate diameter (D, = 5.1 cm), and high temperature region of the flame for the smaller (D, = 1.9 cm and D, = 2.5 cm) substrate diameters. The extent of the radially flat profile will be quantified shortly.

Figures 1 la and b shows the axial temperature profile for the two burner-substrate separation distances for all three substrate diametersand compares the profiles to a I-D computational model. The axial temperature profile for the experiments was calculated by averaging the temperature over 10 radially spaced pixels at each axial position. The computational mode1 used for comparison of experimental results is similar to the Sandia SPIN code with the details given elsewhere (Glumac and Goodwin, 1996). The GRI-Mech combustion mechanism contains 177 reactions and 32 species. In comparison the mode1 and FRS temperature profiles are similar, but the computational profiles for both separation distances show a flatter temperature profile within the gap, while the experimental temperature profile decreases in the gap linearly as the substrate is approached. Initially it was believed that this may be due to inaccuracies in measuring the background intensity, but measurements of the background were verified by introducing helium into the chamber. Helium has a small Rayleigh scattering cross-section, leaving only the background signal. Therefore, it is thought that the discrepancy may be due to the fact that the mode1 neglects radial heat transfer and radiation. This is partly confirmed by the fact that the difference between the experiment and model is reduced, for shorter substrate- burner separation distances. Further investigations are needed to confirm this observation.

The axial temperature profile is given in Figure 12 for varying flow rates. Both computations and experiments suggest that the flame temperature over the central portion of the gap increases by slightly over 100 K as the flow rate is doubled. Again the computational mode1 shows a flatter temperature profile within the gap, while the experimental temperature profile decreases in the gap linearly as the substrate is approached for the reasons discussed previously.

In order to-compare the radial uniformity of the temperature profile above the substrate the diameter (D,) of the radially uniform temperature region is defined by the distance between the temperature at the center of the flame and where it changes by lo%, (higher or lower). The radially uniform diameter at an axial distance 2 mm below the substrate is plotted in Figure 13 for all stagnation-flow flames studied. For the two smallest substrates (with diameters smaller than and roughly equal to the burner), the uniform region is roughly 8.5% of the substrate diameter, while, for the largest substrate (60% larger than the burner), the uniform region is actually slightly larger than the substrate diameter. Additionally it should be noted that the flat portion of the largest substrate diameter terminates at a lower temperature while D, terminates with a higher temperature for the smaller two substrate diameters (D, = 1.9 and 3.2). As the distance between the substrate and burner is increased there is a general trend that D, decreases slightly for the two smaller diameters, but slightly increases for the largest substrate diameter. As the flowrate is increased Dr is observed to slightly also slightly increase.

In order to further study the stagnation-flow flame and extend the capability of FRS thermometry. the feasibility of simultaneously recording the velocity field using PIV was investigated. This is possible since the scattering from particles can be absorbed by the filter. Since particles had to be introduced through the burner the smaller diameter copper burner (CH,-B) was used for this study. Figure 14 shows an example of the PIV velocity field overlaid with the IRS temperature field for a stagnation-flow flame arrangement. Both instantaneous (144 and average (14b) of 100 instantaneous temperature/velocity fields are presented. The substrate used in this study was 3.2 cm in diameter and located 3.2 cm above the burner. The velocity vectors were calculated using standard space-correlation techniques between the initial and time delayed PIV image. The PIV analysis program was developed by Innovative Scientific Solutions Inc. For the instantaneous images, it was found that the scattering from the PIV particles could saturate the FRS image. This results in a relatively low seeding density which is problematic for accurate PIV measurements reducing the spatial resolution of the measurement, and allows velocity vectors to be calculated in the flame only. In order to calculate the instantaneous temperature field, a threshold was set and the temperature was interpolated around regions where particles exist. Although the instantaneous temperature/velocity field image appears uniform, large regions of this image had to undergo this threshold/interpolation process; and therefore current results are somewhat suspect. With a

6


few improvements to the system this problem may be reduced. First, the line center absorption depth of the filter could be increased by using a different absorption line with increased absorption Also It should be noted that investigators have reported that the line center absorption measured with similar Nd:YAG lasers are less than theory would predict [Forkey et al., 19951. Solving this discrepancy would help the filter better absorb the strong particle scattering. Also it may be possible to employ cameras with a greater dynamic range which may reduce particle saturation and blooming problems or use smaller PIV particles to reduce their scattered signal. The instantaneous velocity and temperature field, however, shows the general trends expected for a stagnation-flow flame. An instantaneous structure at the air flame boundary on the left side of the flame, captures a region where the velocity vectors indicate the entrainment of cooler ambient air and the temperature subsequently is reduced (R = -0.7 and X = 0.9). It should be emphasized that these are preliminary results and much more work is needed to make simultaneous FRS and PIV an accurate technique.

For the average PIV and FRS image (Fig. 14 b) the two techniques could be incorporated independently. This allowed the particle seeding density for the PIV images to be increased, so that a smaller correlation region could be used and particles could be introduced into the ambient air so that the velocity vectors could be calculated there as well. Again the temperature and velocity fields show expected trends associated with a stagnation-flame flow with the axial velocity reducing as the substrate is approached, and the radial velocity going from zero at the center of the substrate and increasing with the radial distance. In the near future, the velocity field will be measured using PIV for the FRS stagnation- flow flames described previously.

CONCLUSIONS

Filtered Rayleigh Scattering (FRS) was demonstrated for two-dimensional thermometry in reacting flows. Temperatures were recorded in laboratory flames containing particles and/or near surfaces using molecular Rayleigh scattering. Hydrogen- air flames created using a Hencken burner, and premixed methane-oxygen-nitrogen flames were investigated. The accuracy of the FRS measurements was tested by comparing FRS-derived temperatures with calculated values and temperatures recorded with coherent anti- Strokes Raman spectroscopy (CARS). For these flames, the FRS method gave temperatures within 2% of the

expected value (from measurement and/or calculation). Instantaneous temperature measurements were demonstrated in Methane-air flames in buoyantly driven flames and average temperature fields were measured in stagnation-flow flames. Also the feasibility of simultaneous FRS temperature field measurements and PIV velocity field measurements were demonstrated.

ACKNOWLEDGMENT

The Authors would like to acknowledge the Air Force Research Laboratory and the National Science Foundation with Dr. John Foss for their support in funding this study. Also the assistance of the staff at Rutgers University and Dr. William Weaver at Innovative Scientific Solutions is greatly appreciated.

REFERENCES

Bertagnolli, K.E., and Lucht, R.P. (1996). “Temperature Profile Measurements in Stagnation-Flow, Diamond-Forming Flames Using Hydrogen CARS Spectroscopy,” Twenty-Sixth Syt7zposillt?l (Internahonal) on Cotnbustion, pp. 18251833.

Cattolica. R.J., and Stephenson, D.A. (1983). “Dynamics of Flames and Reactive Systems,” Bowen, J.R., Manson, N., Oppenheim, A.K., and Soloukhin, R.I. (Eds.), Progress in Astronautics and Aeronautics.

Dibble, R.W., Long, M.B. and Masri, A. (1985). “Dynamics of Flames and Reactive Systems,”

Bowen, J.R., Leyer, J.C., Soloukhin, R.I. (Eds.), Progress in Astronautics and Aeronautics.

Elliott, G.S., Glumac, N., and Carter, C.D. (1997). ‘Two- Dimensional Temperature Field Measurements Using a Molecular Based Technique,” Combustion Science and Technology, Vol. 125, pp. 351-369.

Elliott, G.S., and Samimy, M. (1996) “A Molecular Filter Based Technique for Simultaneous Measurements of Velocity and Thermodynamic Properties,” AIAA Paper, AIAA 96-0304.

Forkey, J.N., Finkelstein, N.D., Lempert, W.R., and Miles, R.B., (1995). “Control of Experimental Uncertainties in Filtered Rayleigh Scattering Measurements,” AMA Paper, AIAA 95-0298.

Fourguette, D.C., Zurn, R.M., and Long, M.B. (1986). “Two-Dimensional Rayleigh thermometry in a Turbulent Nonpremixed Methane-Hydrogen Flame,” Combustion Science Technology, Vol. 44, No. 30.

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Glumac, N.G.. and Goodwin, D.G. (1996). “Diagnostics and Modeling of Strained Fuel-Rich Acetylene/Oxygen Flames Used for Diamond Deposition,” Cornb&iorr and Flume. Vol. 105, pp. 321-331.

Glumac, N.G. (1996) “Flame Temperature Predictions and Comparison with Experiment in High Flow Rate, Fuel-Rich Acetylene/Oxygen Flames,” Combustion Science Technology, Vol. 122, pp.383 -398.

Hoffman, D., Munch, K.U., and Leipertz, A. (1995)“Two-Dimensional Temperature Determination in Sooting Flames by Filtered Rayleigh Scattering,” Optics Letters. Vol. 21, pp. 525.526.

Lao, Q.H. Schoen, P.E.. and Chu, B. (1976). “Rayleigh- Brillouin Scattering of Gases with Internal Relaxation,” The Journal of Chemical Physics, Vol. 64, No. 9, pp. 3547-3555.

Miles, R.B., Forkey, J.N., and Lempert, W.R. (1992). “Filtered Rayleigh Scattering Measurements in Supersonic/Hypersonic Facilities,” AMA Paper, A&4-92-3894.

McMillin, B.K., Palmer, J.L., Seitzman, J., and Hanson, R.K. (1993). ‘Two Lme Instantaneous Temperature Imaging of NO in a SCRAMJET model Flowfield,” AIAA-93-0044.

Pitz, R.W., Cattolica, R., Robben, F., and Talbot, L (1976). “Temperature and Density in a Hydrogen- Air Flame from Rayleigh Scattering,” Combustion and Flame, pp. 3 13-320.

Seitzman, J.M., Kychakoff, G., and Hanson, R.K. (1985). “Instantaneous Temperature Field Measurements Using Planar Laser- Induced Fluorescence,” Opfics Letters, Vol. 10, p. 439.

Seitzman, J.M., Palmer, J.L., Antonio, A.L., Hanson, R.K., Debarber, P.A., and Hess, C. (1993). “Instantaneous Planar Thermometry of Shock Heated Flows Using PLIF of OH,” AfAA Paper, AIAA-93-0803.

Tenti, G., Boley, C., and Desai. R. (1974). “On the Kinetic Model Description of Rayleigh-Brillouin Scattering -from Molecular Gases,” Canadian Journal of Physics, Vol. 52, pp. 285-290.

Yip, S. and Nelkin, M. (1964). “Application of a Kinetic Model to Time-Dependent Density Correlations in Fluids,” Physical Review. Vol. 135, pp. A1241- A1245.


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1 \

-_____-__-__---------- I I I

I

I I

I

I

I

I

I I

I

I I

I

I

-

Figure 1. Schematic of laser and optical arrangement for FRS temperature measurements.


1.1

1 .o

0.9

0.8

0.7

2 0.6 .

0.5

0.4

0.3

0.2

0.1

0.0

Figure 2. Schematic of iodine cell.

A

J

I I I I I I

18787.6 18787.8 18788.0 18788.2 18788.4 18788.6

Wavenumber (cm-‘)

Figure 3. Doppler-broadened absorption lines of I? (in the B?J+’ - X’Z,t system) within the frequency-tuning range of the Nd:YAG laser.

- -


L

Substrate

m Burner

Figure 4. Schematic of stagnation-flow flame.


.

Background Scattering from Surfaces and Particles

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Absorption Filter Profile

Rayleigh Scattering Spectral Profile

5 6 7 8 9

Frequency

Figure 5. Schematic of absorption profile convoluted with molecular Rayleigh scattering.

2300

2100

1900

1700

500

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

SW / wa,,

Figure 6. Intensity ratio for temperatures measured using FRS in reacting flows.

4 1400

1200

5s Iwo e 9 ;; 600 ki z 000 z 400

~ 200

0

b) 1400

Radius (mm) Radius (mm)

c) 1400

1200

1500 K

1250

” v 500’

150 -100 -50 0 50 100 150

Radius (mm) 250

Figure 7. Instantaneous (a) bin by 3 (b) and average (c ) images and temperature profiles taken using FRS above a hydrogen-air flame.

300 d loo ” ” ” ” ” ” ” ’ ” “I’

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

E!.quiv. Ratio

- Avgys +I- -t- htantanc.msEtiiby3

,, ! I I I. , , /I, I I

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Eqiv. Ratio

Figure 8. Average temperature (a) and standard deviation (b) of the temperatures for a hydrogen- air flame operated at various equivalence ratios.

I .

lli


Q N

Q

Temperature Scale

500 1000 1500 2ooo 2500 K

g) CH,-D: D. = 2.5 cm. L = 2.5 cm. b = 1.04

f) CH,-C: D, = 5.0 cm, L = 2.5 cm, @ = 1.08

Figure 10. Filtered Rayleigh Scattering temperature field of axisymmetric stagnation-flow methane/air flames for varying substrate diameters (D,), burner-substrate separation distances (L), and flame equivalence ratio (0).

+

L


a) 2200

2000

1800 1

1600 -

q 1400 -

f 1200 - iij z g. 1000 -

F 800 -

600

400

I A 1.9 cm Substrate . . . .I.. . 3.2 cm Substrate

b) 2200 -

2000 -

1800 -

1600 -

1400 - s f 1200 - -;r; g 1000 -

’ 800 -

600 -

400 -

200 -

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Axial Distance From Burner(cm)

+ 1.9 cm Substrate . . .*. , . 3.2 cm Substrate

,-.a-.- 5.1 cm Substrate ---c-- Computer Model

0.0 0.5 1.0 1.5 2.0 2.5

Axial Distance From Burner(cm)

Figure 11. Axial temperature profiles of a stagnation-flow methane/air flame for burner-substrate separations of 1.3 cm (a) and 2.5 cm (b) measured using FRS for varying substrate diameters and evaluated using a 1-D computational model.

(c)l999 American Institute of Aeronautics & Astronautics $

,

2200 -

2000 -

1800 -

1600 -

E 1400 -

2 1200 - tl

e r-” 1000 -

800 -

600 -

400 -

200 -

I- FRS CH, _ D, 6.3 SLM . . . .* . FRS. CH, . C, 8.9 SLM .-.m-.- FRS. CH, - E, 11.4 SLM ---O--- -A--

I- U- ‘. Model.. CH, - E, 11 4 SL

f

0.0

I

0.5

I I I

1.0 1.5 2.0

Axial Distance From Burner (cm)

I I

2.5 3.0

Figure 12. Axial temperature profiles of a stagnation-flow methane/air flame for burner-substrate separation of 2.5 cm measured using FRS and evaluated using a 1-D computational model for varying flow rates.

m CH,-C:L=1.3cm 0 CH, - C: L = 2.5 cm 177771 CH,-D:L=2.5cm EXil CH, - E: L = 2.5 cm 1

3.2

Substrate Diameter (cm)

5.1

Figure 13. Diameter of the uniform temperature region 2 mm above the substrate for various operating conditions.

3

L

.


a> 3.5 -

r

8 1.5 ii E

--

P 1

0.5 1 0 -2

-2ords

Temperature Scale

2500K

I I I I I I , , , , , , , ( , , , , ,

-1 0 1 2 R [cm1

- 2ords

0~“““““““““‘I -2 -1 0 1 2

R km1

Figure 14. Instantaneous (a) and average (b) simultaneous temperature field measurements made using Filtered Rayleigh Scattering and velocity vectors measured using Particle Image Velocimetry (Ch, - B: o = 1.1, T, = 2104 K).

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