NASA Contractor Report 178245 

Appl ication of CARS to Scrarnjet Combustion 

(YASB-CR- 178245) APPIICATXCI CE CABS TO N87-20400 

SCBABJET CCBBCSIXCN €ha1 Kejor t (Systeas 

Fesearch Labs-, Inc.) 32 F. CSCL 07D 

Unclas 

63/25 45354 

Richard R. Antcliff 

Research Applications Division 

Systems Research Laboratories, Inc. 

2800 Indian Ripple Road 

Dayton, OH 45440-3696 

Contract NAS1-16562 

April 1987 

Natlonal Aeronautrcs and 

Space Adminlstratlon 

Langley Research Center 

Hampton. Virginla 23665-5225 
Preface 

This final report was prepared by the Research Applications Division of 

Systems Research Laboratories, Inc., under NASA Contract NAS1-16562, 

with Dr. G. Burton Northam as NASA Project Monitor. The research was 

conducted by Dr. Richard R. Antcliff. This report describes efforts 

performed during the period 12 March 1981 through 2 October 1986. 

i ii 
iv 
t I 

TABLE OF CONTENTS 

Section Page 

INTRODUCTION ............................ 1 

THEORETICAL BACKGROUND ....................... 1 

CARS SYSTEM ............................ 3 

Laser System ......................... 3 

Signal Generation ....................... 5 

Referencing .......................... 5 

Nitrogen Detection ...................... 7 

Oxygen Detection ....................... 8 

System Calibration ...................... 10 

COMPUTER CONTROL .......................... 12 

Data Reduction ........................ 13 

CARS Signal Errors ...................... 14 

CARS DATA-REDUCTION ERRORS ..................... 15 

SUBSONIC DIFFUSION BURNER ..................... 16 

COMPUTATIONAL FLUID DYNAMICS .................... 18 

SUMMARY .............................. 22 

REFERENCES ............................. 24 

V 
LIST OF ILLUSTRATIONS 

Page Figure 

1 

2 

3 

4 

5 

6 

7 

a 

CARS Schematic. 

mirror, F - filter, A - polarization analyzer, 

T - telescope, 1/2x - half-wave plate, 1/4x - quarterFocal configuration. Four laser beams enter from left 

DC - doubling crystal, R - dichroic 

wave plate, CL - cylindrical lens ............. 

and six exit at right 

4 

................... 6 

Polarization configuration required fororejection of 

nonresonant background. 0 = 30 . 8 = 60 

Calibration data from flat-flame burner 

......... 9 

.......... 11 

Shadowgraph of combustion produced by coaxial diffusion 

burner .......................... 17 

CARS data and CFD calculation along centerline ...... 19 

jet diameters downstream ................. 20 

22.1 jet diameters downstream ............... 

CARS data and CFD calculations from radial profile 5.5 

CARS measurements and CFD calculations from radial profile 21 

vi 
INTRODUCTION 

Physical probes have provided combustion researchers with temperature 

and pressure information for many years. However, interference with the 

flow properties and degradation in hostile environments limit the use of 

probes. 

tions. 

(CARS), has the additional advantages of (1 ) high conversion efficiency, 

(2) a laser-like beam for high collection efficiency, fluorescence, and 

luminosity discrimination, and (3) high spatial and temporal resolution. 

These advantages must be weighed against the increased complexity of a 

CARS system and its limitations. These limitations include the presence 

of an interfering nonresonant background, the need for optical access, 

and the requirements for stability of the optical system. CARS has been 

applied to many complex environments including plasmas,’ internal combustion engines ,394 shock tubes ,5,6 full-scale combustors, ’-13 and a 

supersonic combustor. 

Optical diagnostic techniques generally overcome these 1 imi taOne of these techniques, coherent anti-Stokes Raman scattering 

14 

This report describes a system which has been developed at NASA Langley 

Research Center for simultaneous measurement of temperature and nitrogen 

and oxygen number density. The system has been described, in part, previously 15-18 as specific subsystems were developed. This system has 

been utilized to obtain a data base for comparison with computationalfluid-dynamics (CFD) calculations. These data have been included to 

illustrate the capabilities of the system. 

THEORETICAL BACKGROUND 

A CARS signal is generated as two laser beams at frequency o1 and one 

laser beam at frequency o2 are mixed to produce a fourth laser-like beam 

at the CARS frequency w3 = 201 - 02. The CARS signal is dependent upon 

the polarizability of the molecule. 

19 as a power series 

The polarizability can be written 

1 
where E is the electric field strength and x(’) is the susceptibility. 

It is the third-order terms which describe CARS and other nonlinear 

Raman techniques. 

shown2’ to be 

The intensity of the CARS signal, 13, has been 

where I, is the intensity at frequency wl; C is the speed of light; q3 

is the refractive index at w3; and L is the interaction length. This 

equation assumes that the required momentum phase matching has been 

satisfied and that the beams have the same focal diameter. 

linear susceptibility is a complex termz1 which has been simplified for 

20,22 normal CARS conditions 

The nonwhere N is the number density of the species probed; A 

tion difference between the states studied; do/dn is the Raman cross 

section and assumed to be constant; wr is the frequency of a Raman 

active rotational or vibrational state; r is the Raman halfwidth; and 

(3) is a collection of susceptibility terms which are generally a 

slowly varying function of frequency. The xi3) term can become important 

under resonance23 conditions, when near other rotational or vibrational 

 transition^,'^ or when the species density is low. 

is the popula- v,J 

v,J 

XB 

25 

From Eq. (3) it can be seen that as the difference between w1 and w2 

approaches a Raman resonance, the signal increases dramatically. Normally w1 is held fixed, and w2 is scanned to generate a CARS spectrum. 

However, in turbulent environments this is impractical since the sample 

changes rapidly. An alternative makes use of a broadband dye 

laser for w2. Utilizing an array detector allows the complete spectrum 

to be acquired in a single 10-ns laser shot. 

2 
CARS SYSTEM 

The CARS system at NASA Langley employs a Nd:YAG laser, broadband dye 

lasers, nonplanar BOXCARS beam a1 ignment, a reference signal generated 

in ambient air, and both photomultipl ier-tube (PMT) and photodiode-array 

detection. Figure 1 is a schematic diagram of the optical layout and 

will be referenced in the following description. 

Laser System 

The main pump laser was a Quanta-Ray DCR-1 Nd:YAG laser. This laser 

is normally operated at 10 Hz and emits Q 200 mJ in a 10-ns pulse at 

532 nm. The laser has been updated with new pump cavities and O-ring 

seals. The 532-nm output was produced with a Quanta-Ray HG-2 

temperature-control led frequency doubler. The temperature control is 

helpful in eliminating repeated tweaking of the crystal angle as the 

crystal heats up. The 532-nm output was separated from the residual 

1064-nm output by means of a Pellin-Broca prism. The residual output 

was directed through a second frequency doubler to obtain an additional 

532-nm laser beam. This portion of the 532-nm beam was used to pump the 

580-nm dye-laser oscillator. 

Two broadband (Q 200 cm") dye lasers were employed in this system. The 

oscillator of these dye lasers consists of a flowing dye cell oriented 

at Brewster's angle relative to the dye-laser optical axis. The oscillator was pumped slightly off axis in a cavity formed by a total and a 

30% reflector. A single amplifier stage was used for each dye laser. 

Several combinations of dye-laser concentrations and pump-laser energies 

were used in the oscillators and amplifiers to ascertain the optimum 

beam quality, power output, and stability. No advantage was found over 

using a simple 50/50 split of the laser energy between the oscillator 

and amplifier and the same dye concentration in both. 

The nitrogen-dye-laser (606.5 nm) concentration was Q 7.5 x 10'' molar 

rhodamine 640 in methanol. The oxygen-dye-laser (508 nm) concentration 

was - 5 x The output frequency of 

these lasers can be adjusted by altering the Concentration of the dye. 

molar rhodamine B in ethanol. 

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Signal Generation 

A nonpl anar BOXCARS 27928 beam configuration was used to maximize the 

spatial resolution of the system while maintaining optimum signalgeneration efficiency. 

about the focus. The 532-nm "pump" laser was split into two equal beams 

and positioned vertically on the focusing lens. The two dye-laser beams 

were positioned vertically but placed to one side of the "pump" beams. 

The dye-laser beams should be slightly outside an imaginary circle on 

the lens having a diameter equal to the "pump"-beam separation and a 

midpoint at the center of the lens. 

beams were spatially separated from the input laser beams, and the input 

beams could be readily trapped. The crossing and focusing element was a 

plano-convex 50-cm 1 ens. 

Figure 2 shows the laser-beam configuration 

With this configuration the signal 

A small nitrogen jet was used to obtain the experimental interaction 

length. This jet, surounded by an argon sheath, was stepped through the 

interaction length and the signal recorded by measuring the signal to 

10% on either side of the peak. The effective signal-generation length 

of Q 1 mn was determined. The interaction volume was assumed to be a 

cylinder having a diameter of 250 urn. 

Referencing 

Measurements obtained from a stable environment (e.g., ambient air) 

showed considerable fluctuation in the signal intensity. This fluctuation was due to several factors including shot-to-shot pump-laser fluctuations (both in intensity and frequency), variations in the broadband 

dye laser output due to mode competition, and movement of the beams 

causing changes in the interaction length and in the detector response. 

This fluctuation presented a problem in making concentration measurements since such measurements are dependent on the signal intensity. To 

alleviate this problem a referening system was established to compensate 

for variation in the signal-generation effi~iency.~' A 20% beam splitter was placed immediately after the crossing and focusing lens. This 

splitter, placed at 45 deg., formed a second focal volume in ambient 

air. The nitrogen and oxygen signals generated in this reference volume 

5 
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6 
and the signals generated from the data (flame) volume were detected 

simultaneously. A flat glass plate was placed immediately after the 

splitter in the reference leg to compensate for a path difference in the 

sample leg caused by the splitter. 

adjusted to produce optimum correlation between the sample and reference 

signals. This was accomplished through use of a real-time program which 

displayed the correlation as the system was adjusted. 

tions were better than 10% for the nitrogen signals and better than 4% 

for the oxygen signals. 

The reference leg was optically 

Typical correlaNitrogen Detection 

The complete nitrogen Q-branch spectrum was generated with a broadband 

dye laser. An intensified-si1 icon-photodiode (ISPD) array was used in 

this system to capture a complete spectrum with each laser shot. The 

electronic scanning of this array of diodes was controlled by a minicomputer as described below. 

the detector to disperse the input beam and, thereby, reject stray laser 

light. 

A 1-m monochromator was used in front of 

A study was made to determine the experimental dynamic range of the 

ISPD.18 Using calibrated sources the actual dynamic range of the 

instrument was found to be 

small measured range. First, detector noise limited the lower measurable limit to about 200 counts. Secondly, the spherical lens normally 

used to focus the CARS signal onto the detector was found to cause local 

saturation of the microchannel plate intensifier since the full detector 

height was not utilized. This dynamic range was insufficient for tracking the large signal-intensity variations associated with turbulent combu s ti on. 

100. Two major factors contributed to this 

Two modifications were made to the system to increase this dynamic 

range. 

chromator was replaced by a cylindrical lens. This lens produced a line 

image on the detector, thereby utilizing the full detector height. This 

improvement reduced the 1 oca1 saturation of the microchannel plate 

First, the spherical lens at the entrance to the detector mono7 
intensifier and improved the linear dynamic range by approximately a 

factor of five. 

external splitter arrangement as suggested by Goss.~’ The nitrogen CARS 

signal was directed through a series of splitters to produce three separate signal beams having intensities of Q l, 19, and 80% of the original 

signal. 

position and opened fully. 

onto the slit plane. The signals followed the path of the monochromator 

and were imaged side by side on the detector. 

intensity signals to be recorded simultaneously by the diode array 

detector and increased the dynamic range by a factor of 100, which was 

sufficient for these studies. The reference nitrogen signal was also 

imaged onto the horizontal slit. Thus, four separate spectra were 

simultaneously recorded by the array detector. 

The second improvement was the implementation of an 

The monochromator entrance slit was rotated to the horizontal 

The three signals were imaged side by side 

This allowed the three 

Oxygen Detection 

RCA 8575 Photomultipliers (PMTs) were used to detect the oxygen signals 

since only the integrated intensity of the signal was required. 

surements of the oxygen density are complicated by the presence of a 

nonresonant background signal at low oxygen densities. In this system 

the nonresonant background signal has been nearly eliminated by use of 

polarization ~election.~’ 

nant and nonresonant signals are different due to the electronic origin 

of the nonresonant signal. To take advantage of this difference, the 

polarization of the oxygen dye laser was rotated 60 deg. relative to the 

pump polarization. This was accomplished with a Karl Lambrecht 1/2-wave 

Fresnel Rhomb and a polarizing prism. A zero-order 1/4-wave retarder 

was also placed in the beam path to minimize elliptical polarization 

introduced by multiple optical elements. A polarizing prism was placed 

in the pump beam to improve the polarization characteristics of the 

beam. 

MeaThe polarization characteristics of the resoWith the scheme discussed above, the polarization of the resonant signal 

is the same as that of the pump beams (Fig. 3). The nonresonant signal, 

however, was rotated 30 deg. from the pump beam. The nonresonant signal 

was blocked by placing an analyzer orthogonal to the nonresonant 

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polarization axis. 

this configuration since the analyzer and resonant-polarization axis 

were separated by 60 deg. 

Some rejection of the resonant signal occurred in 

Bandpass filters were used to block stray light from the PMTs. 

initial complications were found due to the large bandpass of these filter~.~~ A rotational hydrogen line (5 - 7) was passed by these broad 

rejection filters and detected by the oxygen PMTs. 

eliminated through the use of narrower bandpass rejection filters, 

Some 

This signal was 

System Calibration 

The accuracy and precision of the CARS measurements were demonstrated by 

use of a flat-flame burner. This burner had been studied previously and 

found to be characterized by a we1 1 -behaved temperature prof i 1 e. The 

burner, produced by McKenna Industries, Inc., was composed of a 7.94-cmdiam. sintered-stainless-steel plug housed in a stainless-steel jacket. 

The burner was cooled by a water coil within the plug. The burner was 

operated with the air flow held constant at 100 g/min and the hydrogen 

flow adjusted over the stoichiometric fraction range of % 0.1 - 1.0. 

The water flow rate and temperature rise as well as the hydrogen and air 

flow rates were measured and recorded by the computer system. These 

experimental values were used to calculate the expected temperature and 

33 species concentrations in the flow. 

Figure 4 shows the comparison of the CARS and calculated values for temperature, nitrogen number density, and oxygen number density. The CARS 

values shown represent the mean of 100 data shots. The oxygen measurements have a minimum detectable limit of % 1 x 1017 molec./cc due to the 

interaction of the nonresonant and resonant suscepti bi 1 i ty which could 

not be completely eliminated because of incomplete polarization of the 

laser beams. 

The relationship between the intensity in the data legs and reference 

1 egs was also required for accurate calculation of species concentration. This was accomplished by acquiring data with ambient air in both 

the data and the reference sample volumes. 

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11 
COMPUTER CONTROL 

A complete description of the computer control of the Reticon detector 

has been given previously;16 therefore, only a summary will be given 

here. The multichannel detector used in this system was an EG&G PAR 

1420 Reticon. This detector contains 1024 individual diode elements, 

the center 700 of which were preceded by a fiber-optic-coupled, 

proximity-focused, microchannel plate intensifier. An EG&G PAR 1218 

Detector Controller was used for electronic control of the scanning and 

data acquisition of the detector. A Reticon interface was specially 

built to communicate with the diode-array controller. This interface 

handles separate input and output data lines, signal drivers for longdistance operation, and the inverted logic required by the controller. 

Scanning of the Reticon was controlled by random-access memory (RAM) 

within the 1218 controller. This memory provided each individual detector element (pixel) with predefined scanning instructions. These 

instructions were calculated by the MODCOMP and downloaded into the 

1218. This instruction set fast scans over the unintensified portion of 

the detector and provided normal analog-to-digital conversion over the 

center 700 pixel s. 

An assembly-language program was written to initiate the Reticon scanning. At the beginning of each scan cycle, a signal was sent from the 

1218 to fire the laser. 

plished between laser shots. The first was the data-acquisition scan, 

Three complete scans of the Reticon were accomand other two were used to clean the pixels of any remaining signa 

before the next laser firing. During these "clean scans" the data 

transferred to magnetic tape for processing. 

Collection of photomultiplier data was also triggered by the laser 

were 

pulse 

and, therefore, occurred simultaneously with the Reticon scans. 

Kinetics 1500 CAMAC Crate was used to house the PMT collection electronics. The PMT signals were sent to a LeCroy 2249SG 12-Channel Analog-toDitigal Converter. 

mable Dual Gate Generator. These signals were routed to the MODCOMP via 

an IEEE interface and stored on magnetic tape with the Reticon data. 

A 

This unit was triggered with a LeCroy 2323 Program12 
1 

An additional program was run simultaneously with the data-acquisi tion 

program to produce a real-time display of several system parameters. 

particular, burner flow conditions and PMT output were displayed. The 

Reticon output was displayed on an oscilloscope. 

In 

Data Reduc t i on 

To obtain the temperature, the experimental nitrogen spectra were compared with a library of precalculated theoretical spectra. These spectra were calculated in 20-K steps and stored on disk. The experimental 

data were retrieved from tape and corrected with a background data scan 

and a detector response function. The computer then selected the largest unsaturated sample from the three nitrogen data signals. 

were compared with a library of precalculated theoretical spectra to 

obtain the experimental temperature. 

The data 

Once the temperature had been obtained, the concentrations of nitrogen 

and oxygen were extracted. The number density N of a species at temperature t has been shown3' to be 

where N300 is the number density at 300 K; It is the experimental integrated intensity corrected for reference fluctuations; and 1300 is the 

integrated intensity at 300 K, obtained from a calibration run in room 

air. 

tral 1 i brary. 

The calculated values were generated from the precalculated specThe experimental nitrogen integrated intensity was obtained by summing 

the spectral channels selected by the fitting routine. Oxygen density 

calculations were simp1 ified by the use of photomultiplier tubes which 

yield integrated intensity directly. A time of Q 2.5 s was required for 

analysis of each measurement on the MODCOMP computer. The arithmetic 

mean, standard deviation, and 95% confidence 1 eve1 were calculated for 

each data set (100 laser shots) for comparison with CFD calculations. 

13 
CARS Signal Errors 

Errors must be considered which were associated with signal generation 

and signal detection. 

system is the fluctuation in the laser outputs. First, the YAG laser 

intensity varies considerably from shot to shot. Since this laser is 

used to pump the dye lasers, this variability is transferred to their 

output. Also, temperature changes in the doubling crystals effected a 

long-term change in the intensity output. A temperature-stabilization 

unit was installed on the primary doubling crystal to reduce this 

effect. 

in this system to compensate for this type of fluctuation. The spectral 

character of the broadband dye lasers was dependent upon the fluorescence and absorption of the dye used. However, because of the competition among the many available modes, the output was very structured and 

variable. This type of fluctuation cannot be tracked adequately by 

referencing and constituted a major source of noise. 

The most prevalent error associated with the CARS 

In addition, referencing of the generation efficiency was used 

In the current system, four laser beams were focused and crossed to form 

the sample volume. When sampling is conducted in the center of a turbulent environment, considerable beam steering may occur prior to the 

focus. This steering causes random misalignment of the beams and subsequent random decrease in signal intensity. As the signal beams exit 

through the sample, beam steering may occur again. 

move the beam on the detection optics and alter the collection 

efficiency . 

This steering may 

The size and position of the sample volume was also critical to the 

interpretation of the data. 

and products form and move downstream. These eddies have a broad size 

range. 

of the temperature and densities will occur, and true point measurements 

will not be possible. In addition, if the sample volume is positioned 

such that it measures two different flows (i.e., fuel and coaxial air), 

a weighted averaging will occur. 

exists between sample-volume size and absolute signal intensity. 

In the burner studied, eddies of reactants 

If the sample volume is larger than the eddy size, an averaging 

It should be noted that a tradeoff 

14 
The nonresonant signal can be a source of error. The nitrogen analys 

software assumed that the nonresonant signal was relatively low and, 

therefore, handled in the data reduction. The oxygen system, because 

the relatively low oxygen density, utilized polarization rejection of 

S 

of 

the nonresonant background. This technique requires excel lent polarization characteristics in the lasers and optics for achievement of complete rejection. 

acteristics of the laser beams were introduced by the large number of 

optical elements through which the laser beams had to pass. Therefore, 

the possible incomplete rejection of nonresonant background limited the 

minimum oxygen level detected. 

In this system distortions in the polarization charThe narrow fi 

hydrogen 1 ine 

signal. This 

density cal cu 

ter used before the oxygen PMTs to reject an interfering 

(see above) may have distorted the lineshape of the oxygen 

distortion could have caused an error in the oxygenation since it is dependent upon a known lineshape. The 

errors associated with the Reticon detector used in the nitrogen leg 

have been well documented.18 These errors were found to be due mainly 

to saturation of the image intensified on the front end of the photodiode array. 

CARS DATA-REDUCTION ERRORS 

The theoretical description of the CARS process was continuously being 

refined to fit the experimental data more accurately. These improvements are generally not needed for such low-resolution studies. However, they may introduce finite errors and, therefore, are discussed 

here. 

a simple function of constituent densities which were adiabatically calculated from the experimental temperature. The nonresonant values of 

 rad^^^ were used. These values have been disputed by Recently, 

more complex fits which include the nonresonant background as a variable 

have also been attempted with success.36 This system also assumed that 

the Raman linewidth follows a simple 1/T function. Rahn has shown that 

this assumption is an oversimpl ifi~ation.~~ The theoretical spectra 

must be convolved with the system laser and detector linewidth functions. 

In this system the nonresonant-background term was assumed to be 

The convolutions of Y~ratich~~ have been used for this system. 

15 
Recently, Teets3' and Katol ica4' introduced an improved convolution 

which more accurately takes into account the nonresonant interference. 

The 1 i brary interval (20 deg. ) also imposes a lower 1 imi t on the reduced 

data . 

SUBSONIC DIFFUSION BURNER 

The burner used in the present studies consisted of two coaxial tubes. 

The inner tube, which carried the fuel, had a 4.7-mm inner diameter and 

a 6.4-mm outer diameter. This tube was tapered at the exit plane to a 

lip thickness of 0.51 mm. The outer tube, which carrried air, had an 

inner diameter of 24.5 mm. The length of the burner was Q 45 cm. Typical flow rates were 100 m/s in the air tube and 15 m/s in the fuel tube. 

The fuel consisted of a mixture of 80% hydrogen and 20% nitrogen. The 

addition of nitrogen to the fuel was necessitated by the use of nitrogen 

as the temperature probe species. 

Figure 5 displays a spark shadowgraph of this burner under the abovestated flow conditions. This obviously very turbulent flow provided a 

good test of the dynamic range of the CARS system. Fast photography of 

this flow reveals the formation of large eddys which break up further 

downstream. 

The subsonic burner was mounted on a motorized X-Z translational system. 

The Y-axis (along the laser beams) was adjustable but not motorized. 

This system effectively allows the sample volume to move through various 

positions of the flame without alignment difficulties. 

The normal acqu 

air. After the 

was made. Each 

During the back 

sition sequence began with a calibration run in ambient 

burner had been started and stabilized, a background run 

data point was the accumulation of 100 laser shots. 

round run the dye-laser beams were blocked to allow only 

stray light to enter the detectors. The burner was then positioned by 

means of the motorized translational stages, and a CARS data point was 

obtained. One additional background point was obtained for approximately every five data points. 

16 
17 ORIGINAL PAGE TS 

OF #W)R Ollh .rfY 
An axial profile and two radial profiles were obtained above the coaxial 

burner. The axial profile began as close to the jet exit as physically 

possible and extended 35 fuel-jet diameters downstream (Fig. 6). 

radial surveys were located at 5.5 and 22.1 fuel-jet diameters downstream (Figs. 7 and 8). 

and extended 5 fuel-jet diameters to the outer portion of the flame. 

The 

These radial profiles began at the center line 

COMPUTATIONAL FLUID DYNAMICS 

The major application of the CARS results to date has been in the validation of computational -fluid-dynamics (CFD) computer-model ing codes. 

Considerable effort has been placed on generating codes for predicting 

complex combustion environments. 

data which can be used to refine the numerous adjustable parameters 

found in these codes. 

data. The first, a modification of a CHARNAL code,41 solves parabolized 

Navier-Stokes equations using a marching finite-difference algorithm and 

includes a two-equation turbulence model (k - E). Combustion was modeled by hydrogen-oxygen equilibrium chemistry, with the nitrogen being 

treated as inert. 

fraction, turbulent kinetic energy, static temperature, and static pressure at the initial station located 0.01 fuel-jet diameters downstream 

of the exit plane. Initial velocity and turbulence intensity profiles 

were obtained with a hot-wire anemometer using cold-air flows. Initial 

profiles of static temperature and pressure were assumed to be uniform 

over both jets at 300 K and 1 atm. Equilibrium chemistry assumes that 

the local mixture reacts instantaneously to an equilibrium mixture of 

products; this assumption is typically used with low flow rates. 

The second computational method used was the TEACH code, which is a 

modified version of the REACT2D4* code. The code solves the elliptical 

time-averaged Navier-Stokes equations using the k-E turbulence model for 

two-dimensional reacting flows. The eddy-breakup combustion model of 

Magnussen and Hjertager43 was used; this model relates the combustion of 

fuel to the breakup and mixing of turbulent eddies and the mean concentration of reacting species. A solid wall located at 4 jet diameters was 

used to approximate the air boundary. 

18 

CARS measurements provide experimental 

Initially, two separate codes were used with the 

Input to the code consisted of velocity, fuel mass 

The law of the wall velocity 

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21 
profile was used. At the outlet, axial gradients of all variables 

except velocity were assumed to vanish. The TEACH code iterated until 

the nondimensional residual was minimized. 

In general, the correlation between the CARS data and the data generated 

by the CFD codes was very good. The major differences were in the flamespreading rate and the peak temperature. These deviations are readily 

explainable by the assumptions built into the CFD codes. 

may contribute to this deviation--an overprediction of the heat release 

and an overprediction of the mixing rate. The heat release may be 

over-predicted for two reasons. 

adequately model the real system which has finite reaction rates. Secondly, these codes do not include a complete coupling model between 

turbulence and the chemical reaction. The mixing rate may have been 

overpredicted for three additional reasons. 

can cause numerical smearing; this effect was suspected to be minor 

since the flow was aligned along the mesh and refinements to the grid 

had only small effects upon the results. Secondly, the turbulence model 

itself, even for nonreacting flows, overpredicted the mixing of the jet. 

Thirdly, as the jet reacts, a low-density "buffer region" forms in the 

shear layer; the turbulence model did not account for the dynamic 

effects of this variable density. 

Two factors 

First, the equilibrium model may not 

First, numerical diffusion 

The CHARNAL code appears to furnish more accurate predictions of the 

mixing as indicated by the relatively close correlation between the 

experimental and calculated data in the axial profile and the lower 

predicted spreading rate in the radial profiles. However, the peak temperatures calculated by the TEACH code more closely match the experimental data. The lower peak temperature may be the result of the use of 

the eddy-breakup model. This model will weakly link the chemistry and 

turbulence, thereby preventing the system from obtaining the "equil ibr i um" temperature Val ue . 

SUMMARY 

A nonintrusive diagnostic system has been developed around a coherent 

anti-Stokes Raman spectroscopy instrument. This system is capable of 

22 
measuring temperature and nitrogen and oxygen number densities in turbulent combustion environments. High temporal and spatial resolution, 

luminosity discrimination, and a large dynamic range are important 

advantages of this system. 

gated with this system. 

Turbulent subsonic flow has been investi23 
. c 

REFERENCES 

1. N. Hata; A. Matsuda; K. Tanaka; K. Kajiyama; N. Maro; and K. Sajiki: 

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2. W. F. Lynn; P. P. Yaney; L. P. Goss; and S. W. Kizirnis: "Plasma 

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3. G. C. Alessandretti; and P. Violino: "Thermometry by CARS in an Automobile 

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4. D. Klick: K. A. Marko; and L. Rimai: "Broadband Single-Pulse CARS Spectra 

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of New York Press, Albany, New York, July 1981 3 P- 132. 

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AIAA-85-1106. * 

11. E. J. Beiting: "Multiplex CARS Temperature Measurements in a Coal-Fired 

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p. 1684. 

12. B. Attal; M. Pealat; and 3. P. Taran: "CARS Diagnostics of Combustion," 

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Jan. 14-16, 1980, AIAA-80-0282. 

24 
13. M. Alden; and S. Wallin: "CARS exDeriments in a Full Scale (10 X 10 ml 

Industrial Coal Furnace," Applied Optics, Vol. 24, No. 21, 'November i, 

1985, p. 3434. 

14. T. J. Anderson; I. W. Kay; and W. T. Peschke: "CARS Feasibility 

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JANNAF Combustion Meeting, Pasadena, CA, October 7-10, 1985, CPIA 

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presented at the 22nd 

15. R. R. Antcliff; and 0. Jarrett, Jr.: "Comparison of CARS Combustion 

Temperatures with Standard Techniques" in "Combustion Diagnostics by 

Nonintrusive Methods," eds. J. A. Roux and T. 0. McCay, Vol, 92 of 

Progress in Astronautics and Aeronautics 1984, p. 45. 

16. J. R. Posenau; and R. R. Antcliff: "Multichannel Detector Control with a 

MODCOMP Computer," presented at the Americas Annual MUSE Meeting, 

Ft. Lauderdale, FL., Dec. 4-9, 1983. 

17. R. R. Antcliff; 0. Jarrett, Jr.; and R. C. Roqers: "CARS System for 

Turbulent Flame Measurements,' Journal of Piopulsion and Power, Vol 1, 

No. 3, May-June 1985, p. 205. 

18. R. R. Antcliff; M. E. Hillard; and 0. Jarrett, Jr.: "Intensified Silicon 

Photodiode Array Detector Linearity: 

Raman Spectroscopy," Applied Optics, Vol. 23, No. 14, July 15, 1984, 

p. 2369. 

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Application to Coherent Anti -Stokes 

19. W. M. Tolles; J. W. Nibler; 3. R. McDonald; and A. 8. Harvey: "A Review of 

20. R. N. DeWitt; A. B. Harvey; and W. M. Tolles, "Theoretical Development of 

the Thi rd-Order Suscepti bi 1 i ty as Re1 ated to Coherent Anti -Stokes Raman 

Spectroscopy (CARS), 'I NRL Memorandum Report 3260, Apri 1 1976. 

21. H. Lotem; R. T. Lynch; and N. Bloembergen, "Interference Between Raman 

Resonances in Four-Wave Difference Mixing," Physical Review A, Vol. 14, 

No. 5, November 1976, p. 1748. 

22. L. P. Goss; and P. W. Schreiber: "Assessment of the Application of CARS to 

Combustion Diagnostics," Proceedings of the International Conference on 

LASERS '80, 1980, p. 220. 

23. J. F. Verdieck; R. J. Hall; and A. C. Eckbreth: "Electronically Resonant 

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Methods," eds, J. A. Roux and T. D. McCay, Val 92 of Progress in 

Astronautics and Aeronautics 1984, p. 58. 

24. L. A. Carreira; and R. R. Antcliff: "Applications of Resonance Enhanced 

CARS," in"Advances in Laser Spectroscopy," Vol. 1, Chap. 6, 1982, p. 121. 

25 
25. F. Moya; S. Druet; and M. Pealat: "Flame Investigation by Coherent 

Anti -Stokes Raman Scattering," presented at the AIAA 14th Aerospace 

Sciences Meeting, Washington, D.C. , January 26-28, 1976, AIAA-76-29. I 

26. W. B. Roh; P. W. Schreiber; and J. P. E. Taran: "Single-Pulse Coherent 

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August 1, 1976, p. 174. 

27. Y. Prior: "Three-Dimensional Phase Matching in Four-Wave Mixing," Applied Optics, Vol. 19, No. 11, June 1, 1980, P. 1741. 

28. 3. A. Shirley; R. J. Hall; and A. C. Eckbreth: "Folded BOXCARS for 

Rotational Raman Studies," Optics Letters, Vol. 5, No. 9, September 1980, 

p. 380. 

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Coherent -Anti -Stokes Raman Spectroscopy Technique," Journal of Energy, 

Vol. 7, No. 5, May 1983, p. 389. 

30. L. P. Goss; 0. D. Trump; B. G. McDonald; and G. L. Switzer: "10-Hz 

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Combustion Studies," Review of Scientific Instruments, Vol. 54, No. 5, 

May 1983, p. 563. 

31. L. A. Rahn; L. J. Zych; and P. L. Mattern: "Background-Free CARS Studies 

of Carbon Monoxide in a Flame," Optics Communications, Vol. 30, No. 2, 

August 1979, p. 249. 

32. R. R. Antcliff; 0. Jarrett, Jr.; R. C. Rogers; and T. Vanoverbeck: 

"Multispecies in Turbulent Combustion," presented at the 23rd JANNAF 

Combustion Meeting, Hampton, VA, October 20-24, 1986. 

33. V. R. Mascitti: "A Simplified Equilibrium Hydrogen-Air Combustion Gas 

Model for Use in Air Breathing Engine Cycle Computer Progress," NASA 

TN D-4747, 1968. 

34. W. G. Rado: "The Nonlinear Third Order Dielectric Susceptibility 

Coefficients of Gases and Optical Third Harmonic Generation," Applied 

-- Physics Letters, Vol. 11, No. 4, August 1967, p. 123. 

35. R. J. Hall: "Theoretical Modelina of the CARS SDectra of Combustion 

Molecules, I' Proceedings of the international Conference on Lasers, 

published by S.R.S. Press, McL ean, VA 9 1982, P. 726. 

36. R. J. Hall; and C. R. Boedecker: "CARS Thermometry in Fuel-Rich Combustion 

Zones," Applied Optics, Vol. 23, No. 9, May 1, 1984, P- 1340. 

37. L. A. Rahn; A. Owyoung; M. E. Coltrin; and M. L. Koszykowski : "The J 

Dependence of Ni trogen 'Q' Branch Li newi dths, I' Proceedi ngs Of the VI Ith 

International Raman Conference, pub1 i shed by North Ho1 land, NY, 1980, 

P* 694. 

- 

26 
38. M. A. Yuratich: "Effects of Laser LInewidth on Coherent Anti-Stokes Raman 

Spectroscopy," Molecular Physics, Vol. 38, No. 2, 1979, p. 625. 

39. R. Teets: "Accurate Convolutions of CARS Spectra," Optics Letters, Vol. 9, 

No. 6, June 1984, p. 226. 

40. ti. Kataoka; S. Maeda; and C. Hirose: "Effects of Laser Linewidth on the 

Coherent Anti -Stokes Raman Spectroscopy Spectral Profile," Applied 

Spectroscopy, Vol. 36, 1982, p. 565. 

Hydrogen-Air Jets in Local Chemical Equilibrium (A Guide to the CHARNAL 

Computer Program) ,I' NASA CR-2407, June 1974. 

41. D. 6. Spalding; B. E. Launder; A. P. Morse; and G. Maples: "Combustion of 

42. L. M. Ciapetta: "Users Manual for a TEACH Computer Program for the 

Analysis of Turbulent, Swirling Flow in a Research Combustor," United 

Techno1 ogies Research Center Report R83-915540-27, 1983. 

43. B. F. Magnusen; and 8. H. Hjertager: "On Mathematical Modeling of 

Turbulent Combustion with Special Emphasis on Soot Formation and 

Combustion," presented at the Sixteenth Symposium (International ) on 

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27 
Standard Bibliographic Page 

. Report No. 

.. Title and Subtitle 

2. Government Accession No. 

NASA CR-178245 

APPLICATION OF CARS TO SCRAMJET COMBUSTION 

. II 

3. Recipient's Catalog No. 

5. Report Date 

April 1987 

6. Performing Organization Code 

17. Key Words (Suggested by Authors(s)) 

coherent anti-Stokes Raman spectroscopy, 

CARS, nonintrusive diagnostics, non1 inear Raman spectroscopy, combustion 

diagnostics 

'. Author(s) 

R. R. Antcliff 

18. Distribution Statement 

Unclassified - Unlimited 

I. Performing Organization Same and Address 

Systems Research Laboratories, Inc. 

Research Appl ications Division 

2800 Indian Ripgle Road 

Dav t on. OH 4544 -3696 

National Aeronautics and Space Administration 

Washington, DC 20546 

2. Sponsoring Agency Name and Address 

.5. Supplementary Notes 

Langley Technical Monitor: G. Burton Northam 

Final Report 

8. Performing Organization Report No. 

6657 Final 

10. Work Unit No. 

11. Contract or Grant No. 

NASl-16562 

13. Type of Report and Period Covered 

Contractor ReDort 

14. Sponsoring Agency Code 

16. Abstract 

A coherent anti-Stokes Raman spectroscopic (CARS) instrument has been devel oped 

for measuring simultaneously temperature and N2 - 02 species concentration in 

hostile flame environments. A folded BOXCARS arrangement was employed to obtain 

high spatial resolution. Polarization discrimination against the nonresonant 

background decreased the lower limits of 02 detectivity. The instrument has beer 

primarily employed for Val idation of computational fluid-dynamics computer-model 

codes. 

hydrogen diffusion flame with good results. 

Comparisons have been made to both the CHARNAL and TEACH codes on a 

., - -. 

Unc 1 ass i f ied I Unclassified I 34 I 

For sale by the National Technical Information Service, Springfield, Virginia 22161 

NASA Langley Form 63 (June 1985)