dual-pump coherent anti-stokes raman scattering measurements of nitrogen and oxygen in a laminar jet...

Dual-pump coherent anti-Stokes Ramanscattering measurements of nitrogen and oxygenin a laminar jet diffusion flame

Robert D. Hancock, Frederick R. Schauer, Robert P. Lucht, and Roger L. Farrow

Dual-pump coherent anti-Stokes Raman scattering ~CARS! has been demonstrated for the simultaneousmeasurement of gas-phase temperature and concentrations of molecular nitrogen and oxygen. A po-larization technique was used to vary the relative intensities of the two CARS signals and expand thedynamic range of the relative concentration measurements. Detailed temperature and oxygen molefraction measurements were performed in the stabilization region of a hydrogen–nitrogen jet diffusionflame. These results indicate that there is a region below the nozzle exit where significant amounts ofoxygen are found on the fuel side of the peak flame temperature profile. © 1997 Optical Society ofAmerica

Key words: CARS, dual-pump CARS, three-laser CARS, diffusion flames.

1. Introduction

Coherent anti-Stokes Raman scattering ~CARS! is alaser diagnostic technique that has been used exten-sively for measuring gas temperature and majorspecies concentrations in harsh combustion environ-ments. Several different multiple-species CARStechniques have been developed since the inception ofCARS in the early 1970’s.1–4 Early applications ofthe CARS technique allowed for the detection of onlyone species at a time. However, it was soon foundthat if two species had closely spaced resonant fre-quencies, then both could be detected with a singlebroadband Stokes laser.5–9 However, in many com-bustion environments the two species of interest ~i.e.,N2 and O2! are spectrally separated and a more elab-orate CARS systems is required. In the mid 1980’s,Eckbreth and co-workers at the United TechnologiesResearch Center demonstrated dual-broadbandCARS, which uses two broadband laser sources in

R. D. Hancock, F. R. Schauer, and R. P. Lucht are with theDepartment of Mechanical and Industrial Engineering, Universityof Illinois, 1206 West Green Street, Urbana, Illinois 61801. R. L.Farrow is with the Combustion Research Facility, Sandia NationalLaboratories, Livermore, California 94550. R. D. Hancock andF. R. Schauer are also with the Aero Propulsion and Power Direc-torate, Wright-Patterson Air Force Base, Ohio 45433.Received 12 August 1996; revised manuscript received 27 No-

vember 1996.0003-6935y97y153217-10$10.00y0© 1997 Optical Society of America

conjunction with two narrow-band pump laser beamsat the same frequency.10–12 This technique allowsmore than two species to be probed simultaneously.More recent applications of this technique includedual-broadband rotational CARS for O2 concentra-tions and temperatures13 and a combination of dual-broadband rotational CARS with vibrational CARSfor simultaneous measurements of temperature andconcentration of fuel, O2, and N2.14 Another multi-species CARS technique is the dual-Stokes technique,in which two pump beams and two Stokes beams areused to generate CARS signals for two or more spe-cies simultaneously.15–17Dual-pump CARS was first introduced by Teets,18

who used two pump beams at different but fixed fre-quencies v1 and v2. The technique was further de-veloped by Lucht,19 who used a tunable dye laser asthe second pump beam for the detection of N2 and O2in a pressure cell and the detection of propane and N2in a fuel-injected engine. A short time later, Luchtet al.20 demonstrated pure rotational and vibrationaldual-pump CARS and the use of polarization tech-niques to vary the relative intensities of the twoCARS signals.The dual-pump CARS technique permits two spe-

cies, independently of their vibrational Raman reso-nances, to be probed simultaneously. The spectrafor the two species are generated with the same threelaser beams in precisely the same spatial location andinstant of time. This helps to reduce the effects ofbeam steering in high-temperature gradient regions

20 May 1997 y Vol. 36, No. 15 y APPLIED OPTICS 3217

in flames. Furthermore, the CARS signals from thetwo species can be generated at nearly the same fre-quency, and therefore both species can be detectedwith a single array detector. Furthermore, the fre-quency resolution and separation of the two CARSspectra generated with dual-pump CARS can be ad-justed to optimize temperature and concentrationmeasurement accuracy. The relative signal levels ofthe two species can be varied by use of polarizationtechniques.This paper describes the further development and

application of dual-pump CARS. The use of polar-ization techniques to vary the relative CARS signallevels from the Q branches of the two species is dem-onstrated. The effect of tuning the narrow-band dyelaser ~v2! to shift the frequencies of the CARS signalsfor the two species relative to one another is investi-gated. Single-laser-shot experiments were per-formed to characterize the precision ofmeasurementsof the relative mole fractions of two species. Finally,the application of the dual-pump CARS technique toaH2–N2 diffusion flame to obtain temperature andO2mole fraction information in the stabilization regionof the flame is described.

2. Dual-Pump CARS Equations

The energy-level diagram for the dual-pump CARStechnique is shown in Fig. 1. The left-hand side ofthe figure illustrates that a CARS polarization is es-tablished for N2 when the frequency difference v1 2vs corresponds to a vibrational Raman resonance of2330 cm21. The induced vibrational Raman polar-ization scatters the incident v2 beam to produce anti-Stokes radiation at frequency vas 5 v1 2 vs 1 v2.The right-hand side of the figure illustrates that, forO2, the frequency difference v2 2 vs corresponds to avibrational Raman resonance of 1556 cm21. The vi-brational Raman polarization of the O2molecule scat-ters the incident v1 beam to produce the anti-Stokessignal at a frequency of vas 5 v2 2 vs 1 v1.Through the proper selection of input laser beam fre-

Fig. 1. Energy-level schematic for dual-pump CARS measure-ments of oxygen and nitrogen.

3218 APPLIED OPTICS y Vol. 36, No. 15 y 20 May 1997

quencies and polarizations, CARS signals for virtu-ally any two species with Raman-active resonancescan be obtained simultaneously.The polarization dependence of dual-pump CARS

can be determined by the calculation of the tensorelements of the CARS susceptibility, as described fortwo-laser CARS by Rahn et al.21 Assuming perfectphase-matching and monochromatic lasers, theelectric-field amplitude for the anti-Stokes signal isgiven by Nibler and Knighten22:

Eas , @x1122~3! e1~e2 z es! 1 x1212

~3! e2~e1 z es!

1 x1221~3! es~e1 z e2!#E1E*sE2, (1)

where Eas 5 Easeas is the electric-field vector and thetensor elements xijkl

~3! ~2vas, v1, v2, 2vs! are derivedfrom formulas given by Owyoung.23 The tensor el-ements are

x1122~3! 5

124 @s 1 2a~v2 2 vs! 1 b~v1 2 vs!#, (2a)

x1212~3! 5

124 @s 1 2a~v1 2 vs! 1 b~v2 2 vs!#, (2b)

x1221~3! 5

124 @s 1 b~v2 2 vs! 1 b~v1 2 vs!#, (2c)

x1111~3! 5 x1122

~3! 1 x1221~3! 1 x1212

~3! , (2d)

where s is the nonresonant electronic component ofthe susceptibility and a~vk 2 vs! and b~vk 2 vs! arecomponents of the Raman susceptibility. For an iso-lated Raman vibrational line,

a~vk 2 vs! 5 KnJJ9@~a9!2dJJ9 2245 bJJ9~g9!2# , (3a)

b~vk 2 vs! 5 KnJJ9@215 bJJ9~g9!2# , (3b)

KnJJ9 5DN~n 1 1!

2mv0~v0 2 vk 1 vs 2 iG!21, (3c)

where a9 and g9 ~both in units of square meters! arethe mean and the anisotropy of the polarizabilityderivative, respectively, bJJ9 is the Placzek–Teller co-efficient, m is the reduced mass ~in kilograms!, v0 isthe resonant frequency ~in inverse seconds!, and G isthe Raman linewidth ~HWHM! ~in inverse seconds!.KnJJ9 has units of coulombs squared times squaremeters per joules squared. For Q-branch transi-tions, DN is the population difference for the twolevels ~n, J! and ~n9, J9! coupled by the Raman tran-sition.When all the input laser beams are parallel polar-

ized, the resulting CARS intensity is given by

Ias , I1Is I2ux1111u2. (4)

For the polarization arrangement shown in Fig. 2,the anti-Stokes intensity transmitted through theCARS analyzer ~half-wave plate and polarizer! is

Ias , uEasu2 , @~s 1 2a2 1 b1!cos uas

1 ~s 1 2a1 1 b2!sin uas#2I1Is I2, (5)

where ak 5 a~vk 2 vs! and bk 5 b~vk 2 vs!. TheRaman components are related by the depolarization

ratio r 5 by~2a 1 2b!.23 N2 and O2 vibrationalQ-branch lines are nearly isotropic so r ' 0 and b '0. Therefore approximation ~5! becomes

Ias , @~s 1 2a2!cos uas 1 ~s 1 2a1!sin uas#2I1Is I2. (6)

Calculation of dual-pump CARS spectra requiresconvolution of the laser linewidths with the molecu-lar susceptibility. The CARS spectral signal inten-sity Ias~vas! ~in watts per square meter times inverseseconds! at frequency vas is given by a triple convo-lution19:

Ias~vas! 5 k * ux~v1 2 vs!

1 x~v2 2 vs!u2I1~v1!I2~v2!Is~vs!

3 d~v1 1 v2 2 vs 2 vas!dv1dv2dvs, (7)

where k is a proportionality constant and Ik~v! is thespectral intensity ~in watts per square meter timesinverse seconds!. In many CARS experiments, in-cluding this one, narrow-linewidth injection-seededNd:YAG lasers are used, and the frequency band-width of pump beam 1 is small compared with typicalRaman linewidths. Therefore we can write

I1~v1! 5 I1g1~v1 2 v10! 5 I1d~v1 2 v10!, (8)

where I1 is the intensity ~in watts per square meter!,g1~v1 2 v10! ~in seconds! is a normalized line-shapefunction, and v10 is the central frequency of the spec-trum for pump beam 1. In addition, because we nor-malize each experimental CARS spectrum bydividing by a nonresonant background spectrum, wecan analyze the CARS spectra assuming that thespectral intensity of the Stokes laser is constant:

Is~vs! 5 Isgs~vs 2 vs0! 5 Isv. (9)

Substituting Eqs. ~8! and ~9! into Eq. ~7! and perform-ing the integration over dv1 and dvs, we obtain

Ias~vas! 5 kI1I2 Isv * ux~vas 2 v2!

1 x~vas 2 v10!u2g2~v2 2 v20!dv2. (10)

Fig. 2. Polarization scheme for extending the dynamic range ofdual-pump CARS.

The right-hand side of Eq. ~10! can be written as

Ias~vas! 5 kI1I2 IsvH* ux~vas 2 v2!u2g2~v2 2 v20!dv2

1 ux~vas 2 v10!u2 1 * @x~vas 2 v2!

3 x*~vas 2 v10! 1 x*~vas 2 v2!x~vas 2 v10!#

3 g2~v2 2 v20!dv2J . (11)

Equation ~11! gives us the CARS signal intensity at aparticular frequency vas. However, the broadbandCARS signal is detected with a spectrometer andarray detector. The detected CARS signal intensityat a frequency v9as corresponding to a particular pixellocation is given by

Sas~v9as! , * Ias~vas!gd~vas 2 v9as!dvas

5 kI1I2IsvH* ux~vas 2 v2!u2g2~v2 2 v20!

3 gd~vas 2 v9as!dvasdv2 1 * ux~vas 2 v10!u2

3 gd~vas 2 v9as!dvas 1 2 ReF* x~vas 2 v2!

3 x*~vas 2 v10!g2~v2 2 v20!

3 gd~vas 2 v9as!dvasdv2GJ , (12)

where gd~vas 2 v9as! is the detector response functiondetermined by, among other factors, the spectrome-ter resolution and the pixel dimension on the detectorarray. The detector response function is typicallydetermined from spectral-fitting analysis of room-temperature N2 spectra. The CARS analysis pro-gram25 CARSFT was modified to perform theconvolution given in Eq. ~11! and used to fit the ex-perimental spectra.The nonresonant background susceptibility was

calculated from the estimated flame composition andtemperature at the CARS probe volume and was thenused as an input parameter for the CARS spectral-fitting calculations. The nonresonant backgroundsusceptibility is fairly constant and can be accuratelycalculated for the steady flames produced with theHencken burner. However, the nonresonant back-ground susceptibility will vary across the flame frontfor a jet diffusion flame. This background can becalculated by use of temperature and mole fractioninformation for the flame obtained with our compu-tational fluid dynamics code. This code has beendeveloped to look specifically at these types of jetdiffusion flames. These calculations are of course


somewhat suspect in the stabilization region belowthe nozzle, for which we have assumed a diffusion-type flame structure even though we observe O2 onthe fuel-rich side of the peak temperature region, butthe variations in the nonresonant susceptibility donot significantly affect our determination of the O2mole fraction.

3. Experimental Facility

Dual-pump CARS measurements of O2 and N2 wereperformed in room air, in a Hencken calibrationburner, and in a H2–N2 jet diffusion flame. TheHencken burner produces a flat, uniform, steady, andnearly adiabatic flame for a wide range of flow con-ditions. This burner is useful for the developmentand testing of laser diagnostics because the flame isso well characterized.26 The dual-pump CARS sys-temwas first used to make detailed measurements inthe Hencken burner and then to probe the stabiliza-tion region of the H2 jet diffusion flame.A schematic drawing of the dual-pump CARS sys-

tem is shown in Fig. 3. The 532-nm frequency-doubled output of a Continuum, injection-seeded, Nd:YAG laser was used as a pump beam at 532 nm andto pump both narrow-band and broadband dye lasers.The frequency bandwidth of the 532-nm pump beamwas 0.0045 cm21 FWHM. The broadband dye laserradiation ~vs! was centered spectrally at ;607 nmand had a FWHM of 140 cm21. The narrow-banddye laser wavelength was varied throughout the ex-periment but was optimized for ;553.4 nm ~v2 518 070 cm21! and was readily tunable over severalnanometers. The FWHM of the narrow-band dyelaser was ;0.1 cm21.The polarization of the laser beam out of the Nd:

YAG laser was vertical. In the set of experiments inwhich polarization techniques were employed, aFresnel rhomb was placed in the 532-nm ~v1! beam torotate it to horizontal and another was placed in the607-nm ~vs! laser beam to rotate it 45° from the hor-izontal. The narrow-band dye laser beam ~v2! po-larization remained vertical.A three-dimensional folded BOXCARS phase-

Fig. 3. Schematic diagram of the dual-pump CARS system: M’s,mirrors; 1y2’s, half-wave plates; GP’s, Glan polarizers; BS’s, beamsplitters; FR’s, Fresnel rhombs; P’s, prisms; A’s, apertures; T’s,beam traps; L’s, lenses.


matching geometry was used to generate the CARSsignal.27 The three laser beams were crossed andfocused in the flame, producing a diagnostic volumein which over 95% of the CARS signal beam wasgenerated in a region less than 2.0 mm long and ;50mm in diameter. The focal length of the achromaticlens was 250mm, and the distance between the beamaxes at the lens was ;17 mm. The resulting CARSsignal was focused into a 1-m double spectrometer inwhich it was spectrally dispersed and detected with a512 3 512, backilluminated, unintensified CCD ar-ray ~Photometrics!.28Most of the dual-pump CARS spectra obtained

were the average of 100 laser shots. Averaging wasacceptable in both the Hencken burner and jet diffu-sion flames because they are both extremely stable inthe spatial regions investigated. In order to achievethe highest possible accuracy and precision for theCARS temperature measurements, a high signal-to-noise ratio for the CARS spectra was maintained andthe broadband dye laser spectral profile was fre-quently monitored. An example of a dual-pumpCARS spectrum of N2 and O2 is shown in Fig. 4.This spectrum was obtained in room-temperature airand is the average of 100 laser shots. The lasershots were averaged on the CCD array over a 10-sexposure by use of a Nd:YAG laser operating at 10Hz.A Hencken burner29 was used to provide well-

characterized flames for use in the development ofthe dual-pump CARS diagnostic system. TheHencken burner has an array of fuel tubes that areplaced in a stainless steel honeycomb structure. Airflows through the honeycomb structure, and the re-sulting flames are flat, uniform, steady, and nearlyadiabatic for sufficiently high flow rates. There is alarge volume above the 36.5 mm 3 36.5 mm squarehoneycomb surface where the flame temperature isradially and axially uniform. Temperature mea-surements in H2–air flames were collected at the3.81-cm axial location directly above the center of theburner. The air flow was held constant at 60.6

Fig. 4. Room-temperature spectrum obtained by dual-pumpCARS. The least-squares-fit O2 mole fraction is 0.21. The the-oretical curve was calculated with CARSFT.

SLPM ~0 °C! ~SLPM indicates standard liters perminute!, and the H2 flow was varied to produce theequivalence ratio flames desired. A detailed de-scription of temperature measurements for equiva-lence ratios ranging from 0.5 to 2.5 for H2–air flamesis given elsewhere.26The jet diffusion flame apparatus consisted of a

2.54-cm-diameter axisymmetric fuel tube that con-tracts to a 1-cm-diameter sharp-edged nozzle. Thefuel tube is surrounded by a 15.24-cm-diameter co-flow of air at 0.48 mys, which helps to remove roomair disturbances. The diffusion flame investigatedin this experiment consisted of 15.0 SLPM of H2 and6.25 SLPM of N2. The resulting calculated averagepeak center-line axial velocity was 4.9 mys at roomtemperature. The jet and annulus air velocity pro-files were also measured with a hot-wire anemometerand found to be nearly top hat and within 5% ofvolume-based calculations over the center 8 mm ofthe jet.

4. Polarization Techniques

In some applications of dual-pump CARS the signalfrom one of the species may be significantly largerthan for the other. If the same detector is to be usedfor both species, then saturation of the detector couldoccur for one of the species while the signal from theother is lost in the noise. An example of this is for N2andO2 in room-temperature air or air-fed combustiondevices. There is 3.76 times as much N2 as O2 inroom air, and because the signal is proportional to thesquare modulus of the CARS susceptibility, the N2signal will be ;15 times larger than the O2 signal.Another situation may be in a combustion environ-ment in which the concentration of one species maybe much less than that of another species of interestin various regions of the flame. The polarizationtechnique can be used to balance the mean intensityof the CARS signals for two species, provided that thesignal intensities are high enough to overcome thesignal reduction of a factor of 4 resulting from thepolarization arrangement described. If themean in-tensities for the two species are approximately equalin the mean then fluctuations about the mean can bemore easily measured. This polarization techniquewas first proposed by Lucht et al.20 but is demon-strated here first. However, this polarization tech-nique was not used for the calibration burner andlaminar jet diffusion flames in order to maintain thelargest signals possible.Figure 5 illustrates how the O2 signal in room-

temperature air can be increased relative to the N2CARS signal. Spectra are shown for uas of 0°, 30°,and 90°. When uas approaches 290° or 90°, the O2signal is almost completely rejected, and when theangle is near 0° or 180°, the N2 signal is greatlyreduced and the O2 signal becomes dominant. It isdifficult to remove all of the N2 signal completelybecause of accumulated errors in the polarizer set-tings or birefringence in elements of the optical sys-tem.

5. Measurements in the Calibration Burner

A major advantage of the dual-pump CARS tech-nique is that the wavelengths of the CARS signals forthe two species are nearly the same, allowing theCARS spectra to be imaged onto a single linear arrayor CCD camera at the highest possible resolution.The two signals can be shifted relative to one anothersimply by changing the wavelength of pump 2 ~v2!.An example of this is shown in Fig. 6, in which the N2signal is shifted relative to the O2 signal when thewavelength of pump 2 ~v2! is scanned from 553.4 to555.4 nm. The overlapped spectra interfere coher-ently, and the shape of the full spectrum is sensitiveto the frequency shift between the spectra and to theCARS linewidths of the two species. These spectrawere obtained in a fuel-lean ~f 5 0.236! H2–air flameproduced with the Hencken burner.26 The temper-ature predicted by the NASA Lewis equilibriumcode30 for this flame was 1025 K. The three spectrashown produce a mean temperature of 1020 K andwere all within 7 K of this mean. The average O2mole fraction for the experimental data was 0.196,whereas the NASA Lewis code predicted 0.153.Spectra from another fuel-lean ~f 5 0.393! flame alsoshow good temperature agreement but the experi-mental data give an O2 mole fraction of 0.156,whereas the NASA Lewis equilibrium code predicts0.118. Close inspection of the O2 spectra indicatesthat the lower experimental rotational level peaks in

Fig. 5. Effect of varying the angle of the transmission axis of thepolarizer in the CARS signal channel. When polarization tech-niques are used with dual-pump CARS, the relative amount ofsignal transmitted into the spectrometer for each of the two speciescan be controlled by passing the CARS signal through a half-waveplate and polarizer. The angular setting of the CARS signal chan-nel polarizer is indicated for each spectrum. The theoreticalcurves were calculated with CARSFT.


the n 5 0 to n 5 1Q-branch transition are larger thanthe CARSFT code results. A high-temperature O2CARS spectrum fromDreier and Schiff31 also shows asimilar discrepancy between experiment and theoryfor the same spectral region. It appears that betteragreement between the experiment and the modelcould result with improved modeling of the high-temperature O2 linewidths.

Fig. 6. Location of the N2 CARS signal relative to the O2 CARSsignal can be varied by changing the frequency of pump 2. Pump1 was held constant at 532 nm, and the broadband dye laser wascentered at 607 nm. Pump 2 was varied to the wavelengths in-dicated on the spectra. All three spectra were collected at thesame conditions in a fuel-lean ~f 5 0.236! H2–air flame producedwith a Hencken burner and fit with the CARSFT code. The NASALewis equilibrium code calculates a temperature of 1025 K and anO2 mole fraction of 0.153.


6. Single-Laser-Shot Measurement Statistics

CARS has been commonly used to provide single-shottemperature information in combustion environ-ments. Broadband dual-pump CARS can be used toobtain the entire vibrational spectrum for N2 and O2in a single 10-ns laser pulse. This is advantageousin turbulent flames or other unsteady situations inwhich averaging the CARS signal over numerous la-ser shots is unacceptable.We performed single-shot dual-pump CARS mea-

surements of O2 and N2 in room-temperature air.The average peak amplitudes of the N2 and O2 single-shot spectra were 55,000 and 6000 counts respec-tively. Under atmospheric laboratory conditions thetemperature is known and can be held constant dur-ing the least-squares-fitting process. The single-shot spectra illustrate that the absolute magnitude ofthe N2 and the O2 CARS signals will vary from shotto shot but that the variation in the relative signallevels of N2 and O2 is drastically reduced by compar-ison. In other words, even though there were fluc-tuations in either the spatial mode structure orfrequency spectrum of our dye laser that gave bi-modal results for the raw signal probability densityfunctions ~pdf ’s!, the two signals were well corre-lated, and the pdf of the ratio of the signals was nearGaussian and had a much reduced width. Conse-quently the relative concentrations of N2 and O2 canbe measured with good precision. This same type ofcorrelation will apply at flame temperatures.A pdf plot of the square root of the area under the

O2 signal is shown in Fig. 7. The x-axis value isnormalized by the mean value of the integrated O2signal; a value of 1 would represent a signal equal tothe mean. Figure 8 shows a pdf for the N2 signal.Bimodal distributions exist for both the O2 and theN2. This could possibly be due to intensity fluctua-tions in one or more of the laser beams. These pdf ’sindicate that the CARS signal for each species fluc-tuates as much as 50% from shot to shot. However,as illustrated in Fig. 9, if the ratio of the two signalsis obtained and plotted in the same manner, thespread of the data is dramatically reduced. The

Fig. 7. Histogram ~pdf ! of the square root~SQRT! of the integrated area of the O2

CARS signal. The standard deviation di-vided by the mean is 18.8%.

Fig. 8. Histogram ~pdf ! of the square root~SQRT! of the integrated area of the N2

CARS signal. The standard deviation di-vided by the mean is 15.4%.

Fig. 9. Histogram ~pdf ! of the ratio of thesquare root ~SQRT! of the integrated areas ofthe O2 and the N2 CARS signals. The stan-dard deviation divided by the mean is 7.9%.

standard deviation divided by the mean for the ratioof the signals results in a value of 0.079, or ;7.9%,whereas for the O2 and the N2 pdf ’s shown in Figs. 7and 8 this value is 18.8% and 15.4%, respectively.As expected, this illustrates that the fluctuations inthe N2 and the O2 single-shot CARS signal intensitiesare correlated because both signals result from theinteraction of the same three laser beams. Figure10 illustrates the resulting mole fraction distributionwhen all of the 300 CARS spectra were fit with theSandia CARSFT code. The mean O2 mole fraction wasfound to be 0.200 with a standard deviation of 0.009.The standard deviation divided by the mean resultsin a value of 4.5%.The good correlation between the signals from the

two species in dual-pump CARS can be comparedwith the results of Bengtsson et al.,14 who observedlower correlation between the vibrational CH4 CARSsignal and the pure rotational N2yO2 signals ~relativestandard deviation of ;10%! because the vibrationalCH4 CARS signal and the pure rotational signalswere not generated by the same three laser beams.This technique was also found to be sensitive to theintensity split among the various beams used to gen-erate the CARS signals. However, other results bythe same authors13 for O2 concentration and temper-ature measurements made with dual-broadband ro-tational CARS show a good correlation between theO2 and N2 signals. We believe that this is because,as with dual-pump CARS, the same three beams areused to generate the two CARS signals. However,dual-pump CARS is applicable to many more mole-cules of combustion interest because either pure ro-tational or vibrational resonances can be probed.

7. Application to a Jet Diffusion Flame

Dual-pump CARS was used to perform simultaneousN2 temperature and O2 mole fraction measurementsin the stabilization region of an axisymmetric H2–N2jet diffusion flame. The stabilization region of a lam-inar diffusion flame is of great interest to many re-searchers in the combustion community, and flamestabilization is obviously a subject of great practicalas well as scientific interest. It is also a challenging

Fig. 10. Histogram ~pdf ! of the O2 mole fraction from least-squares fits made with the Sandia CARSFT code. The mean O2

mole fraction was found to be 0.200. The standard deviation di-vided by the mean is 4.5%.

region in which to perform measurements because ofthe high gradients. Because of the high spatial gra-dients, it is also an excellent region for demonstratingthe applicability of our dual-pump CARS technique.Again, because both signals come from the samethree laser beams, there is no possibility of spatialmismatch in theN2 andO2 profiles, andwe can there-fore confidently assert that we have measured O2 onthe fuel-rich side of the peak temperature region ofthe flame. It has been observed that under a widerange of flow conditions in H2 jet diffusion flames, theflame will attach to the outside of the fuel tube sev-eral millimeters below the exit of the nozzle. Closevisual inspection of these flames would indicate thatas the fuel exits the nozzle some of the fuel diffusesaway from the nozzle and down the outside of thenozzle. The modeling results of Fukatani et al.32show that at the lowest part of the H2 flame, theupward-flowing air and the H2 diffusing downwardmeet and react like premixed gas mixtures. Taka-hashi and Katta33,34 show similar modeling resultsfor CH4 flames and observe air passing through anarrow dark space between the flame and nozzle,producing a premixed region on the fuel side of theflame. In order to investigate these predictions, weused dual-pump CARS to probe this region in detailto determine the temperature distribution and thedegree of O2 penetration of the flame zone. Threespectra from the21.27-mm axial height are shown inFig. 11. The spectrum of Fig. 11~a! is from the5.334-mm radial location nearest the nozzle, the spec-

Fig. 11. Dual-pump CARS spectra showing the presence of O2 onthe fuel side of the flame zone. All three spectra were obtained1.27mm below the lip of the nozzle. Spectrum ~a! is a point on thefuel side of the flame zone, ~b! is at the peak temperature region ofthe flame zone, and ~c! is on the air side of the flame zone.


trum of Fig. 11~b! is from the 5.842-mm radial loca-tion where the flame temperature is at a maximum,and the spectrum of Fig. 11~c! is at the 6.858-mmlocation outside the flame. These spectra illustratethat O2 is present in significant concentrations on thefuel side of the peak temperature region of the flame.Temperature and O2 mole fraction profiles for a

H2–N2 diffusion flame are shown in Figs. 12 and 13.These two figures show a detailed set of measure-ments that characterize the temperature and the O2mole fraction profiles in the stabilization region of theflame. Figure 14 shows contour plots of this sameset of data. The top half of the figure shows temper-aturemeasurements obtained in a rectangular regionapproximately 5mmwide by 3mmhigh. The nozzleis slightly contoured and is clearly marked in thefigure. Hot temperatures correspond to black, andthe peak temperature at each of the measurementheights is indicated. Slightly above the nozzle thepeak flame temperature is 2100 K, and at 2.794 mmbelow the nozzle the peak flame temperature has

Fig. 12. Dual-pump CARS temperature profiles in the stabiliza-tion region of a H2–N2 jet diffusion flame.

Fig. 13. Dual-pump CARS O2 mole fraction profiles in the stabi-lization region of a H2–N2 jet diffusion flame.


dropped to 490 K. The peak flame temperature ismarkedwithwhite dots in the figure. The lower halfof Fig. 12 shows the O2mole fraction information thatcorresponds to the temperature measurement regionin the upper half of the figure. Interestingly, O2 isfound on the fuel side of the peak flame temperatureline, indicating a region of premixed H2 and O2. TheO2 concentration gradient is fairly high between20.5and 22.0 mm, indicating that the O2 that has pene-trated the peak temperature region below the nozzledoes not survive past the lip of the nozzle. In theboundary layer near the outside of the nozzle wall theaxial velocity is low. This would allow H2 to diffusedown along the outside of the nozzle quite easily.Furthermore, because the flame temperatures arelow in this region of the flame the O2 can diffusethrough this region without being consumed becauseof chemical reaction.35 Based on examination of theexperimental spectra, modulation was still observedfor O2 mole fractions of ;1%.

8. Summary and Conclusions

A dual-pump CARS system has been developed andused to obtain gas-phase temperature and molecularN2 and O2 mole fractions. We have shown how therelative position of the O2 and the N2 CARS spectracan be adjusted to provide maximum resolution anddetection. We have also demonstrated the use ofpolarization techniques to vary the relative intensityof the two CARS signals, resulting in an improve-

Fig. 14. Dual-pump CARS temperature and O2 mole fractionmeasurements below the nozzle for flame 1. The peak flame tem-peratures are indicated for each radial profile. Note that some O2

is located on the fuel side of the peak temperature line for the lowerradial profiles.

ment in the dynamic range for the measurement sys-tem. An advantage of the dual-pump CARStechnique is that the temperature and the concentra-tion of two species can be measured simultaneously.Another advantage is that the frequency resolutionand separation of the two CARS spectra can be ad-justed to optimize temperature and concentrationmeasurement accuracy. Furthermore, dual-pumpCARS is not very sensitive to beam steering and de-focusing or alignment errors because relative inten-sities do not depend on beam overlap. Adisadvantage of dual-pump CARS is that it does nothave the capability of measuring more than two spe-cies simultaneously. We have shown that dual-pump CARS can be a useful technique innoncombusting or combusting environments inwhich temperature and mole fraction information fortwo species needs to be obtained simultaneously.Temperature and O2 mole fractions measurementshave been obtained in the stabilization region of aH2–N2 diffusion flame. These measurements indi-cate that O2 penetrates the flame and is found on thefuel side of the peak flame temperature profile.

Financial support for this research was provided inpart by Systems Research Laboratory, Aero Propul-sion and Power Directorate of Wright Laboratory~U.S. Air Force!, and the U.S. Air Force PalaceKnight Program. Roger Farrow was supported bythe U.S. Department of Energy, Office of Basic En-ergy Science, Division of Chemical Sciences.

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dual-pump coherent anti-stokes raman scattering measurements of nitrogen and oxygen in a laminar jet...

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