recent advances in laser diagnostics for temperature and species concentration in combustion

Eighteenth Symposium (International) on Combustion The Combustion Institute, 1981

R E C E N T A D V A N C E S IN LASER D I A G N O S T I C S FO R T E M P E R A T U R E A N D S P E C I E S C O N C E N T R A T I O N I N C O M B U S T I O N

ALAN C. ECKBRETH

United Technologies Research Center East Hartford, Connecticut 06108

Laser spectroscopy is assuming an ever increasing role in combustion research due to the capability to provide remote, in-situ, spatially and temporally resolved measurements of important parameters. Laser diagnostics should facilitate improved understanding of a variety of combustion phenomena which, in turn, should lead to improved efficiencies and cleanliness in practical systems, In this paper, laser techniques appropriate to spatially-precise measurements of temperature and species concentration in combustion will be reviewed. The review will encompass spontaneous Raman scattering, Rayleigh scattering, coherent anti-Stokes Raman spectroscopy (CARS), stimulated Raman gain/loss spectroscopy and laser excited fluorescence. An overview of theory and implementation is presented for each spectroscopy and application to measurements in combustion systems is stressed.

Introduction

With the increasing availability of laser sources, laser spectroscopy is assuming an ever broadening role in the diagnostic probing of the hostile, yet easily perturbed, environments characteristic of combustion processes. Physical probes, due to their intrusion, can seriously perturb the fundamental flame behavior they seek to investigate, are often limited in their spatial resolution and temporal response, and, in addition, are confronted with survival at high temperatures and pressures. Laser spectroscopic techniques, on the other hand, offer the potential for the remote, nonperturbing, in-situ examination of combustion phenomena. They are capable of simultaneous high spatial (10 3cm~ or less) and temporal (I0 -8 to 10 -8 sec) resolution, although, in practice, compromises in either or both scales may sometimes be necessary. During the last decade, very intensive research and development have occurred in the field of laser diagnostics, primarily in the areas of demonstrating feasibility, examining accuracy and extending applicability and sensitivity. As the seventies ended, laser spectroscopy was increasingly being applied as a measurement tool. In the next decade, combustion research should benefit greatly from the past decade of diagnostic development.

Many of the physical phenomena being exploited for diagnostic utilization are not new. The elastic scattering of light was understood nearly a century

ago in Rayleigh's research I to explain the blue of the sky. In 1978, the fiftieth anniversary of the Raman effect 2 was observed. It is, of course, the unique properties of lasers as light sources, namely, their power, coherence and spectral purity, which permit these techniques to be practically utilized or others to be discovered and then applied. The power in a laser beam permits the exploitation of very weak processes formerly not practical for probing or it leads to new, nonlinear phenomena not observed at low intensities. Large amounts of energy can be delivered in very short periods of time permitting "instantaneous" measurements of medium properties and observation of rapidly fluctuat- ing phenomena. The coherence, or beam-like nature of the radiation, results in efficient delivery of all of the source power to the probing location and focussing into very small volumes leading to very high spatial resolution. Spectral purity allows very high resolution of molecular or atomic features, with resolutions as large as two orders of magnitude higher than that available with the finest monochro- mators. In addition, the laser emission is tunable, in some instances quite rapidly.

In this paper, laser techniques appropriate to measurement of temperature and species concentration in combustion will be reviewed. The utilization of lasers for anemometry and fuel droplet/soot particle sizing will not be discussed here in order to restrict the scope and focus of the paper. Further- more, the laser spectroscopic techniques to be de-

1471

1472 COMBUSTION DIAGNOSTICS

scribed will be restricted to those which are in- herently spatially precise, e.g., scattering, wave-mixing, in contrast to those which are essentially line- of-sight, e.g., absorption. With spatially precise techniques, measurements are made at a point; the measurement location is then translated to map out the field, point by point, which, of course, can be quite tedious. Although the spatially precise techniques can, in principle, be employed to examine a field by relaxing the resolution, their capabilities are generally greatly limited in this mode. An attrac- tion of line-of-sight techniques is their potential, when properly implemented, for field mapping. These techniques lend themselves readily to computer assisted tomography, although experimental demonstrations are still in their infancy. In the next section of the paper, a brief background into the various diagnostic processes to be reviewed will be given. Then the specific techniques will be reviewed. Spontaneous Raman scattering will be treated first because it is probably the most highly understood, developed and applied of the laser techniques. Because of its limitations, it serves as an interesting baseline with which to compare the other processes to be reviewed, Bayleigh scattering, nonlinear optical techniques such as coherent anti- Stokes Raman spectroscopy (CARS) and stimulated Raman gain/loss spectroscopy, and laser excited fluorescence. By its nature, such a review is limited in the depth with which each area can be treated. The paper by design will avoid discussing nuances and intricacies of the various diagnostic approaches and only an overview of the basic physics underlying the implementation of each phenomenon will be given. Emphasis will be placed, rather, on the types of measurements that can be performed in combustion systems. Much of the progress and history in this area is documented in the proceedings of several symposia 3-6 and reviews v-9 devoted to laser spectroscopic diagnostics of combustion.

Background Before beginning the survey of laser diagnostics,

it is instructive to review the origin of the various physical phenomena being exploited. In view of the increasing exposure that combustion researchers wil likely have to laser spectroscopic techniques in the coming years, an acquaintance with the nature of the processes would seem beneficial. In this vein, a classical picture will suffice here, ~~ although detailed explanations generally need to invoke quantum mechanical treatments. All optical phenomena are governed by Maxwell's equations which can be manipulated to yield the wave equation

[~Tx (Vx) + (1/c 2) 02/Ot21 F. (?,t)

= - (4~r /c2)d2 /~ t2P(~ , t ) (1)

where E is the electric field of the incident electromagnetic light wave, c is the speed of light in vacuum and P is the generalized electric polarization. Often several monochromatic or quasi-monochromatic field components are involved and E and P can be expanded into Fourier components. The induced polarization can be expressed as power series of l~ (to,), namely

(1) (2)

v(to,) = ~'(to,l E(to,) + ~ (~o, =o,, + tok): i,k

E Ito,/E/to~/+ ~ ~"/to, =to, + ~o~ +to,t: j.k.t

~ ( o ~ , ) ~ ( o , , ) E ( t o , ) + ... (2)

X ~) is the linear susceptibility of the medium, the • are the nth order nonlinear susceptibilities, so named because they express how susceptible the medium is to being polarized. The effects of the nonlinearities in the polarization become manifest only at very high laser intensities as will become clear shortly. The induced linear polarization modi- fies the propagation of the light wave through the medium, accounted for by introduction of the refractive index. Dispersion and absorption phenomena accompany the real and imaginary parts, respectively, of the complex refractive index usually modeled classically by an electron oscillating in a harmonic potential well. Raman and Rayleigh scattering arise from the oscillating polarization induced through the linear susceptibility. Recall that an oscillating polarization, e.g., an electric dipole, pro- duces electromagnetic radiation. Rayleigh scattering arises from the induced polarization oscillating at the same frequency as the incident radiation. In general, the polarizability and, hence, the induced polarization, are dependent upon the nuclear positions of the molecule and are thus modulated by the rotation and/or vibration of the molecule. This leads to Raman scattering which is frequency shifted from the incident light by the rotational/vibrational frequency and which may be interpreted as the beat frequency between the incident radiation and nuclear motions. ~

The higher order polarizations are considerably weaker and to an approximation, the succeeding polarizations may be expressed as ~2

P~+~/P("~ = E/E~, (3)

where Eu, is the intra-atomic electric field and typically of order 3(10 ") V/cm. Even at high laser intensities of 109 W/cm z, the ratio of succeeding polarizations is small, about 10 -3 . Thus, nonlinear phenomena were not experimentally observed until the early sixties when giant laser pulses were produced by Q-switching. Today such lasers are com-

LASER DIAGNOSTICS FOR TEMPERATURE AND SPECIES CONCENTRATION 1473

monplace and readily available commercially. Non- linear processes in actuality are very strong at high laser intensities due to the presence of resonance denominators in the nonlinear susceptibilities. In isotropic media such as gases, there are no second order effects due to inversion symmetry. The lowest order nonlinearities in a gas are thus third order in incident electric field strength and arise through the third order nonlinear susceptibility. Important examples of third order processes for combustion diagnostics are CARS and stimulated Raman gain/loss spectroscopy.

Spontaneous Raman Scattering

Raman scattering is the phenomenon of inelastic collision processes between photons of light and molecules in either the solid, liquid or gaseous phases. It has been physically understood for some time and detailed theoretical treatments are readily available. '~ Raman scattering has probably received more attention for combustion diagnostics than any of the other approaches described herein based upon the pioneering investigations of Lapp'* and Leder- man 8 in the late sixties and early seventies. Laser Raman techniques offer a number of advantages for combustion diagnostics. 3-9 In brief, only a single laser is required to monitor all of the species of interest. The laser can operate at any wavelength without the necessity of being tuned to resonances of the molecules being probed. Visible wavelengths are favored, however, because the Raman cross sections scale as h ~4, where hR is the Raman scattering wavelength. With proper detection, many species can be monitored simultaneously. Quite importantly, the Raman scattered intensities are unaffected by collisional quenching. Absolute calibration is readily achieved by comparing the scattered signal from the species of interest with that of nitrogen. Unfortunately, spontaneous Raman scattering is very weak. It is the weakness of the process, despite all of its advantages, that often limits applicability and leads to utilization of alternative, and generally more complex, approaches. In Raman scattering, the incident photons may either lose or gain energy from the target molecules. In the former event, termed Stokes scattering, the molecule becomes excited, and in the latter, termed anti-Stokes, the molecule is deexcited providing, of course, that it was in an excited state prior to the interaction. Typically, the fraction of molecules involved in the scattering process is so small that there is little worry about perturbation to the molecular energy distribution. Since each molecular species possesses a characteristic set of energy states, the spectral distribution of Raman scattering is uniquely determined by the incident laser wavelength and the species from which scattering occurs. If no change in vibrational

quantum number occurs, the scattering is termed rotational Raman, otherwise it is termed vibrational- rotational or vibrational for short. Due to the small energy changes that accompany purely rotational Raman scattering, the spectra reside in close proxim- ity to the laser line. In multicomponent gas mixtures, the rotational Raman spectra of the varous gases overlap making it difficult, but not impossible, to detect any one species. ~a-L6 Rotational Raman scattering has not been as widely used in flames as vibrational Raman due to the fact that the latter is well displaced spectrally from the exciting line and interferences between vibrational Raman spectra are rare. This is quite evident in Fig. 1. As can be seen, with optical multichannel detection, all major flame constituents with a Raman shift between 900 and 2400 cm- ' were monitored in a single, 20 nsec, laser pulse. 'r In principle, all of the major species could be monitored simultaneously.

In Fig. 2, a typical spontaneous Raman measurement setup is schematically depicted. Laser light of power/energy, Q~, is directed through the medium being probed. The Raman scattering is collected at an angle 0 to the incident beam (typically 90 ~ over a solid angle ft and an extent I along the length of the laser beam and analyzed in an appropriate spectrally selective instrument. It should be stressed that Raman scattering is not intensity dependent. Focussing of the incident laser beam is generally employed, but only to enhance the spatial resolution of the measurement. The strength of the scattered Raman radiation Qr, is given by

Qr = Q,n - - II l~ (4)

where n is the number density of molecules in the appropriate initial quantum states for scattering to be observed, aa/df~ is the Raman scattering cross section dependent on the laser polarization and viewing direction, and ~ is the total optical collection efficiency. Tabulations of Raman cross sections are widely available. 3'9 In atmospheric pressure flames, Qr/Q, is typically of O(I0-~4). Species density measurements t3-28 derive in essence from Eq. (4) and are restricted, typically, to the major species, i.e. > -0.1%. Depending on the concentration level, density measurements are typically accurate to within 5-10%. The spectral distribution of Raman scattering is essentially a display of quantum state populations, with appropriate weighting. Hence, temperature measurements can be performed from the shape of the spectral distribution in a number of ways, e.g., from spectral band contours, '3''4''~'z4'z6'29 line intensity '3 or band peak height ratios9 ~ In terms of measurement accuracy, contour fitting is preferable to the latter approach. 3' A common technique is to ratio the anti-Stokes to


2,o~,.,,,,:, N C() C2

b

24oor I I I 90Ocm-1 N 2 CO 0 2

a

2,oo~'_' I 01 i ~'oo~-~ N2 2 Ether

Fro. 1. Multiple species Raman detection at various locations in a one atmosphere, two-stage diethylether air flame. (Reprinted with permission, Ref. 17).

Stokes vibrational scattering produced by a single pulse, zl'z'~'z7 Such an approach is independent of laser power/energy and fluctuations therein, and most importantly of number density. Depending on the measurement period, temperatures are typically

~~_~DE[7_..X ~ R f DETECTOR ~ '~ ..,.."

COLLECTION" ~ COMBUSTOR

Oi

FIc. 2. Schematic of a typical spontaneous Raman scattering measurement arrangement.

accurate to between 2 and 5%. Most of the work on species measurements in

flames and jets utilizing Raman scattering has been directed toward detection of majority species such as Nz , 14'16'17'19 28 0 2 , 1 4 ' 1 7 " 2 0 ' 2 2 ' 2 ; 3 ' 2 6 H2,14'22'23

17 2 2 23 7 20 2 2 24 27 7 20 22 2~ 22 23 27 CO, " ' CO, " ' ' HzO, ' ' ' CH 4, ' ' CzHz, C~Hs, and NO. In air fed combustion processes, temperature measurements are generally performed from Nz 1~'14'16'21'z~ ,~1 since it is the dominant constituent, which maximizes the signal strength, and is present nearly everywhere despite the extent of chemical reaction. Temperature measurements have also been made fitting spectra from Oz, 14'1628'31 Hz, 1~~4 CO, 18 COz, ~6 and N O Y An example of the capability of spontaneous Raman scattering for probing steady atmospheric pressure, premixed hydrocarbon-air flames 2'~ for comparison with theoretical models is displayed in Fig. 3. These measurements were made with a spatial resolution of 4Oix or less and a precision of + 0.01 mole fraction. Similar theory-experiment comparisons using Raman scattering have been reported by several investigators. 14,24 ,26 ,28 I

In Raman scattering, laser selection depends primarily on the temporal resolution required of the measurement and the severity of the combustion environment from a luminosity standpoint. In steady state flames and flows with little natural luminosity, cw lasers are often used and Raman scattered photons are literally counted. 7.~3A4,lS'z~ The laser beam is generally chopped and photons are accumu- lated employing gated detection which also permits background radiation, either naturally occurring or laser induced, to be sampled and subtracted. The Raman signal is generally enhanced by employing simple double pass schemes for both the laser and scattered photons, 7 placing the experiment inside the laser cavity, 1:3'z~ or using multipass cells, az In turbulent flows, such approaches can lead to mea-


........... } Theoretical i i I

, , o , ~ Experimental " (o) Temp. .... �9 ................ .............

0 . 2 " ..""~ �9 ~ o �9 - 2 0 0 0 e- "(,) 0 " / o o o

( - )H20

Y 0

_r (~ C H .." ,

1.0 2 . 0 3 .0 4 .0 5 .0 6 .0

Distance Perpendicular to Flame Front (mm)

FI(;. 3, Comparison of experimental (Raman) and theoretical temperature and major species profiles for a stoichiometric methane-air flame. (Reprinted with permission, Ref. 23)

surement errors in the true time-averaged properties of the medium. ~ Average density measurements can be made if the detection bandwidth is made sufficiently broad, but temperature measurements cannot, unless the unsteadiness is well characterized. When the count rate becomes sufficiently high during the temporal resolution period, i.e., near unity, 33 species probability distribution functions (pdf) 34 and power spectra 3~ can be obtained. To date these approaches have been successfully applied in relatively cold flows using CH 4 which has a very large Raman cross section. It is not clear at this time how applicable such approaches will be at flame densities with the normal flame constituents.

Pulsed lasers are employed for either one or both of two reasons, luminous environments need to be probed and "instantaneous" measurements of medium properties are desired. In the former instance, one need only be concerned with peak laser power; in the latter, laser energy is of paramount importance. If the medium is turbulent, single pulse measurements are desirable since they lead to determination of the pdf from which true medium average properties and the magnitude of turbulent fluctuations can be ascertained. Analogous to the situation of time- averaging cw Raman data in a turbulent medium, '~ similar difficulties occur when averaging pulsed Raman data, 36 namely, true medium averages may not be obtained, particularly with respect to temperature. Quantitatively, these effects hinge strongly on the magnitude of the fluctuations. For small fluctuations, such effects are unlikely to be serious. This is also true if cycle to cycle fluctuations are small in repetitive processes.

Pulsed lasers and time gating are often employed as a means to overcome background radiation since the cw laser Raman signal, Q~, cannot be accurately

extracted from the background radiation, Q~, for Q r / Q , '/2 ratios of order unity or less, due to the statistics of the photon detection process. ~ It should be stressed that the unwanted background radiation when using a cw laser may include either or both naturally occurring flame luminosity and lased-induced emissions, typically fluorescences. 2~ By selection of an appropriate pulsed laser, peak Raman powers in excess of the background radiations can be obtained enabling measurements to be made. Sources include mode- locked, cavity dumped argon ion lasers, 38 pulsed N 2 lasers, :~ and frequency-doubled neodymium: YAG (2 • Nd). ~2~'24'2~ With pulsed sources, very high levels of background luminosity can be overcome, at least analytically. 7'9 However, fluorescent interferences are still often problematical. 3'~ In addition, in the presence of soot or dust particulates, laser induced or modulated particulate in- candescences ~'~'4~ pose a very serious broadband interference if the focal flux exceeds about 10" W/era 2 which is often the case. Unfortunately, these incandescent interferences are generally in phase temporally with the laser pulse and are not easily discriminated against.

With sufficiently large pulse energies, single pulse measurements become possible. The lasers employed for this purpose include Q-switched ruby, ~7'41'42 dye lasers, ~'"'z'~ and 2 • Nd : YAGJ 7"4:~ If the density is sufficiently high, low energy/pulse N 2 lasers ~ and even cw lasers 15.:~3 3~ can be employed to obtain time resolved information. With a free running, spiking ruby laser, real time turbulence studies are possible. 2~ What is required in all cases is a statistically significant number of photons within the pulse or temporal resolution period. In combustion situations at or near atmospheric pressure,


laser pulse energies on the order of several tenths to a few Joules are required. Even with these energies, the Raman levels correspond generally to just several thousand photons from the dominant constituents. In Fig. 4; temperature probability distribution functions are displayed at several locations in a turbulent hydrogen-air flame, 2'5 fifty diameters downstream of the fuel tube. Temperatures were deduced from the Raman anti-Stokes to Stokes ratio in N 2 using a pulsed dye laser.

Raman scattering can also be employed for two dimensional field mapping by recording the scattered signal from a sheet of light. Hartley ~ demonstrated this approach, termed "Ramanography," by photographing the image intensified Raman light from a high pressure gas mixture. Because of the weakness of Raman scattering, the technique has seen relatively little use. Recently, an improved version has been demonstrated in a multipass laser approach which incorporates a two-dimensional optical multichannel analyzer. 44 Average concentration profiles have been obtained in cold binary gas jets of CH 4 into N 2. With improvements in experimental apparatus, it is hoped that time resolved measurements of the turbulent field will be possible.

Spontaneous Raman scattering has been successfully applied to a number of practical devices,

0.30 x l d = 5 0

0.100"20 14.5 mm

16

o

0.10 = 13 mm ~ ~

~ 0 12

~ O.lO t no ~ o" .~

0.30

0.20 1 ~ - - ~ m r = Imm

O- m ~ 0 300 800 1600 2400

TEMPERATURE T (~

FIG. 4. Probability density functions (histograms) of temperature for a H2-air turbulent diffusion flame at various radial positions fifty nozzle diameters downstream of the fuel tube. (Reprinted with permission, Ref. 25).

although its utilization is sometimes restricted to certain fuels, stoichiometries and operating conditions. At Sandia, cw laser Raman scattering has been employed to map out fuel /air distributions in an internal combustion engine prior to ignition, 45 as illustrated in Fig. 5. There time-averaged equivalence ratio measurements are summarized as a function of radius and crank angle for a motored, nonfir- ing, stratified charge engine with propane fuel. In an earlier study, a6 measurements during combustion showed strong laser induced interferences and were precluded for fuel-rich conditions. Measurements were possible for other engine operating conditions. Pulsed Raman spectroscopy 43 has been applied to perform single pulse temperature and density measurements in a homogeneous charge, gas-fueled combusting engine using a frequency-doubled neodymium laser. At AEDC, pulsed Raman measurements have been made in a variety of combusting and noncombusting tunnels. ~' At AFAPL, averaged temperature measurements have been performed using a pulsed N 2 laser system in the combustion plume of an afterburning J85-5 turbojet engine and in a combustion tunnel housing a bluff centerbody combustor9 In the latter instance, high laser induced backgrounds, attributed to laser-excited fluorescences, were generally encountered using propane as the fuel. Temperature measurements required from 3 to 16 minutes each, the time increasing with the level of background which tended to follow the equivalence ratio. Temperature uncertainties ranged from +4% at low background levels to as high as 95% at the richer conditions examined. By way of contrast, it is interesting to note that single pulse (15 nsec) CARS measurement have been performed in the same facility fueled with JP-4 and a shale derived JP-8 fuel. a7 Recently, Raman measurements have been attempted 42 in backscatter using a pulsed ruby system in a 10 • 20 m, 300 MWe pulverized fuel utility boiler. In the initial experiments, laser modulated soot incandescence was found to be equally as strong as the Raman signal. In experiments in a swirled, turbulent methane diffusion flame, 5 Raman measurements were precluded by strong laser modulated soot incandescence interferences. These interferences are now known to exhibit very fast risetimes and no phase lag in the interference has been found even with 10 -~ sec pulse excitation. ~ If u v lasers are used in an attempt to minimize this type of interference, 7 laser excited fluorescent interferences may well be enhanced.

Near-resonant Raman scattering, in principle, could lead to a substantial increase in the Raman cross sections, and, concomitantly, to large increases in the Raman intensities. In near-resonant Raman scattering, the laser frequency is tuned close to an electronic resonance of the molecule being probed resulting, in some instances, in several orders of


CA

x R '~

r

Fic. 5. Precombustion equivalence ratio distribution as a function of crank angle and in-cylinder location. Four different views are shown for clarity.

magnitude signal enhancement. Unfortunately, the major species of combustion interest possess strong absorption bands only near or below 2000 A, making them inaccessible to such enhancement. Near-resonant detection of minority species is mitigated by their low concentrations, compromises in laser energy due to linewidth condensation and spectral selectivity, and the requirement to probe a selected vibrational-rotational state. This area has seen very little diagnostic application in combustion.

Rayleigh Scattering

Rayleigh scattering is the elastic (i.e. unshifted) scattering of light quanta from molecules, and hence, is not specific to the molecules causing the scattering. Thus, the technique cannot be used for individual species concentration measurements, but only for total number density. The latter can be done accurately only if the Rayleigh cross sections for the individual constituents comprising the mixture

are approximately the same. Despite the lack of species specificity, Rayleigh scattering is of interest since its cross sections are approximately three orders of magnitude stronger than vibrational Raman scattering. Rayleigh scattering has been used to map temperature and number density profiles in a H2-air f l a m e . 49 Temperature is ascertained by a high resolution scan of the Doppler width of the Rayleigh scattered light from a frequency stabilized, single mode Ar § laser. One complication in this approach is the presence of Brillouin scattering (i.e., from density waves) at the higher densities in the precombustion zone gases. Total density is obtained from the intensity of scattering assuming a known composition.

If the scattering cross section is maintained nearly constant from reactants to products by judicious selection of the fuel mixture, then temperature can be inferred from density measurements using the gas law. This approach has been employed in studies of both turbulent premixed 5~ and diffusion ~ flames. Laminar flame speeds have also been determined


.Zone of N uctuating

TURBULENT CH4/AIR FLAME TEMPERATURE

F1 ame~_~ __ ~|

. i: l [ l ~ IM ~LVI#$

:iii iiiiii!i!!!i!i, 't o, iiii!!!iiii ',l :::::::::::::::::::::::::::: i.ll

::C :.iiii ~

II :H'/A~ :11

~t~ IN ~tv~Ns

F~(;. 6. Temperature probability distributions determined by Bayleigh scattering at various axial locations in a premixed, turbulent methane/air flame.

from Rayleigh scattering measurements of temperature performed in this way. 5z In the turbulent diffusion flame work, pdf's and power spectra were generated from the time series of temperature in a manner similar to that used in Raman studies. 33-35 In Fig. 6, temperature pdf's are shown at various locations in a premixed CH 4 / air turbulent diffusion flame using Rayleigh scattering from a cw Ar § laser. 5~ In isothermal jets, variations in Rayleigh intensity are attributable to composition changes and can be used to study mixing of gases with different Rayleigh cross sections. 53

From a practical viewpoint, Rayleigh diagnostics suffer from Mie scattering interferences and spuriously scattered laser light. Mie cross sections are ten to twenty orders of magnitude stronger than Rayleigh processes and thus the environment must be virtually free of particulates. The technique is thus restricted to very clean situations. Rayleigh scattering is also commonly employed for apparatus calibration.

Coherent Anti-Stokes Raman Spectroscopy (CARS)

Coherent anti-Stokes Raman spectroscopy has recently come to prominence for combustion and gas phase diagnostics based upon the pioneering investigations of Taran and his coworkers at ONERA in Francef 54 The effect was originally discovered in the early sixties by Maker and Terhune '~5 and re- mained essentially in the province of nonlinear optics until Taran's application of it for gas phase diagnostics. In the early to mid seventies, a number of feasibility experiments were performed. ~e Diag- nostically, there were two important demonstrations. Broadband CARS generation in a single pulse was obtained illustrating the feasibility for single pulse, i.e., nearly instantaneous, measurements of medium properties. 57 Crossed-beam phase matching in gases was developed leading to high spatial resolution measurements. ~8

CARS permits the diagnostic probing of the high interference environments typical of practical combustion processes for several reasons. First, in contrast to spontaneous Raman phenomena, CARS is a fairly strong process leading to signal levels, at atmospheric pressure and above, several orders or magnitude larger than those for Raman scattering. Second, the CARS signals are coherent, i.e., beam- like, leading to high signal and low interference collection efficiencies. Third, discrimination against laser induced interferences is further improved because the CARS signals are upshifted in frequency from the lasers employed. Thus, CARS offers signal to interference ratios many orders of magnitude higher than spontaneous Raman scattering and is capable of probing practical combustion environments over a broad range of operating conditions. T M As with Raman scattering, CARS appears best suited to thermometry and major species concentration measurements, i.e. >0.1 percent. It is thus complementary to laser-induced fluorescence which is more appropriate to trace radical concentration measurements.

The theory and application of CARS are welt explained in several very good reviews which have appeared recentlyf 2'"3 Briefly, as seen in Fig. 7, incident laser beams at frequencies o~ and t% (often termed the pump and Stokes beams, respectively), are mixed appropriately and interact through the third order nonlinear susceptibility of the medium, Eq. (2), to generate a polarization field which pro- duces coherent radiation at frequency ~% = 2t% - t%. When the frequency difference (to~- t%) is close to the frequency of a Raman active resonance, t%, the magnitude of the radiation at t%, then at the anti-Stokes frequency relative to t%, i.e., at c% + o~, can become very large. For efficient signal generation, the incident beams must be so aligned that the three wave mixing process is properly phased. Phase matching occurs in gases when the input laser


�9 Approach

P "

a) 1 / /

�9 Energy level diagram

(Ol (03

f(ov

�9 Spectrum

Scanned

+- I --d, Broadband

/-x, I (~ (Ol (o3

Fic. 7. Coherent anti-Stokes Raman spectroscopy (CARS).

beams are aligned parallel or collinear to each other. In many diagnostic circumstances, collinear phase matching leads to poor and ambiguous spatial resolution because the CARS radiation undergoes an integrative growth process. This difficulty is cir- cumvented by employing crossed-beam phase matching, such as BOXCARS, 5s shown in Fig. 7, or a variation thereof. 64-66 In these approaches, CARS generation occurs only at the beam intersection and very high spatial precision is possible.

Measurements of medium properties are performed, as in Raman scattering, from the shape of the spectral signature and/or intensity of the CARS radiation. The CARS spectrum derives from the square of the absolute value of the third order nonlinear susceptibility, and thus, CARS spectra can exhibit constructive and destructive interference effects. Constructive interferences occur from contributions made from neighboring resonances. De- structive interferences result when the resonant Raman transitions interfere with each other, or, more commonly, with the nonresonant background susceptibility of the electrons and remote resonances. As the concentration of the species of detection interest becomes weaker, the "signal," i.e., the resonant contribution from the desired species, fades into essentially a nonzero baseline level generated

nonresonantly from the electrons. Eventually, the modulation of this background becomes impercepti- ble and the species can no longer be detected. This was once thought to be a fundamental limitation of CARS in regards to species detectivity, but several investigations 67'6s have shown that the nonresonant contributions can be suppressed or cancelled by proper orientation of the mixing wave field and CARS viewing polarizations. For most molecules of combustion interest, all of the foregoing spectral effects are readily handled numerically. CARS computer codes to synthesize spectra have been developed and validated experimentally for a number of molecules.

The CARS spectrum can be generated in either one of two ways, as seen in Fig. 7. The conventional approach is to employ a narrowband Stokes source which is scanned to generate the CARS spectrum piecewise. This approach provides high spectral resolution, limited only by the linewidths of the lasers employed, strong signals and eliminates the need for a spectrometer. Much interest centers today on coherent spectroscopy because of the very high resolutions attainable. Tunable Stokes wave CARS generation can be used in laminar flames or periodic phenomena when properly synchronized, ~~ but it is not appropriate for nonstationary and turbulent combustion diagnostics due to the nonlinear behavior of CARS on temperature and density. The alter- nate approach, appropriate to time-resolved diagnostics, employs a broadband Stokes source, s7 This leads to weaker signals due to partitioning the energy over a broad spectral region, but generates the entire CARS spectrum with each pulse permitting instantaneous measurements of medium properties if broadband detection is employed.

Although CARS has no threshold per se and can be generated with cw lasers, high intensity pulsed lasers are required to provide a statistically significant number of CARS photons in each spectral resolution interval and to generate CARS signals in excess of the various sources of interference. Pump laser selection for gas phase diagnostics generally involves a choice between ruby and frequency- doubled neodymium (2 x Nd) with the latter being preferable due to its high pulse repetition frequency and more favorable spectral range. Pulsed, nitrogen laser-pumped dye laser systems are commercially available for CARS studies; due to their low pulse energies, they are most suited to gas phase spectroscopic investigations ~ and laminar flame studies. The Stokes laser is typically a tunable dye laser, generally pumped by splitting off a portion of the pump laser.

Considerable emphasis in CARS investigations in gases has been placed on spectroscopic studies in an attempt to understand the spectra produced and/or to exploit the high resolution capabilities of CARS. Understanding the CARS spectroscopy


of various molecules is key to their use for diagnostic purposes. At room temperature, N2, 5'a" O2, 5'~9 n2, s CH4, ~6 and C2H269 have been examined. Spec- troscopic studies at high temperatures have generally been directed toward thermometry which derives from analyzing the shape of the CARS spectrum. To this end, CARS generation has been examined in flame N 2 , s '6a '65,7~ H 2 , 63'7~ C O , s '65 '7~ O 2 , 7z

and H20. v3 CO 2 also appears promising for thermometryY a4 but its spectrum has not yet been quantitatively described. CARS generation has been successfully demonstrated in highly sooting flames 7~'70 and employed for detailed temperature field mapping in a laminar, propane diffusion flame, TM as shown in Fig. 8. There, radial profiles of N 2 temperature at several heights above a simple tube burner are displayed. At flame temperatures, CARS measurements can be made to within a 25-50 K accuracy.

A very unique feature of CARS is its potential for species concentration measurements over limited concentration ranges from the shape of the spectrum. In the presence of a significant nonresonant susceptibility from the background gases, the CARS spectrum from the specie of interest will consist of a modulated nonresonant profile, the depth of modulation being density dependent. This occurs in gas mixtures when the molecular species occur at concentrations typically in the range from 0.1 to 20%. 71'74 Such an approach has been used to follow CO "a and O J z decay in flame zones and has been verified by microprobe sampling in a fiat flame/~ In situations where the nonresonant susceptibility is small relative to the resonant susceptibility of the species of interest, concentration measurements are performed from the absolute intensity of a portion of or all of the spectrally integrated CARS signature. 77 In the presence of a strong nonresonant contribution, the background must of course be first cancelled, e.g. using polarization approaches a7"~s for the CARS spectrum from the species of interest to

be detectable. Due to the strong intensity depen- dence of the CARS signal strength, density measurements are generally performed by comparing the CARS signal from the measurement volume to that generated simultaneously in a reference cell con- taining a gas at a known temperature and pressure. The reference cell normalizes for pulse to pulse laser power fluctuations, serves as a calibrating standard, and accounts for small optical misalignments. These measurements are not easily performed and are the object of much current research. Concentrations measured in this way are probably accurate to within 10-20%. To circumvent difficulties associated with external reference cells in sooting and turbulent environments, in-situ reference generation has been proposedJ 8 Using external reference cells, CARS has been used to map H 2 densities in a premixed natural gas/air flame 54 as seen in Fig. 9. The measurements were made in a qualitative way ne- glecting to include temperature dependent variations in linewidth and population distribution in the data analysis. Nevertheless, they illustrate the capabilities of CARS for flame mapping. In a novel approach, spatially resolved CARS has been demonstrated from a line through a CH 4 jet TM to measure time averaged densities. CARS can also be used for flow visualiza- tion somewhat akin to Ramanography. Using collinear and collimated beams, CARS has been employed to photograph small H 2 supersonic jets into the atmosphere, s In atmospheric pressure flames it is difficult to make CARS concentration measurements below the several tenths of a percent level s'9'74"5~

with the exception of H 2. To overcome this limitation, there has been interest in electronic resonance enhancement, st The third order susceptibility dis- plays resonant enhancement when the laser pump or CARS frequency approaches an electronic transition. This is an area of active research and its utility for combustion diagnostics is not clear at this time.

The potential of CARS for practical application was realized in 1979 with measurement demonstra-

," ',~ ;' ', , ~, ,., 2ooo J~ , ,~ / ~ ~ , "t , ~ , 4s

~ " ~ / ~ ; , / , " ~ " I ' ~ / ~,

/ I I \ l I I I 18 HEIGHT ABOVE 500 ~ ~ I '~ BUR~','ER-mm

RADIAL POS~ TlON~nrn

FIC. 8. Radial temperature profiles determined by CARS in a laminar, propane diffusion flame. (Re- printed with permission, Ref. 76).

E 200(

tram)

~o

0 . ,

/~_ Flonwm ~im~- _-_,/

Fx(;. 9. Hydrogen density distribution measured by CARS in a horizontal natural gas flame. (Reprint- ed with permission, Ref, 54).

LASER DIAGNOSTICS FOR TEMPERATURE AND SPECIES C O N C E N T R A T I O N 1481

tions in a variety of practical devices, an internal combustion engineY the exhaust of a kerosene fired burnerY and liquid-fueled combustors housed in combustion tunnels. 47'81 In the internal combustion engine work, both motored and gasoline-fired situations were examined and CARS spectra of N 2 and propane were obtained. A narrowband Stokes source was scanned synchronously with the engine cycle to generate the CARS spectrum piecewise but, nevertheless, temporally resolved vis-a-vis the engine cycle. Measurements were not possible during some parts of the engine cycle, due perhaps, to severe refractive distortion of the laser beams. In (47) temperature and species concentration measurements from N 2 and O~ were made in a bluff center body combustor fueled with propane and various liquid fuels. In Fig. 10 is shown the axial variation of major species and temperature in the combustion tunnel with propane as the fuel. In (61) N 2 thermometry was demonstrated in a swirl burner and in the exhaust a ]T-12 combustor can. Single pulse thermometry was demonstrated in both studies 47'6~ and averaged CARS temperatures were generally in good agreement with thermocouple measurements. In the one case, 6~ delicate instrumentation was housed in a control room adjacent to the burner test cell and the CARS signals were piped out using fiber optic guides.

Stimulated Raman Gain/Loss Spectroscopy

Besides CARS, there are other third order nonlinear processes 1~ which may be exploited for f/ame

80, 2250

diagnostics. Unlike CARS which is a signal generat- ing process, the other approaches generally involve modulation in some form of a probe laser beam. In the Raman induced Kerr effect (RIKES), 82 a pump laser induces a polarization rotation in the probe laser beam. Although commonly mentioned as a potential approach for combustion diagnostics, there have been few gas phase demonstrations of RIKES to date and it will not be considered further here. In stimulated Raman gain/loss spectroscopy, a pump beam induces gain in a probe beam at the Stokes frequency or loss in a probe beam at the anti-Stokes frequency via the imaginary part of the third order susceptibility. This approach has several advantages over CARS. No phase matching is required. The spontaneous Raman spectrum is, in effect, recorded making spectral interpretation more straightforward. In and around atmospheric pressure, the gain/loss is generally quite small, (10 -5 ) to (10-4), and the technique is not readily multi- plexed in a broadband sense, although several probe lasers may be employed to register information simultaneously at several spectral locations. At high pressures, e.g., >100 atm, the gains may be large enough to permit broadband probe lasers and broadband detection. For laminar flame probing, Raman gain/loss spectroscopy is slightly more sensitive than CARS with nonresonant susceptibility can- cellation. *~ Due to its high spectral resolution capabilities and spectral character, Raman gain/loss is well suited to gathering fundamental spectroscopic data in flames, s3 Temperature measurements have been performed in premixed fiat flames from the ratio of vibrational-rotational Raman line intensities ~~ and single pulse measurements of methane gas densities have been demonstrated. 84

7~ t 6O

~5o ~2c

N 2

1'o 2o 30 ,'o io ~o DISTANCE ON CENTERLINE FROM COMBUSTOR FACE (cm)

F~c. 10. Axial profiles of nitrogen, oxygen and temperature using CARS in a bluff-body stabilized propane diffusion flame. (Reprinted with permission, Ref. 47b).

2000

1750

15OO~

1250

1000 ~

750

5OO

7~ 50

Laser Induced Fluorescence Spectroscopy

In general, the aforementioned scattering and wave mixing techniques are incapable of measuring species in very low concentrations, i.e., ppm levels. Laser induced fluorescence has received considerable attention recently in this regard, particularly for measurements of flame radical concentrations at ppm or sub-ppm levels. ~

Experimentally, laser fluorescence measurements are made in a manner very similar to Raman scattering (Fig. 2) with the exception that the laser must be tunable. Fluorescence is the spontaneous emission of radiation from an upper electronic state excited in various ways; here attention is restricted to excitation via absorption of laser radiation tuned to coincide with a molecular resonance. The fluorescence may be at the same wavelength as the exciting wavelength, termed resonance fluorescence, or shifted in wavelength, generally to the Stokes side, i.e., longer wavelengths. This occurs when the tool-


ecule returns to a state other than that from which it originated. Resonance fluorescence can suffer interference from elastic scattering processes such as Mie or Rayleigh scattering and it is generally preferable to view shifted radiation, particularly when soot is present.

There are several basic criteria which must be satisfied if fluorescence measurements are to be performed on a given molecule. First, the molecule must have a known emission spectrum. Certain molecules, particularly polyatomics, may dissociate when electronically excited, prior to spontaneous emission. Second, the molecule must have an absorption wavelength accessible to a tunable laser source, i.e., roughly between 2000 ]k to 1.5 Ix. Third, the rate of radiative decay, the Einstein A coefficient, of the excited state must be known for quantitative measurements since the fluorescence power is proportional to this rate. Fourth, the fluorescence efficiency needs to be evaluated. Besides spontaneous radiative decay, the electronically excited state may also be deexcited due to collisions, a process termed quenching which reduces the quantity or efficiency of the fluorescence. The upper state may also disappear via chemical reactionY a situation that must be avoided or minimized for quantitative measurements. Analytic quenching corrections to fluorescence data appear feasible only in well characterized flames where temperature and major species concentrations are known together with the deactivation rates of the excited state for various collision partners as a function of temperature, s6 Using this tack, hydroxyl profiles have been mapped through an atmospheric pressure, premixed flame front using laser induced fluorescence with a spatial resolution of i00 IX, as shown in Fig. i1. The fluorescence measurements are in excellent agreement with those determined by laser absorption indicating that quenching effects were properly accounted for. In addition, the measurements displayed excellent agreement with a theoretical model of the flame zone.

In less well characterized media, such analytic corrections are probably quite inaccurate and various approaches have been proposed to evaluate or circumvent the effects of quenching. In one approach, sT the quenching rate is determined in the postflame equilibrium gases by comparing the measured fluorescence intensity with that estimated from an equilibrium calculation of the species density of interest. The quenching rate is then scaled according to gas kinetic collision theory as different flame zones are probed. Key to this method is the use of a very narrow detection bandwidth. In another proposed approach, ss the exciting laser pulse is abruptly terminated and the fluorescence decay monitored from whence the quenching rate can be deduced. A practical difficulty in a flame is that the quenching rate can be very large leading to very

10 -2

10 -3

10 -4

e~

10 5

Luminous Region

_ I

�9 \ I �9 ! I

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Burned Gases

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I

Stoichiometric CH4-Air Flame

�9 Laser Fluorescence o Laser Absorotion

- - - - - Theory

lg -6 I I I 0.d 0.8 1.2

DISTANCE ALONG GAS FLOW (cm)

Fie. 11. Hydroxl concentration as a function of distance along gas flow in a stoichiometric methane- air flame. Laser fluorescence and absorption measurements are compared to theoretical predictions. (Reprinted with permission, Ref. 86).

rapid decay, requiring nanosecond or subnanosec- ond temporal resolution which is not a trivial task experimentally. This method has been recently employed to measure OH in a low pressure flame. 89 It can also be shown that, if very short laser pulse excitation is employed, i.e., the laser pulse length is much less than the quenching time, and the fluorescence is collected only during the pulse, that the fluorescence is independent of the quenching rate. s9 Very high response times are required as above and detectivity is limited since a considerable portion of the total fluorescence is not monitored.

Of the approaches to circumvent the requirement for quenching corrections, saturated laser fluorescence 9~ has probably received the most attention. In saturated fluorescence, the incident laser intensity is made sufficiently large so that the absorption and stimulated emission rates are much greater than the collisional quenching rate which can then be neglected. Another advantage of working in the saturation regime is that the fluorescence signal is maximized. For a given minimum signal detectability level, saturation thus provides the highest species detection sensitivity. Saturated


fluorescence is not without its complications and difficulties. High laser intensities are required to achieve saturation, but are difficult to obtain at certain wavelengths, e.g., NO at 2265 ,~. For many molecules of interest, flashlamp-pumped dye lasers do not possess the requisite spectral intensities to achieve saturation. Furthermore, their long pulse lengths 0(10 -~ sec), permit more excited state chemistry to occur leading to measurement errors. Dye lasers pumped by an appropriate harmonic of a Q-switched neodymium laser generally possess the requisite intensities to saturate and the short pulse lengths, 0(10 -~ sec), minimize the potential for laser induced chemistry. However, on the time scale of a 10 nsec laser pulse, rotational equilibration does not necessarily occur complicating the data analysis. 9='9~ Rotationally nonequilibrated spectra have been observed in a number of cases. ~ If rotation is completely frozen or completely relaxed, data reduction is fairly straightforward. In between these limits, the fractional population feeding the directly pumped transition has to be estimated. Another problem with saturation concerns the focal intensity profile, i.e., nonsaturation in the wings of the fo- cussed laser beam. This has been examined both analytically ~ and experimentally.~-gv The temporal distribution of the laser intensity can also affect the shape of the saturation curve. 9~ With these uncertainties, saturated fluorescence currently appears accurate to within a factor of two s with more sophistication in data treatment likely to improve this ultimately to perhaps +25 percent.

Much of the work in laser fluorescence during the seventies has been directed toward spectroscopic investigations of the various species of interest with ultimate application to atmospheric monitoring, aerodynamics or combustion. Applications have become more prevalent in recent years. Considerable attention has been directed at Na ~5 ~o3 because of its energy level simplicity, ease of excitation at Rhodamine dye laser wavelengths and frequent use as a tracer. In sodium seeded flames, near-resonant Rayleigh scattering has been demonstrated to be effective in overcoming fluorescence trapping in optically thick flames. 9~ Other atomic species have also been examined including Li, "~'~~176 Zr, ~~ TI, ~~176 and In, ~~176 the latter two in connection with two-line atomic fluorescence thermometry. Pb and Ga have also been suggested in this regard.~~ Fluorescence from Cu, Mg and Cr has also been observed in flames. ~c~ Probably the most widely investigated molecular species is NO~ ~''~ because of its convenient and broad absorption spectrum through the visible. Hence, it is easily excited by a variety of laser sources. Of these studies, only one observation of NO~ fluorescence in the post flame region has been reported. ~~ A major reason for interest in laser fluorescence resides in its capability for detecting flame radicals such as

OH,..8O.ST,89.,o. Ho C2,H*.H2 CH,~.H,,.3.~,4 C N in,H4

NH, Hs'llo NH2, H7 SH ~r and SO. sT Interest is grow- ing in monitoring hydrocarbon species '~8-'~~ via laser induced fluorescence. Laser induced fluorescence has also been observed in a variety of stable molecules such as CH20, T M I2, ~2 NO, 6'~23 CO, .24 N2, T M O~ ~25 and a variety of barium ~26 and sulfur ~7 compounds. Two s of the NO studies were performed in flames. In one, 6 saturation was attempted but could not be achieved for intensities up to 6(10~)W/cm2cm-L Several of these investigations examined two photon excitation which generally permits use of a visible laser obviating the need to frequency double into the u v . Two photon absorption cross sections, however, are generally six to seven orders of magnitude smaller than single photon cross sections making saturation unlikely. Several investigations have examined saturation phenomena, often in Na, 9~-9r but also in Li, ~5 OH, 6 C2, Hz CH, 6'H4 CN, "'H4 and I2. ~2=

Besides species concentrations, fluorescence can be employed for temperature determinations as well. Atomic seeds can be used and temperature measured from the ratio of the fluorescence intensities of two transitionsJ ~176 In this technique, termed two-line atomic fluorescence for obvious reasons, both fluorescence transitions originate from the same upper electronic state negating quenching effects. In a different approachY fluorescence is used to measure the population of an upper electronic state in the seed atom which can be shown to be temperature dependent. At low densities and temperatures characteristic of hypersonic flow, temperature can be extracted from the temperature broadened resonance fluorescence linewidth. ~~ The rotational transition intensity distribution in molecular fluo-

lO -~

~ - Kexp=3318 exp(-31,797/T ) 3/i/4 /

L_ Keq= 178.9 exp(- 26,421 / T ) " ~ z x

4 / I / 6

I I I I I I I0-~ 4.6 5-0 5.4 5.8

10a/T (K -I)

Flc. 12. A test of the equilibration of SO 2 + 2 H z = S H + O H + H 2 0 in fuel rich H2/O2/N 2 flames with 1% H~S. Experimental points are based on laser fluorescence measurements of SO 2, SH and OH. (Reprinted with permission, Ref. 87).


rescence is commonly employed for temperature measurements, typically from a flame radical such as OH. z8.~o9.~ 1o In flame radicals in which vibrational energy transfer can occur during the pulse, temperature can be extracted from the fluorescence intensities initiating from two different vibrational states in the upper electronic state by invoking detailed balance. 6

The utilization of laser fluorescence has been primarily to kinetic studies in flames 88'87 or flow reactors. ~ In an extensive study of sulfur chemistry in flames, ~7 fluorescence was employed to measure hydroxyl and four sulfur bearing species in a series of 10 atmospheric pressure, stoichiometric and fuel rich H J O z / N 2 flames doped with small fractions of H2S. Exemplary of the data in this regard is the test for equil ibration displayed in Fig. 12 wherein SO 2, SH and OH were measured using laser induced fluorescence. Fluorescence from sodium tracers has been used to monitor flame turbulence 1~ and to measure velocity, temperature and pressure in hypersonic flows. ~o2

Summary

Recent advances in spatially precise laser diagnostics for temperature and species concentration measurements in combustion systems have been reviewed. Spontaneous Raman scattering and coherent anti-Stokes Raman spectroscopy (CARS) are well suited for thermometry and major species concentration ( ->0 .1%) measurements, but the former is restricted to probing relatively clean flames. CARS appears capable of broad practical utility. Rayleigh scattering is not species specific and can only measure total number density, and thus temperature in isobaric situations. Its use is restricted to very clean, particle free situations. Laser induced fluorescence is most suited for concentration measurements of flame radicals at trace concentrations (<1000 ppm). Temperature can be measured as well often by introduction of a seed material. These techniques should facilitate improved understanding of combustion phenomena which could lead to enhanced efficiencies and cleanliness in energy and propul- sion systems. With the world fossil fuel situation as it is, the integration of advanced laser diagnostics into combustion research comes at a very propitious time.

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COMMENTS

B. T. Zinn, Georgia Tech, USA. Your paper clearly demonstrated the ability of the various laser diagnostic techniques to measure concentrations of low molecular weight species in various flames. I would appreciate it if you would comment on the potential extension of these diagnostic techniques in the identification and concentration measurement of the concentrations of high molecular weight species (e.g., aromatic hydrocarbons) in combustion situations (e.g., a sooting flame).

Author's Reply. The spectroscopy of high molecular weight species is, of course, quite complicat- ed and much more research is required in this area. I think it is doubtful whether optical techniques will be able to differentiate one molecular species from another in a given class in the manner of mass spectroscopy. For example, in CARS examinations of hydrocarbon molecules, D. R. Williams and I. A. Stenhouse at ACRE Harwell find little difference in the spectra from various molecules for carbon atom numbers greater than eight. It will probably be possible to distinguish categories of molecules from one another in different regions of a flame. A. D'Alessio (C.N.R., Naples) is proceeding along this track using laser induced fluorescence. Aromatic hydrocarbons can be seen in various flame regions, but the exact molecular species cannot be identified.

S. N. B. Murthy, Purdue University, USA. In constructing pdf of scalar quantities from Raman scattering measurements, it is important to pay attention to the minimum number of data points that are required in various regions. Has this been done in some of the data you presented?

Author's Reply. The number of data points in the various pdf 's or histograms displayed is probably marginal from a statistical standpoint. The sample size is often constrained by expediency or experimental constraints such as the laser firing rate, extent

of computerized automation, etc. My purpose in showing these data was to indicate what can or has been done. With the ever ongoing advances in instrumentation, I don' t believe it will be difficult to gather statistically significant data samples in reasonable periods of time.

F. Robben, Lawrence Berkeley Lab., USA. You appeared to dismiss the possibility of using contin- uous Raman with a high power argon laser for time resolved diagnostics of turbulent combustion. I believe that with proper statistical analysis of long time series of Raman (and Rayleigh) scattering, the power spectrum, probability density duration and various cross-correlation functions can be obtained. I believe that more effort should go into this type of analysis, as it has several advantages over pulsed, high power laser Raman diagnostics.

Author's Reply. I don't dismiss the possibility; the text discusses work in this area. Time restraints restricted the amount of attention devoted during the verbal presentation. I am skeptical about the utility of this approach with present state-of-the-art lasers using Raman scattering in flames. The Raman demonstrations to date have employed molecules with high Raman cross sections (i.e., CH4) in low temperature situations. At flame temperatures, the scattering from N 2 would be about two orders of magnitude lower and I think the frequency response would not be adequate for turbulence studies. Furthermore, application would probably be restricted to flames with modest levels of luminosity. Rayleigh scattering is probably a better candidate for this approach as you and your colleagues and R. W. Dibble (Sandia) have demonstrated. However, in this case, only "'clean" flames can be examined. I think it is incumbent for researchers interested in this technique to quantitatively explore regimes of applicability, i.e., frequency response as a function of the type of scattering employed, pressure,

1488 C O M B U S T I O N D I A G N O S T I C S

temperature , molecu la r species, in ter ference level, etc.

R. Teets, General Motors Research Labs, USA. CARS spect ra have a compl ica ted d e p e n d e n c e on R a m a n l i newid th s w h i c h can in tu rn d e p e n d on f lame compos i t ion , temperature , and pressure . C an you c o m m e n t on h o w this may effect CARS mea- s u r e m e n t s of t empera tu re and concent ra t ion?

Author's Reply. There are two app roaches to quant i ta t ive CARS diagnost ics . In one, a r igorous theoretical t r ea tmen t is p u r s u e d requ i r ing de ta i led knowledge o f molecu la r energy level spac ings , line- wid ths , a n d so on. In the other, in the absence of the necessa ry mode l l i ng data, one u s e s an empir i - cal procedure . T h i s may range f rom ga the r ing a " l ibrary" of spec t ra for re levant exper imenta l condi- t ions w h i c h are u s e d to reduce data, or resor t ing to emp i r i c i sm in the numer ica l models . An example of the latter was the use of a jud ic ious ly selected, cons tan t R a m a n l inewid th to deduce t empera tu res f rom N 2 CARS spect ra prior to l i newid th data be- c o m i n g available. L i n e w i d t h data is n o w avai lable for N 2 as a f unc t i on of rotational q u a n t u m n u m b e r and t empera tu re a n d exhibi ts good ag reemen t wi th analyt ical l i newid th models . Us i ng these models , l i newid ths can be ca lcula ted for other d ia tomics wi th fair certainty. For po lya tomic species , little l inewid th data is cur ren t ly available. A l t h o u g h the CARS spect ra are l inewid th dependen t , numer i ca l sens i t iv i ty s tud ie s indicate no acute sensi t iv i ty , e.g., + 25% var ia t ions in l inewid th resul t typical ly in smal l spectral charges . With cal ibra ted empi r ic i sm, CARS m e a s u r e m e n t inaccuracies are p robab ly ac- ceptable. As f u n d a m e n t a l data become avai lable for d i f ferent molecu la r species, m e a s u r e m e n t accuracy will improve.

Felix Weinberg, Imperial College, England. It

appears to me that, in the f ield of l amina r p remixed f lames, d iagnos t i c m e t h o d s have ou t s t r i pped the des ign of bu rne r s sui table for m e a s u r i n g f lame profiles. T h e p r o b l e m lies in the need to obta in several po in t m e a s u r e m e n t s wi th in the s t ruc ture of a f lame on ly f ract ions of a m m thick. E v e n the t iniest f lame f luc tua t ion is de t r imenta l s ince a smal l error in locat ion co r r e sponds to a large change in temperature, concen t ra t ion or velocity. F l a m e s on mos t bu rne r s are not suf f ic ien t ly stable. O n po rous p lug burners , refract ion causes l ight beams to be refracted into the sinter , m a k i n g the impor tan t ini t ial react ion zones inaccess ib le to optical probing. A l t h o u g h this cou ld be a l levia ted by increas ing the f low veloci ty to the b u r n i n g velocity, the f lame t hen becomes uns table . Other bu rne r s that have been d i s c u s s e d in the p resen ta t ion seem to involve s tabi l iza t ion by cold sur faces and hence q u e n c h i n g in the vic ini ty of the zone u n d e r s tudy. In the ques t for a l ternat ive bu rne r s I w o u l d sugges t that two o p p o s e d porous p lug bu rne r s in counte r f low unde r cond i t i ons of f low veloci ty app roach ing b u r n i n g veloci ty could p roduce a very s table sys t em wi th all r eg ions accessible to optical probing .

Author's Reply. The work I desc r ibed by the group at the Univers i t e de Sciences et T e c h n i q u e s de Lille is re levant in this regard. T h e y p ropaga ted the laser b e a m pe rpend icu la r to the b u r n e r surface and recorded spat ia l ly resolved R a m a n spect ra t h rough the f l ame front in a single, 20 nsec pulse. Th i s approach, in principle, can overcome the f lame stabi l i ty p r o b l e m you descr ibed. CARS genera t ion from a long a l ine has also been demons t ra t ed . I f the l ine were pos i t ioned pe rpend icu la r to the f lame front, m e a s u r e m e n t s in a s ingle pu l se cou ld resolve f lame zone s t ructure. These approaches are not trivial to i m p l e m e n t however . With a spat ia l ly-s table flame, t ime averag ing can be emp loyed to e n h a n c e spectral and, thus , m e a s u r e m e n t qua l i ty and permi t m e a s u r e m e n t s wi th h ighe r sensi t ivi ty. For funda- menta l f l ame s tudies , spat ial ly s table f l ames are certainly preferable and your sugges t i ons a long these l ines are mos t helpful .

recent advances in laser diagnostics for temperature and species concentration in combustion

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