laser diagnostics applied to combustion systems

Twentieth Symposium (International) on Combustion/The Combustion Institute, 1984/pp. 1149-1176

LASER DIAGNOSTICS APPLIED TO COMBUSTION SYSTEMS*

S. S. PENNER, C. P. WANG** AND M. Y. BAHADORI Energy Center and

Department of Applied Mechanics and Engineering Sciences University of California/San Diego, La Jolla, CA 92093

Laser-diagnostic techniques have been successfully applied to combustion systems. Optical- penetration problems occur with heavy particulate loadings unless lasers at long wavelengths (~5 Ixm) are employed.

We present summary tables showing laser techniques that have been or may be applied for measurements of population temperatures, mean hydrostatic pressure, partial densities in gas mixtures, species concentrations, density gradients, velocity components, particle sizes, flow visualization, and velocity-density correlations.

1. Introduction

We have published a description of nonintrusive diagnostics for measurements on coal-combustion systems. 1 For heavy particulate loadings, micro- wave and acoustic techniques show the best penetration capabilities, while long-wavelength infrared lasers may be less generally useful but are prefer- able to lasers operating at shorter wavelengths. X- ray holography is suitable for development aimed at particle sizing in coal combustors. Applications have been made of prompt 3'-ray emission with neutron excitation in order to determine atomic constituents.

Emphasis here is on measurements of molecular parameters with good spatial resolution that pro- vide insight into rates and mechanisms of combustion reactions. In Table I, we present a compilation of selected lasers that were readily available as of January 1984. It should be noted that advanced de- velopments are contemplated leading to much higher laser powers (e.g., ~-10 MW instead of 5 kW for HF at a beam divergence of - 1 0 -6 instead of 10 -3 rad). For very large laser-power densities, nonlinear effects, which we neglect in this paper, will become dominant. Power outputs of cw gas lasers are seen to range from -10 -1 to - 2 • 104 W with

*Preliminary versions of this paper were presented at the 1983 Fall Meeting of the American Physical Society, San Francisco, CA, November 1983, and the June 1984 laser meeting of the AIAA, Snowmass, CO.

**Also Aerophysics Laboratory, Aerospace Cor- poration, E1 Segundo, CA 90045.

beam divergence angle of ~-i mrad at discrete wavelengths between 325 and 10,600 nm. Pulsed gas lasers with energies between 10 -2 and 104 J were also in use in the long ultraviolet, visible and infrared (near 10 Ixm). Tunable and semiconductor lasers have been widely applied in the visible, ultraviolet and infrared.

Detector data are listed in Table II. The laser intensities are generally so high that detector performance is not a limiting factor, although excep- tions have been noted and small signal-to-noise ratios may become a problem. For laser diagnostics, the construction of array detectors with many detector elements (100 • 100 and larger photodiode arrays) have begun to play an important role. Ide- ally, a laser measurement would have the following features: (a) a pulsed laser illumination system of ns duration to produce measurable signals for "fro- zen" combustion systems; (b) short times (~-10 -r s) of measurement (e.g., with multi-wavelength CARS or LIF 21 with short fluorescence lifetimes); (c) a technique for repeating the observations at high pulse-repetition rates (e.g., ~10 ~ s-I); (d) use of a detector array for three-dimensional observation, imaging and data storage of the entire multiphase, reacting flow field. These facilities will permit simultaneous measurements of important physical variables as functions of time.

The growth of applications to combustion systems is exemplified by the data of Fig. 1, which show that ~20% of papers at recent symposia have involved laser diagnostics. The literature on the use of lasers in flame measurements is more extensive by about a factor of 6 and includes other combustion journals and journals dealing with optics, applied spectroscopy, quantum electronics, fluid mechanics, plasma physics, etc.

1149

1150 COMBUSTION DIAGNOSTICS

TABLE I Survey of representative available lasers (January 1984)

Laser type

Gas

CW

Pulsed

Solid-state

Tunable

Semi- conductor

Medium

Wave- length,*

nm

HeCd 325; 442 HeNe 633; 3,392 HeXe 3,508 Ar 448; 514 Kr 647 C02 10,600 CO 4,800-8,500 HF 2,600-3,600

Cu-vapor 510 Au-vapor 628 N2 337 CO2 10,600 KrF 249 XeCI 308 XeF 351

Ruby 694 YAG 1,060 Glass 1,060 Alex-

andfite 700-818

Dye 340-1,200 Dye 340-1,200 FEL 400-10,000

GaAs 830-910 Lead

salt 3,000~30,000

Power output,

W

0.1 0.1 0.1

40 10

20,000 10,000 5,000

500

0.1

0.001

Pulse energy,**

l

0.015 0.010 0.1

10,000 100 100 100

0.5

10-~

10

400 650

1,000

Beam Beam Pulse divergence, diameter, rate,**

mrad mm pps

0.8 1 1.0 1 1.0 2 0.6 2 0.6 2 1.0 66

Re~rences

2 3 4

5 ,6 6 7 8 9

- - 80 6,000 10 - - 80 6,000 10 - - 10 20 11 0.3 350 single 12 4.0 100 single 13 5.0 100 single 14 5.0 100 single 14

single 15 10 16

1 15

1 17

1.0 5 20 11 1.0 2 - - 6 - - - - - - 18

*Harmonics, especial ly doub l ed frequencies , are usual ly obtainable.

**Simultaneous peak values of pulse energy and pulse rate are not usual ly obta inable .

2. Summary Tables on Diagnostics Applied to Combustion Systems

Papers on laser diagnostics and laser interactions in the Combustion Symposia z3-zs are summarized in Table III. Summary tables showing measured variables, physical parameters used, laser procedures employed, measurement characterization, special features, and performance characteristics are presented in Tables IV to XII.

Abbreviations have been used in the tables. The following are some of the symbols employed in Ta- bles IV to XII: p = mean hydrostatic pressure = Yipi, Pi = partial pressure of species i, Tel = electronic temperature defined by the equilibrium populations in electronic energy levels, Trot = rotational temperature determined from equilibrium populations in rotational energy levels, Tvib = vibrational temperature determined from equilibrium

populations in vibrational energy levels, Tro.vi b = rotational temperature determined from equilibrium populations in rotational energy levels for a particular vibrational energy level. Abbreviations in Table III and column (3) of Tables IV to XII: MS = Mie scattering, RS = Rayleigh scattering, LRS = laser Rayleigh scattering, SRS = spontaneous Raman spectroscopy, SRRS = spontaneous resonant-Raman spectroscopy, SRGS = stimulated Ra- man gain spectroscopy, IRS = inverse Raman spectroscopy, LA = laser absorption, LRA = laser resonance-absorption, LRA-DS = laser (near-) resonance-absorption derivative spectroscopy, CRS = coherent Raman spectroscopy [o~s = detected output frequency, to1 and toe (<to1) are input frequencies], CSRS = coherent Stokes Raman spectroscopy (to s = 2~o2 - to1), CARS = coherent anti- Stokes Raman spectroscopy (cos = 2tOl - 002), HORSES -= high-order Stokes effect spectroscopy

LASER DIAGNOSTICS APPLIED TO COMBUSTION SYSTEMS

TABLE II Photo-detector performance data ~

1151

Wavelength

0 .3 -1 Ixm

0.5 ~m (photo-multi- plier)

1 - 51~m

3 Ixm (In As, 77 K)

5 p.m and longer

10.6 I~m (Hg Cd Te, 77 K)

Photon-Noise Limited Region

Theoretical NEP, Actual NEP,

W W

2hvB/~q

10 -18 for 1 Hz 10 -17 for 1 t lz

Amplifier-Noise Limited Region

Theoretical Actual D*, cm- D*, crn- Actual NEP,

Hzl/2-W - I Hzl/2-W -1 W

(4kT/RA) l/z

2 • 1012 7 • 1011 1.7 • 10 - lz for 1 Hz and I cm z

Background-Noise Limited Region

Theoretical Actual D*, cm- D*, cm- Actual NEP,

Hzl/2-W -1 Hzl/2-W -1 W

(4Pl3hv /'q) 1/2

5 • 101~ 2 • 101~ 5 • 10 -1] for

1 Hz and 1 cm 2

NEP = noise equivalent power (W), A = detector area (cm2), B = detector bandwidth (Hz), h = Planck constant, v = light frequency, ~q = quantum efficiency, k = Boltzmann constant, T = detector temperature, R = load resistance, P8 = background radiation power per unit area, D* = (A • B)I/2/NEP cm-Hzl/~-W 1.

(e.g., cos = 3co2 - 2col), HORAS = higher-order anti-Stokes spectroscopy (e.g., cos = 3601 - 2co2), RIKES = Raman-induced Kerr-effect spectroscopy (to1 - co2 = tOm, COrn = molecular frequency), OHD- RIKES = optical heterodyne detection RIKES, LIF = laser-induced fluorescence, FT-IR-L = Fourier

40

g

g 2 0

i ~o

g

P. .

/

/ " (161)

(1

2O

1

lO

1972 1974 1976 19r8 1980 1982

year

FIG. 1. Number of papers on laser diagnostics applied to combustion systems (solid curve) and the corresponding percentages (dashed curve) as a function of time for the 14th (1972) to 19th (1982) Com- bustion Symposia; z~-2s numbers in parentheses correspond to the total number of papers presented at each symposium.

transform infrared absorption measurements using lasers, OSMS = optical Stark modulation spectroscopy, LIC = laser-induced chemiluminescence, LMS = laser micro-schlieren measurements, LS = laser shadowgraphs, LMI = laser-Moir6 interfero- metry, LDA (or LDV) = laser-Doppler anemometry (laser-Doppler velocimetry), E-LIB = emission from atomic ions following laser - induced breakdown. Symbols used in column (4) of Tables IV to XII: * = the technique has been applied to flames, av = a technique that yields data averaged over the line of sight, c = a conventional procedure, c f = a conventional procedure that may be facilitated by the use of lasers, co = a technique that requires the coherence properties of lasers, cp = a conventional procedure, in principle, but that is really only made possible in practice by using lasers, sp = a technique that yields data with high spatial resolution.

The ultimate in achievable sensitivity in laser diagnostics is probably attainable by using intracavity modulation. A first step in this direction has been made with the introduction of ICLAS (intracavity laser absorption spectroscopy) and the development of gasdynamic lasers. Similarly, adaptive optics may play a useful role in future diagnostics.

The utilization of any technique that has not been previously applied successfully to flames can be expected to represent a challenging research prob-

T A B L E I I I

P a p e r s o n l a s e r d i a g n o s t i c s a n d l a s e r i n t e r a c t i o n s p u b l i s h e d in t h e I n t e r n a t i o n a l C o m b u s t i o n

S y m p o s i a z~-~

Page Number in the Symposium

14th 15th 16th 17th 18th 19th

(Holographic) inter- ferometry 53, 217, 1503 799 821, 1183, 1211 1213 787, 1511

LMS, LS 177, 211, 1119, 655, 731, 857, 747, 777, 1725 245, 267, 563, 1755 359 1365 903, 1503 1283

LMI 303

LDA, LDV 699 553, 573 79, 105, 411, 569, 619, 777, 1747

315, 327, 467, 945, 1201, 1295

81, 579, 921, 931, 961, 993, 1021, 1031, 1041, 1117, 1543, 1837, 1949

245, 259, 349, 359, 375, 403, 413, 423, 433, 477, 1077

Miscellaneous, e.g., lasers used to monitor the feed rate, trigger a spark, measure the burning rate, etc. 99, 189, 367 351 195, 1385, 1871 503

LA, LRA 857 671, 819, 997 543 415, 425, 801, 89, 449 811, 853, 1767

Laser light scatter- 941 1415, 1427 245, 619, 663, 155, 279, 985, 921, 961, 1021, 311, 359, 423, ing/extinction, 681, 695, 1365, 1383 1091, 1117, 459, 477, LRS, MS 1711, 1193 1127, 1137, 487, 691,

1471, 1489, 1359, 1395, 1501 1413

Laser beam attenua- 1333 1193 563, 611, 1017, 1149, 1257 655 tion 1375

LIF 871 497, 505, 647, 23, 485, 1471, 11, 73, I07, 797, 867, 1511, 1559, 311, 1359 957 1567

Pyrolysis, photolysis, chemiluminescence 1317 1459 299 11, 73, 107

SRS 307, 993 1471, 1521, 459 1533, 1583, 1703

CARS 789 975 1471, 1533 259

SRGS, IRS 1471, 1533

Tomography 1041, 1501

Optoacoustic measurements 797

Laser technology. 203 935

Hilbert transform cinematography 489

Laser ignition 1297, 1401 21, 489 747 1183 1709

L A S E R D I A G N O S T I C S A P P L I E D T O C O M B U S T I O N S Y S T E M S 1153

T A B L E I V

M e a s u r e m e n t s o f ( p o p u l a t i o n ) t e m p e r a t u r e s

(1)

Measure- ments of

Population temperatures (Trot, T~,~, Tr~vib, Tro-v~b-el, etc.)

(3)

Procedures

(2)

Physical parameters

used

Equilibrium population distributions assuming LTE

LA, LRA, LRA-DS; SAS (saturated absorption spectros- copY)

SRS, SRRS; yields best detectivity 33

(4) Measurement

characterization

cf, ac,* for LA and LRA; cf, sp,* for SAS

cp,sp,* for SRS

Special features

Tunable lasers are used In for LRA and laser-derivative spectroscopy. LRA and LRA-DS in- crease detection sensitivity by factors of 103 to 106 for particular energy levels above the values obtainable with correlation spectroscopy. SAS is a variant of Doppler-free spectroscopy and yields spatial resolution, z9

Generally used to measure In Tro.vtb.el for the electronic ground state. The overall efficiency for production of tl~e spontaneously emitted Raman frequency is -10 -8 for SRS and -10 -2 for SRRS. The flow-field is recon- structed with ramanog- raphy (multiehannel, pulsed SRS with 20 ns excitation). 34 The de- sign and performance of a photon-counting, two-channel spectroscopic system is described in Ref. 35; the system is capable of simultaneous dynamic measurements of either the concentrations of two gas species in cold flows or of the temperature and concentration of a single gas species (e.g., N2) in a flame. The observed signals were 103 times stronger than those typically observed from the N z Q-branch using a 1-W cw laser; see also Ref. 36.

Representative performance

characteristics

shock-tube studies 3~ on Na, 02, N~O, and Kr mixtures, (NO) = (0.7- 2.3) • 10 -8 mole/cm 3 for t -< 250 p.s and 2384 -< T -< 3850 K. In studies of reaction between OH and vi- brationafly excited CO in a fast-flow discharge reactor, 31 LRA measurements yielded "I'~ -< 2000 K with Trot = Ttr = 298 K; for a review, see Ref. 32.

SRS, the measured temperatures (e.g., for Nz) were T <- 2800 K (2400 --- T, K -< 2800 data are doubtful) for Hz/air diffusion flames; 37 T -< 2000 K for CH4/air and C3H8/ air flames 38 with an accuracy of _+50 K. Prob- ability density functions and correlations of T and concentrations were measured in turbulent diffusion flames 39'4~ for vibrational Raman spectra. Pulsed (using a cavity- dumped Ar+-ion laser) spontaneous Raman- scattering measurements were reported 41 in high-temperature (-2500 K), arc-heated Ng. Even when the flow was seeded with 100-p.m diameter glass beads, S/N was suffi- ciently large for measurements. Using the Stokes Q-branch of N~, the temperature was measured (900-1450 K) in a diluted exhaust stream of a CH4-burning eombustor, when seeded with 5-tzm diameter fly-ash particles (<-6000 era-3). 42 No

laser-induced interference was detected. End-gas temperatures were measured 43 at the center of an engine- type combustion chamber with SRS for both knocking and non- knocking operation with stoichiometric n-bu- tane/air (600-1470 K).


TABLE IV (continued) Measurements of (population) temperatures

(1)

Measure- ments of

(2) Physical

parameters used

Four-wave mixing spectroscopy (yields 33 better

S/N than SRS)

(3)

Procedures

Non-linear optical techniques that generate CRS: CARS, ASTERISK RIKES, OHD- RIKES; also SRGS and IRS. Single- pulse measurements ( - 1 0 -8 sec

resolution) are made in CARS using a broad-band dye laser to excite many Raman resonances. A nearly frequency-indepen- dent, non-resonant background signal limits CARS in temperature deter- minations.

(4) Measurement

characterization

co,sp,* for CARS

Special features

In CRS, the stable molecular states have the same parity. Generally, four laser frequencies are involved: two laser frequencies (VLl and yr.3) causing induced absorption, one laser frequency (vL~) for induced emission, and a spontaneously emitted, phase-coherent frequency vs = 1)LI + PL;~ -- Vt2. Signal strength is greatly enhanced at resonance when vs corresponds to a molecular frequency. Phase- matched frequencies are involved in CARS

with VLl = VL3; improved background suppression is obtained

when ~PLI ~g" PL3 > PL2 as in ASTERISK, RIKES, OHD-RIKES, for which phase matching is replaced by the use of polarized laser beams. Using resonance CARS, the detection sensitivities were improved by factors of 10 ~ to 10 s, de-

pending on the resonance conditions; 44 CARS detectivity is of the order of 1015 cm -3.

A CARS instrument was used in Ref. 45 to measure instantaneous properties at 20 Hz in temporally-fluctuating media; the instrument was remotely operated. Reflected, broadband CARS (RBBCARS) was used in simultaneous multimode, pressure-induced, frequency-shift measurements in shock- compressed (up to 1.53 GPa) organic liquid mixtures; ~ the method has the potential of yielding changes in molecular structure and constituent species identities in chemically- reacting, shock-compressed materials.


characteristics

Temperatures of 800- 2000 K were obtained with CARS on N2 in stabilized, lean, 2-D, laminar, premixed CH4/air flames. 47 Res- olutions of - 1 . 2 5 cm -1 in CARS on heated N2 and - 2 . 7 cm I in col-

linear, phase-matched CARS on flame N 2 were obtained in a single 10-ns pulse and showed good agree- ment with computer- generated predictions. 4s

Temperatures (900 2000 K) were measured in a jet-engine exhaust using CARS. 45 With

CARS on N2 in an internal combustion engine, 49 655-2600 K

were measured. High- resolution, cw CARS spectra of the vl Q- branch of CH4 were obtained in an under- expanded supersonic (M -< 8.2) jet below 31.5 K and 2 torr; ~~ in the interaction volume (<10 -8 era3), < 3 x

1019 molecules contrib- uted to the signal for a single rotational line. A space- ( - 0 . 1 mm) and time- ( - 1 0 ns) resolved CARS measurement is described in Ref. 51. Continuum resonance CARS excitation in a gas (lz) was reported in Ref. 52. CARS and resonant-CARS from C z produced by laser abla- tion of soot particles (RECLAS) were used 53

for simultaneous soot- particle and gas-temperature measurements; the ratio of the Q(3)/ Q(5) Hz lines in a laminar HC/a i r diffusion flame yielded 2150 K. RIKES has been applied to organic liquids for a single dye-laser pulse. 54 CARS was

used to detect O and O e in Hz/O~ and CH4/ 02 flames, s5 Saturation

of the molecular CARS signal (due to stimu-

LASER DIAGNOSTICS APPLIED TO COMBUSTION SYSTEMS 1155


(1)

Measure- ments of

(3) (2) Physical

parameters used

Other techniques

Procedures

SRGS and IRS are nearly unaffected by luminescence, scale linearly with species density and pump-laser power; resolution is limited by the laser line-width; probe- laser amplitude fluctuation yields shot noise in cw operation but {he laser line-width resolution is obtained in quasi-cw tests.

LIF; P (planar) LIF maybe used for flow- field reconstmction.

(4) Measurement

characterization

cp,sp;* for IRS

cp,sp,*; collisional quenching interferes with fluorescence at elevated pressures 21

Special features

Unlike CARS, phase matching of incident and generated fields is not required; ultra-high spectral resolution (-0.002 cm 1) is obtained with the direct production of l~man spectra; there is less background fluorescence with IRS (anti- Stokes-shifted probe) than with SRGS (Stokes-shifted probe) because the probe-laser frequency is greater than the high-power pump-laser frequency in IRS, whereas the reverse is true for SRGS. In an IRS study of the N2 vibrational Q-branch at 10 atm, S / N ~ 71; S/N = 450 when the scattered light was measured for a dye laser and the cw laser was blocked. ~

Temperatures are prefera- bly measured by using seed materials with transitions having strong, known temperature dependence (e.g., In, Ga, Tl, Pb, Ba, 12, OH, NO) and employ- ing two or more resonance laser wavelengths.


characteristics

lated Raman scattering) may limit sensitivity. For pulse widths of 6- 7 ns, the combined detection e~cieney of the photomultiplier and dispersing optics was 0.01 at an S/N of ~ i 0 wbeta T~ib.o2 = 2980 -+ 110 K(Tadiabatic = 3074 K) for an H2/O2 flame and Tvib.O2 = 3300 • 150 K (Tadiabatic = 2916 K) for a C H 4 /

02 flame.

In experilnerrts on supersonic velocities in nitrogen flows using IRS, 57 velocity uncertainties were less than 5% and uncertainties of temperature and density about 10~; measured temperatures were 175 and 300 K. Flow velocities of 145 • 30 m/s were measured in subsonic molecular nitrogen flows using SRGS. s8 VMues of S/N c( Ppump x

1/2 Pprobe AI)-I/2 ~ 100

should be attainable in combustion environ- ments. 56 Representative IRS studies? a 2 MW pump laser, 6 ns exposure, 10 pulses per sec with 300 mW probe laser; sensitivity was improved by -104 when quasi-cw rather than cw operation was used. ~ Fully resolved spectra of N2 were obtained in a CH4/air flame using IRS; c'~ the Q(18) line of the Ne (v = O t o y = 1 band) yielded 1800 K.

A typical value of OH density, obtained from the signal intensity at ffroi)ort = 1300 K, was 6 • 1013 cm 3 in

H202 decomposition at 40 tort. 61 Using a rapid-scanning, frequency-doubled ring dye laserfi 2 T = 1930 • 100 K was obtained


TABLE IV (cont inued) Measurements of (population) temperatures

(1)

Measure- ments of

(2) Physical

parameters used

(3)

Procedures

(4) Measurement

characterization

Special features

FT-IR-L

OA (optoacoustic) and OLD (optoacoustic laser-beam deflection method)

cf, av or sp with inversion,*

cf, a,,*

; Systems with heavy particulate loading may be studied by using suffi- ciently long wavelengths. Small particles (100/~ to 100 p,m diameter) produce noisy, slowly-varying, background absorption. ~ Particle luminosity does not interfere as in SRS, LIF or LA. The spectra of CO, NO, CO2, SO2, C2H2, and C2H4 were well resolved but higher resolution is needed for HzO.

Probe laser(s) are used to monitor the OA signal produced by a strong, pulsed laser input (e.g., Nd: YAG laser). The sound speed (i.e., mean molecular weight and heat capacity) must be known.


characteristics

in OH fluorescence measurements; laser tuning rates of -20,000 c m - l / s permitted single sweep lineshape measurements in less than 100 ~.s. The large scanning range (5 cm -1) covered several adjacent spectral lines. The use of 1900 ppm of NO in a CHa/air flame 63 yielded T within -+10% at a spatial resolution of 0.8 mm x 0.8 mm • 0.2 mm. Use of an XeC1 excimer laser on a CH4/air flame with 50 Ixg of In /ml yielded 2150 K (-+200 K) 64 whereas CARS gave 2125 + 75 K for the same system. XeCI will excite some OH lines directly. 2L~

This is a preferred method for large particle loadings. The FT spectrometer ~ had a frequency range of 3500 cm -1, 0.88 cm -1 resolution, 0.03 em -1 frequency repeatability, single scan rate of 300 cm-1/s, S/N = I0,000. The S/N ratio was degraded by a factor of 1.25 for HC/air flames relative to a static gas sample; it was further degraded by a factor of 5 for coal/gaseous fuel/air flames, Rotational temperatures of CO2 = 1546.5 -+ 47.1 K in coal combustion. 67 Us- ing absorption, a partial pressure of 9.3 x 10 -2 atm (with -+30% uncer- tainty) was obtained for CO2 in coal combustion. 67

T = 100 to 2000 ~ C were measured in a C3H8/ air flame with a spatial resolution of 1 em x 0.15 cm x 0.01 cm. 68



(1)

Measure- ments of

(~) Physical

parameters used

(3)

Procedures

LRS

(4) Measurement

characterization

ef, rp, *; temperatures are obtained from absolute values of Rayleigh-scattered signals.

Special features

See Table VI for details.


characteristics

1550 ~ T, K ~ 1850 were measured in LRS in lean C~H4/air flames with traces of organic peroxides or inhibi- tors, 6~ The effects of atoms and free radicals were neglected, For turbulent, premixed, cn4/air flames (6 = 0.61) and turbulent-jet diffusion-flames (38% CH4 + H~), concentrations and temperatures were measured 104 times per see with frequency-response measurements of 15 kHz; TM

Tmea. -< 2000 K with rms fluctuations 2475 K, Mie scattering and flame luminescence were sources of error.

lem. For the benefit of readers who are not con- versant with laser diagnostics, we show in Figs. 2- 9 diagrams defining measurement principles for selected procedures.

Fig. 2. Laser-Resonance Absorption Spectroscopy, LRA (cp,av,*)

LRA improves on correlation spectroscopy. Las- ers must be closely matched to line-absorption peaks. With frequency scanning, more sensitive line-cen-

ter derivative spectroscopy may be employed. The following frequency shifting was used in Ref. 86 for line-center absorption measurements on NO:

pu lsed ha rmon ic pu lsed dye Nd; YAG l ase r separa to r 532 nm, 220 mJ laser 1

J R a m e n 280 nm, 13 mJ f requency 560 nm, 60 mJ ~ h i f t e r doub le r

226 nm, 60 ~J absorp t ion measurements in the ( 0 ,O ) -band �9

of the NO A 2 ~ X 2 ~ t r ans i t ion

(1)

Measurements of

Mean hydrostatic pressure p and partial pressures Pi for the species present in the flame

Table V Measurements of mean hydrostatic and partial

Physical parameters

used

Measurements with tunable lasers of spectral line profiles, combined with temperature measurements. Optical collision cross-sections must be known for all species present.

Doppler-free saturation and other two-photon saturation spectroscopy to probe spectral line shapes.

(3)

Procedures

LA, LRA, FT-IR-L

Doppler-free, high-resolution, two-photon spectroscopy or other saturated absorption spectroscopy.

(4) Measure-

ment character-

ization

c f , a v

c p , s p

) r e s s u r e s

Special features

Measurements of this type have not been made and would not be expected to be accurate in regions of active combustion because optical collision cross- sections involving atoms and radicals are not generally known.

Since Doppler broadening is eliminated or modified, the collision-broadened line profiles and line-center absorption are more readily determined and will yield pressure estimates,

1158 C O M B U S T I O N DIAGNOSTICS

TABLE VI M e a s u r e m e n t s o f partial and total spec i e s d e n s i t i e s

(1)

Measure- ments of

Partial densities of chemical species and total density

(2)

Physical parameters

used

Partial densities are obtained from species- concentration measurements, provided the temperature and pressure are known. If the concentrations are known, the total density may be measured directly by using Ray- leigh scattering, provided Rayleigh scattering cross sections are available for each species,

(3)

Pro- cedures

LRS

(4) Measure-

ment character-

ization

cf, sp,*

Special features

While many Rayleigh-scattering cross sections are available for stable species, these are not generally known for atoms and free radicals. Hence, density measurements with Ray- leigb scattering cannot be made very accu- rately in regions of active combustion. Sys- tematic measurements of Rayleigh-scattering cross sections are needed for these species and may be readily obtained from shock-tube studies.


characteristics

In premixed H2-air combustion in a heated turbulent boundary layer, 71

C~0'~/0~ -< 9~ and ~/0= = 0.25-1.0. LRS was used 7~ for simultaneous measurements of density at two points in stabilized, V- shaped, premixed, turbulent C2H4/air flames (d~ = 0.6); densities normalized with respect to the unburnt gas were 0.25 to 1.5. Density fluctuations were measured TM in the heat-re- lease zone of a rod-stabilized C2Ha/air flame propagating into a grid-induced turbulent flow field (r = 0.5-0.75, upstream velocity = 250-684 cm/s); the photomultiplier output was am- plified by an electrometer with a band-pass from dc to 1.9 kHz (3 db corner frequency); 5000 samples were acquired at each h~ation at a constant rate of 4000 or 2000 s -1. An imaged Ray- leigh scattering technique was used to measure turbulent flame thickness in an operating engine; TM spatial resolution = 0.1 mm and time resolution = 0.01 ~s

. (using a multi-element detector and a pulsed laser); probability distributions were employed to describe variations in flame thickness; see also Refs. 75 and 76.

(1)

Measure- ments of

Species concentrations

TABLE VII Measurements of species concentrations (compare Table Iu

(e)

Physical parameters

used

Species concentration measurements are generally performed on the assumption that equilibrium population distributions obtain at LTE. Without this assumption, absolute intensity measurements must be performed and

(3)

Pro- cedures

LA, LRA; SAS

(4) Measure-

ment character-

ization

cf, ac,*; cf, sp, * with SAS

Special features

LRA (with tunable lasers) is an improvement over correlation spectroscopy; with derivative LA near the line center, ppb can be measured; LA is especially useful for shock-tube studies (e.g., Ref. 30).


characteristics

In atmospheric, premixed, laminar CH4/air flames, 77 Xolt = 3 x 10 -4 to 4 x 10 -3 for d~ = 0.86. Abso- lute vibrational populations of CO were found to be ~1014 to ~1012 cm -3 for v = 1 to 4, using IR resonance absorption measurements; 31 see also Ref. 32.


TABLE VII (continued) Measurements of species concentrations (compare Table IV)

(1)

Measure- ments of

(3)

Pro- cedures

SBS, SRRS

(2)

Physical parameters

used

data reduction requires knowledge of (a) transition probabilities and (b) spectral line shapes. An exception occurs for CARS because the shape of the spectrum depends on concentrations (~0.5 to 30%).

CARS, AS- TERISK, BIKES, OHD- BIKES

SRGS, IRS cp,sp

LIF; P cp,sp,* (planar) LIF

(4) Measure-

ment character-

ization

cp,sp,* for SRS

co,sp,* for CARS

Special features

Theoretical quantum-limit detectivities have been compared for CARS, IRS and SRS and show that the CRS methods tend to be greatly su- perior in luminous en- vironments or when high resolution is required, 3'3 CARS is preferred at high p while IRS is better at low p because of deactivation by quenching. 3a See the explanatory remarks made in connection with temperture measurements For species- concentration determi- nations, it is sufficient to monitor outputs from individual spectral lines, although measurements on many lines are needed to verify LTE

LIF is especially useful for measurements of trace concentrations (e.g.. NH, OH, CH, Cz). Rotational and vibrational relaxation as well as total emitted radiation following excitation of a rovibrational band, which yield spontaneous emission rates (only at very low p) and concurrent quenching rates (at higher p), are measurable

Multiple LIF emissions are useful for complete flow-field visualization; spatial resolution re-


characteristics

cry2 -< 36 x 10-6, cltg -- 10 x 10 -6 and CH20 ~ 3 X 10 -6 mole/em 3 for Hvai r diffusion flames. 37 Using single- laser shot excitation in 20 ns, major species were detected simultaneously in different parts of flames using multichannel pulsed Ra- man spectroscopy; a4 the ap- paratus had a resolution of about 300 spectral and 10 spatial elements. See Ref~ 78 for engine studies using SRS and probes.

In a CARS experiment, ~ the C2 radicals formed in a mi- crowave discharge and in an acetylene welding torch were detected under triple- resonance conditions. C 2 concentrations were ~5 x 106 cm-3 in the discharge and ~1 • 10 It cm -3 in the flame; the detection sensitivity was ~10 s em-3; see Ref. 45 for CARS concentration measurements in augmented jet-engine exhausts. Highly sooting flames 48 showed coherent spectral interference in electronic resonance CARS on C2, produced by laser- induced vaporization of soot; CO was difficult to detect below the 1-2% levek

See Refs. 58, 60 and 79.

Measurements in C2H2/O2 and C2Ha/NaO flames yielded (Ref. 80): (CH) = 103 -+ 50 and (CH) = 380 -+ 150 ppm; NO fluorescence saturation was not observed. In H2/N2/O2 flames (4/1/6) containing Xs (as H2S) of -<1.0 mole %, number densities were: 81 (OH) -< fl • 10 II, (SO) -< 1016 , (SOz) -< 2 x 1016, (SH) -< 3 x 10 Is, and (S~) -< 6 x 10 Is cm -3. In trace sodium fluorescence measurements (Na < l0 II era-3), 82 interference from particle scattering was sup- pressed (10,000:1) by pumping the D2 line and monitoring DI; the flames were turbulent, free-jet,



(1)

Measure- ments of

(2)

Physical parameters

used

(3)

Pro- cedures

(4) Measure-

ment character-

ization

E-LIB

OSMS

c p , s p , *

cp,sp,*

Special features

quires Abel inversions.

Ionic emission is a sensitive tool for monitoring elemental compositions. The high power densities achievable with lasers are useful in producing breakdown. The LTE assumption may become inapplicable.

Frequency-dependent absorption of laser radiation with a probe laser


characteristics

C3H s-diffusion flames. A tunable dye laser was used

to excite OH in a flame thickness of 0.5 mm and showed 700 ppm of OI1 using absolute emission measurements, a3 (OH) = (0.3- 3.8) • 1021 m -3 in an

O2--C31t8 flame study, s'l Relative CH concentration profiles were obtained at 20 torr in a CH4/O2 flame (~b = 1.06) using LIF from the [A2&(v ' = O) --'* X 2 ~ l ( v " =

0)] band; ~ quenching of the AeA excited state, its pressure dependence, and effective quenching cross section of 6 ~,2 were obtained. For an H 2 / N 2 0 flame study, see Ref. 86. For an OH concentration measurement in boundary- layer combustion of a lean H J a i r mixture (~b = 0.2) flowing over a heated (1170 K) platinum plate, see Ref. 87. For a review of LIF, see Ref. 88. Applications of picosecond laser spectroscopy to the measurement of collisional quenching rates by time-resolved LIF in flames were reported in Ref. 89; the measured collisional quenching lifetime following excitation of the R2(4)A2~,+(v ' = 0) *-- X21](v" = 0) OH transition was 1.8 ns in the burned gas region of an atmospheric pressure, premixed CH4/air flame. LIF measurements for triatomie gases have been performed recently.

See Ref. 90 for an overview and cited references, which show the following results: less than 1 ~g of Na and K were detected in a coal gasifier; 91 detection limits (in ppm) for elements in air were: ~ CI (20), F (40), P (1.2), S (200), Be (0.0006), As (0.5), and Na (0.006); see also Refs. 93-95.

A narrow-bandwidth (cw) phase-fluctuation laser-heterodyne interferometer was



(1) (2)

Physical Measure- parameters ments of used

(3)

Pro- cedures

LIC

FT-IR-L

Resonant multipho- ton optogalvanic detection

(4) Measure-

ment character-

ization

cp,sp

Special features

was performed, while optical Stark modulation was produced by a strong laser field (e.g., in Na, Nz, H2).

Concentration measurements in chemiluminescence require absolute calibrations since the assumption of LTE becomes inapplicable.


characteristics

used in Ref. 96; sensitivities of 10 -8 cm -1 were obtained. The detection limit was 5 ppb NH3 in air. Stark-modulated IR absorption spectroscopy was used to obtain spatially-resolved measurements of gas-species densities; 97 the deteetivity decreases with increasing gas density. Quantitative, single-shot ( -10-30 ns) measurements were made; concentrations of free radicals were of the order of tens of ppm Spatially-resolved OSMS was applied to combustors by monitoring transitions between electronic energy levels; 9s a detectivity of ~300 ppm was estimated for OH and - 4 ppm for band-integrated OH fluorescence measurements in a sooting flame; Na of -0 .1 ppm was reported; see also Refs. 99 and 100.

cf, av or sp with The same requirements See the corresponding entries inversion tech- apply as for tempera- in Table IV. niques ture measurements.

co,sp,* This technique differs que from E-LIB through the use of resonant multi-photon transitions and may be applied for measurements of H and 0 in flames. See Ref. 101.

Resonant 2-photon excitation was followed by 1-photon ionization and detection of H in an atmospheric Hz/air flame by observing the ionization signal as a function of dye-laser wavelength.l~

Gas and glass lasers have been used for LRA before, e.g., the Br2 continua were studied with the 6328 ,~ line from an He-Ne laser (Ref. 148), bound- free transitions in C12 (Ref. 149) and in NOC1 (Ref. 150) with a frequency-doubled ruby laser at 3471 /~, CH4 at 3.392 txm with an He-Ne laser (Ref. 151) and CH20 at 3.508 Ixm with an He-Xe laser (Ref. 4). General procedures and attainable precision are discussed in Ref. 152.

Fig. 3. Spontaneous Raman Scattering, SRS (cp,sp,* )

A schematic diagram illustrating the principles involved in SRS is shown.

Inelastic collision between a photon of energy hVL and a molecule populates the virtual energy level hvL above the ground state. In elastic Rayleigh scattering, emission occurs at VL. In Raman (R.)


(1)

Measure- ments of

Density gradients

(2)

Physical parameters

used

! Changes in index of refraction with density al- : low identification of density gradients

TABLE VIII Measurements of density gradients

(3) (4) Measure-

ment Pro- character-

cedures ization

LMS cf, av,*

LS cf, av,*

LM1 l ef, av,*

Special features

Pulsed LMS systems are useful for making time- dependent measurements.

I The use of lasers yields shadowgraphs with very sharp definition because the light intensity varies at constant h, which makes use of achromatic lenses un- necessary.

When a laser light source is used with two parallel gratings that are rotated while maintaining a constant relative angle, the Moir~ pattern consists of straight fringes. Prior beam passage through a flame modhqes the Moird pattern according to the gradients in index of refraction encountered by the laser radiation.


characteristics

The schlieren method has been applied to studies of flame structure in an engine 1~ and to interactions in turbulent 1~ combustion.

For applications of shadowgraphs to spark ignition engines, including the use of double passes, see Refs. 105-110.

Temperature maps (300 to 1700 K) were obtained TM

for axially symmetric, premixed CH4/air flames (mole ratio of CH4 to air = 1:3). Measured density gradients were -<2,77 x 10 -4 g/em 4 in flow over a 2-D diamond airfoil in a supersonic wind tunnel. 112 Deflection mapping of a candle flame with a resolution of 5 • 10 -s rad and lens mapping with a resolution of 10 -2 rad were demonstrated in Ref. 113. Temperatures of a premixed HJO2 flame were mapped by Moir*! defiectometry 114 (T = 300 1600 K for a typical radial temperature profile).

TABLE IX Measurements of velocity components

(l)

Measure- ments of

Velocity components

Physical parameters

used

Doppler effect and optical mixing spectroscopy

(3)

Pro- cedures

LDA

(4) Measure-

ment character-

ization

CO, sp, *

Special features

Experimental implementa- tion may involve any one of the following procedures: (i) local os- cillator-heterodyne arrangement; (it) dual- beam optical arrangement, (iii) symmetrical heterodyne arrangement. The sign of the velocity component is obtained, for example, by using an acousto- or electro-optical modulator to pre-shift one of


characteristics

Velocities have been measured from a few mm/s to M - 2; typical spatial resolution 2100 V-m; typical frequency bandwidth 210 kHz.

In an LDA experiment, It5 both agglomerate and gas velocities were measured with a burst counter by eliminating either large or small amplitude pedestal signals. A carbon agglomerate stream (10-75 ~m) was injected along the center- line of a fiat-flame burner


(1)

Measure- ments of

(2)

Physical parameters

used

Time of flight between two or more spatial points

Emission from Doppler- shifted spectral lines

Doppler-shifted Raman spectroscopy

TABLE IX (continued) Measurements of velocity components

(3) (4) Measure-

ment Pro- character-

cedures ization I

LDA co,sp.*

cp,sp LIF

IRS cp,sp

Special features

the incident frequencies. The beat signal for a positive value of v then corresponds to fe + fD and, for a neg- ative v, to fp - fD, where fe = preshifted frequency and fD = Doppler-frequency shift. The frequency bandwidth is higher than the bandwidth used in the time-of- flight technique (cf. the next entry); for high particle concentrations, this procedure is pref- erable to the time-of- flight technique.

Transit times of tracer particles are measured between two focused laser spots. Correlation functions are used for high particle number densities. This two-spot technique is the Four- ier transform of the dual-beam technique in LDA and is the preferred procedure for low particle concentrations.

Doppler-shifted absorption was used (to eliminate the need for an un- shifted reference signal) with L1F detection. Two sets of counter- propagating beams were used to measure LIF in a two-dimensional subsonic flow, for spectral lines of known shape. Na is a suitable species for LIF at high and 12 at low temperatures. A narrow bandwidth laser is frequency-tuned across a spectral line, causes measurable changes in spectral fluorescence, and thereby defines the lineshape and Doppler broadening.

IRS absorption lines are frequency-shifted by the Doppler effect.


characteristics

in the presence of a com- bustible gas mixture (C/H mass ratio = 2.96-3.10) doped with 0,3-p,m AI203 particles. Average errors for agglomerate-velocity defects were 10%; the gas velocities were 1,47 to 1,54 m/s; see also Ref. 116,

Velocities have been measured for M < 1; typical spatial resolution =100 p,m; typical frequency bandwidth =100 Hz; see Ref. 117 for a pro- posed experiment using a laser time-of-flight veloci- meter (LTV) operating on the principle of laser-resonance fluorescence (LRF); see also Ref. 118.

Doppler-shifted absorption with LIF detection of 12, excited with an Ar+-ion laser at 514.5 nm, was used in an iodine-seeded, under- expanded jet flow. Fluores- cence was detected at 10,000 points in the supersonic flow with a 100 • 100 element photodiode array camera. The accuracy was =-+5 m/s and was determined by the 12 line width. 119 For a survey, see Ref. 21. The fluorescence was detected lz~ as in Ref. 119. Velocities were de- rived from four frames. The measured mean velocities of 5-50 m/s agree with other velocity data.

Supersonic molecular-flow velocities were measured for N2 in a wind tunnel; 57 v = 471 -+ 13 m/s, with uncertainties of <5%.


TABLE X Measurements of mean particle sizes and particle-size distributions

(1)

Measure- ments of

Mean particle size and particle-size distributions

(2)

Physical parameters

used

Mie theory for spherical particles, assuming known optical properties, Nearly all of the published studies involve the assumption that the particles are monodisperse. Absorp- tion (extinction) must be used in addition to scattering if number densities are to be obtained.

(3)

Procedure

Angle-dependent light scattering

Laser near-forward scattering for single-particle monitoring

Laser-Fraunhofer diffraction measurements are used in commercial instru- ments and require corrections for multiple scattering, beam steering and variable detectivity,

Sizing with a variable- frequency grid

(4) Measure-

ment character-

ization

c,sp,*

c,sp,*

c,sp,*

c,sp,*

Special features

This simple procedure is preferred for dilute, monodisperse particulate systems. Combined particle- size and number- density determina- tions require the use of both light- scattering and light-attenuation measurements.

Near-forward scattering is favored as an experimental technique because these scattered intensities are only weakly dependent on particle shape and refractive index. Single particle counting (SPC) becomes readily fea- sible in dilute particulate systems. To obtain ensem- ble averages from SPC, it is necessary that the inter- particle distance ~4 x particle diameter and number densities :~ 101~ s em -3

(with d in /~m).

The far-held Fraunho- fer diffraction pattern is superimposed on the geometrical image. The light-energy distribution in the focal plane is used to determine particle sizes. Only particles ~-2.5 h are measurable.

The particle image is scanned across a variable-frequency grid. The visibility becomes vanishingly small when the particle diameter approaches the local grid spacing.


characteristics

For C3H8/O2 and C 2 H J O g flames, 121 the soot number densities, particle sizes, and concentrations were (3-75) • 109 cm a, 6-27 nm, and (0.7-11.0) • I0 -8 g/cm 3, respectively; see also Refs. 122-125.

Individual particles in the size range 0.25-100 p.m at number densities up to 106 em -3 were measured; lz6 see also Refs. 127 and 128.

An accuracy of about 2% was obtained for the Rosin-Ra- mmler size param- eter and +O. 1 for the exponent in a study of droplet- size distribution; 129 measured sizes were 5-500 ~m.

This imaging technique is best suited for 10 Ixm and larger particles. 127 Velocities and their correlations were obtained by measur- ing incandescent particle transit times through the entire grid.

LASER DIAGNOSTICS A P P L I E D TO COMBUSTION SYSTEMS

TABLE X Measurements of mean particle sizes and particle-size distributions

1165

(1)

Measure- ments of

(2)

Physical parameters

used Procedure

Diffusion-brnadening spectroscopy in which the Brown- tan motion is used to obtain velocities and hence diame- ters. In the Ray- leigh limit of Mie theory, the scattered light distribution is indepen- dent of optical properties and particle size and, therefore, scattering will not yield useful sizing.

CARS

(4) Measure-

ment character-

ization

co,sp,*

Co,sp

Special features

Optical properties are not needed for monodisperse systems. The technique is only applicable to particles smaller than about 0.5 p,m. This technique has been used to measure the two parameters that are needed to characterize a log- normal distribution.

When soot particles are heated, the observed CARS intensities of C2 depend on particle- size range; the CARS signals are electronically enhanced at selected dye-laser frequencies.


characteristics

Particles "~23 nm were observed in CH4/O2 flames, 5 mm above the burner; they grew to 60-120 nm, de- pending on the equivalence ratio, 14 mm above the burner; la~ see also Ref. 131.

Theoretically, CARS intensities of C2 depend on laser- incidence angle and on particle size. 132

TABLE XI Flow-visualization techniques

(1)

Measure- ments of

Flow-field visualization and mapping of species concentrations, density gradients, temperatures, etc.

(2)

Physical parameters

used

Changes in flow velocities, ! densities (as measured, e.g., by Rayleigh scattering), species concentrations (~ measured by LA or LRA), population temperatures (as measured by LA, LRA, L1F, light emission, etc.). Spatial variations are determined by em- ploying appropriate inversion procedures for measurements that yield averages over the line of sight,

(3)

Procedure

Holography

Rayleigh scattering

(4) Measure-

ment character-

ization

CO,Sp,*

ef, sp,*

Special features

While complete field reconstruction is tedious, a great deal of useful information is obtainable.

The elastically scattered light mea- sures densities directly in the Rayleigh limit.


characteristics

Holographic interfero- metry has been used to obtain the temperature 13,3,134

and fuel-vapor concentration distributions; TM visual rec- ords were obtained for events in bitu- minous coal burning. 135

For C2H4/air mixtures (~ = 0.35) in reacting turbulent boundary layers, 1~

(O - Pb)l(Po - Oh )

and rms [15/(0o - Pb)] densities were <-1.2 and (-0.05)


TABLE XI (continued) Flow-visualization techniques

(1)

Measure- ments of

(2)

Physical parameters

tlsed

(3)

Procedure

Mie scattering

LIF

Speckle velocimetry

(4) Measure-

ment character-

ization

cf, sp,*

cp,sp,*

CO,Sp

Special features

With uniform particulate distributions, the digitized scattered signals may be used to monitor gas concentrations directly provided the particle number density mea- sures the local density directly.

Species concentrations, velocities, and temperatures may be measured directly. Useful excited species are 12, OH, NO. See Refs. 21, 83, 88, 119, and 138-140.

A double-pulsed light source and sequential recording are used to obtain sequential images. Image processing involves laser illumination of a photograph, which produces Young's fringes in the back focal plane. A camera is finally used to record a Fourier transform of the illuminated region,


characteristics

-0.4, respectively,

using Pb/Po = 0.225 (from Ray- leigh scattering), where Po = free- stream density and Pb = average den- sit),.

Gas densities were measured for ducted, premixed combustion in spark-ignited C3Hs/air mixtures.136 The pdf for scattered light yielded t~" c"/~Uo -< 0.1~6 and ~ / ~ _< is .

2-D measurements of flame temperatures were obtained using two-fine LIF on OH, TM which is applicable to Tvi b ~- 1800 K; the signal levels were too low at lower temperatures. A twice Q-switcked (At = 40 to 180 Ixs) Nd:YAG laser was used to pump a dye laser, which was frequency-doubled and to~ed to an OH absorption line in a planar flame region, The OH LIF was observed and provided irff, ormation about small-scale turbulent flame structure. 139

2-D velocity fields were used as an approximation. 141 Time delays between successive pulses were 150 Ixs and the maximum displacement was ~200 ~m; velocities exceeding 100 m/s could be probed.

LASER DIAGNOSTICS APPLIED TO COMBUSTION SYSTEMS

TABLE XI (continued) Flow-visualization techniques

1 1 6 7

(1)

Measure- ments of

Flow fields, etc.

(2)

Physical parameters

used

Density changes, etc.

Gradients of index of refraction produced by temperature gradients.

(3)

Procedure

Laser tomography

LMI

(4) Measure-

ment character-

ization

c~sp,*

cf, sp.*

Special features

LA or LRA is used to make many measurements. Inver- sion yields a two- dimensional prop- erty field. Line-intensity ratios and line profiles may be used to obtain temperature and concentration fields.

Rays from a collimated light source are deflected by gradients in index of refraction.


characteristics

Local and instantaneous velocities of the flame front were obtained 142

in turbulent, premixed flames of

C3Hs/air diluted with Nz (e.g., 6 = 0.9). The spectral density of the flame velocity = 0 to 0.4 (max) to 0.1 cmZ/s for v = O to 3 to 13 Hz. Droplet sizes and concentrations were measured 143

in axisymmetric sprays of kerosene. Cumulative size distributions were obtained for diam- eters <-160 i~m.

The experimental procedure is the same as in Ref. 111 and was applied to an H2--O~ flame. Temperature profiles were con- structed from LMI. TM

scattering, we observe either the Stokes line at VL- - (AE/h) or the anti-Stokes line at VL + (AE/h). If AE is a rotational, vibrational or ro-vibrational energy, the rotational, vibrational or rovibrational R. spectrum is observed. If Vu is different from AE/h, non-resonant R. scattering

occurs and the line intensity is I l oc [VL ---+ (AE/h)]4NIP~I, where Ni is the number density of molecules in the lower (l) energy level and P2u l is the transition probability (related to cross section) for energy change between upper (u) and l energy levels. At equilibrium, level populations are given

TABLE XII Measurements of correlations in turbulent flows

(1)

Measure- ments of

Velocity-density correlations in turbulent , flows, etc.

(g)

Physical parameters

used

Simultaneous measurements are needed of two or more of the parameters listed in the preceding tables.

(3)

Procedure

As appropriate, for each of the parameters involved.

(4) Measure-

ment character-

ization

As specified, for each of the parameters involved.

Special features

Each correlation measurement requires separate discussion because experimental problems are likely to be encountered that are not involved in individual measurements.


characteristics

For representative experiments, see Refs. 104 and 144- 147.


v i r tua l leve~ w i th .ne rgy hY L

]1 f

v i r tua l level w i th energy hVL+AE

energy level ~E above ground level

ground mo lecu la r energy level

by the Boltzmann equation. Since the Nl are proportional to |l, measurements of intensity ratios yield (equilibrium) population ratios and hence temperatures. Absolute It and known transition probabilities or cross sections are needed to determine absolute number densities. Because ground populations are much greater than those in excited levels, R. Stokes lines are much more intense than anti-Stokes lines. Intensity ratios of R. Stokes to anti-Stokes lines may be used to measure temperatures. For incident intensity Io at low and moderate tempera-

- 3 - 4 tures, IRayleigh sca t te r ing/ Io ~ 10 to 10 ~ (105 to 10 ~) • I . . . . . . . . . . t R . . . . t t e r i ng / Io . If VL -- AE/h, resonant R. scattering occurs at intensities that are larger by ~103 than for non-resonant R. scattering. For applications of SRS, see Refs. 36, 41, and 153- 157.

Fig. 4. Coherent Raman Scattering, CRS (co,sp,*)

The diagrams illustrate the difference between spontaneous and coherent R. scattering. The stable (c and d) and excited (e) electronic molecular states have the same and opposite parity, respectively.

l . . . . . ! ...... l !

SRS CRS

Nonlinear optical techniques that generate coherent R. radiation are represented by the four-wave mixing processes involving phase-matched laser frequencies VEt (induced absorptions from c to the virtual state vt), YEa (induced absorptions from d to v2), and vu2 (induced emission from vt to d). t~aA59 In general, Vs = rE1 + VL3 -- VLZ. For phase- matched input frequencies, the field at Vs is always phase-coherent with the incident fields at VEt, rE2, and VL3. However, the coherent R. intensity at vs is greatly enhanced at resonance when rE3 -- VLZ

= v~.. For vL1 = rE3, coherent anti-Stokes R. scattering (CARS) obtains. 16~ Scattered intensities with CRS are ~109 greater than with SRS. t~3 Sev- eral CARS configurations t~ have been used. Im- proved sensitivity in CARS may result by using internal referencing.

Fig. 5. Stimulated Raman Gain Spectroscopy, SRGS, and Inverse Raman Spectroscopy, IRS (co,w,*)

These techniques, unlike CRS techniques, involve the use of a pump laser with much higher power output than the probe laser. The pump laser is tunable while the probe laser is at a fixed frequency. In both SRGS and IRS, resonances are observed at the larger pump or probe frequency di- minished by the characteristic frequency (Av) of the Raman-active medium. In SRGS, the tunable pump laser is at higher frequency than the probe laser. The observed intensities at Vprobe as a function of pump frequency show stimulated gain for Vp,mp -- Vprobe = AV. In IRS, the tunable pump laser is at lower frequency than the probe laser. The observed intensities at Vprobe as a function of pump frequency then show decreases when stimulated gain o c c u r s for Vprob e - - Vpump = AV. A l t h o u g h four-wave mixing processes are involved, it is not uncommon to represent SRGS and IRS by showing only the first three frequency changes. Compared with CARS,

T I ' 1Ji)u m p t"p umo

~Pr~ ( . . . . . . . ) ( . . . . . . . ) yp . . . . ! ~

IRS [ For SRGS Vprob e and Vpump

lipump (tunable) are interchanged]

SRGS 58Ae~ and IRS ~,57,59,6~ have the advan- tages that phase matching of incident and generated fields is not required, ultra-high spectral resolution (0.002 cm -t) results and R. spectra rather than complex quadratures of the spectra are produced. There is less observed background fluorescence in IRS than in SRGS because, in IRS, the probe-laser frequency is greater than the frequency of the pump laser, whereas the reverse is true for SRGS.

Fig. 6. Velocity Measurements (co,sp,* )

Laser-Doppler anemometry (LDA) or velocimetry (LDV) depend on the influence of the Doppler effect of scattering centers on optical mixing spec-

167 169 170 troscopy. - Three arrangements are shown.


For turbulent flows, the signs of velocity components are needed and an acousto- or electro-optical modulator is used to pre-shift the beat signal such that positive v correspond to fp + fD while nega- tive v correspond to f e - f o , where fp is the preshifted frequency and fD is the measured Doppler

fl~,a . o .

.p . , , . , . t2 ,

T . , . ym~o. ,c . I h~

shift. In speckle velocimetry (CO,Sp), 141'171 the double-pulsed output (B) from a 30-nsec ruby laser with a delay time of ~150 Ixsec illuminates the diagnostic region, after passage through spherical (L1) and cylindrical (L2) lenses. A converging lens (L3) focuses the image region on a recording camera. The double exposure produces sequential images (spec- kles) on the photographic receiver, which is then processed by laser illumination of the photograph to produce Young's fringes in the back focal plane. A camera is focused on the plane, where the Four- ier transform of the illuminated region (0.5 mm in diameter) is located. Speckle velocimetry has been applied to low-velocity latex flows with some measure of success. It will be dimcult to use in combustion systems, where signal-to-noise ratios may be too small for velocity mapping.

Fig. 7. Particle Sizing Using Light Scattering or Light Extinction (c,sp,* )

Light scattering by uniform spherical particles has been inte~reted according to the Mie theory for many years 172 to obtain particle radii from theoretical curves for the angular distribution of scattered laser radiation by inferring the ratio of particle ra- dius (a) to laser wavelength (h).173

Ang~tar d i l t r i b u l i o n o f scattorld laser *htensiliml Us. of �9 , , r i , ba . - t , eQ~l . ey

Sizing of individual particles may be performed with a variable frequency grid (CO,$p). 127'174'175 AS

the particle image (for d -> 10 Ixm) is scanned, the intensity of the transmitted signal shows variable visibility. The visibility becomes vanishingly small when the particle diameter approximates the local grid spacing. The technique was calibrated at room temperature with alumina and pulverized coal particles against a commercial off-line analyzer. For individual hot, incandescent particles, better S/N may be obtainable in emission than with a laser background. For dense particulate loadings, interference from particles and gaseous emitters near the center of the diagnostic region will degrade the accuracy of particle sizing.

Laser-Fraunhofer-diffraction for particle sizing 129 (co,sp,*) is accomplished by using a parallel beam of light to illuminate a cylindrical, particle-laden region. Each particle deflects some of the incident radiation. The far-field Fraunhofer diffraction pattern is superimposed on the geometrical image. A lens of focal length f focuses undeflected light on the axis in the focal plane, while the diffracted light surrounds the central spot. Particle movements do not move the diffraction pattern. For a small particle deflection s, the angle of deflection is s/f . The diffraction patterns formed by rectangular apertures or circular discs are inversely proportional to the characteristic lengths involved, provided these are much larger than h. The system functions as a Fourier transform lens for multi-variate droplet or particle sizes. The light energy (not the intensity) distribution was used in the focal plane to determine particle sizes. Measurable particle-size limits are ~ 2.5 h; multiple scattering, non-uniform particle shapes, poor S/N for partially transparent particles and for particle-size distributions are problem areas.

11" for 1 Fm beam par t l c fe par t ic les expander f i e l ~ ~

para l le l J mono- F o u r i e r chromatic t ransform detector in the Hght lens focal p lane

of the lens

L a s e r - F r a u n h o f e r - d i f f r a c t i o n for p a r t i c l e s i z i n g ,

Fig. 8. Laser Diffusion-Broadening Spectroscopy 1 3 0 1 3 1 1 7 6 1 7 9 for Particle Sizing ' ' -

The experimental arrangements are shown for homodyne (either the beam along kl or the beam

incident p//~- ks - k2 beam ~" ~ detector

1170

/~2 is blocked off) and for heterodyne spectroscopy (both beams are present). The scattering angle is 0. Homodyne spectroscopy has been the preferred technique for sizing small (-<0.5 Izm) particles in flames. The scattered laser radiation is shifted by the Brownian motion of small scattering centers. The observed spectral distribution of the scattered laser radiation is analyzed to determine particle sizes. For monodisperse particles, the optical properties of the scattering centers do not need to be known; for polydisperse particulate distributions, these physical data are needed. A combination of scattering and transmission measurements is generally required to determine particle-size distributions and number densities of the scattering centers. 179 For the small particle sizes for which diffusion-broadening spectroscopy is most suitable, the relation between particle diffusion coefficient and particle diameter is well approximated by the Epstein limit of kinetic theory.

Fig. 9. Flow Visualization

Density gradients may be measured with a (pulsed) laser-micro-schlieren system (cp,sp,*). loa The 10-mm laser beam was expanded, recollimated and imaged on a translucent screen, where it was recorded with a polaroid camera during a single pulse.

o f f c e n t e r s p a r k p lug

�9 J~ collimated camera recollimating enlarging J-l. laser beam

t r a n s l u c e n t a p e r t u r e f l a m e s c r e e n f r o n t

Laser micro-schl ieren optical s y s t e m �9

Moir~ deflectometry (cp,sp) may be used for flow visualization. TM The experimental arrangement u4 (C.B., collimated beam; f, flame; G1, G2, gratings; S, mat screen; T.V.C., vidicon camera; M, video monitor; T.V.L.S., TV line selector; OSC, oscillo- scope) involves a light source and two parallel grat-

COMBUSTION DIAGNOSTICS

ings (Ronchi rulings) that are rotated with a constant angle 0 relative to each other. The resultant Moir~ pattern consists of straight fringes. If the beam passes through the flame and is distorted before en- tering the gratings, the Moir~ pattern will be modified. Gradients of the index of refraction may then be evaluated from deformations of fringe patterns. High mechanical stability is not required. The fringe spacing and contrast are adjusted by changing 0.

Quantitative, simultaneous, multiple-point measurements of species concentrations, velocities and temperatures are made by using planar laser-induced fluorescence (LIF). 63'137'180 Temperatures are obtained for strong transitions. Seed materials are 12, OH and NO (e.g., 1900 ppm of NO in a premixed CH4-air flame63). The spatial resolution was 0.8 mm x 0.8 mm x 0.2 mm. NO was measured to 30 ppm in individual exposures, e3

3. Comments on Selected Developing Procedures*

Laser diagnostics are a rapidly developing field of research. Any new application is likely to represent a challenge and difficulty in execution.

While total gas densities have been measured successfully with Rayleigh scattering, 181 Mie scattering from particles interferes if LDV measurements are made simultaneously. Velocity-density correlations in turbulent flows are accordingly obtainable only with some difficulty 1~ and applications to large-scale turbulent flames may require improved techniques.

CARS has been developed as a powerful analyt- ical tool. 44,Sz,53,158-16~ Flame applications are described especially in Refs. 44, 182 and 183. Res- onance CARS 44,52,SaAs6-19z with greatly increased detectivity has not yet been applied to flames. Ap- plications to particle-laden flames may be facilitated by developing long wavelength IR-IR CARS, which

*For elaboration of this discussion, see a forth- coming article by the present authors in the Journal of Quantitative Spectroscopy and Radiative Trans- fer.

2 2 5 n m burner beam WEX dump

532nm

Q u a n t a - R a y laser system Ret'conl c a m e r a

C = cy l i nd r i ca l tens

F = f i l ter

V i s u a l i z a t i o n of c o m b u s t i o n s p e c i e s .

C.B.

Im

Im

IB

F G 1 G 2

I I I I

M o i r e d e f l e c t o m e t r y .


represents a challenging experimental program. Long-wavelength infrared holography has been

applied in Tokamak devices 193 and will be facilitated by the use of a Schottky diode detector array. 194 With fast thermoplastic recording materials, electro-optical detectors, and digital electronic cam- eras, 19 it should yield greatly improved opportuni- ties for flame visualization.

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laser diagnostics applied to combustion systems

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