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A Comparison of Suction Pyrometer and CARS Derived

Temperatures in an Industrial Scale FlameP. M. J. HUGHES

a, R. J. LACELLE

a& T. PARAMESWARAN

b

aEnergy Research Laboratories, CANMET, Energy,Mines and Resources Canada, Ottawa, Onta

K1A 0G1, Canadab

Atlantis Scientific Systems Group Inc, Ottawa, Ontario, K2C 0P9, Canada

Published online: 15 May 2007.

To cite this article: P. M. J. HUGHES , R. J. LACELLE & T. PARAMESWARAN (1995): A Comparison of Suction Pyrometer and CARDerived Temperatures in an Industrial Scale Flame, Combustion Science and Technology, 105:1-3, 131-145

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Combust. Sci. and Tech., 1995Vol. 105,pp. 131-145Reprints available directly from the publisherPhotocopying permitted by license only0 995 OP A (Overseas Publishers Association)Amsterdam B.V. Published unde r license byGordon and Breach Science Publishers SAPrinted in Malaysia

A Comparisonof Suction Pyrometer and CARS DerivedTemperatures in an Industrial Scale FlameP. M. J. HUGHES and R. J . LACELLE Errergy Research Laboratories, CANMET,Energy, Mines atrd Ressurces Canada Ottawa, Ontario CanadaK IA OG IT. PARAMESWARAN AtlantisScientificSystems Grouplnc. Ottawa, Ontario,Canada K2COP9

(Received December 21,1993; in final form December 6 ,1994)ABSTRACT-This report compares temperatures measured with the suction pyrometer and coherentanti-Stokes Raman spectroscopy (CARS) techniques in turbulent, swirling, coal an d No. 2 oil flames. Theflames were generated in a h orizontally fired, cylindrical (1.0 m d ia. by 5.0 m long), axi-symme tric, tunnelfurnace (firing rate 1,000 MJ/h). A broadband USED-CARS system was employed to make several, onethousand shot acquisitions in the furnace before and after the suction pyrometer measurements. Theseexperiments indicate t hat the CA RS technique can s-uccessfully be applied to practical flames to maketemperature measurements. It is demonstrated that these measurements are more meaningful and morereliable than temperatures mea sured using a suction p;yrometer.K e y Words: CARS, temp erature, measuremen t, suction py rometer, flame, furnace.

I N T R O D U C T I O NTlhe measurement of temperature plays a major role in combustion diagnostics. Theuse of physical probes such as thermoc ouples for this purpose is well known. In recentyears optical diagnostic methods have also become popular in combustion research.Coherent anti-Stokes Ra man spectroscopy (CARS) is an optical technique which isemerging as a powerfu'l metho d for mea suring tempe ratures in hostile environm ents. Inth~e xperiments described in this work, temperatures measured in industrial flamesusing the CARS technique a:rle compared with those obtained with a conventionalplnysical pro be called a suction pyrometer.

SUC TION PYRO MET ER MIEASUREMIENT 'TECHNIQU EA suction pyrometer may be described as an aspirating thermocouple. Typically itconsists of a bare or coated thermoc ouple of suitable size. Th e thermocoup le is enclosedin a tubular passage which serves as a refractory lined radiation shield. Gas from thetest point is drawn into this passage and the enclosure and the thermocouple areallowed to come to thermal equilibrium. Figure 1shows a schem atic of the probe usedin these experiments. 'The com bustio n gases at the test point are draw n int o the probethrough the 15.88mnn opening. These gases then pass over the 2.38 mm diameter


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P.M. J . HUGHES et a1

C e r m i c Tubes1 ead (Oia 2.38mm)

15.88 mm O iaEnd View A- A Side View 0-0

FIGURE I Schematic or the suction pyrometer (dimensions in mm).

t h e r m o c o u ~ l e .As the suction rate increases. the convective heat transfer coefficientfrom the gas to the thermocouple an d t o the enclosure increases. The gas f low rate isincreased until the temDerature recorded by the th er m oc ou ~l e ecomes constant. Thisreading, at the limiting suction rate, yields the temperature of the gas entering theprobe.Suction pyrom eters ar e generally bulky. Thei r size as well as the draw ing of the gasescan perturb the test environment and affect the measured temperatures. This can bea significant problem in regions where there are steep gradients of temperature,compo sition or velocity. Th e size of the prob e also limits the spatial resolution t hat c anbe obta ined. These p;obes may be suitable for meas uring -me an gas temperatures;however, they are not recommended for use in the near burner regions of practicalflames. In these regions the turbulent fluctuations can be extensive due to rec irculatinggases.

C A R S M E A SU R E M E N T T E C H N I Q U ECARS is an optical method which is gaining acceptance as a useful tool for themeasu rement of temp eratu re in flames. The re are man y review articles which describethe principles of CARS and discuss their application to combustion diagnostics.(Andersen and Hudson, 1977; Druet and Taran , 1981; Hall and Eckbreth 1984;Eckbreth er al., 1988;Greenhalgh , 1988; Attal-Tretout et al., 1990). In brief ,CARS is anoptical process in which tw o pho ton s of frequency w , from a pum p laser combine witha single photon of frequency w, from anoth er (Stokes) laser , through the third ordernonlinear electr icsusceptibili ty of th e test medium, to yield an out put CA RS beamat a rrequency 2 1 3 , - o , . The intensity of the CARS signal, which has coherentlaser-like properties, is greatly enhanced when the frequency difference w , - w, equals


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A COMPARISON OF SUCTION PYROMETER AND CARS TEMPERATURES 133

a Raman frequency of the medium. Since the CARS frequency is greater than theexciting pu mp frequency, and the intensity enhancement occurs du e to resonance witha R ama n transit ion this technique is referred t o as Coherent anti-Stokes Ra ma nSpectroscopy. In the C AR S process x ' ~ ' elates the macroscopic polarization of themedium to the cube of the incident electric field and makes CARS a very sensitivetechnique. ,yI3' is dependent on the medium properties, such a s temperature and so i t isthrough this susceptibility that the gas temperature can be determined.- -CA RS has several potential advantages over conventional measurement techniquesused in combustion environments. Since the probe used is optical, CARS is non-intrusive and the measurements can be extended to regions where there are steepgradients without dis turbing the f low. The CARS interaction occurs over a smallregion an d in a short t ime scale. This leads to g ood spatial and temporal resolution. Inaddition, when pulsed lasers are used, many temperature m easurements can be ma desuch that the turb ulen t statistics of the flow field can be d etermined.The appl icatio n of CA RS to different types of practical flames has been dealt with bymanv authors . Nitrogen is the maior species in air-fed combustion and CARStherm ome try is generally don e by recording the Q branch spectrum of N,. T o cite someexamples, as early as 1979 remote non-intrusive C AR S measurem ents of tem perat urein an internal combustion engine were reported by Stenhouse et al . (1979). Ald in an dWallin(1985) have show n that CA RS can be applied to the hostile environment faced ina coal fired boiler. In these and o ther early attem pts the frequency w, of the Sto kes laserwas scanned so as to excite the various molecular resonances of nitrogen. ScanningCA RS is useful for stable media; however, the acquisition of a CA RS measu rement c antake of the order of minutes. When the medium properties are fluctuating a t the rate ofthe order of KH z, i t is more approp riate to use a broadband dye (Stokes) laser so thatthe entire CAR S spectrum can be generated in a s ingle laser shot. Temperature a ndspecies concentration measurem ents in a jet engine exhaust using broad ban d CAR Swere first dem onstra ted by Eckb reth et al . (1984). Beiting (1986) has reported broad-band CA RS derived tem peratures in a coal fired MHD generator. CARS tem peratureswere recorded by Spisberg et al . (1987), in a plasma hea ted with a ir flow seeded withcoal powder. These and other such experiments (Ferrario et al., 1983; review byGreenha lgh (1988); Hanco ck et al . (1989); Sjunnesson er ol., 1992) have proved that, th eCA RS method can be usefully applied to industrial flames even when particulates arefoun d in th e flow field.Early in the development of CARS it was recognized that proper phase matchingbetween the input and output beams is essential . Various arrangements such ascollinear, BOX CARS (Eckbreth, 1978) and USE D-CA RS have been employed t osatisfy the phase matching requirement a nd provid e go od spatial resolution. CollinearCARS has the disadvantage of generating the CA RS signal along the complete lengthofove rlap of the probe beams. As a result, special effort must bem ade ,su ch a s tubing onthe sourc e and detection sides of the flow field (Alden a nd Wallin, 1985), to excludea C AR S signal from o utside the region of interest. Also, collinear C AR S suffers frompoor spatial resolution since the beam s overlap alo ng the full beam length inside thefurnace. The BOXCARS technique has improved spatial resolution over collinearCARS; however, since three separate laser be ams a re used, it is quite susceptible tobeam steering du e to density gradients in the flow field. USED-C ARS , as implemented


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134 P.M. . HUGHES et 01.in this study (and discussed later) has all of the advanta ges of BOXC AR S and colline arCA RS an d do es not suffer from the sa me degree of sensitivity to beam steering asBOXCA RS. Another variant of the CAR S techniques is to reduce the influence ofa s trong nonresona nt background by employing suitable polarization arrangeme nts(Rahn et al., 1979; Attal-Trttout et a/., 1980; Eckbreth an d Hall, 1981). Hav ing col-lected a dependable set of experimental CARS spectra the desired temperature orconcentration is extracted from this dat a by carefully fitting them with theoreticallycalculated spectra (Greenhalgh et al., 1988 and the references therein).With pro per h andling of the measu red signals (Attal-Tretou t et al., 1990; Meier et al.,1991) the C AR S technique can be applied to collect average and s ingle pulse da ta(Stufflebeam et al., 1989; Fa rro w a nd Rahn, 1985) from prac tical flames. These singleshot CARS can be used to present the statistics of the fluctuations in the measuredquantity as a probability dis tr ibution function (P DF). The mean value of the measure-ment an d the P D F are useful in making comp arisons with predictions from flowcombustion s imulation programs.This report deals with the analysis of CARS measurements taken in practical coaland No. 2 oil (a light oil) flames in the tunnel furnace facility at the C A N M E T EnergyResearch Laboratories (ERL). These measurements were made wit'h a conventionalUSED-CARS (Eckbreth et al., 1984; Lang et al., 1990; Davis et al., 1981) system,designed and developed at ER L (Hughe s and Parame swaran, 1987; Hughes, 1981). Th eCA RS derived temperatures are com pared with temperatures measured using a suctionpyromete r. Th e experiments described here were condu cted in a region of the furnacewhere the temp erature, composition an d velocity profiles were relatively flat-far awa yfrom recirculation zones to ensure that there would be good corresponde nce betweentemperatures measured with both techniques.

E Q U I P M E N TTunnel FurnaceThe furnace used in these experimen ts is shown in Figu re 2. Figure 3 is a schem atic ofthe entra nce region of the furnace. The interior of the furnace is approx ima tely 5.0'mlong and 1.0m in diameter. For these experiments, the initial metre length of thefurnace was l ined with a refractory to create a n adiabatic region t o enhance thedevolatilization of the fuel on entering the furnace. Th e remaining 4.0 m of the furnace issurrou nded with a cooled wall to simula te a therm al load o n the gases in the furnace.The gas tem perature measurements were mad e on the long axis of the furnace, in theadiabatic region, about 0.8m from the burner.At one end of the furnace the fuel (coal o r No. 2 oi l )and pr imary a i r en ter through thecentral 3.18cm pipe (Fig. 3). The secon dary air enters through the 7.94cm d iameterannu lus aro un d the primary pipe. T he secondary flow of air is given a swirl com pon entvia fixed vanes set at 45'. In one of the expe rime nts with No. 2 oil as a fuel, a tubeinserted about 8.0cm off axis carried the fuel into the furnace at the entrance of thequarl. In this case the oil was atomized mechanically. In other expe riments where theN o. 2 oil was introduced along the central axis of the furnace, the atomization was


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A COMPARISON O F SUCTION PYROMETER A ND CARS TEMPERATURES

SECTION

FIGURE 2 Schematic ofth e CC RL tunnel furnace facility.

EntrancePlane of

FIGURE 3 Schematicof the entrance region ofthe CCRL tunnel furnace facility.

accomplished thro ugh she ar with an air jet. O n entering the furnace the gases passthrough a 28" expansion(quar1) and th en thr ough the cylindrical portion of the furnace.The CARS ExperimentFigure 4 is a schem atic of the optical arran gem ent showing the placement of the opticson either side of the tunnel furnace. At th e heart of the CA RS optics is the frequency


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P. M. . H U G H E S et a/ .

F I G U R E 4 Schematic of the CARS optics used in the tunnel furnace

doubled ND -YAG laser (2 x ND-YAG). This laser provides the pum p photons for theCA RS interaction at the test point in the furnace and pu mps the oscillator an d amplif iercel ls in the dye laser . ~h o d a m i n e 40 G was used i n t h e dye laser osci lla tor an damplifier to give a spectrally broad Stokes beam for the single shot CARS measure-ments . The 2 x ND-Y AG laser converts most of the 1 .06p m radiation t o 532 nm w itha measured spectral width o f0. 4c m- ' . Th e residual 1.0 6pm is split off by the harmonicseparator (HS) and converted t o 532 nm in the harmonic doubler (H D ) before beinginserted into the dye laser oscillator t o create the Stokes laser beam. The harmo nicsepa rato r (HS ) also reflects the 532 nm radiation past th e dye laser where part (29%) isdraw n off to amplily the Stokes laser beam. The rem aining 532 nm beam is directed tothe CAR S experiment. T1 and T 2 are Galilean telescopes for the s izing of the p um p a ndStokes laser beams. The pum p (532 nm ) beam passes through a half wave-plate anda glan-laser polarizer (H P& GLP) to allow for control of the intensity at the test pointin the furnace. The pum p beam is then sent through ano ther half wave-plate ( H P) toensure tha t its plane of polariza tion is parallel to th at of the Stokes beam on e nteringthe furnace. Both the Stokes and p um p beams are then directed to the dichroic mirrorD for combination. Since the USED -CARS configuration is employed in this experi-ment this dichroic mirror reflects the p um p beam a nd transmits the Sto kes beam. Thisallows the Stokes beam to be propagated down th e hole in the doughn ut shaped pum pbeam as is shown in Figure 5. The U SED-CA RS beam configuration is a comp act an deasy to align system when th e pum p an d measured beams must be transmitted overlong distances.Theco mbine d pu mp and s tokes beams are brought t o a focus at the test point by lensLI (Fig. 4) . Th e zone of overlap of the two prob e beams was measured t o be abou t


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A COMPARISON O F SUCTION PYROMETER AN D CARS TEMPERATURES

I rTest P o i n t

FIGURE S Schematic ol the orientation of the probe beams for the U SED-CA RS configuration

200pm diameter by 6.5cm long. The CARS signal is generated in this volume and ispropagated as a coherent radiation emanating from the focal point of lens L1. Thespatial distr ibution of the CA RS beam with respect t o the pu mp beams is shown inFigure 5 (Davis et al., 1981).Th e lens L2 on th e other s ide of the furnace collimates thepum p and C AR S beams. The beams then pass throu gh the optical separator an d f ibreop t i ccoup le r (0SFO C Fig . 4). Th e OS F O C is a collection ofoptics which separates theCARS beam from the probe beams and inserts it into a fibre optic. The fibre opticcarries the CA RS signal to a holographic grating spectrometer. In the spectrometer theCAR S signal is dispersed and imaged on to an optical multichannel analyzer where thespectral profile of the CA RS signal is digitized an d sen t to a co mp uter for analysis. Th eYAG laser oscillates at 10 Hz an d s o a CA RS mea sureme nt is mad e every 0.1 s. Eachmeasurement is averaged over th e 9 ns "on-time" of the laser. Th e spatial averaging ofthe CA RS measurement is over the 200 pm diam eter by 6.5 cm long focal volume.

F U R N A C E T E S T C O N D I T I O N S A N D M E A S U R E M E N T D E T A I L STh e run conditions for the three f lames studied a re show n in Tab le 1. Th e protocol forthese tests included a four hou r warm-up time to ensure th at s tab le run conditions hadbeen achieved a nd the refractory temperatures were foun d to be constant. F or tes ts1 an d 2 the primary air flow rates an d the excess oxygen in the exhau st stack weremeasured. Th e secondary air flow rates were calcula ted from the analysis of the fuel andassuming complete combustion. The fuel for test num ber 1 was carried to the furnacethrough a pipe inserted through the quarl. For this test, atomization of the fuel wasaccomplished by mechanical sh ear of the oil jet. In test num ber 3 the oil was carried t othe furnace through a nozzle at t he centre of the fixed vane swirl generator. In this testthe atomiz ation of the fuel was accomplished by sh ear of air and oil jets in a speciallydesigned nozzle.


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P. M. J . HUGHES er a/.TABLE l

Test Fuel Fuel Feed Primary/Atom . Secondary Air ExcessNumber Rate Air Flow Rate Flow Rate Oxygen( W h ) (kg/h) (ka/h) %1 oil 17.13 13.95 412' 12.12 coal 56.00 26.42 786. 8.03 oil 34.30 32.60 920 10.1

Calculated

Several measurements of gas temperature w ere made with the CA RS system after therefractory wall temperatu res were relatively const ant. The suction py rom eter wa s theninserted through a d own stream port at a n angle to the central axis . Th e head of thepro be was positioned to be at the p oint where the CAR S system focal volume waslocated. Each such measurem ent required a 15 minu te stabilization period t o ensur ea correct observation with the suction pyrometer. When the suction pyrometer wasremoved a C AR S temperature measurement was ma de immediately to check for anydrift in the ga s field temperatu re. O ne C AR S experiment consisted of 1,000 single shotmeasurem ents(in tes t 3, 500sh bt sets werecollected) which werecollected over a periodof 1.67 min. With suc h a system, each C AR S acquisition (or shot) gives a tem peratu reaveraged ov er 9 ns an d each set of CA RS m easurements (1000 or 500 shots) will givea tem peratu re averaged over 1.67 min. (or 0.83 min. for 500 shots).

R E S U L T S A N D D I S C U S S IO NThe CA RS temperatures were determined from a comparison of the experimental an dtheoretical spectral profiles of the captured CA RS signals. In air-fed co mbu stion,nitrogen is the predom inant species in the gas field and its CAR S spectrum is used fordetermining gas temp eratu re. Since the repeated generation of theoretical N, CA RSspectra duri ng a fit cycle can be time consumin g, a set or library of theoretical spectraover a range of temp eratu res was first calculated with a com put er program developedespecially for this purpose. Expe rimental CA RS spectra ar e fitted w ith this library t oget best fit temperature s, relative frequency shifts, conc entrat ion an d i nstrum ental slitparameters. A nonlinear fitting progra m based on a modified Ga uss-N ewto n technique(M arq uar dt, 1963) was used. Fur ther details of this software package ar e describedelsew here(P aram esw aran and Snelling, 1988). It should be noted that, since a U SE D-CA RS beam configuration was used, the fitting proce dure also included the effect ofa v ariable nonreso nant susceptibility. Th e reason for this is to minimize the im pact ofpu mp beam field statistics on the C AR S signal (Lang and Wolfrum, 1990).In a typical CARS experiment, background, non-resonant (in an argon cell) andresonant room temperature nitrogen CARS spectra are recorded for calibrationourDoses. Th e resonan t N , CA RS sDectra are then m easured in t he flame. These flame. ,.spectra ar e background subtracted an d divided by the non-resonant spectrum . Th eroom t emp eratu reCA RS spectrum is used to determine the instrument function for the


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A COMPARISON O F SUCTION PYROMETER AND CARS TEMPERATURES 139CA RS optical arrange ment. The f lame spectra are then f i tted with theoretical spectrawhich tak e into accoun t the effect of the instrumen t function.

Experimental CA RS signals generally have a cer tain percentage of noise associatedwith them. This can come from dark noise, shot noise an d sho t to sh ot var iabili ty of theprobe laser beams (CARS noise). The effect of noise can be included in the fittingprocess by assigning appropriate weights to the observed counts measured by theintensified photo diode array (IPD A) detector (Snell ing et al., 1987). Another experi-mental difficulty can ar ise from the non-lineari ty of the I PD A. Th e f i t ting program s cancorrect the da ta for this non-lin earity by using the experimentally observed functiona lrelationship between observed and true counts. The model used for generating thetheoretical CA RS spectra in these analyses takes accoun t of collisional narr ow ing(Hall, 1983) and cross coherence (Kataok a e t a / . , 1982; Teets, 1984) an d the fits weredone with appropriate corrections for detector non-lineari ty (Snell ing et al., 1989,Parame swaran, 1990). Th e theoretical C AR S spectra were convolved with a Voigtinstrument function determined from the ro om tempera ture CAR S spectrum. Theseconvolved sp ectra were then com pared with the experimental spectra to determine theflame temperature.Figure 6 shows a typical comparison between a measured N,CAR S spec t rum (shotnumber 993) and the best fit theoretical spectral profile. Detector non-linearitycorrection a nd w eights based on 6.97% CA RS noise were used in the analysis of thesespectra. By performing a similar analysis on each s ho t in a set of da ta, the statistics ofthe measurements can be determined. A sample of the probabil i ty d istr ibution function(P DF ) for a set of measurements in the oil f lame is shown in Figure 7. To n this plot isthe fitted temp eratu re of the first spectrum in the set. This first spectru m is derived fromthearithm etic sum of the subsequent spectrain the set. T,,,, is the av erage of the fittedtemperatures of each shot in the set of CAR S spectra.

AREA NORMA LIZED CA RS SPECTRA L PLOT; FILE 543 020 1 0Comparison of Measured SingleShot With Theory: Coal Flame

0.02 -0.01 -no60 no70 no80 nwo ntoo 21110 21120 also n14 0 21150

Frequency (ern*)

FIGURE 6 Sample comparison of measured and th eoretica lCARS spectral profiles for an o il flame.


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P. M . J. HUGHES er a1

CARS POF; FILE S4302008Oil Flame:CARS Noise and Detector Non-Linearity

TSOEV - 137 K I I I

800 1000 1200 1400 1600 1800 2000Temperature (K l

FIGURE 7 Sample PDF of the CARS measurements in the oil flame.

Table 2 sho ws a comparison of the gas temperatures measured with the CA RStechnique (T,,,,) and with the suction pyrometer (T,,). T,,,, is the average tempera-ture of each set of CA RS measureme nts. T he theoretical temperature(T,,) is calculatedusing the heating value of the fuel and the am ou nt of excess air from the stack analysis.In each case the theoretical temperature assumes complete combustion.In Table 2 it can be seen that , for tests 2 and 3, T,,,, is quite close t o T,,. In test 1 thesuction pyrometer tem perat ure is significantly different from the CA RS temperature. Itwas suspected that there was an error in the procedure for the use of the suctionpyrometer. This was confirmed in test number 3 where the suction rate through theprobe was increased above the rate normally used. As can be seen in Table 2, themeasured gas tempe rature is higher than that measu red in test numb er 1 and muchcloser to the CA RS tem perature. As was stated earlier, the test point in the furnace waschosen such that the suction pyrometer should not dis t urb the flow, and hence thetemperature should be close to that measured with the CAR S technique.

I t can also be seen th at the gas temperature is closer to the theoretical temperature inthe oil flame as opposed to that measured in the coal flame. It is possible that

TABLE 2Comparison of CARS and Suction Pyrometer Derived Temperatures '

I oil 1518 136.7 1296 17032 coal 1375 208.5 1394 17563 oil 1525 291.1 1565 1639


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A COMPARISON OF SUCTION PYROMETER AN D CARS TEMPERATURES 141

com bustio n is stlll continui ng at the point m easured in the co al flame. All of the heatrelease has not occurred a nd therefore the gas tem perat ure is significantly far from thetheoretical maximum, T,,. The combustion of coal occurs in a two s tep process:devolatil ization an d ch ar b urnout. The devolati lization occurs within a few ms afterentering the furnace. After sufficient mixing with air the volatiles b urn an d provide theintense radiant f lux required for char burn out. The cha r can take 1 to 2 seconds tocompletely burn. It is likely that the char bu rno ut stage has not been completed at thepoint where the temperatures were measured. Therefore the theoretical maximumtemperature for coal will not have been attained at the point where the m easurementswere taken.In CA RS experiments the energy in the pum p beam can be sufficiently high as toinduce population changes in the vibration bands of the p robe m olecule (Gierulskiet a/., 1987 an d Snelling e t a/., 1989). This is called stim ulated R am an p ump ing an d canresult in artificially high fitted temperatures for the CA RS spectra. This effect can beidentified by the presence of a "hot-band" in the roo m tem pera tureC AR S spec tra. Thisband was not found to be present in a ny of the room temperature spectra. Whereas theinfluence of pum pen erg y was not dete rmined in this study, an estimation of theeffect itwill have on the hot CA RS spectra can be m ade by com paring the s ignal s trength in theregion of the 2-1 hot ban d t o that in the region of the funda men tal band for the roo mtem pera ture spectrum (Snelling et a/., 1989). In these C AR S experiments, this ratio wasless than 0.003 and thus th eer ror d ue to s timulated Ram an pumping, if any, is less than35 K. This value is much smaller than the s tandard deviations of the measured CA RStemperatures shown in Table 2.Another problem that c an cause concern, with the use of a mu ltimode pum p laserwith the US ED-CA RS set-up, is thecorrelation between the two pum p photo ns(La ngeand Wolfrum, 1990). Fo r non-Ga ussian pum p laser statistics this correlation ca n causean enhancement of the ratio of the nonresonant to resonant intensities and result insystematic errors in the derived tem perature an d nitrogen mole fraction. This correla-tion may be eliminated by introducin g a suitable delay between the tw o pum p beams(Fa rrow a nd Rah n, 1985; Lana e and Wolfrum, 1990). When this is not possible it ismore sensible to fit the ratio x~~ , , / ~ , , ,nitrogen mole fractio n~no nreso nan; usceptibil-i ty) as another parameter, (Hall and Boedecker, 1984).This appro ach was followed in. .the analysis of thed ata here to avoid tem perat ureerr orsdue to p ump beam correlation.As stated earlier, the measurement location was chosen such that the temperaturemeasured by the suction pyrometer would not be subject to errors caused by thepresence of the probe. In addition the measurement location was chosen to besufficiently close to the burner to challenge the CARS measurement technique. TheCA RS spectra collected in the coal flame were subject, to a m inor degree, to theinfluence of the pa'rticles in the measurem ent zone. O n the ave rage only ab ou t 30spectra were rejected, due to an intractable signal as a result of particle incandescencefrom each of th e 1000 sho t spectra l files collected in th e coal flame. In th e oil flames,however, there were virtually no spectra rejected. This may be because the m easure-ment location was sufficiently far away from the burner so that there were no oildroplets suspended in the flow. This oil vaporized very quickly and bu rned with a cleanflame near the bu rner leaving no solid material to interfere with th e CA RS beams in themeasurem ent zone. At this measureme nt location , there is very little volatile material.


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P. M. J. HUGHES et a/.TABLE 3

Variation of the Furnace Temperature With Time of DayFuel File Number Time Collected TcARsOil S4302006Oil S4302007Oil S4302008Oil S4302009Coal S4302010Coal S4302011Coal S4302012Coal S43020 13Coal S4302014

Fo r this reason the interference in the CA RS spectral profile caused by C , abso rptio n(Bengtsson et a / . ,1990) was not noticed in an y of the signals collected in the co al o r oilflames.Ts, is the stand ard devia tion of the fitted temp eratures in the set of CAR Smea sureme nts from the mea n tempera ture (T,,,,) and is equivalent to the rms value ofthe f luctuatingcomponent of temperature. I t is interesting to no te that T,, is smalle r intest num ber 1 than in the other tw o tests. This is consistent for all of the C AR Smea sureme nts in the coal an d oil flames. In test nu mbe r 1 the nozzle used to spray th eoil into the furnace was smaller than the pipe carrying the fuel for the othe r tw o tests. Inad dit ion th e flow rate of fuel in test 1 was much smaller than that for test 3. Because ofthese factors the length scale for turbulence gene ration for test num ber 1 is expected tobe smaller than that for the other two tests. Therefore, the rms value of temperaturefluctuations will be smaller.The furnace was allowed to run for about four hours to ensure that the gastemperature would be constant th roughou t the t ime required to collect the CA RS andsuction pyrometer measurements. Tab le 3 shows the average CARS temperature at thesame location at various t imes throughou t the da y of the experiment. This table showsthat the gas temperatu re in the furnace was increasing duri ng both the coal an d oil firedexperiments. After the measurements in the oil flame were completed, the Fuel wasswitched to coal and the furnace was allowed to come to equilibrium (as monitored inthe furnace refractory). The suct ion pyro mete r measurements were made just before thedata f iles ~4302 008 nd ~43 02 01 2 ere acquired with the CA RS equipment in the oiland coal flames respectively. F igure 8 shows how the tem perature varied in th e furnaceduring the acquisition of the suction pyrometer measurements. The period when thesuction pyro meter measurem ents were taken is also show n in Figure 8. Since the flametemperature was changing over the 15 minute measurement period it is difficult toassign a precise suction pyrom eter tempera ture.

C O N C L U S IO N S A N D R E C O M M E N D A T IO N SSome of the advantages of CARS derived temperatures over those measured witha suction pyrometer in coal an d oil flames have been demo nstrated. T hese experiments


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A COMPARISON OF SUCTION PYROMETER AND CARS TEMPERATURES 143TEMPERATURE VARIATION AT TEST POINTDurhg 0 0 and CoalMsawrmnwnls

FIGURE 8 Variation ofthe furnace temperature at the test point.

were designed to ensure that e rror s resultingfrom the use of a suction pyrometer wouldbe minimized. Th e suction pyrometer was not used an y closer than ab ou t 1 m from theburner to avoid the influence of the probe o n the flow field. Th e C A R S measurementtechnique does not di stur b the flow and will give more m eaningful measurements th anthe suction pyrometer. Each C A R S measurem ent (or shot) is virtually instan taneo us(averaged over 9 ns) and with pulsed lasers in a sh ort tim e ( I to 2 minutes) it is possibleto collect sufficient shots to measure a mean temperature for the flow field ata particular location. The suction pyrometer can take as much a s 15 minutes to m akea measurement.Due to the pulsed n ature of the probe laser beams, the C A R S technique ca n give themean an d fluctuatingcom ponent of temperature in a f lame. The suction pyrometer cannot do th is . A knowledge of the temperature fluctuations is very useful for thevalidation of simulation cod es for furnaces. For example, the prediction of the amo un tof thermal NO, formed in turbulent air breathing co mbu stion is highly depende nt onthefluctuatingcomponent of temperature(Ma1teet a / . , 1979 and Hayhurst et al., 1980).C A R S can provide reliable measurem ents of the fluctuati ngco mpo nent of temp eratur ein practical flames for comparison with prediction codes. It is not sufficient to usea mean value such as determined with the suction pyrom eter.Plans are being made t o apply the C A R S technique t o coal and oil flames in the nearburner region of the sam e furnace. Because of the steep gradie nts found here, onlyoptical techniques such as C A R S can be used. It is likely that particles will havea greater influence on the C A R S signal due to the higher particle load ing in localizedregions near the burner. In addition the volatile matter mav begin to affect the" . -measured C A R S signals. Thes e volatiles may: cause mor e beam steering, have a stro ng-er influenceon the nonreson ant susce ~tibil i t v. nd dis tort the C A R S s ~ e c t r a l r o f i l evC, absorption. F uture experiments will continue to use U S E D - C A R S to minimizebeam steering, XmOle/~,,,itting to tak eac cou nt of a variable nonreson ant susceptibilityan d cond itiona l fitting to eliminate the effect of C, absorption.


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144 P. M. J. HU GH ES e t a l.Whereas in the present experiments the CARS temperatures were compared to

suction pyrometer measurements and theoretical flame temperatures, predicted tem-peratures from c omputation al f luid dynamic (CFD) simulation progr ams will be usedas a basis of comparison for the CARS technique. These prog ram s have been devised topredict the mean and fluctuating com pone nt of the field values for velocity, composi-tion and temperature inside a furnace. It was demonstrated in these experiments thatthe gas s tream te mperature increased abo ut 100"K in about 45 min. In futu re experi-ments greater care will be taken to ensure that the furnace has achieved equilibrium.A C K N O W L E D G E M E N T STh e authors would like to express their thanks to Dr. D. R. Snelling and his research team o lt he N ationalResearch Council for their assistance in the development of the CA N M ET CA RS equipment and analysissoftware.

R E F E R E N C E SAndersen, H. C., an d Hudso n, B. S.(1977).Coh erent Ant i-Stok es Ram an Scattering. Mol. Specr..5, 142, Lon gD. A. (Ed.) Chem ical Society, New Yo rk.Attal-Tretout, B., Bealat, M., and Taran, J . P. (1980). CA RS D iagnostics of Combustion. Jou r. of Eneryy.

4, 135.Attal-Trttout, B., Bouchardy, P., Magre, M., Pealat, M., and Taran. J . P. (1990). CARS in Combustion:Prospects and Problems. Appl. Phys., B. 51, 17.Alden, M., and Wallin, S. (1985). CA RS E xperiments in a Full Scale (10 x [Om) Industrial Coal Furnace.Appl. Opt., 24, 3435.Beitina, E. J. (1986). Multiplex CAR S Tempe rature M easurements in a Coal-Fired M H D E nvironment.~ i ~ l .pt., 25 (lo) , 1684.Bengtsson, P. E., Alden. M., Kroll, S., an d Nilsson, D. (1990). Vibrational C AR S Ther mo me try in SootyFlames: Ouantitat ive Evaluation of C, A bs or ~t io n nterference. Combust. Flame. 82. 199.D avis. L . C . , ~ a r k o . . A,, a n d R i m a i , ~ . * ( 1 9 8 1 ) . ' ~ n ~ u l a ris tri bu tio n of ~ o h e r e n c ~ a m a nmission inDegenerate F our-Wave Mixing with Pum ping by a Single Diffraction Coupled Laser Beam: Configur-atio ns for High Spatial Resolution. Appl. Opr., 20 (9), 1685.Druet, S. A. J. , and Ta ran , J . P. E. (1981). CA RS S pectroscopy. Pray. Quanrum Elecr.. 2, 1.Eckbreth. A. C., and Hall, R. J. (1981). CAR S Concentration Sensitivity W ith and W ithout NonresonantBackground Suppression. Comb. Sci. and Tech.. 25, 175.Eckbreth, A. C., Dobbs, G . M., Stufflebeam. J. H., and Tellex, P. A. (1984). CA RS T emp erature an d SpeciesMea surem ents in Augm ented Jet Engine Exhausts. Appl. Opt., 23, 1328.Eckb reth, A. C. (1988). Lase r DiaynosricsJor Combustion Te mpe rature and Species. Aba cus Press, Cam -bridge, Mass.Farrow, R. L. and Rahn, L. A. (1985). Interpreting Coherent Anti-Stokes R aman Spectra M easured withMultimode Nd: YAG Pump Lasers. J . Opr. Sac. Am. B.,2,903.Ferrario. A., Garbi, M ., and Malvicini. (1983). Real-Time CARS Spectroscopy in a S emi-Industrial Furna ceIn Technical. Diyesr, Conference on L asers and Electro-oprics .Optical Society of America, WashingtonD. C., Baltimore, MD., May 17-20, paper WDZ.Gierulski. M., Noda. T., Yamam oto, G.. Marow ski, G., and S lencrka. A. (1987). Pump-Induced PopulationChanges in B road band Cohe rent Anti-Stokes Raman Scattering. Opt. Lett., 12,608.Gree nhalg h, D. A. (1988). Qu anti tati veC AR S Spectroscopy. In A dvances in Non-Linear Specrroscopy ClarkR. J. H.. and Hester R. E. (Ed.) Wiley, New Y ork.Hancock, R. D.. B oyack. K. W. and H edm an, P. 0 . (1989). Coherent Anti-Stokes Rama n Spectroscopy

(CA RS ) n Pulverized Coal Flam es. In Advances in CoalSoecrroscopy M euzela ar H. L. C.(E d.). Plenum

.- -.Hall. R . 1.. and Eckbreth, A. C. (1984). Coherent Anti-Stokes Rama n S pectroscopy (CARS): Application toCombustion Diagnostics In: Laser Applicaliuns.Ready, J. F., and Erf, R. K.(E ds.)5,2 13 Academic Press.


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A C O M P A R I S O N O F S U C T I O N P Y R O M E T E R A N D C A R S T E M P E R A T U R E S 145Hayhurst, A. N., and Vince, I. M. (1980). Nitric Oxide Fo rmati on lrom N, in Flames: The Importance or'Prompt ' No Progr. Energy Combusr. Sci., 6,35.Hughes, P. M., and Paramesw aran, T. (1987). An O ptical Diagnostic System for the Measurem ent of Gas

Tem peratur e and Species Concentration C A N M E T R eport no. CM-87-5E.Hughes, P. M. (1988). Practical Aspects of CA RS in C ombustion Research E R L Report no. 88-17 (OP ).Ka taok a, H., M aeda. S., an d Hirose, C. (1982). ElTects of Laser Line W idth on th e Coherent Anti-StokesRaman Spectroscopy Spectral Profile. Appl. Spect., 5, 565.Lang, B., an d Wolfrum , J. (1 990). Th e Impac t of Laser Field Statistics in Determ ination of Tem peratur e an dConcentration by Multiplex USED CARS . Appl. Phys. B.. 51. 53.Malte, P. C., and Rees, D. P. (1979). Mechanisms and Kinetics of P ollutant formation during Reaction ofPulverized Coal In Puluerized Cool Combus tion and Gosificotio n L. D. Smoot and D T. Pratt. (Eds.). 183Plenum Press, New Y ork.Marquardt , D. J. (1963). An A lgorithm for Leas! Squares Estim ation of Non-Linear Parameters. J. Soc.Indust. App. Moth .. 11 (43). 1.Meier. W.. Plath, I., an d Stricker, W. (1991). Th e Application of Single Pulse CA RS for Tem per atur eMeasurem ents in a Turbulent Stagnation F lame. Appl. Phys. B.. 53. 339.Paramesw aran, T., and Snelling, D. R. (1988). Th e Calculation o f Theoretical A nti-Stokes Raman Spectra.N R C Techn ica l Report , NR C no. 29810, TR-GD-013.Parameswaran, T. (1990). Coherent Anti-Stokes Raman Spectroscopy (C ARS ) Software Development-.Project Report for EMR.Rahn. L. J.. Zych, L. I., and Mattern, P. L. (1979). Background-free CARS S tudies of Carbon Monoxide Ina Flame. Out. Comm..30. 249.Sjunneson, A., ~e n ri k so n , ., and Loistrom. C. (1992). CA RS Measurem ents and Visualization of ReactingFlows in a BluR Body Stabilized Flame A l A A P a p e r 92-3650, AIA/SAE/ASME/ASEE 28th JointProp ulsion Conference and Exhibit, July 6-8. Nashville, TN., USA.Snelling. D. R.. Smallwood, G. J. . Sawchuck, R. A., and Parameswaran. T. (1987) Precision of MultiplexCARS Temperatures Using Both Single-Mode and Multi-Mode Pump Lasers. Appl. Opt., 26 (I) , 99.Snelling. D. R.. Smallw ood, G. J.. and Paramesw aran, T. (1989). ERect of Detector N on-Linearity a nd ImagePersistence on CAR S Derived T emperatures. Appl. Opt., 28 (15). 3233.Spisberg, P., Cahem. C., and Deschamps. P. (1987). CA RS Temperature Measurem ents in a Plasma HeatedCoa l Com bus tor Journal de Physique, Colloque C 7, Supplement au n I2 ,48,757.Stenhouse. A.. Williams. D. R.. Cole. J. B.. a nd S words. M. D. (1979). CA RS M easurem ents in an Inte rnal.Combustion Engine. ~ ~ ~ 1 .pt. , 18,3819.StuWebeam, J. H., and Eckbreth, A. C. (1989). CARS Diagnostics of Solid Propellant Combustion atElevated Pressure. Comb. Sci. and Te ch.. 66. 163.Teets, R. E. (1984). Accurate Convolutions of coher ent Anti-Stokes R aman Spectra. Opt. Let. , 9,226

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