CHALLENGES TO LASER-BASED IMAGING TECHNIQUES IN GAS TURBINECOMBUSTOR SYSTEMS FOR AEROSPACE APPLICATIONS
Randy J. Locke*,Dynacs Engineering Company, Inc.
2001 Aerospace Parkway, Brook Park, OH 44142
Robert C. Andersonf, Michelle M. Zaller*, Yolanda R. Kicks5,NASA Lewis Research Center
21000 Brookpark Rd. Cleveland, OH 44135
Increasingly severe constraints on emissions, noise andfuel efficiency must be met by the next generation ofcommercial aircraft powerplants. At NASA LewisResearch Center (LeRC) a cooperative research effortwith industry is underway to design and test combustorsthat will meet these requirements. To accomplish thesetasks, it is necessary to gain both a detailedunderstanding of the combustion processes and aprecise knowledge of combustor and combustorsubcomponent performance at close to actualconditions. To that end, researchers at LeRC areengaged in a comprehensive diagnostic investigation ofhigh pressure reacting flowfields that duplicateconditions expected within the actual enginecombustors. Unique, optically accessible flametubesand sector rig combustors, designed especially for thesetests, afford the opportunity to probe these flowfieldswith the most advanced, laser-based optical diagnostictechniques. However, these same techniques, tested andproven on comparatively simple bench-top gaseousflame burners, encounter numerous restrictions andchallenges when applied in these facilities. Theseinclude high pressures and temperatures, large flowrates, liquid fuels, remote testing, and carbon or othermaterial deposits on combustor windows. Results areshown that document the success and versatility of thesenonintrusive optical diagnostics despite the challengesto their implementation in realistic systems.
Introduction
The next generation of aircraft powerplants will operateat conditions resulting in much higher overall
combustor inlet temperatures and pressures comparedwith current designs.1 A thorough understanding ofcombustion phenomena and combustor subcomponentperformance at actual operating conditions is critical tothe successful design and construction of thesepowerplants. Advances in non-intrusive opticaldiagnostic methods and test rig designs have now madeit possible to acquire two-dimensional optical data fromwithin combustors and flame tubes which closelysimulate actual engine conditions. Performingexperiments of this type, however, requirescircumventing or otherwise overcoming inherentproblems not typical of bench-top experiments. Theseproblems, both logistical and technical, involve not onlythe diagnostic techniques, laser beam delivery, and dataacquisition, but the test rigs themselves. Frequentcombustor configuration changes place an additionalburden on the diagnostic techniques requiring a robustdesign and the ability to adapt to multiple test rigs andfrequent component modifications. The techniquesmust also accommodate other standard, less flexiblemeasurement techniques such as gas sampling by probeextraction. Foremost, these techniques must be able tosuccessfully translate from the bench-top to thepowerplant; in other words, they must be capable ofremote operation and of performing dependably in afrequently hostile environment.
Many mature optical diagnostic techniques have beenused successfully on bench-top or laboratory scalesetups and have had a significant impact on combustionstudies.2"4 Raman techniques, such as coherent anti-Stokes Raman spectroscopy (CARS), have been usedfor years to study combustion phenomenon and toelucidate information concerning species andtemperature.5 However, the point-wise and alignment-critical nature of CARS places severe limitations uponits application in an environment where there is a highdegree of vibration, large temperature fluctuations, andthe test cell is inaccessible during operation. Onlyrecently have advances in the development of spatiallyresolved Raman spectroscopy been applied to
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combusting flowfields,6 although it has yet to bedemonstrated in an actual aero-combustor. Similarly,degenerate four-wave mixing7 (DFWM) and transientgrating spectroscopy8 (TGS) have shown great promiseas diagnostic tools for high pressure combustingflowfields. However, with problems similar to thoseexperienced by the CARS method, these too have yet tobe successfully used in an actual combustorenvironment.
Laser-induced fluorescence (LIF), planar laser-inducedfluorescence (PLIF), and recently, analogpredissociative techniques have been used successfullyto examine a wide range of combustion processes. Thetwo-dimensional nature of PLIF has made it the morepromising and useful of the two for aerospace gasturbine combustor applications. Additionally, its multi-species selectivity, flow field imaging capabilities, andpotential quantitative nature make it a favorablecandidate for flame studies.
PLIF has been used previously to probe laboratory scalelow pressure and atmospheric pressure gaseous flamesfor species concentration and distribution,9'10 velocity11,and temperature12 measurement. Shock tube studies byPLIF methods13 have also enjoyed significant successincluding temperature and species measurement.Recently, laboratory scale high pressure gaseous flamesnear 1.0 - 4.0 MPa,14'15 and spray flames approaching1.0 MPa16 have been successfully examined via PLIFimaging. PLIF Measurements made using opticallyaccessible ground based power systems17, diesel18 andspark ignition19 (SI) engines have been critical tounderstanding the combustion processes in thesesystems. Recent two-dimensional fluorescence imagingmeasurements which simulate proposed gas turbineconcepts but at atmospheric pressures have also beensuccessfully performed20.
While these investigations have produced significantresults and have added greatly to our understanding ofcombustion processes, these experiments have onlysimulated certain aspects of the combustor and itsoperating conditions. What was needed was a means tononintrusively examine the combusting flowfield, and toobserve the performance of combustor subcomponents,such as the fuel injector and swirlers, operating atanticipated conditions of pressure and temperature,using jet fuels. The test rigs necessary to allow this typeof testing present many challenges to successfulapplication of optical diagnostics. Conditionsapproaching 6.1 - 10.1 MPa in pressure, 2255 K intemperature, and mass flows approaching 17 kg/s areunder consideration for future tests. Furthermore, thevery nature of this type of testing requires remote
operation of all aspects of the diagnostics procedures.This includes laser operation, data/image acquisition,and test rig operation. Optical accessibility to thecombustion and fuel injection zones is also required,necessitating the application of complex windowcooling technologies to prevent degradation andpotential melting of window materials.
At NASA LeRC, efforts have been underway to adaptand implement existing nonintrusive optical diagnosticsmethods to examine the realistic, reacting flow fieldsgenerated by advanced fuel injector designs. Twooptically accessible flametubes capable of operation upto 2.0 MPa in pressure and 2033 K in temperature withflow rates up to 3.68 kg/s are presently in use. A third,much larger housing designed for 6.7 MPa operation,has been delivered and installed but has not yetundergone optical testing. For these tests, opticalaccessibility is the key. Window materials must be fullycapable of withstanding the above conditions whileremaining optically clear for the acquisition ofmeaningful data and images.
Due to testing costs and strict scheduling constraintswhich precluded more time consuming individualtesting, each diagnostic had to be integrated with thetest rig in such a way as to allow simultaneous dataacquisition. The optically accessible rigs at LeRC weredesigned to allow implementation of a large number ofoptical diagnostic methods. PLIF and Planar Miescattering were chosen as the primary methods sinceadequate laser energy exists to make an acceptable lasersheet and the same UV excitation wavelength can beused for both. Furthermore, these techniques, inaddition to having high temporal and spatial resolutionand high sensitivity, allow an opportunity to examineboth intermediate and stable species.
Hardware
The test facility at NASA LeRC delivers nonvitiated au-to the two unique, optically accessible combustor testrigs utilized for this series of experiments and describedin detail elsewhere.21"22 The first, pictured in Figure la,is a 21.6cm x 21.6cm radially-staged gas turbinecombustor. This rig, called a sector rig, is designed totest larger, multi-component injector systems. Thesecond test rig, shown in Figure Ib, measuring 7.6 cm x7.6 cm, is a flame tube designed to test singlecomponent injectors or small, multi-componentsystems. Typical rig operating conditions range frominlet temperatures of 533 K - 866 K, inlet pressures of0.55 MPa - 2.03 MPa, and mass flows of 0.16 kg/s to0.77 kg/s for the flame tube, and 1.13 kg/s - 3.63 kg/sfor the sector rig. Equivalence ratios ((|>) range from
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0.30 to 0.60. Initially JP-5 or JP-8 jet fuel was used fortesting but Jet-A is now used for all tests.
The window housings, which are identical for each testrig, are equipped with UV grade fused silica windowsmeasuring 38 mm axially, 51 mm radially, and 13 mmthick. To counter the heat generated in the combustingflow stream, the inner surface of the windows arecooled with a thin film of nitrogen. The nitrogen flowcomprises less than 10% of the aggregate combustormass flow and maintains a typical window innersurface temperature less than 977 K.
Window breakage has not been a major problem.Breakage, which typically occurs in the form of shearcracks, has been experienced only on the larger sectorrig. Cracks have not occurred during a test run but onlyafter the shutdown procedure has been completed andthe rig is cooling down. Since breakage does nothappen often and not always at the same location, itcannot be directly attributed to any one cause.However, it is assumed that the breakage occurs as aresult of uneven cooling of the window mounts and thecombustor thereby causing uneven stresses.
Window deposits are a recurring problem at certain testconditions. While testing specific fuel injector designs,it became necessary to periodically remove deposits thataccumulated on the windows. This is accomplished byfirst retracting the ICCD cameras out of the way, thenremotely sweeping a 50 ml sheet of focused, 532 nmoutput provided by a Continuum "Surelite" Nd:YAGlaser over both detector windows. In this manner, thedeposits are ablated from the interior window surfaces.Cleaning the laser beam insertion window was found tobe unnecessary since the continued panning of theincident 281 nm laser sheet effectively keeps thiswindow clear.
A direct result of test rig heating is axial growth. Eachrig demonstrates this characteristic to a varying degree.This growth is always in the upstream direction sincethe downstream segment of the rig is anchored to thetest bed. Growth of up to 7-8 mm has been observed.Tracking and correcting for this growth with theincident lasers and cameras under remote control was anadditional challenge during these experiments. This isachieved by scribing the combustor housing just belowone of the imaging windows in millimeter increments.The output beam from a helium neon laser was thenspotted at the origin of this ruling and monitored with avideo camera throughout the test run. As the rig growsor shrinks with changing conditions, this beam spot isobserved to drift along the scaled ruling. The cameraand laser sheet delivery system are then shifted an equal
amount in the same direction, thereby maintaining theoriginal, pre-lightoff alignment.
Various fuel injector designs have been fitted into eachtest rig. These injectors are positioned such that theinjector exit plane projects approximately 5 mm intowindow viewing area, thereby providing a referencepoint for the resultant images and rig coordinate system.The rig coordinate system defines x as the azimuthal orhorizontal direction with positive x to the right whenlooking upstream. Z is the axial coordinate, with z = 0defined as the injector exit plane and positive z in thedownstream direction. Y is the radial or verticalcoordinate with positive values above the rig centerline.
Optical Setup
Figure 2 presents a cutaway of the test facility and theexperimental layout which is described in detailelsewhere.23 The 10 Hz, 532 nm output from aContinuum model ND-81C, Nd:YAG laser pumps a ND60 dye laser, the output of which is doubled by a UVXultra-violet wavelength extension system. The resulting281 nm output, maintained at approximately 20 ml forthe experiments described herein, is delivered through90 mm acrylic tubes to the test cell by a series ofremotely controlled high damage threshold mirrors.Since this laser provided a divergence of only 5 mrads,the laser output is allowed to freely expand over thebeam path which ranges from 12 meters for the flametube to 25 meters to the sector rig. The laser beam, priorto entering the test section, is formed into a sheet by af = 3000 mm cylindrical lens. The resultant sheet sizeat the laser focal volume is approximately 22 mm by0.3 mm. A second beam path, not shown in the figure,has recently been added to allow simultaneous twocolor experiments or the use of a second YAG laser forwindow cleaning operations.
Due to the distances involved, and to safetyconsiderations demanding that the test cell beinaccessible to personnel during all test runs,positioning of the laser sheet and cameras by remotecomputer control is necessary. For the nearer flametube rig, this requires controlling, effected by a ParkerHannifin Compumotor model 3000, up to four axes onthe laser sheet positioning table suspended immediatelyabove the windowed test section (see figure 2). Controlof two Aerotech 3-axes positioners, each holding anICCD camera, is accomplished via two Unidex model11 controllers. For the more distant sector rig, aCompumotor model 4000 provides control for anadditional 4 axes which are required for laser sheetdelivery into that test section.
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The complexity of this motion control system wasreduced by writing a computer program to coordinatethe simultaneous positioning of both the laser sheet andcamera detection systems. The LabWindows/CVIsoftware development tool from National Instrumentswas used to accomplish this task. The final programallows the user to select which laser beam and detectorconfiguration is to be utilized for each test run from apossible ten combinations. The program also providesa high degree of flexibility by allowing the user tospecify the type and orientation of each stage mountedin the test cell and how these stages are to be connectedto the motion controllers. The program controls thedistance and direction that each designated stage movesand allows the user to define an origin. The program,through keyboard command, positions the laser sheetanywhere within the insertion window with respect tothe defined origin in terms of a rectangular coordinatesystem. The program records the user's coordinates,test conditions, as well as origin for future test runs or incase of a power failure.
Figure 3 illustrates the typical diagnostic setup used forthis series of experiments. Fuel or OH PLDF and planarMie scattering signals are collected normal to theincident laser sheet by gated and intensified 16 bitICCD cameras from Princeton Instruments, each with a384 x 576 pixel array. The intensifies, adjusted toprovide a 75 ns gate, are synchronously triggered withthe laser pulse. Each camera uses a Nikon 105 mmf/4.5 UV Nikor lens focused on a plane coincident withthe incident laser sheet. The PLIF camera is equippedwith both a Schott WG-305 filter and a narrow bandinterference filter centered at 315 nm with a FWHM of10.6 nm yielding a transmission efficiency of 16%.The Mie scattering camera is equipped with a narrowband interference filter centered at 283 nm with aFWHM of 2 nm and a transmission efficiency of 6%.Both single shots and on chip-averaging of successiveimages may be obtained using this detection system.
Figure 3 also shows the placement of a two-componentAerometrics Phase/Doppler particle analyzer (PDPA).This instrument is used to measure the light refractivelyscattered by fuel droplets (30° forward scatter). ThePDPA system is mounted onto a large 3-axis Accudexpositioner from Aerotech. The two-line 488 nm and514 nm output is supplied by an argon ion laser fromCoherent and delivered by fiber optic to the transmitterunit. The transmitter and receiver are aligned 15° fromthe horizontal plane to maximize the number ofmeasurement sites within the test section. The focallengths for both receiving and transmitting optics are500 mm. The transmitter beam separations are 40 mm.Droplet size, velocity and number density distributions
may be obtained by shifting the data acquisition sitealong the propagation direction of the transmitter.Since the PDPA device makes point measurementswithin a small region, the probe volume must betraversed under remote control throughout theaccessible flowfield to characterize the spray.
Originally each of these diagnostics were alignedseparately, which led to difficulties in spatiallycorrelating the data acquired from each method.Subsequently a single, universal alignment tool is usedfor each method. This constitutes a flat plate measuring19 mm x 96 mm which is scribed on both sides with ametric ruling, and arrows indicating both the flowdirection and the laser sheet path. The plate is insertedthrough the lower window location which was fittedwith a spark plug for these experiments, and extendsthrough the flow path a measured distance from the fuelinjector exit plane. For reference, the center line of thecombustor is also scribed into the plate. Holesmeasuring 3 mm in diameter are located along thiscenter line for the purposes of aligning the PDPAinstrument. Following alignment, the plate iswithdrawn and the spark plug replaced.
Image Acquisition and Processing
Image collection is accomplished using PrincetonInstrument's Winview software. The collected imagesare transferred to an SGI Indigo workstation forprocessing. Processing and image analysis on the SGIis accomplished using PV-WAVE from VisualNumerics, Inc. The gray scale images from the camerasare converted for display using a pseudocolor scaleconsisting of 25 color plus black (low intensity) andwhite (high intensity) where each color represents aspan of 10 counts in a linear span of 255. The colors inthe pseudocolor scale have been chosen to make iteasier to see details in the less intense portions of theimages. Image processing includes removal of noisespikes, background subtraction, and, in some cases,correction for laser sheet energy distribution.
We have also developed an additional, unique imageprocessing capability that allows us to obtain viewslooking upstream into the fuel injectors. These views,called end-on or "cross flow" views, were developed toexamine the fuel spray pattern or patternation of eachfuel injector studied. In this process, forty-one side-view images are acquired at 1 mm increments across theflow field at each test condition. A computer programthen configures these 41 images into an image stack.The program interpolates the region between each ofthe 41 individual images thereby filling in the gapsresulting in a smoothed 3-D image block. The image
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block can then be sliced in any desired orientation.Figure 4 illustrates this process for a lean direct injectordesign installed in the sector rig. In this figure the flowexits the page to the right. The left side of the imageshows, for the sake of simplicity, only a few selectedside-view fuel fluorescence images in the z-y plane.The right side of the figure shows a few of the resultantcross flow views obtained in the manner described. Theimages in an image stack are scaled together so that thehighest signal level represents the 99th percentile. Theimages are displayed in this manner in order toaccentuate the lower light level structures that wouldotherwise be lost in the glare of the higher intensityfeatures.
Because the injectors are positioned with their injectorexit planes projecting approximately 5 mm into thewindow viewing area, a large amount of incident laserlight scatter from the injector face is encountered. Thisscatter is intense enough to allow passage of a small butsignificant portion through the selective filters of thedetectors. This scatter is eliminated by placing anexternal beam block over the top of the laser sheetinsertion window effectively blocking any light fromhitting the face of the fuel injector.
Since direct measurement of the laser sheet intensity inthe z direction is not practical, another technique hasbeen developed to correct for the fall off in laser sheetpower at the upstream and downstream edges. Weassume that the average intensity in the cross-flowimages over an area enclosing the jet should be constantas we move downstream over the relatively short axialrange we can see. This is a reasonable assumptionbecause the viewable distance is relatively short(approximately 40 mm). This assumption leads to the
conclusion that any variation in this average is due tolaser sheet energy changes. In our recent work, we havechosen to correct the xy-images by a factor whichcauses the within-jet average to be a constant over arange of z values.
The left-hand image in figure 5 shows a typical sideview, fuel fluorescence image obtained on the flametube rig for a two-circuit fuel injector concept. The testconditions were: Piniet= 1.6MPa, Tinlet=680K,4>totai= 0.31, and X,.M = 281.5 nm. This image illustratesone of the challenges encountered in these experimentsand brings out one of the many benefits. The problemillustrated here is the obscuration of the fluorescencesignal by the buildup of soot on the detector windows.This deposition is seen here as a dark mass in theupstream (left) center position, just downstream of theinjector location. The benefit illustrated is the abilityto illuminate the detailed flowfield structure. Thisimage, taken at the center line of the injector clearlyshows the spray from both inner and outer fuel circuits.This imaging data has been used to calculate the fullfuel spray angle at condition and subsequently found toagree quite well with the theoretical values at most testpoints. The soot buildup at low power condition, whileproblematic, was easily removed by the ablation methoddescribed earlier.
Main
Pilot
Side View Cross FlowFigure 5. Comparison of side-view and cross-flow viewsacquired for a two-circuit injector installed in theflametube. Test conditions: X,M = 281.5 nm, Tinia = 682K, Pbte = 1.6 MPa, <JHotai = 0.304
The right-hand image in figure 5 presents a cross flowimage derived using the above described technique for aposition 14 mm downstream from the fuel injector exitplane. The advantages of the cross flow display arequite obvious. Actual flow structure and fuel spraysymmetry are clearly illuminated. In the fuel PLIFimage, two concentric rings of fuel, either vapor and/orliquid, are seen to be coming out of the page andexpanding away from the two-circuit injector with ahigh degree of uniformity.
Figure 6 shows the fuel PLIF cross flow image shown in
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figure 5 (right) compared with a planar Mie scatteringcross flow image acquired simultaneously at the sameaxial position. The fuel PLIF image again shows twoconcentric rings of fuel from the pair of injectors. Incontrast, the planar Mie image shows only a single ringemanating from the inner fuel circuit, the outer ring isabsent. The reason for this is due to the outer fuelcircuit's apparent greater efficiency at vaporizing thefuel spray, which explains the lack of droplet scatteringcenters in this region. This comparison offers a meansby which to address critical fuel vaporization issueswhich have been examined by other methods24'25 butwith limited success.
Fuel PLIFz = 14mm
Mie Imagingz=14mm
Figure 6. Comparison of fuel PLIF and Planar Miescattering cross-flow images acquired at the same axialposition for a two-circuit injector installed in theflametube. Test conditions: XexC = 281.5 nm, Tuii« = 682K, Pmia = 1.6 MPa, <t>«xai = 0.304
Another issue brought forward by considering figure 6is the question of possible extinction effects. The rightside in the PLIF image and the left side of the planarMie scattering image, each being the side opposite theirrespective cameras, appear to show some decrease insignal intensity. This affect may be attributed to theextinction of the incident light sheet or to the extinctionof the induced emission or scattering by the interveningflowfield. Since this phenomenon is not ubiquitous, butonly appears thus far in test runs involving highpressures and large mass flows, we have as yet notinvestigated the extinction question thoroughly.Obviously, with tests scheduled to begin using the highpressure 6.08 MPa rig where extinction effects may bemore severe, the investigation of these effects should beaccomplished.
Tests were performed in which PLIF, Planar Miescattering, and PDPA were each attempted. These testswere originally performed simultaneously with goodresults. However, due to the close tolerances inpositioning the three different diagnostics (see figure 3),
a few instances of unwelcome collisions between thevarious optical components occurred. In thesecollisions, optical alignment was invariably lost for oneor more of the detectors requiring shutdown of the rig inorder to re-enter the test cell to realign the systems.Subsequently, the PDPA diagnostics were run only aftermoving the two ICCD cameras a safe distancedownstream following their data acquisition run.
high fuel
b. Mie Imaging
» f
• O
• l"
.l
PDPA along -106* KmPDPA along -15'iraFu«l PUF along -105* DmFud PUF along -15* liraMtabng-iWlMM* along -15* Ira
I, *v»i. J _^ft*
c. 3-d drop-One plot
Figure 7. Comparison of fuel volume distribution asmeasured by PLIF, Mie Scattering and PDPA acquired atthe same axial location. Lines in the images indicate thepath along which PDPA measurements were made.Conditions: XOK = 281.5 nm, Tide, = 616 K, Pwa = 558kPa, (JHooi = 0.445
Figure 7 shows a comparison of the data acquired at thesame axial location, 12.7 mm from the fuel injector exitplane, for each of the three techniques; PDPA, PLIFand planar Mie. For this series of experiments theflametube was equipped with a two-circuit fuel injectorwith only the pilot operating under the following flowconditions: 7^,= 616 K, Pinlet= 558 kPa, <j) = 0.445,^0= 281.5 nm. The white lines labeled -105° and -15°on the PLIF and Mie images denote the path alongwhich PDPA mass flux data was acquired. The graph atthe bottom of the figure plots both the pixel intensityalong the -105° (open symbols) and -15° (solid
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symbols) lines from the PLIF and Mie images and themass flux data acquired from PDPA measurements.From the plot, close agreement between the threedifferent techniques is seen to exist along the -105° line.Along the -15° line, the agreement holds only for thedata along the positive x axis. Along the minus x axis,the PDPA data falls off sharply. There are two possiblecauses for this observation; first may be the obscurationof the PDPA receiver due to the severe beam insertionangle involved. The second potential cause may be thatof extinction of the scattered light from the far side ofthe flametube by the intervening flow. This data falloffhas been observed prior to this on high pressurevaporizing sprays26. The approximate 1-2 mm shiftbetween the maxima for the three techniques on each ofthe plotted -105° and -15° lines is a vestige of usingseparate alignment methods. This has subsequentlybeen eliminated by using the same alignment toolpreviously described for each of the diagnostics.
Conclusions and Future Considerations
We have demonstrated at NASA Lewis that variousrelatively mature laser diagnostic techniques cansuccessfully be applied simultaneously to the realisticflowfields of high pressure and temperature aero-combustor test rigs. Images have been obtained throughPLIF and planar Mie scattering measurements providingheretofore never before seen views of the combustionprocess and fuel injector operation at actual conditions.Our methods are evolving with experience and changesare continuously being made to improve or adaptdiagnostic techniques to these large scale rigs. Forexample a remotely controlled motorized filter wheel isplanned to be used on the ICCD cameras for all futureimaging. The filter wheel, which holds up to five, two-inch diameter filters, will effectively increase ourdiagnostic capabilities by increasing the number ofspecies and other flow parameters that can be examinedin a single test run. Additionally, it will allow exactcomparison between PLIF and Planar Mie scatteringresults thereby eliminating the need for corrections suchas magnification, and pixel response variations,required when two different cameras are used.
Another example of continuous improvement is theadvent of a new 1.0 nm FWHM narrowbandinterference filter making it possible to record OH PLIFby eliminating a majority of the interference offluorescence signals in the region of the fuel injectorexit plane. Until recently fuel fluorescence wasexamined rather than OH fluorescence because thisinterference precluded good OH PLIF measurements.
process by which cross flow views are generated. Thiswill speed up one of the most time consuming aspectsof the analysis. Additionally, topographical and three-dimensional plotting of image pixel intensity, whichprovides an easier means to examine the flow path, arealso being automated.
The means to make corrections for laser sheetinhomogenieties such as those recently reported27 arealso being investigated. However, difficulties due tomandatory remote operation, and the fact that both thelaser sheet and the resultant planar fluorescence andscattering must both pass through different windows ofquestionable transparency makes this a daunting task.
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10American Institute of Aeronautics and Astronautics
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