1

US - Ukraine Workshop on Innovative Combustion andAerothermal Technologies in Energy and Power Systems

Kiev, Ukraine (May 21-24, 2001)

LASER DIAGNOSTIC MEASUREMENTS IN A LEAN-PREMIXEDLABORATORY-SCALE GAS TURBINE COMBUSTOR

Thomas H. FletcherPaul O. Hedman

Chemical Engineering DepartmentBrigham Young University

Provo, Utah 84602

ABSTRACTSignificant progress has been made in

computational fluid dynamic (CFD) modeling of gasturbine systems, particularly in the compressor andturbine regions. Models of the combustor section arenot used with the same degree of confidence. One ofthe major reasons for the lack of confidence in thecombustor models is the lack of data for evaluation,especially at high pressure. Detailed profile dataobtained in an atmospheric laboratory-scale premixedcombustor are now available for four swirling leanpremixed conditions. Laser-Doppler anemometry (LDA)was used to measure velocities, planar laser-inducedfluorescence (PLIF) was used to monitor OH and CH,and coherent anti-Stokes Raman spectroscopy (CARS)was used to measure temperature and major speciesconcentrations. All of these techniques generatedmultiple points, so that both mean values andprobability density functions are available at eachlocation. The Reynolds' stresses <u'v'> are alsoavailable in both mean and PDF form. These dataprovide a good means to evaluate current CFDcombustion models. In the future, we hope to be ableto obtain similar data at high pressure.

INTRODUCTION

Background. This study was part of a U.S.Department of Energy's Advanced Turbine Systems(ATS) program to develop and commercialize ultra-highefficiency, environmentally superior, and costcompetitive gas turbine systems for base-loadapplications in the utility, independent powerproducers, and industrial markets.

The LSGTC simulates many of the key combustorcharacteristics of commercial gas turbines. The use ofadvanced optical diagnostics permits near-instantaneous, non-intrusive measurement of suchquantities as velocity (LDA), temperature (CARS), flameshape (PLIF), and species concentrations (PLIF andCARS) that are essential for model validation. LDAmeasurements have been made in clean combustionenvironments. The CARS instrument has been used inmany different flames ranging from clean, laminar,

premixed flames to particle-laden (pulverized coal),highly turbulent, diffusion flames. PLIF has recentlybeen used to obtain instantaneous two-dimensionalimages of OH and CH radicals in a turbulent, swirling,premixed natural gas/air flame. The data also giveinsight into the physical processes governing theoperation of practical gas turbine combustors. Thisinsight has provided considerable direction to themodeling of the combustion process, and a databasesuitable for model subcode evaluation and verification.

In addition to the experimental work performed underthis contract, other funding also supported research atBYU on advanced gas turbine models. A list ofpublications is available on the web (www.et.byu.edu/~tom/gas_turbines/Relevant_Publications.html)

Objectives. The objective of the study was to modifythe existing laboratory scale gas turbine combustor(LSGTC) and use three laser diagnostic instruments(PLIF, LDA, and CARS) to make instantaneous in situmeasurements with in the flame zone of the swirlstabilized combustor. The instantaneous in situmeasurements provide insight into the combustioncharacteristics of the burner, and also provide a set ofdata for model verification and comparison. Pastexperience at BYU/ACERC has shown that themodeling of combustion behavior is more accurate andproceeds more rapidly when coupled with pertinent,foundational experimental research. Also, comparisonsof model predictions to actual data are necessary tovalidate the accuracy of the code predictions.

Approach. A 2-dimensional, laboratory-scalecombustor was designed to "specifically reproducerecirculation patterns and LBO processes that occur ina real gas turbine combustor" (Sturgess, et al., 1992).The combustor was designed to closely model the flowand combustion processes that occur in a real gasturbine combustor, but in a simpler, near 2-dimensionalaxisymmetric geometry. The model combustor was alsodesigned with optical access so laser-based diagnosticinstruments could be used to measure the variouscombustion and flow field characteristics associatedwith the combustion. The laser-based diagnostics used

2

in the program included: 1) two-component laserDoppler anemometry (LDA) to obtain gas velocity; 2)coherent anti-Stokes Raman spectroscopy (CARS) toobtain gas temperature and limited speciesconcentrations; and 3) planar laser inducedfluorescence (PLIF) to obtain 2-D images of OH and CHcombustion intermediates. All of the laser instrumentsprovided sets of multiple instantaneous measurementsthat could be interpreted in terms of mean andstandard deviation properties. The local probabilitydensity functions (PDF) at up to 200 local in situlocations in the flame zone of the combustor wereobtained for all three types of measurements. Inaddition to the specific in situ measurementsmentioned, film and video cameras were used toexamine flame structure and flame stability.

There were four separate experimental test conditionsexamined during the course of this study. Completedata sets from measurements of PLIF, LDA, and CARSwere obtained at medium and high swirl levels for twolean premixed natural gas stoichiometric ratios (φ= 0.65and φ= 0.80). The high swirl case at φ= 0.80 was themost stable flame condition tested. The medium swirlcase at φ= 0.65 was very near the lean flammability limitfor this burner, and consequently was very unstable.Example experimental results from the most stable case(HS, φ= 0.80) and the least stable case (MS, φ= 0.65)are included in the following sections of this paper.Some comparisons to model predictions are alsoincluded.

EXPERIMENTAL APPARATUS

Swirl is often used in a gas turbine combustor tostabilize the flame. Figure 1 presents an illustration ofthe flow vortex that results from such stabilization. Thepremixed fuel is rotated rapidly as it is passed throughthe swirl injector. This swirling flow field creates astrong central vortex, and at least 2 major recirculationzones, a central recirculation zone, a side recirculationzone. The central recirculation zone causes a reversalin the flow velocity in the center part of the combustor,which provides a stagnation region that can stabilizethe flame. The bluff body of the combustor creates aslide recirculation zone with rotation between the wallsof the combustor and bluff base. Depending on thelevel of swirl and the stoichiometry of the flame, one oranother of these recirculation zones will provide themajor source of flame stabilization.

An atmospheric pressure laboratory-scale model of agas turbine combustor (LSGTC) was provided to thislaboratory by Wright-Patterson Air Force Base(Roquemore, 1992) as part of an Air Force study on thecombustion characteristics of a practical Pratt &Whitney injector in a atmospheric pressure model gasturbine combustor. Extensive CARS gas temperaturemeasurements and LDA velocity measurements were

obtained in the LSGTC with various practical, non-premixed injectors of particular interest to researchersat Wright-Patterson Air Force Base and Pratt & WhitneyAircraft Co. (Hedman, et al., 1995, Hedman andWarren, 1995, and Warren and Hedman, 1997).

This test facility was modified for use in this program toinclude a pressurized natural gas feed system and ageneric turbulent, swirling premixed natural gas/airinjector. Figure 2 shows a schematic of the combustor.The air and fuel are premixed externally to thecombustion chamber and the mixture is fed into the fueltube. Uniform mixing of gaseous fuel and air is assuredby: 1) the length of the tube from the mixing point tothe burner, and 2) by the sonic disrupter that was usedin a previous program to control combustion instability.The sonic disrupter is a small cone with many smallholes in its conical surface. The premixed fuel/air flowsthrough the cone from the outside, forming a number ofjets that impinge in the center of the cone. Theimpinging jets ensure that the fuel/air feed is completelymixed.

The air supply system provides air flows up to 4000slpm. An airflow rate of 500 slpm was used throughoutthis study. This flow rate was sufficient to getreasonable combustion simulation but simplified theoperation of the facility.

A natural gas compressor was used to pressurize amanifold of conventional gas cylinders up to a pressureof 2000 psia. The pressure from this blow-down naturalgas system was reduced with pressure regulators, andthe flow rate was controlled with a simple needle valve.Flow rates were measured with a rotameter. Flow ratesof natural gas were adjusted to give the appropriatefuel equivalence ratio for the particular test condition ofinterest.

The LSGTC provided by Wright-Patterson Air ForceBase was modified to incorporate the generic turbulent,swirling, premixed natural gas/air injector of this study.The central fuel tube from the original design was usedto feed the fuel/air mixture to the burner. The newburner design was configured to easily integrate intothe existing combustion chamber and to takeadvantage of fuel/air feed systems that were already inplace.

The new premixed burner, shown schematically inFigure 3, was designed to connect directly to theexisting fuel tube. Provision was made to preventflame flashback into the premixed fuel tube by installinga brass flame arrester. The flame arrester is a block ofsolid brass that was drilled in a hexagonal pattern withsixty separate, 0.120 inch diameter holes. The highgas velocity (at least 19 m/s) through these holes

3

ensures that the flame can not propagate back into thefeed tube.

Since the flames of interest in this study were tosimulate those of a swirl stabilized practical gas turbinecombustor; provision was also made in the injector toswirl the premixed flow. This was done by installing acylindrical brass swirl block where eight rectangular slotsof 0.125 inch width were machined at a prescribedangle. Three different swirl blocks were fabricatedwhere the angles from the vertical were 30, 45, and 60degrees. The three swirl blocks give theoretical swirlnumbers (Beér and Chigier, 1972) of 0.43 (LS),0.74(MS), and 1.29(HS). For this study, only mediumswirl (MS), and high swirl (HS) injectors were used.

PLANAR LASER-INDUCED FLUORESCENCE (PLIF)MEASUREMENTS OF OH

Planar laser-induced fluorescence (PLIF) is a valuabletool for studying combustion phenomena. Flamepropagation is partly governed by the diffusion andtransport of important combustion intermediates likeOH. Laser-induced fluorescence has the potential ofmeasuring trace amounts of these short-livedcombustion species at near instantaneous temporalresolution (10 ns). Likewise, because PLIF is laser-based, probing of the burner is non-intrusive. Laser-induced fluorescence measurements are often made atthe focal point of a converging excitation laser.However, fluorescence can also be capturedinstantaneously over a two-dimensional area. The two-dimensional area captured resembles an instantaneousphotograph-like image of the concentration of thecombustion species being probed using a thin sheet oflaser light tuned to the appropriate wavelength. ThePLIF images provide near instantaneous images of theturbulent flame structure, including informationregarding the location of flame fronts and size ofturbulent eddies. This information is very useful inanalyzing burner designs, understanding combustionphenomena, and validating combustion computermodels.

Details of the PLIF instrument have been reportedpreviously (Hedman, et al., 1998). For this study, anexcitation wavelength of ca 283 nm was used for theOH radical. The 283 nm UV laser light was generatedby frequency doubling 566 nm laser light from atunable dye laser (Spectra Physics PDL-3) with a KD*Pcrystal in the wave extender (WEX). The tunable dyelaser used a Nd:Yag laser as a pump source. The 10ns UV laser beam from the WEX was directed to theLSGTC and formed into a thin two-dimensional sheet oflaser light about 0.5 mm thick and ca 100 mm in width.The fluorescence images of the OH radical at ca 308nm was captured on an electronic camera that uses anintensified charge coupled display (ICCD) with an

electronic shutter speed of 100 ns. The ICCD camerawas setup orthogonal to the 2-D laser sheet and wasfitted with an appropriate filter (to block unwanted light)and a Nikon quartz f/1.2 lens. Two hundred fifty sixseparate “instantaneous” PLIF images were capturedfor each of the four test conditions and saved to thecomputer for later data analysis.

Figure 4 presents four example instantaneous imagesfor each of the four test conditions. There are fourimages presented for each test condition. Thoseidentified with the letter A represent the minimumintensity image collected. Those identified with theletter B represent a near average intensity image.Those identified with the letter C represent themaximum intensity image collected. Those identifiedwith the letter D were included because of the oddity ofthe image collected. The numbers associated witheach image are just the sequential numbers in the setof 256 images. The capture of each image wassufficiently separated so that there was no stochasticinformation available from image to image. Howeverthe set of 256 images has been analyzed to determinemean and standard deviation at each pixel in theimage. The software used to generate the imagenormalizes the image to the maximum pixel count (red).Consequently, red in one image corresponds to adifferent pixel count than red in another image. Theactual pixel counts are available, and have been usedto provide PDFs of the pixel count distribution at givenlocations within the flame. LIF theory indicates that thepixel count is proportional to the actual concentration ofOH. Unfortunately, calibration of the pixel counts intoactual concentrations was beyond the scope of thisstudy.

The stochastic nature of the flame is readily apparentfrom the images presented. The effect of swirl and fuelequivalence ratio on flame structure and stability arereadily apparent. The OH images flames at high swirlshow increased stability and more intense images atboth high and low fuel equivalence ratios. The imagesat medium swirl and φ = 0.80 are also intense andstable. The flame at medium swirl and φ = 0.65 wasvery unstable, and was near lean blowout. In fact theflame at these operating conditions would occasionalextinguish for no apparent reason.

The collection of 256 separate images for each testcondition have been used to determine pixel by pixelmean and standard deviation images for each of thefour test conditions. Figure 5 presents examples of themean PLIF OH images for the HS, φ = 0.80 (moststable flame), and MS, φ = 0.65 (least stable flame).The average images provide a source of data on meanflame structure that can be compared directly to codepredictions. These OH image for the HS, φ = 0.80 caseshow that the highest concentrations of OH in the

4

central recirculation zone directly above the injector.This suggests that the recirculation of thesecombustion intermediates back into the ignition zone isan important factor in stabilizing this flame. The highestOH concentrations for the MS, φ = 0.65 case (albeit ata much lower concentration) show in the centralrecirculation zone directly above the injector and also inthe side recirculation zones. This suggests that atlower swirl and fuel equivalence ratio the siderecirculation zones have a significant role in flamestabilization. This is consistent with observations drawnby Murray (1998) from the LDA velocity measurements.

Examples of OH PDF distributions for the HS, φ = 0.80case are presented in Figure 6. The distributions arebased on the count distributions obtained at thelocation of each of the selected pixel sites as illustratedon the companion map of the mean image. The peakand average counts for each selected location areincluded on each of the PDF distributions. Theaverage counts range from 181 (Location a) to 1652(Location g). The peak counts range from 600(Location a) to 4633 (Location g). It is evident that thedistributions are not Gaussian (a common modelingassumption) but may be better described by a betadistribution. The shape, count level, and PDFdistribution are all useful when comparing to modelpredictions.

LDA GAS VELOCITY MEASUREMENTS

An important part of this ATS study has been themapping of velocity profiles for the premixed naturalgas/air burner. These velocity measurements weremade using a conventional two-color four-beam LaserDoppler Anemometer (LDA) system in forward-scattermode. This LDA configuration allows the simultaneousmeasurement of either axial and tangential or axial andradial velocity components. The details of the LDAinstrument have been provided elsewhere (Hedman, etal., 1998; Murray, 1998)

Due to the nature of LDA data acquisition, the datacontains a distribution of velocity measurements foreach measurement location. These velocitydistributions were used to determine: 1) mean (u, v, w)and standard deviation velocities (u’, v’, w’), 2)Reynolds stresses (u’v’ and u’w’), and 3) the probabilitydensity function (PDF) at multiple locations in the flame.The local flow characteristics at multiple locationsthroughout the burner have allowed creation of iso-contour maps of the flow characteristics in the burnerflame that was optically accessible. Optical accessallowed most of the interesting flame zones to becharacterized.

The velocity data yield considerable insight into the flowstructure inside the burner flame. This information

coupled with coherent anti-Stokes Raman spectroscopy(CARS) temperature and species data and planar laserinduced florescence (PLIF) data give insights of thereaction mechanisms and flame structures in thepremixed natural gas/air flame used for this study. Inaddition, the data have been used as a benchmark forcombustion modeling testing and evaluation.

Complete data sets (complete radial profiles at agreater number of axial locations) were obtained for themedium swirl, and high swirl injectors at φ = 0.65 and φ= 0.80. Consequently, four complementary sets ofinstantaneous axial & radial velocities, and axial &tangential velocities have been obtained for thepremixed, natural gas combustor at fuel equivalenceratios of 0.65 and 0.80 for both the medium (SN =0.74) and high (SN = 1.29) swirl burners. The twocomponent velocity data (either axial & radial, or axial &tangential) were collected at approximately 130separate radial/axial in situ locations and 200 in situaxial/tangential locations within the flame zone. Fourthousand instantaneous data points were collected ateach location. Each data set provides mean andstandard deviation axial, radial, and tangential velocitydata, as well as two component Reynolds stress basedon axial/radial and axial/tangential data sets. Axial,radial, and tangential probability density functiondistributions are also available for each test conditionsand in situ location where data were collected. Onlyexample results are presented in this paper. Completeresults are available in Murray (1998) and Hedman, etal. (1998).

Data sets were taken at axial locations of 10, 20, 30,40, 50, 75, 100, 125, 150, 200, and 250 mm abovethe base of the combustor. The basic flow field in thecombustor was illustrated using interpolated iso-contours of the mean and standard deviation of eachcomponent of velocity (axial, radial, and tangential),and Reynolds stress. The PDFs provided insight intothe turbulent characteristics throughout the combustor,and across the shear zones within the flame structure.

Example results from the LDA velocity measurementsare presented in Figure 7 for the mean axial iso-velocitydata. As with the PLIF images, the example contourplots are limited to the HS, φ = 0.80 (most stable) caseand the MS, φ = 0.65 (least stable) case. The contourson the left side of each figure were generated byduplicating the data from the right side of each figure.Limited data were obtained at the negative locationsnear the centerline (-6 mm, and -3 mm). The negativelocations were included in the test matrix to determinethe degree of symmetry in the results. Generally, thevelocity fields were very symmetric about the centerline. The combined images were superimposed on aschematic of the combustor to provide a spatialreference frame for the flow field.

5

The visual flame at the HS operating condition, Figure7(A) was well attached, with the base of the funnelcloud created by the vortex structure being drawn intothe nozzle and out of sight. The sharp axial velocitypeak near the injector exit is readily apparent, as wellas the high velocity regions that border the centralrecirculation vortex created by the swirling flow. Theaxial velocity decays rapidly above the injector due inpart to dissipation in the shear layer and the stagnatingeffects of the combustion reaction. That the flow isdominated by recirculation zones is clearly evident inthe channel-like regions of positive axial velocity beingbordered by zero velocity contours. The splitting of theflows along the walls into portions of upward anddownward moving gas is confirmed by the contourslocated between the large central recirculation zoneand the smaller outside recirculation zone. It can beseen that the axial velocity is highest at the midpointbetween the recirculation zones and then decaysoutward in both directions as part of the flow movesupward and the other part is drawn down into theregion of negative axial velocity near the wall.

Similar observations about the flow structures can bedrawn from the axial velocity contour plot for themedium swirl injector, Figure 7(B). The flow patternsare very similar, but the flame structure is somewhatbroader, and the velocities in the recirculation zonesare of a somewhat lower magnitude. The visible flamewith the MS injector did not enter the injector like theHS flame was observed to do.

Multiple (ca 4000) velocity measurements (axial, radial,and tangential) at many in situ locations (ca 200) in aflame zone allow local PDF information throughout theflame structure to be determined. The measured PDFinformation provides considerable insight into theturbulent characteristics of flow, and provides, as doesthe mean and standard deviation velocity data,important results needed for model development andvalidation.

Figure 8 presents some example axial velocity PDFsgenerated from the LDA data at φ = 0.80 for the HSburner. The PDFs presented in this figure werecalculated from about 4000 axial velocitymeasurements taken at an axial location of 80 mmabove the injector and at radial locations of 2, 8, 14,20, 26, 32, 38, and 44 mm from the burner centerline.These results provide PDFs from the centralrecirculation zone (negative mean velocities) throughthe shear layer into the zone of positive axial flow. Atthis axial location, the axial velocity distribution wasnearly Gaussian near the centerline (negative flowvelocity). But as one moves from inside the vortex,through the shear zone and into the surroundingoutside gases, the distribution begins to becomedistorted, reflecting the flow structure outside the shear

region. In fact, the distribution becomes bimodal as theLDA diagnostic volume straddles the shear zone (v = 0m/s). As one moves past the shear zone, thedistribution once again begins to show a moreGaussian distribution in the positive axial flow thatexists in this region. At a position lower in thecombustor, the shear layer was much thinner, and thetransition through the shear layer would first favor thenegative velocity side then the positive side. The bi-modal PDF distributions show characteristics of bothsides of the shear layer.

CARS TEMPERATURE MEASUREMENTS IN THELSGTC

Coherent anti-Stokes Raman Spectroscopy (CARS) isan advanced laser-based diagnostic technique thatallows in situ measurement of gas temperature andselected species concentrations. The CARS instrumentat this laboratory has been used in several previousstudies to obtain spectral data for the N2, O2, CO, andCO2 species in different flames (Boyack and Hedman1990; Dawson and Hedman, 1996, Flores, 2001). TheCARS technique utilizes the Raman shift that resultswhen a particular gas molecule is energized by a highpower laser. The Raman shift is attributed to theinelastic scattering that occurs between the photons ofthe laser beam and the orbiting electrons of themolecule. With the inelastic scattering, energy can beexchanged in either direction between the photons andelectrons. This results in a spectral signature that isquantum shifted either up or down from the pump laserfrequency. These spectra, which contain informationabout temperature and species concentration, arereferred to as the Stokes or anti-Stokes signals.Comparison of the measured spectra with computedspectra from quantum theories (Farrow, 1995) allowsthe gas temperature and/or species concentration tobe determined.

The Raman signal is relatively very weak, and isscattered in 4π steradians. Thus, Raman scattering islimited in signal strength. CARS utilizes the Ramaneffect but combines a laser beam tuned to the Stokesfrequency of a particular molecule with two laser beamsat the pump frequency. This nonlinear wave mixingprocess is illustrated in Figure 9 for temperature (basedon nitrogen) and CO concentration. The nonlinearwave mixing, illustrated in the wave vector diagram,produces a laser like beam at the anti-Stokesfrequency that contains the spectral information. Thislaser-like beam, which may be 1000 times stronger thana Raman signal, can be focused into a broad bandspectrometer where the spectra can be obtained andstored for later computer analysis.

The ACERC/BYU CARS instrument has been used tocollect temperature and species concentration data for

6

the LSGTC. As with the PLIF and LDA measurements,four test conditions [SN = 0.74 (MS) and 1.29 (HS) atfuel equivalence ratios (φ) of 0.65 and 0.80] wereselected for this study. However, only exampletemperature results are included in this paper. As withthe velocity measurements, the results are presentedas iso-contour plots of gas temperature. Approximately1000 separate instantaneous CARS measurementswere made at various in situ locations within thecombustor. As with the PLIF and LDA data, the resultshave been reduced in terms of mean temperature,standard deviation temperature, and temperature PDFsat selected locations within the combustor.

It is customary with velocity data to divide the standarddeviation values by the local mean velocity todetermine the local turbulent intensity. This was notpossible with the velocity data reported above becauseof the recirculation zones bounded by zero velocitycontours. Turbulent intensities based on thesevelocity measurements would produce regions wherethe turbulent intensity is indeterminate. However, thisrestriction does not apply to the CARS temperatureresults. Consequently, iso-contour plots of thenormalized standard deviation in temperature

( ′ T ′ T T ) provide useful insight into the regions ofsignificant temperature changes.

As with PLIF and LDA results, example gastemperature contours and temperature turbulencecontours for the HS, φ = 0.80 (most stable) case, andMS, φ = 0.65 (least stable) case are presented inFigure 10. The iso-contour maps were created frommultiple instantaneous measurements at about 200separate in situ locations within the flame zone. Thedifferences between the most stable case and the leaststable case are striking. The differences between peak(1636 K and 1472 K) and minimum (402 K and 323 K)temperatures for the two cases are thought to beprimarily a result of the different fuel equivalence ratios.The differences between normalized standarddeviations in temperature (60% versus 39%, and 5.5%versus 4.4%) are thought to be caused by acombination of the different fuel equivalence ratios andswirl numbers. The difference in flame structurebetween the two cases is reflected in the contour plotsof both the mean and normalized standard deviation intemperature. The contour plots show that the highestmean temperature and standard deviations for the HScase (most stable) are much closer to the burner inlet,which suggests a stronger vortex structure and a morestable flame in this case. This is consistent with theLDA velocity results and the OH PLIF results presentedabove.

The ability to collect multiple instantaneous in situ dataat many locations within the flame zone of a practicalcombustor allows the PDF distributions to be obtained

throughout the flame zone. These PDF distributionsare of interest to code developers and are useful incharacterizing turbulence models, and in verifying codepredictions. Example OH PLIF PDFs and axial velocityPDFs were earlier in the paper for the HS, φ = 0.80case. Figure 11 presents example temperature PDFsfor this same case at the locations indicated on thecompanion mean temperature contour plot. The PDFdata show a wide range, which tends to confirm highlevels of turbulence as seen in the velocity PDF data,and in the visual images shown in the PLIF data. Theshape of the temperature PDFs are not quite Gaussianin most locations in the flame, but like the velocity datashown in Figure 8, show a bimodal distribution inregions that straddle the shear zone between the innerand outer recirculation zones.

SUMMARY AND CONCLUSIONS

A laboratory-scale gas turbine combustor (LSGTC) andassociated laser-based diagnostic instruments wereused to make in situ combustion measurements in alean premixed natural gas flame that closely simulatesflames that would exist in a utility gas turbine engine.Use of advanced optical diagnostics permitted near-instantaneous, non-intrusive measurement of velocity(LDA), temperature (CARS), and instantaneous flameshape (PLIF of OH) that are essential for modelvalidation.

Four separate experimental test conditions wereexamined during the course of this study. Completedata sets from measurements of PLIF, LDA, and CARSwere obtained at medium and high swirl levels for twolean premixed natural gas stoichiometric ratios (φ =0.65 and φ = 0.80). The high swirl case at φ = 0.80was the most stable flame condition tested. Themedium swirl case at φ = 0.65 was very near the leanflammability limit for this burner, and was very unstable.

The stochastic nature of the flame is readily apparentfrom the PLIF images presented. The OH imagesflames at high swirl show increased stability and moreintense OH images at both high and low fuelequivalence ratios. The images at medium swirl and φ= 0.80 are also intense and stable. The flame atmedium swirl and φ = 0.65 was very unstable.

The OH PLIF image for the HS, φ = 0.80 case showsthe highest concentrations of OH in the centralrecirculation zone directly above the injector. Thissuggests that the recirculation of these combustionintermediates back into the ignition zone is animportant factor in stabilizing this flame. The highestOH concentrations for the MS, φ = 0.65 case show inthe central recirculation zone directly above the injectorand also in the side recirculation zones. This suggeststhat at lower swirl and fuel equivalence ratio the side

7

recirculation zones have a significant role in flamestabilization.

Examples of PDF distributions for the HS, φ = 0.80case were presented. The OH PLIF distributions arenot Gaussian (a common modeling assumption) butmay be better described by a beta distribution.

Velocity measurements were made using aconventional two-color four-beam Laser DopplerAnemometer (LDA) system in forward-scatter mode.The visual flame at the HS operating condition was wellattached with the base of the funnel cloud created bythe vortex structure being drawn into the nozzle andout of sight. A sharp axial velocity peak existed nearthe injector exit. High velocity regions also borderedthe central recirculation vortex created by the swirlingflow. The axial velocity decays rapidly above theinjector due in part to dissipation in the shear layer andthe stagnating effects of the combustion reaction. Theflow is dominated by recirculation in the channel-likeregions of positive axial velocity bordered by zerovelocity contours. The flows split along the walls intoportions of upward and downward moving gas. Theaxial velocity is highest at the midpoint between therecirculation zones and then decays outward in bothdirections as part of the flow moves upward and theother part is drawn down into the region of negativeaxial velocity near the wall.

The measured velocity PDF information providesconsiderable insight into the turbulent characteristics ofthe flow. The axial velocity PDF distribution was nearlyGaussian near the centerline (negative flow velocity).However, the distribution became distorted near theshear region. The PDF distribution becomes bimodalas the LDA diagnostic volume straddles the shear zone(v = 0 m/s). As one moves past the shear zone, thedistribution once again begins to show a moreGaussian distribution in the positive axial flow thatexists in this region. The bimodal PDF distributionsshow characteristics of both sides of the shear layer.

The CARS instrument was used to collect temperatureand species concentration data for the LSGTC. Thestandard deviations of the gas temperature normalizedby the local mean gas temperatures were used todetermine principal reaction zones. The differencesbetween peak temperature (1636 K and 1472 K) andminimum temperature (402 K and 323 K) for the twocases is thought to be primarily a result of the differentfuel equivalence ratios. The differences betweennormalized standard deviations in temperature of 60%versus 39%, and minimum temperature turbulencelevels of 5.5% versus 4.4%, are thought to be causedby a combination of the different fuel equivalence ratiosand swirl numbers. The temperature contours for theHS case (most stable) are much closer to the burner

inlet, which suggests a stronger vortex structure and amore stable flame in this case. This is consistent withthe LDA velocity results and the OH PLIF results.

The temperature PDF data confirms high levels ofturbulence as seen in the velocity PDF data, and in thevisual images shown in the PLIF data. The shape ofthe temperature PDFs are nearly Gaussian in mostlocations in the flame, but show a bimodal distributionin regions that straddle the shear zone between theinner and outer recirculation zones.

ACKNOWLEDGEMENTS

This study was supported by theDOE/METCCooperative Agreement No. DE-FC21-92MC29061 through Subcontract No. 93-01-SR014with SCERDC (Dr. Daniel P. Fant, and Dr. Lawrence P.Golan, Project Directors). Students contributing to theexperimental measurements included Jason Haslam,Robert Dawson, Robert Murray, Stewart Graham,Wayne Timothy, and Daniel Flores. Additional supportfrom ACERC, and BYU is acknowledged.

REFERENCESBeer, J. M., and Chigier, N. Combustion Aerodynamics,

Applied Science Publishers, London (1972).Boyack, K. W., and Hedman, P.O., "Dual-Stokes CARS

System for Simultaneous Measurement ofTemperature and Multiple Species in TurbulentFlames," Twenty-Third Symposium (International) onCombustion, The Combustion Institute, Pittsburgh,PA (1990).

Dawson, R. W. and Hedman. P.O., “A Technique forDetermining Instantaneous N2, CO, O2, CO2, andH2O Species Concentrations Using Multiplex CARSin Premixed Gaseous Flames,” presented at theWestern States Section/Combustion Institute, PaperNo. 96F-087, The University of Southern California,Los Angeles, California (October 28-29, 1996).

Farrow, R. L., Personal Communication: CARSFTComputer Code for Calculating Coherent Anti-StokesRaman Spectra, Sandia National Laboratories,Livermore, California (1995).

Flores, D. V., “A Study of Premixed Natural Gas-AirCombustion Based on CARS and LDAMeasurements on a Laboratory-Scale Gas TurbineCombustor,” Ph.D. Dissertation, ChemicalEngineering Department, Brigham Young University,Provo, Utah 84602 (In Preparation 2001).

Gupta, A. K., Lilley, D. G. and Syred, N. Swirl Flows,Abacus Press (1984).

Hedman, P. O., Sturgess, G. J., Warren, D. L., Goss, L.P., and Shouse, D. T., "Observations of FlameBehavior from a Practical Fuel Injector UsingGaseous Fuel in a Technology Combustor," Journal

8

of Engineering for Gas Turbines and Power, Vol.117, pp 441-452 (July 1995).

Hedman, P. O., and Warren, D. L., "Turbulent Velocityand Temperature Measurements from a Gas-FueledTechnology Combustor with a Practical FuelInjector," Combustion and Flame, Vol. 100, pp 185-192 (1995).

Hedman, P. O, Smoot, L. D., Brewster, B. S. andFletcher, T. H., “Final Report-Combustion Modelingin Advanced Gas Turbine Systems,“ AdvancedCombustion Engineering Research Center, BrighamYoung University, Provo, Utah 84602, U.S.Department of Energy, Morgantown EnergyTechnology Center, Cooperative Agreement No. DE-FC21-92MC29061, Subcontract No. 93-01-SR014,Clemson University Research Foundation SouthCarolina Energy Research and Development Center,Clemson, South Carolina 29634-5702 (February 28,1998).

Murray, R. L., “Laser Doppler AnemometryMeasurements in a Turbulent, Premixed NaturalGas/Air Combustor,” M.S. Thesis, ChemicalEngineering Department, Brigham Young University,Provo, Utah 84602 (April 1998).

Roquemore, W. M., Personal Communication, Wright-Patterson Air Force Base, Dayton, Ohio (1992).

Sturgess, G. J., Sloan, D. G., Lesmerises, A. L.,Henneghan, S.P., and Ballal, D.R., "Design andDevelopment of a Research Combustor for LeanBlowout Studies", ASME Journal of Engineering forGas Turbines and Power, Vol. 114, pp 13-19 (1992)35th International Gas Turbine and AeroengineCongress and Exposition, Brussels, Belgium (June1990).

Warren, D. L., and Hedman, P. O., "Differential Massand Energy Balances in the Flame Zone from aPractical Fuel Injector in a Technology Combustor,"Journal of Engineering for Gas Turbines and Power,Vol. 119, pp 352-361 (April 1997).

Figure 1. Flow patterns in a confined, bluff-base, premixed,swirl-stabilized combustor (adapted from Gupta, et al., 1984).

QuartzWindow

490 mm

1.54 m

240 mm

540 mm

200 mmFuel/airMixtureInlet

Fuel/airTubeInsideBurner

Combustion Chamber

Support Base

Premixed Injecto r

152 mm

Figure 2. Schematic of premixed combustor (adapted fromSchmidt and Hedman, 1995).

9

Figure 3. Schematic of premixed injector and the mediumand high swirl block inserts (adapted from Schmidt andHedman, 1995).

C-030B-105A-062 D-069

(C) Test Series 3, MS Injector, = 0.65

C-247B-113A-058 D-196(D) Test Series 4, HS Injector, = 0.65

C-221B-101A-001 D-189

(A) Test Series 1, MS Injector, = 0.80

C-207B-126A-008 D-072

(B) Test Series 2, HS Injector, = 0.80

Figure 4. Example instantaneous OH PLIF images at eachtest condition. A: near minimum image; B: near averageimage; C: near maximum image; D: image with odd structure.

200

150

100

50Axi

al P

ositi

on, m

m

200

150

100

50Axi

al P

ositi

on, m

m

Figure 5. Example of mean PLIF images of OH in the ATSburner (premixed natural gas/air, air flow = 500 slpm). Left:high swirl, φ = 0.80; Right: medium swirl, φ = 0.65.

10

Z = 50 mmR = 00 mmPk Cnts = 4633Avg Cnts = 1652

b

c d

e

g h

a

15

10

5

01 20

Z = 70 mmR = 00 mmPk Cnts = 2119Avg Cnts = 740

20

(c)

1000 Detector Counts

12

8

4

0 1 2 301000 Detector Counts

16

4

(g)

16

12

8

4

01 20

Z = 60 mmR = 00 mmPk Cnts = 2489Avg Cnts = 1032

(e)

1000 Detector Counts

0.40 0.2 0.6

10

0

20Z = 80 mmR = 00 mmPk Cnts = 600Avg Cnts = 181

(a)

1000 Detector Counts

80

40

0 10

1000 Detector Counts

Z = 50 mmR = 24 mmPk Cnts = 2908Avg Cnts = 330

2

(h)

20

10

01 20

Z = 60 mmR = 24 mmPk Cnts = 2860Avg Cnts = 646

(f)

1000 Detector Counts

1 2 30

Z = 70 mmR = 24 mmPk Cnts = 3232Avg Cnts = 1105

12

8

4

0

16

(d)

1000 Detector Counts

15

10

5

0

Z = 80 mmR = 24 mmPk Cnts = 932Avg Cnts = 327

0 0.40.2 0.6 0.8

(b)

1000 Detector Counts

Figure 6. Example PDFs from OH-PLIF images for the high swirl, φ = 0.80 case.

Figure 7. Example mean iso-axial velocity contours from combined axial/tangential data sets.

11

Figure 8. Example of axial velocity PDF's across flame shear layer for the high swirl, φ = 0.80 case.

Figure 9. Details of CARS Spectroscopy.

z =50mmr =18mmT =1097KSD =282KTurb=25.7%

200016001200800

(h)

z =70mmr =30mmT =1317KSD =293KTurb=22.2%

20001600

1200800

(d)

z =50mmr =06mmT =1341KSD =255KTurb=19.0%

200016001200800

(i)

200016001200800

z =50mmr =30mmT =1063KSD =281KTurb=26.4%

(g)

z =70mmr =06mmT =1578KSD =127KTurb=8.04%

(f)

180014001000

z =70mmr =18mmT =1532KSD =183KTurb=12.0%

(e)

z =90mmr =06mmT =1606KSD =91.2KTurb=5.68%

(c)z =90mmr =18mmT =1626KSD =98.0KTurb=6.03%

(b)

z =90mmr =30mmT =1564KSD =164KTurb=10.5%

(a)

RadialLocation,mm0 20 40 60

250

200

150

100

50

MeanTemperature,K500 700 900 1100 1300 1500

30 302020 10 10

90

70

5030 0618

Tmax1636.3K

Tmin401.85K