[American Institute of Aeronautics and Astronautics 42nd AIAA Aerospace Sciences Meeting and Exhibit...

American Institute of Aeronautics and Space1

Detection of the Onset of Unstable Combustion in Lean Premixed Combustors

Byeong-Jun Lee\

School of Mechanical Engineering, Yeungnam University, Kyongsan, Kyongbuk, Korea

Jong Guen Lee1 and D. A. Santavicca2

Department of Mechanical and Nuclear Engineering Pennsylvania State University, University Park, PA

Abstract

Results are presented from an experimental studyconducted in a laboratory-scale lean premixedcombustor of the transition from stable to unstablecombustion. The transition from stable to unstablecombustion is initiated by a small change inequivalence ratio when operating near a stabilityboundary. Measurements of the pressure in thecombustor, pressure in the mixing section and CHchemiluminescence emission from the entire flameare made as a function of time prior to, during andimmediately after the transition. Cross-correlationsbetween the three possible combinations of theseparameters are then calculated as a function oftime. It is found that the cross-correlations showevidence of the onset of unstable combustionsignificantly before the transition to a limit cycleinstability occurs.

Introduction

Unstable combustion is the result of theclosed-loop coupling between unsteady heatrelease and acoustics. If the heat release andpressure fluctuations are properly phased, theamplitude of the fluctuations increases linearly,until the system becomes saturated and operatesin a limit cycle. There has been considerableresearch on unstable combustion in lean premixedcombustors, and as a result our understanding ofunstable combustion has been significantlyimproved over the past 10 years1-11. Thisunderstanding has provided valuable insights forthe development of both passive12-14 and active15-

22 control strategies for suppressing unstable

combustion. Passive control strategies includechanging the convection time lag, changing thedynamic response of the fuel system and the useof acoustic dampers. The advantage of passivecontrol strategies is that they are relatively simple,while the disadvantage is that their effectiveness istypically limited to a narrow operating range.Active control, although typically more complexthan passive control, has the potential forsuppressing unstable combustion over the entireoperating range of the combustor. The mostcommon active control strategies employ eitherprimary or secondary fuel flow modulation, wherethe objective is to produce out-of-phasemodulation of the heat release that suppresses theinstability. An important practical consideration isminimizing the amount of fuel flow modulation thatis required, since this reduces the NOx penalty andrequires smaller, less expensive actuators. Oneapproach for reducing the amount of fuel flowmodulation required for effective control whenusing secondary fuel flow modulation is tooptimize the secondary fuel injection location22.Another approach is to detect the onset ofunstable combustion and initiate control beforeachieving limit cycle behavior. Most studies ofunstable combustion, however, have focused onthe limit cycle behavior of unstable combustorsand there have been very few studies of the onsetor initiation of unstable combustion23. Theobjective of the research presented in this paper isto experimentally investigate the transition fromstable to unstable combustion for the purpose ofidentifying parameters that can be used as anindicator of the onset of unstable combustion.

Experiment

This study of the onset of unstablecombustion was conducted in the laboratory-scalelean premixed combustor shown schematically in

\ Visiting Scholar1 Senior Research Associate2 Professor

42nd AIAA Aerospace Sciences Meeting and Exhibit5 - 8 January 2004, Reno, Nevada

AIAA 2004-457

Copyright © 2004 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.


Fig. 1. Air enters the combustor through a chokedinlet and then flows through an annular mixingsection before entering an optically-accessibledump combustor. Inside the mixing section, the airflows through swirl vanes, angled at 30 degrees,downstream of which natural gas is injected fromsixteen 0.635 mm diameter holes equally spacedaround the centerbody. The fuel injection holes,which are choked at all of the conditions tested,are located 25 mm upstream of the dump plane,resulting in a partially premixed fuel distribution atthe entrance to the combustor section22. Theupstream end of the combustion chamber is afused silica tube that is 275 mm in length and 110mm in diameter. The downstream end of thecombustion chamber is a stainless steel tube thatis 725 mm in length and 78 mm in diameter. Thepartially-restricted exit of the combustor (19 mm indiameter) provides an acoustically-closedboundary condition. Air flow to the combustor iselectrically heated, and for this study the inlet airtemperature is set to 350ºC. The combustorpressure is nominally 1 atm for all conditionstested. Although the fuel and air flows are choked,this system is still susceptible to equivalence ratiofluctuations because of the fact that the air flowrate at the location of fuel injection is fluctuating intime.

Pressure fluctuations are measured usinghigh frequency response piezoelectric pressuretransducers (PCB model P112A) mounted in thecombustor dump plane and in the mixing sectionat a location 100 mm upstream of the dump plane.(It would be preferable to measure the pressure inthe mixing section at the location where the fuel isinjected, but that was not possible in this case.)

The chemiluminescence emission from the entireflame is detected using a photomultiplier tube. Aninterference filter, centered at 430 nm with a 5 nmbandwidth (FWHM), selectively transmits the CHchemiluminecscence and the correspondingportion of the broadband CO2

chemiluminescence24. Global chemiluminescenceemission has been extensively used as anindication of the overall rate of heat release in bothstable and unstable lean premixed flames24-28.This, however, must be done with caution for tworeasons. One is that the globalchemiluminescence emission depends not only onthe overall rate of heat release, but also on theequivalence ratio24,28. The second reason is thatthe geometry of an unstable flame is significantlydifferent from that of a stable flame at similar flowconditions, making it impossible to apply stableflame calibration data to unstable flamemeasurements. In this study, it is not necessarythat the chemiluminescence measurement be anaccurate indicator of the flame’s overall heatrelease. All that is necessary is that thechemiluminescence be related to the periodicvariations in the combustion process caused bythe instability. In order to capture the highfrequency oscillations associated with unstablecombustion, the pressure and chemiluminescencemeasurements are recorded at a samplingfrequency of 10 KHz.

Results and discussion

The first step was to determine the stableand unstable operating regimes of the combustor.

Choked inlet

Inlet temperature TC

Dump plane PT

Fused silica section combustor(110 mm dia. x 275 mm)

Restricted exit (dia.=19mm) Stainless steel section combustor

(78 mm dia. X 725 mm)

Swirl vanesFuel injection location (16 holes

0.635 mm in diameter)

Mixer PT

Figure 1. Schematic drawing of the optically accessible combustor


This was done by conducting steady stateexperiments over a range of equivalence ratiosfrom 0.5 to 0.9 (in increments of 0.025), a range ofcombustor (unburned) gas velocities (4, 5.5 and 7m/s), with fixed inlet temperature of 350 ºC andcombustor pressure of 1 atm. Once the stabilityboundaries were determined, unsteadyexperiments were conducted at selected operatingconditions. These tests involved operating thecombustor on the stable side of the boundary, andthen slowly increasing (or decreasing) theequivalence ratio until a sudden transition fromstable to unstable combustion was observed, asevidenced by a marked increase in the amplitudeof the pressure fluctuations. The pressure andchemiluminescence measurements were madebefore, during and after the transition from stableto unstable combustion.

Steady state measurements

The stability maps showing the stable andunstable operating regimes for combustor inletvelocities of 4.0, 5.5 and 7.0 m/s are presented inFig. 2, which is a plot of the rms pressurefluctuation measured in the combustor at thedump plane versus equivalence ratio. For eachvelocity, two curves are shown. The curves withthe closed symbols correspond to cases when theequivalence ratio is increased, while the curveswith the open symbols correspond to cases whenthe equivalence ratio is decreased. There isclearly a hysteresis effect, which delays thestability transition as the equivalence ratio is bothincreased and decreased. The hysteresis effect ismost pronounced for a combustor inlet velocity of4.0 m/s, and is only slightly evident at 7.0 m/s.Qualitatively the results are similar for all threecombustor inlet velocities, i.e., instabilities areobserved at low equivalence ratios and at highequivalence ratios, with a region of stablecombustion in between. The rms amplitude of thepressure fluctuations in the unstable regimesincreases with increasing combustor inlet velocityand with increasing equivalence ratio.

The frequency spectra of the pressuremeasurements made in the combustion chamberand in the mixing section, and of the CH*chemiluminescence measurement as a function ofequivalence ratio are show in Figures 3a, 3b and3c, respectively. The gray-scale to the right of thefigure indicates the magnitude of the frequencycomponent in dB. For these measurements, thecombustor inlet velocity was 5.5 m/s and theequivalence ratio was decreased. In the center of

each of the frequency spectra is a black regionfrom an equivalence ratio of ~0.62 to ~0.74, whichcorresponds to the stable operating regime; while,unstable regimes are evident at higher and lowerequivalence ratios. The strongest frequencycomponents, for the pressure and thechemiluminescence fluctuations, are in the rangeof ~330 Hz to ~390 Hz, where the frequencyincreases with increasing equivalence ratio. This

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Figure 2. Stability map as a function ofvelocity, equivalence ratio, φ and direction ofchanging φ. (triangle:4.0 m/s, square:5.5,circle:7.0m/s; filled symbols: increasing φ,open symbols: decreasing φ)

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Figure 3. Frequency components of thepressure and chemiluminescence intensityfor 5.5 m/s and decreasing φ case (a)pressure at dump plane, (b) pressure atmixing section and (c) chemiluminescenceintensity


corresponds to the first longitudinal mode of thecombustor, and the increasing frequency can beattributed to increasing speed of sound due toincreasing average combustor temperature withincreasing equivalence ratio. Frequencycomponents are also observed at the higherharmonics, i.e., as high as the 4th harmonic, wherethe intensity decreases with increasing order.

Figure 4 shows the peak-to-peakfluctuation in the combustion chamber pressure,the peak-to-peak fluctuation in the mixing sectionpressure and the peak-to-peak fluctuation of thechemiluminescence intensity as a function ofequivalence ratio, for the combustor inlet velocityof 5.5 m/s and increasing equivalence ratio case.Also shown in this figure is the time-averagedmean value of the chemiluminescence intensity asa function of equivalence ratio. As mentionedpreviously, the global chemiluminecence emissiondepends on both the rate of heat release and theequivalence ratio. This is evidenced by thenonlinear increase in the meanchemiluminescence emission with increasingequivalence ratio shown in Fig. 4. Also, note thatthere is an increase in the amplitude of thefluctuations between the low equivalence ratioinstability regime and the high equivalence ratioinstability regime and that the increase issubstantially greater for the combustor pressurefluctuation and the chemiluminescence fluctuationthan it is for the mixing section pressurefluctuation.

In addition to characterizing unstablecombustion in terms of the amplitude of the

pressure fluctuation or the amplitude of the heatrelease fluctuation, the correlation between thepressure and heat release fluctuations, which is ameasure of the net gain and damping that resultsfrom the coupling between these quantities, canbe used to characterize the instability. Thiscorrelation is referred to as the Rayleigh Index(R.I.) and is given by the following equation:

where t is time, T is the period of the instability, P'is the pressure fluctuation and q' is the overall heatrelease fluctuation. As noted previously, it iscommon practice to use chemiluminescenceemission as an indicator of the rate of heatrelease, however, there are a number ofuncertainties in doing so, particularly in unstableflames. Therefore, for the purposes of this study,we define a Pseudo-Rayleigh Index (P.R.I.), wherethe rate of heat release is replaced with thechemiluminescence emission in the aboveequation. In this case it is essential that thepressure and chemiluminescence measurementsare made simultaneously. The Pseudo-RayleighIndex as a function of equivalence ratio, for the 5.5m/s combustor inlet velocity and increasingequivalence ratio case, is shown in Fig. 5. Asexpected, the Pseudo-Rayleigh Index is effectivelyzero over the range of equivalence ratios wherecombustion is stable, and is greater than zero atthose equivalence ratios where unstablecombustion was observed. In the two unstableregimes, it is observed that the magnitude of the

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Figure 4. Effects of φ on the mean and peak-to-peak values of the chemiluminescenceintensity (Q), pressure at dump plane (P'dump)and pressure in mixing section (P'mixer) foru=5.5 m/sec and increasing φ case.

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Figure 5. Cross correlation coefficient andPseudo-Rayleigh index as a function of φ foru=5.5m/s and increasing φ cases

∫∫== T0 dt'q'P

T1

.I.R


Pseudo-Rayleigh Index is greater at higherequivalence ratios than it is at lower equivalenceratios, as is consistent with the fact that theamplitude of pressure fluctuations are larger forthe high equivalence ratio instabilities.

As discussed previously, the pressurefluctuations in the mixing section also play animportant role in the instability. Therefore, it isreasonable to expect that the cross-correlationbetween the mixing section pressure fluctuationsand the combustor pressure fluctuations, as wellas, between the mixing section pressurefluctuations and the chemiluminescence intensityfluctuations, would be indicative of the strength ofthe process that is driving the instability. Thegeneral relation for the cross-correlation betweentwo fluctuation parameters is

where Cij is the cross-correlation coefficientbetween parameter “i” and parameter “j” at anygiven time to, w'i is the fluctuation in “i”, w'j is thefluctuation in “j”, w'i,rms is the rms fluctuation inparameter “i”, w'j,rms is the rms fluctuation in “j”, Tis the period of the instability and t is time. Thecross-correlations between the mixing sectionpressure fluctuation, the combustor pressurefluctuation and the chemiluminescence intensityfluctuation are plotted in Fig. 5, along with thePseudo-Rayleigh Index. As shown, there are threepossible cross-correlations between the threeparameters, and all three cross-correlationsprovide a definitive indication of unstablecombustion. Note that the cross-correlationbetween the combustor pressure fluctuation andthe chemiluminescence fluctuation is simply thePsuedo-Rayleigh Index divided by thecorresponding rms fluctuations. The differencebetween the Pseudo-Rayleigh Index (P.R.I.) andthe cross-correlation between the combustorpressure and the chemiluminescence (Cdump,Q) isthat the P.R.I. depends on both the amplitude andthe relative phase of the fluctuations, while theCdump,Q depends primarily on the relative phase ofthe fluctuations.

Transient State Measurements

Based on the stable and unstable regimesshown in Fig. 2, the three operating conditions

listed in Table 1 were chosen for the transientstate measurements.

Case Velocity(m/s)

Equivalence ratio(increase or decrease)

1 5.5 0.625 (dec.)

2 5.5 0.82 (inc.)

3 7.0 0.85 (inc.)

Table 1. Operating conditions for the transient state measurements

Figure 6a is a plot of the simultaneouscombustor pressure, mixing section pressure andchemiluminescence intensity measurementsversus time for the case of a combustor inletvelocity of 5.5 m/s at an equivalence ratio of 0.625as the equivalence ratio is decreased. (Note that t= 0 has no significance.) As indicated by thecombustor and mixing section pressures, thetransition from stable operation to a limit cycleinstability begins at ~350 msec and occurs over aperiod of ~25 msec, which corresponds to ~9cycles of the instability. Figure 6b shows thePseudo-Rayleigh Index plotted versus time, whichwas calculated from the combustor pressure andthe chemiluminescence intensity shown in Fig. 6a.The Pseudo-Rayleigh Index is effectively zero until~350 msec, when it begins to increase. Thiscoincides very closely with the time when thepressure fluctuations begin to increase. Similarly,the Pseudo-Rayleigh Index and the pressurefluctuations reach their maximum value atapproximately the same time, i.e., ~375 msec.Fig. 6c shows the three correlations versus time,which were calculated from the combustorpressure, mixing section pressure andchemiluminescence intensity results shown in Fig.6a. All three correlations show fluctuations aroundzero until ~330 msec, at which time the combustorpressure – chemiluminescence cross-correlation(CPd,q) and the combustor pressure – mixingsection pressure cross-correlation (CPd,Pm) begin toincrease and the mixing section pressure –chemiluminescence cross-correlation (Cq,Pm)begins to decrease. CPd,q and CPd,Pm appear toreach their maximum at ~360 msec, while Cq,Pm

does not reach its minimum until ~400 msec.Therefore the cross-correlations between thecombustor pressure and the chemiluminescenceand between the combustor pressure and themixing section pressure provide an indication ofthe onset of unstable combustion by ~ 20 msec

rms,jrms,i

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Figure 6. (a) Chemiluminescence intensity (Q), pressure at dump plane (P'dump) andpressure in mixing section (P'mixer) for u=5.5 m/s and φ=0.625 case and (b) Pseudo-Rayleigh index and cross correlations. (Pd and Pm represent dump plane pressureand mixing section pressure, respectively)

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Figure 7. (a) Chemiluminescence intensity (Q), pressure at dump plane (P'dump) andpressure in mixing section (P'mixer) for u=5.5 m/s and φ=0.82 case and (b) Pseudo-Rayleigh index and cross correlations. (Pd and Pm represent dump plane pressure andmixing section pressure, respectively)


(~7 cycles) before the pressure fluctuations beginto increase and by ~45 msec (~ 16 cycles) beforethe limit cycle is established.

Figure 7a is a plot of the simultaneouscombustor pressure, mixing section pressure andchemiluminescence intensity measurementsversus time for the case of a combustor inletvelocity of 5.5 m/s at an equivalence ratio of 0.82as the equivalence ratio is increased. As indicatedby the combustor and mixing section pressuresand the chemiluminescence intensity, thetransition from stable operation to a limit cycleinstability begins at ~440 msec and occurs over aperiod of ~50 msec, which corresponds to ~20cycles of the instability. Figure 7b shows thePseudo-Rayleigh Index plotted versus time, whichwas calculated from the combustor pressure andthe chemiluminescence intensity shown in Fig. 7a.The Pseudo-Rayleigh Index is effectively zero untilapproximately 440 msec, when it begins toincrease. This coincides very closely with the timewhen the pressure fluctuations begin to increase,as shown in Fig. 7a. Similarly, the Pseudo-Rayleigh Index and the pressure fluctuationsreach their maximum value at approximately thesame time, i.e., ~490 msec. Fig. 7c shows thethree correlations versus time, which were

calculated from the combustor pressure, mixingsection pressure and chemiluminescence intensityresults shown in Fig. 7a. The combustor pressure– chemiluminescence cross-correlation (CPd,q)fluctuates around zero for ~100 msec, fluctuatesjust above zero for the next 300 msec. Then, itbegins to increase at ~430 msec and reaches itsmaximum at ~480 msec. The combustor pressure– mixing section pressure cross-correlation(CPd,Pm) fluctuates above zero for ~300 msec, andthen begins to slowly increase over the next ~160msec, reaching its maximum at ~460 msec. Themixing section pressure – chemiluminescencecross-correlation (Cq,Pm) fluctuates about zero for~150 msec, and then begins to decrease for ~75msec, when it abruptly comes back to zero andthen begins to decrease again at ~260 msec,reaching its minimum value at ~340 msec. Againthe cross-correlations, particularly those involvingthe mixing section pressure fluctuation, provide anindication of the onset of unstable combustion wellbefore the pressure fluctuations actually begin toincrease. In the case of Cq,Pm, the pre-indicationwas ~150 msec (~60 cycles) in advance of theactual instability.

Figure 8a is a plot of the simultaneouscombustor pressure, mixing section pressure and

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Figure 8. (a) Chemiluminescence intensity (Q), pressure at dump plane (P'dump) and pressurein mixing section (P'mixer) for u=7.0 m/s and φ=0.85 case and (b) Pseudo-Rayleigh index andcross correlations. (Pd and Pm represent dump plane pressure and mixing section pressure,respectively)


chemiluminescence intensity measurementsversus time for the case of a combustor inletvelocity of 7.0 m/s at an equivalence ratio of 0.85as the equivalence ratio is increased. As indicatedby the combustor and mixing section pressuresand the chemiluminescence intensity, thetransition from stable operation to a limit cycleinstability begins at ~440 msec and occurs over aperiod of ~60 msec, which corresponds to ~20cycles of the instability. Figure 8b shows thePseudo-Rayleigh Index plotted versus time, whichwas calculated from the combustor pressure andthe chemiluminescence intensity shown in Fig. 8a.The Pseudo-Rayleigh Index is effectively zero until~460 msec, when it begins to increase. Thiscoincides very closely with the time when thepressure fluctuations begin to increase, as shownin Fig. 8a. Similarly, the Pseudo- Rayleigh Indexand the pressure fluctuations reach their maximumvalue at approximately the same time, i.e., ~500msec. Fig. 8c shows the three correlations versustime, which were calculated from the combustorpressure, mixing section pressure andchemiluminescence intensity results shown in Fig.8a. The combustor pressure – chemiluminescencecross-correlation (CPd,q)fluctuates around zero for~200 msec, fluctuates just below zero for the next230 msec and then begins to increase at ~430msec, reaching its maximum at ~490msec. Thecombustor pressure – mixing section pressurecross-correlation (CPd,Pm) fluctuates above zero for~100 msec, and then begins to slowly increaseover the next ~350 msec, reaching its maximum at~450 msec. The mixing section pressure –chemiluminescence cross-correlation (Cq,Pm)fluctuates below zero for ~150 msec, and thenbegins to slowly decrease until reaching itsminimum value at ~450 msec. Again the cross-correlations, particularly those involving the mixingsection pressure fluctuation, provide an indicationof the onset of unstable combustion well beforethe pressure fluctuations actually begin toincrease. In the case of Cq,Pm, the pre-indicationwas ~250 msec (~90 cycles) in advance of theactual instability.

Conclusions

An experimental study of the transitionfrom stable to unstable combustion in a laboratory-scale lean premixed combustor has been carriedout in order to identify parameters that provide apre-indication of the occurrence of unstablecombustion. Simultaneous measurements of themixing section pressure fluctuations, combustion

chamber pressure fluctuations and CHchemiluminescence fluctuations were made as afunction of time before, during and after atransition from stable to unstable combustion.Cross-correlations among these three parameterswere calculated as a function of time. It is foundthat the cross-correlations, particularly thoseinvolving the mixing section pressure fluctuations,show a pronounced increase before the pressurefluctuations are observed to increase, suggestingthat such measurements could be used to sensethe onset of unstable combustion. A possibleexplanation for this is that before the fluctuationsincrease in magnitude they become phase-synchronized, which causes the cross-correlationsto increase.

Acknowledgements

Support for this research was provided by theAdvanced Gas Turbine Research Program of theDepartment of Energy under Subcontract No. 01-01-SR090. B.J. Lee was supported by CombustionEngineering Research Center in Korea.

References

1 Poinsot, T., Trouve, A., Veynante, D., Candel, S.,Sspositi, E., "Vortex-Driven Acoustically CoupledCombustion Instabilities," J. Fluid Mech., Vol. 177,pp.265-292, 1987.2 S. Sivasegaram and J.H. Whitelaw, "Oscillationsin Axisymmetric Dump Combustors," CombustionScience & Technology, Vol. 52, 1987, pp. 413-426.3 K.C. Schadow, E. Gutmark, T.P. Parr, D.M.Parr, K.J. Wilson and J.E. Crump, "Large-ScaleCoherent Structures as Drivers of CombustionInstability," Combustion Science & Technology,Vol. 64, 1989, pp. 167-186.4 Keller, J.J., “Thermoacoustic Oscillations inCombustion Chambers of Gas Turbines,” AIAAJournal, Vol. 33, No. 12, pp. 2280-2287, 1995.5 Dowling, A.P., “The Calculation ofThermoacoustic Oscillations, Journal of Soundand Vibration, Vol. 180, No. 4, pp. 557-581, 1995.6 Shih, W-P, Lee, J.G. and Santavicca, D.A.,"Stability and Emissions Characteristics of a LeanPremixed Gas Turbine Combustor," Proceedingsof the Combustion Institute, Volume 26, 1996.7 Richards, G.A. and Janus, M.C.,“Characterization of Oscillations During PremixedGas Turbine Combustion,” ASME Journal ofEngineering for Gas Turbines and Power, Vol.120, No. 2, pp. 294-302, 1998.


8 Lieuwen, T. and Zinn, B.T., "The Role ofEquivalence Ratio Oscillations in DrivingCombustion Instabilities in Low NOx GasTurbines," Proceedings of the CombustionInstitute, The Combustion Institute, Vol. 27, pp.1809-1816, 1998.9 A. A. Peracchio and W. M. Proscia, "NonlinearHeat-Release/Acoustic Model for ThermoacousticInstability in Lean Premixed Combustors,"International Gas Turbine & Aeroengine Congress& Exhibition, 98-GT-269, 1998.10 Venkataraman, K.K., Preston, L.H., Simons,D.W., Lee, B.J., Lee, J.G. and Santavicca, D.A.,"Mechanism of Combustion Instability in a LeanPremixed Dump Combustor," Journal ofPropulsion and Power, Vol.15, No. 6, pp. 909-9181999.11 Lee, J.G., Kim, K. and Santavicca, D.A.,"Measurement of Equivalence Ratio Fluctuationand Its Effect in Heat Release During UnstableCombustion," Proceedings of the CombustionInstitute, Volume 28, 2000, pp. 415-421.12 Langhorne, P.J., Dowling, A.P. and Hooper, N.,“Practical Active Control System for CombustionOscillations,” Journal of Propulsion, Vol. 6, pp.324-333, 1990.13 D. L. Straub and G. A. Richards, "Effects of FuelNozzle Configuration on Premix CombustionDynamics," International Gas Turbine &Aeroengine Congress & Exhibition, 98-GT-492,1998.14 R.C. Steele, L.H. Cowell, S.M. Cannon andC.E. Smith, "Passive Control of CombustionInstability in Lean Premixed Combustors,"International Gas Turbine & Aeroengine Congress& Exhibition, 99-GT-003, 1999.15 Schadow, K.C., Gutmark, E. and Wilson, K.J.,“Active Combustion Control in a Coaxial DumpCombustor,” Combustion Science andTechnology, Vol. 81, pp. 285-300, 1992.16 Neumeier, Y. and Zinn, B.T., “ExperimentalDemonstration of Active Control of CombustionInstabilities Using Real-Time Modes Observationand Secondary Fuel Injection,” Proceedings of theTwenty-Sixth Symposium (International) onCombustion, The Combustion Institute, pp. 2811-2818, 1996.17 Yu, K. H., Wilson, K. J. and Schadow, K. C.,“Liquid-Fueled Active Instability Suppression,”Proceedings of the Twenty-seventh Symposium(International) on Combustion, The CombustionInstitute, pp. 2039-2046, 1998.18 Jones, C. M., Lee, J. G. and Santavicca, D. A.,“Closed-loop Active Control of CombustionInstabilities Using Subharmonic Secondary Fuel

Injection,” Journal of Propulsion and Power, Vol.15, pp. 584-590, 1999.19 Lee, J. G., Kim, K. and Santavicca, D. A., “Effectof Injection Location on the Effectiveness of anActive Control System Using Secondary FuelInjection,” Proceedings of the Twenty-EighthSymposium (International) on Combustion, TheCombustion Institute, pp. 739-746, 2000.20 Seume, J. R., Vortmeyer, N., Krause, W.,Hermann, J., Hantschk, C. -C., Zangl, P., Gleis,S., Vortmeyer, D. and Orthmann, A., “Applicationof Active Combustion Control to a Heavy Duty GasTurbine,” ASME Journal of Engineering for GasTurbines and Power, Vol. 120, No. 4, pp. 721-726,1998.21 Hibshman, J. R., Cohen, J. M., Banaszuk, A.,Anderson, T. A. and Alholm, “Active Control ofCombustion Instability in a Liquid-Fueled SectorCombustor,” ASME Paper 99-GT-215, 1999.22 Kim, K., Lee, J.G. and Santavicca, D. A.,"Optimization of the Spatial and TemporalDistribution of Secondary Fuel Injection forEffective Suppression of Combustion Dynamics,"AIAA 2002-4024, Presented at 38th AIAA/ASME/SAE/ASEE Joint Propulsion Conference &Exhibit,7-10 July 2002.23 Poinsot, T., Veynante, D., Bourienne, F.,Candel, S., Esposito, E. and Surget, J., "Initiationand Suppression of Combustion Instabilities byActive Control," Proceedings of the Twenty-Second Symposium (International) onCombustion, The Combustion Institute, pp. 1363-1370, 1988.24 Samaniego, J.M., Egolfopoulos, F.N., andBowman, C.T., "CO2* Chemiluminescence inPremixed Flames," Combustion Science andTechnology, Vol. 109, pp.183-203, 1995.25 Dandy, D. and Vosen, S., “Numerical andExperimental Studies of Hydroxyl RadicalChemiluminescence in Methane-Air Flames,”Combustion Science and Technology, Vol. 82, pp.131-150, 1992.26 Najm, H.N., Paul P.H., Mueller, C.J., andWyckoff, P.S., "On the Adequacy of CertainExperimental Observables as Measurements ofFlame Burning Rate," Combustion and Flame, Vol.113, pp.312-332, 1998.27 Najm, H.M., Knio, O.M., Paul, P.H., Wyckoff,P.S., “A Study of Flame Observables in PremixedMethane-Air Flames,” Combustion Science andTechnology, Vol. 140, pp. 369-403, 1998.28 Lee, J.G. and D.A. Santavicca, "ExperimentalDiagnostics for the Study of CombustionInstabilities in Lean Premixed Combustors,"Journal of Propulsion and Power, Vol. 19, No. 5,pp. 735-749, 2003.

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