[American Institute of Aeronautics and Astronautics 38th AIAA/ASME/SAE/ASEE Joint Propulsion...

American Institute of Aeronautics and Astronautics1

OPTIMIZATION OF THE SPATIAL AND TEMPORAL DISTRIBUTION OFSECONDARY FUEL INJECTION FOR EFFECTIVE SUPPRESSION OF

COMBUSTION DYNAMICS

Kwanwoo Kim1, Jong Guen Lee2 and Domenic A. Santavicca3

Department of Mechanical and Nuclear EngineeringThe Pennsylvania State University

University Park, PA 16802

1 Ph. D. Student, Department of Mechanical and Nuclear Engineering, currently at GE Power Systems2 Research Associate, Department of Mechanical and Nuclear Engineering3 Professor, Department of Mechanical and Nuclear Engineering, AIAA Senior Member

Abstract

Results are presented from an experimental study of theeffect of the spatial and temporal distribution ofmodulated secondary fuel for suppressing unstablecombustion in a lean premixed combustor. Theexperiments were conducted in a laboratory scaleoptically accessible dump combustor operating onnatural gas. By varying the main fuel distribution, twounstable operating conditions with longitudinal modeinstabilities at frequencies near 360 Hz were achieved.Using sub-harmonic injection of secondary fuel thecontrol effectiveness for two different secondaryinjection locations was determined for each of theinstabilities. The instabilities were characterized usingphase-synchronized chemiluminescence imaging andhigh frequency response pressure measurements, fromwhich the Rayleigh Index distribution was calculated.Secondary fuel injection was characterized in terms ofthe spatial and temporal distribution of the secondaryfuel and the flame response function. From an analysisof these results it was shown that the Flame ResponseRayleigh Index could be used to predict the secondaryinjection phase delay required for maximum damping.It was also shown that the most effective control for agiven instability was achieved when the secondary fuelwas injected into a damping region in the RayleighIndex distribution for that instability.

Introduction

Suppression of combustion dynamics in gas turbinecombustors using active combustion control has beensuccessfully demonstrated in a number of laboratoryscale combustors1-5, as well as in a small number offull-scale combustors.6-8 The greatest success has beenachieved using closed-loop control with either main orsecondary fuel flow modulation. The rationale behind

this approach is that the modulated fuel flow producesmodulated heat release which is out-of-phase with theinstability driven oscillations, thereby increasing thedamping and suppressing the instability.

For practical applications it is important to reducethe fraction of the total fuel that is modulated, since thisreduces both the demands on the actuator used tomodulate the fuel flow as well as the NOx penalty dueto locally high equivalence ratios produced by themodulated fuel. Achieving control with the minimumamount of fuel modulation, however, requires a detailedunderstanding of the mechanism whereby control isachieved. As noted above, one mechanism forachieving control is to increase the damping by out-of-phase modulation of the heat release. In this case, onlythe timing of the heat release modulation, and not thelocation, is important. This is the approach used incurrent model-based controllers.9-12 It is also possible,however, for the modulated heat release to perturb themechanisms which are driving the instability, therebyreducing the thermo-acoustic gain and suppressing theinstability. In this case, both the timing and location ofthe heat release modulation can be important. Thissecond method of achieving control is likely to requireless fuel flow modulation, however, its implementationand optimization requires a more detailedunderstanding of the instability and the effect of themodulated fuel on the instability.Several studies have demonstrated the importance ofoptimizing the modulated fuel distribution bothspatially and temporally for effective control. Yu et al.2

found that the injection timing for optimal controlshould be synchronized with the vortex shedding at thedump plane to maximize fuel dispersion. While Cohenet al.13 showed that controlling the spatial mixing of themodulated fuel and the main fuel influenced the controlauthority and emissions. And Lee et al.4 demonstratedin a laboratory-scale dump combustor that secondaryinjection location can have a significant effect on

38th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit7-10 July 2002, Indianapolis, Indiana

AIAA 2002-4024

Copyright © 2002 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.


control effectiveness and that the optimum locationdepends on the nature of the instability. These studiesdemonstrate the importance of optimizing themodulated fuel distribution, both spatially andtemporally, but they do not explain why one spatialdistribution is more effective than another.

The objective of the work presented in this paper isto study in detail the effect of the modulated secondaryfuel, both temporally and spatially, on the instabilityand to determine the mechanism whereby control isachieved. The same cases that were reported in Lee etal.4 are studied in this paper.

Experimental Setup and Diagnostics

Combustor setupThe coaxial dump combustor illustrated in figure 1

was used in this study. The combustor consists of amixing section and a combustion chamber. The flame isstabilized on a bluff centerbody which is flush-mountedwith the dump plane separating the mixing section andcombustion chamber. The centerbody (diameter: 19mm) is mounted co-axially in the mixing section usingvanes which are positioned at an angle of 30º withrespect to the axis and which also serve as axialswirlers. At the inlet of the mixing section the flow ischoked so as to provide a well defined acousticboundary condition. The upstream end of thecombustion chamber is made from a 110 mm diameter(I. D.) fused-silica cylinder and the downstream end isfabricated from a 78 mm diameter (I. D.) stainless steeltube. The overall length of the combustion chamber is1385 mm, which corresponds to a nominal L/D of 12.The combustor exit is partially restricted so that anacoustically closed boundary condition at the exit canbe achieved.14 The fuel is natural gas (96 % methane),which is introduced either directly into the mixing

section, i.e., at a location 25 mm upstream of the dumpplane through 16 injection holes around the centerbody,or well upstream of the choked mixing section inlet.The former injection location results in a non-uniform,partially premixed fuel distribution, while the lattergives a uniform, premixed fuel-air distribution. The airis heated by a 30kW-electric heater and the temperatureof the mixture is monitored by a thermocouple, located40mm upstream of the dump plane, and regulated by atemperature controller to 350ºC.

Operating conditionsThe combustor was run over a wide range of

operating conditions which include combustor inlettemperature, bulk mean velocity, and equivalence ratio.Two unstable operating conditions, which are listed inTable 1, were chosen for this study. These specificconditions were chosen because the observed flamestructure evolution at these operating conditionsshowed distinctly different behavior.

Case I Case IIAverage velocity in combustor

5.25m/sec

Average velocityin mixing section

70m/sec

Inlet temperature 350°CMode of instability(frequency)

Longitudinal mode (360Hz)

Swirl number 0.46 (30° axial swirler)Premixedness of fueland air

Partiallypremixed

Fullypremixed

Amplitude of pressurefluctuations at peakfrequency

Prms/Pmean=5.8%

Prms/Pmean=3.3 %

Table 1. Operating conditions for two unstable cases.

Upstream chokeMixer PT

Inlet temperature TC

Flange PT

Fused silica section combustor(110 mm dia. x 255 mm)

Restricted (dia.=19mm)Stainless steel section combustor (78 mm dia. X 1130 mm)

Downstream PT’s

Swirl vanesMain fuel injection location

Figure 1. Schematic drawing of optically accessible dump combustor


DiagnosticsThe dynamic pressure in the combustor was

measured using a water-cooled piezoelectric pressuretransducer mounted in the dump plane. This signal wasalso used as reference for the active control system.

Phase-resolved CO2* chemiluminescence imagingwas used to characterize the spatial and temporalevolution of the flame’s heat release during unstablecombustion. Chemiluminescence flame emission iscommonly used as a means to record both local andglobal heat release.15-18 By using a PMT fitted with abroad band-pass filter (BG-40: 320-630 nm) andmounted at right angles to the axis of combustor, thechemiluminescence emission from the whole flame isdetected. To investigate the local heat release structure,an intensified CCD (ICCD) camera fitted with the sameBG-40 filter is used. To characterize the unstable flamestructure, fifteen phase-averaged images, eachseparated by 24 phase-angle degrees, are taken. (Eachof the phase-averaged images is an average of 30instantaneous images.) The exposure time of the ICCDcamera is set at 50µsec, corresponding to less than 2%of one instability period. A tomographic deconvolutionprocedure19,20 is used to extract the two-dimensionalheat release structure from the original line-of-sightchemiluminescence images.

Laser induced acetone fluorescence is used toquantify the distribution of secondary fuel at thecombustor inlet by adding a small amount of acetone tothe fuel stream to serve as a fluorescence tracer. The266nm output of a frequency-quadrupled pulsedNd:YAG laser is used as the excitation source and theresulting fluorescence is recorded using the same ICCDcamera used for the chemiluminescence imaging,however, the BG-40 filter is replaced with a band-passfilter (BG-12) and high-pass filter (WG305)combination.21 The measurements were phase-synchronized with the fuel injection cycle, giving asequence of images showing the spatial and temporal

distribution of injected secondary fuel at the combustorinlet during one injection cycle.

Control systemActive control was achieved using modulated

secondary fuel injection on every fourth cycle of theunstable flame, i.e., sub-harmonic injection. Theinjection of secondary fuel is phase-synchronized withthe pressure oscillation in the combustor and the phase-delay relative to the pressure oscillation is varied toachieve maximum suppression of the instability. Apressure transducer mounted in the dump plane is usedas the sensor for the control system. The amplifiedoutput of the pressure transducer is fed into a controlcircuit which determines the negative-to-positive zero-crossing and generates a corresponding reference pulse.The reference pulse is fed into a valve driver (GeneralValve Co., Iota One) where the phase-delay betweenthe pressure signal and the trigger input to thesecondary fuel injection solenoid valve (General ValveCo., Series 9) and the injection duration are set. Theinjection pressure for the secondary fuel, measuredupstream of the solenoid valve, is 150 psig. The amountof secondary fuel per injection can be varied bychanging either injection pressure or injection duration.In a preliminary study on the effect of both parameters,it was found that varying the pulse width results in awider range of fuel flow modulation.

Secondary fuel injectionAs shown in Fig. 2, secondary fuel can be injected

either through twelve injection holes located around theinner circumference of the dump plane (dump planeinjection) or through four injection holes located in theouter wall of the mixing section (upstream injection).Note that only one injection location is employed at atime. In the case of dump plane injection, secondaryfuel is injected directly into the combustor and isexpected to produce a secondary fuel distribution whichvaries both spatially and temporally. In the case of

Figure 2. Schematic drawing showing both main and secondary fuel injection locations (not to scale)

Upstream injection: 4 injection holes ( φφ=1.40 mm) 90o apart

Premixed main fuel

Partially-premixed main fuel

Dump plane injection: 12 injection holes ( φφ=0.34 mm) around the inner circumference of the dump plane


upstream injection, the secondary fuel is injected intothe mixing section approximately 110 mm upstream ofthe dump plane and is expected to produce a secondaryfuel distribution which is relatively uniform spatiallybut varies temporally.

Results

Measurement of control effectivenessThe control effectiveness versus phase-delay for

both instabilities, i.e., case I and II, with both secondaryinjection locations, i.e., dump plane and upstreaminjection, are presented in figure 3. For the purpose ofcomparison, control effectiveness is defined as thesuppression of the rms pressure fluctuation in dB usinga fixed amount of secondary fuel, i.e., a fixed injectionduration. Results are presented for three injectiondurations, i.e., 600, 1000 and 1200 microseconds. Forreference, injection durations of 600, 1000 and 1200microseconds correspond to secondary fuel flow rates

equal to 4.2%, 5.2% and 6.6%, respectively, of themain fuel flow rate.

For the case I instability, dump plane injectionresults in the most effective control with a 21 dBreduction at a phase delay of 1.0 msec, while upstreaminjection has virtually no effect. For the case IIinstability the results are reversed, i.e., upstreaminjection is more effective than dump plane injection.Upstream injection results in a 13 dB reduction at aphase delay of 2.5msec compared to a 1.6 dB reductionwith dump plane injection.

In order to explain this behavior, the detailedcharacteristics of the unstable flames and the secondaryfuel injection are presented in the following sections.

Characterization of main fuel distribution andunstable flame structure

For the case I instability, the main fuel is injecteddirectly into the mixing section through the holes in thecenterbody, resulting in a non-uniform fuel distribution.

phase delay [msec]

0 1 2 3 4

dB reduction -30

-25

-20

-15

-10

-5

0

5

10

p.w.=600 µsecp.w.=1000µsecp.w.=1200µsec

phase delay[msec]

0 1 2 3 4

dB reduction -30

-25

-20

-15

-10

-5

0

5

10

p.w.=600µsecp.w.=1000µsecp.w.=1200µsec

ph ase delay [msec]

0 1 2 3 4 5

dB reduction -40

-30

-20

-10

0

10


phase delay[msec]

0 1 2 3 4

dB reduction -40

-30

-20

-10

0

10

p.w .=600µsecp .w .=1000µsecp .w .=1200µsec

C ase I with d u m p p lane inject io n C a s e I w ith ups t ream in j ec t io n

C ase I I with d u m p p lan e in jec t io n C a s e I I w ith up s tream i n jec t io n

phase delay [msec]

0 1 2 3 4

dB reduction -30

-25

-20

-15

-10

-5

0

5

10

p.w.=600 µsecp.w.=1000µsecp.w.=1200µsec

phase delay[msec]

0 1 2 3 4

dB reduction -30

-25

-20

-15

-10

-5

0

5

10


ph ase delay [msec]

0 1 2 3 4 5

dB reduction -40

-30

-20

-10

0

10


phase delay[msec]

0 1 2 3 4

dB reduction -40

-30

-20

-10

0

10

p.w .=600µsecp .w .=1000µsecp .w .=1200µsec

C ase I with d u m p p lane inject io n C a s e I w ith ups t ream in j ec t io n

C ase I I with d u m p p lan e in jec t io n C a s e I I w ith up s tream i n jec t io n

dB r

educ

tion

dB r

educ

tion

dB r

educ

tion

dB r

educ

tion

Figure 3. Control effectiveness vs. phase-delay for the case I and case II instabilities with dump plane and upstream injection


For the case II instability all of the fuel is mixed withthe air upstream of the choked inlet to the mixingsection, hence, the fuel distribution is uniform and thereis no possibility of equivalence ratio fluctuations due tofeed system coupling. The actual fuel distributions weremeasured using the laser induced fluorescencetechnique discussed previously and are shown in fig. 4.As expected the fuel distribution in case I is uniform,while in case II there is a gradient in the equivalenceratio across the width of the annular mixing sectionwhere the equivalence ratio varies nearly linearly froma value of 0.25 near the inner wall to a value of 0.9 nearthe outer wall.

Figures 5 (a) and (b) show the sequence of phase-averaged two-dimensional deconvoluted CO2*chemiluminescence images during one period ofunstable combustion for the case I and case IIinstabilities, respectively. The pressure measurement atthe dump plane was used as a reference signal. Notethat only the upper half of the image is shown (since thedeconvoluted images are axisymmetric) and the twolines to the left of each figure depict the annularentrance from the mixing section into the combustor.Each image is displayed in false-color where increasingchemiluminescence intensity is from black to white. Inaddition, the measured pressure oscillation for eachinstability is plotted next to the images, where thenumbers refer to the time during the pressure cyclewhen the images were taken. Note that for both cases,the maximum and minimum chemiluminescenceintensity occurs at approximately the same time as themaximum and minimum pressure, respectively,indicating that the pressure and heat release fluctuationsare approximately in phase. It should also be noted thatthe chemiluminescence images for both instabilitiesshow clear evidence of flame-vortex interaction,however, the details of the interaction are noticeablydifferent in the two cases. In case I the flame appears to

be contained within the vortex, whereas in case II theflame appears to be wrapped around the vortex.

The differences between the instabilities at thesetwo operating conditions are more apparent when theRayleigh Index distributions are compared. TheRayleigh Index distribution quantifies the strength ofthe coupling between the local unsteady heat releaseand the local acoustic pressure fluctuations, and isgiven by

where t is time, x and y define the location in a two-dimensional cross-section of the combustor, T is theinstability period, p' is the local pressure fluctuation andq' is the local heat release fluctuation.22 The RayleighIndex distribution identifies regions in the combustorwhere either driving or damping occurs, as indicated bypositive or negative values of the local Rayleigh Index,respectively. Using the local chemiluminescenceintensity variation as an indicator of the local heatrelease and the measured pressure oscillation, theRayleigh Index distribution can be calculated fromthe phase-averaged chemiluminescence images and thepressure oscillation measured at the dump plane. Notethat since the length of the flame zone (<10 cm) ismuch smaller than the acoustic wavelength (~175 cmfor both cases), it can be assumed that the pressure isconstant in phase and magnitude within the combustionzone.

Figures 6 (a) and (b) show the local Rayleigh Indexdistribution for the unstable flames depicted in figs. 5(a) and (b), respectively. In case I (fig. 6 (a)), theRayleigh index distribution shows that there are twodamping regions, one close to the “shoulder” of thedump plane and another along the centerline of thecombustor immediately downstream of the centerbody.A negative Rayleigh index is the result of heat releasewhich occurs when the pressure is at or near itsminimum, which in case I corresponds to images (5),(6) and (7) in fig 5 (a). Inspection of these imagesshows that the maximum heat release at the time ofminimum pressure occurs near the shoulder of thedump plane which is consistent with the fact that this isa region of significant damping. There is also evidenceof heat release along the centerline, immediatelydownstream of the centerbody, which would explain thedamping at this location. Figure 6 (a) also shows thatthe region where the driving is strongest is locatedfurther downstream and close to the wall of thecombustor. A positive Rayleigh Index is due to heatrelease which occurs when the pressure is at or near itsmaximum, which in case I corresponds to images (11),(12) and (13) in fig. 5 (a). Inspection of these imagesshows that the maximum heat release at the time of

equivalence ratio

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

dist

ance

from

edg

e of

cen

terb

ody

[mm

]

0

1

2

3

4

5

6

7

8

9

case Icase II

Figure 4. Combustor inlet fuel distributions for case I and case II operating conditions

∫=T

dttyxqtyxpT

yxR ),,('),,('1

),(


maximum pressure does in fact occur at the location ofmaximum Rayleigh Index.

In case II (fig. 6 (b)), the Rayleigh Indexdistribution shows that there are again two damping

regions. One is located at the same radial position as thecase I damping region, however, about twice as fardownstream of the dump plane. Inspection of images(4), (5) and (6) in fig. 5 (b) shows that at the time of

(a) Case I (b) Case II

Figure 5. Two-dimensional chemiluminescence image sequences for (a) Case I and (b) Case II instabilities (The two lines to the left of each figure indicate the location of the annular entrance from the mixing section into the combustor.)

Figure 6. Rayleigh index distributions for (a) Case I and (b) Case II instabilities (The scale is color-coded)

0 +-

(a) Case I

X [mm]

0 10 20 30 40 50 60 70 80 90 100 110

r [mm

]

05

10152025303540455055

200

200

200

200200

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400

400400

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400400

600

600600

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]

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10152025303540455055

200200

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200200 200

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400400

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0 0

0

00

400400

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-200

-200

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600

600800

000 -200

(b) Case II


minimum pressure the maximum heat release occurs atthis same location. The second damping region islocated immediately downstream of the centerbody, andcan be associated with the region of heat release whichappears in images (5) and (6) at this location.

Characterization of secondary fuel injection system:Measurement of secondary fuel distribution andflame response function

Secondary fuel injection is characterized in termsof the spatial and temporal distribution of the fuel as itenters the combustor and in terms of the so-called flameresponse function.

The fuel distribution is measured using acetonefluorescence, as discussed previously, and the resultsfor the two secondary injection locations are shown infigures 7 (a) and (b). In each figure, the radial fueldistribution, measured 3 mm away from the face of thedump plane, is shown at five different times after thesolenoid valve trigger signal. Both figures show thefluorescence intensity, which is proportional to the fuelconcentration, in arbitrary units versus radial distancefrom the axis of the mixing section. The lines to the leftof each graph define the annular entrance from themixing section into the combustor.

In the case of dump plane injection (fig. 7 (a)), theinjected fuel is concentrated at the injection site,whereas in the case of upstream injection (fig. 7 (b)) thesecondary fuel is distributed across most of the annularmixing section with a maximum concentration near theouter wall. These results also show, in both cases, thatthe duration of the secondary fuel pulse entering thecombustor is approximately 3 milliseconds, whereas thepulse duration of the trigger signal to the solenoid valveis 1 millisecond, therefore there is significant spreadingof the fuel pulse from the solenoid valve to where itenters the combustor.

The flame response function refers to the changein the flame’s heat release due to fuel flowmodulation. The flame’s heat release is measured bydetecting the intensity of the CO2* chemiluminescenceemission, as described previously, and the flameresponse function is the difference between thechemiluminescence intensity with and withoutsecondary fuel flow. It is necessary to measure theflame response function in a stable flame since if theflame were unstable one could not differentiate betweenthe modulation in heat release due to the instability anddue to the modulated fuel flow. In order to achievestable combustion for the purpose of this measurementthe percentage of main fuel injected through thecenterbody was increased to 50% of the total.

The flame response function measurements for thetwo injection locations are shown in figures 8 (a) and(b). Each figure presents the total chemiluminescenceemission due to secondary fuel as a function of timeafter the solenoid valve trigger signal, for threedifferent injection durations, i.e., 600, 1000 and 1200microseconds. In all cases the flame response functionshows a sharp initial increase of about 1 millisecondduration followed by a gradual decrease. In the case ofdump plane injection the sharp increase beginsapproximately 3 milliseconds after the solenoid valvetrigger signal and in the upstream injection case about 5milliseconds.

Discussion of ResultsIn this section the results just presented

characterizing the spatial and temporal evolution of theunstable flame, the spatial and temporal distribution ofthe secondary fuel and the flame response function areused to explain the measured differences in controleffectiveness shown in fig. 3.

Assuming that the instability can be suppressedsimply by out of phase heat addition, the relativeeffectiveness of the two secondary injection locationscan be estimated using the following simulation.

By varying the time delay between the pressuresignal and the valve trigger signal the phasing of theflame response function and the pressure oscillation canbe varied. This is illustrated in fig. 9 where the flameresponse function is superimposed on the pressureoscillation. The effect of the heat release modulationdue to the secondary fuel, for a given time delay, can beestimated by calculating the Flame Response RayleighIndex, which is defined as

∫ ′′=+Tt

tsecondary

o

o

dtqpT

Index Rayleigh Response Flame1

where T is the reciprocal of injection frequency (~90Hz= 360Hz / 4), to is the time delay between the pressuresignal zero crossing and the valve trigger signal, p’ is

upstream injection

Intensity [a. u.]

0 5 10 15 20 25 30 35 40 45 50

r [m

m]

0

5

10

15

20

25

30

∆t=5.0msec∆t=5.5msec∆t=6.0msec∆t=6.5msec∆t=7.0msec

dump plane injection

intensity [a.u.]

0 2 4 6 8 10 12

r [m

m]

0

5

10

15

20

25

30

∆t=3.0msec∆t=4.0msec∆t=4.5msec∆t=5.0msec∆t=6.0msec

(a) (b)

Figure 7. Spatial and temporal distribution of secondary fuel (a) dump plane injection and (b) upstream injection


the measured unstable pressure signal (instabilityfrequency~360Hz) and q’secondary is the modulated heatrelease due to injection, i.e. the flame responsefunction. Figure 10 shows the Flame ResponseRayleigh Index versus the time delay for four cases,i.e., the case I instability with dump plane and upstreamsecondary injection and the case II instability withdump plane and upstream secondary injection. In eachcase the results for injection durations of 600, 1000 and1200 microseconds are shown. The effect of thesecondary heat release is shown to vary as the timedelay between the pressure oscillation and the valve

trigger signal changes, going from maximum dampingto maximum driving and back to maximum dampingover a time interval equal to the period of theinstability, i.e., approximately 2.7 msec.

With dump plane injection in the case of bothinstabilities, the time delay for maximum damping ispredicted to increase from 1.2 and 1.6 msec as theinjection duration increases. The amount of damping isalso shown to change as the injection duration changes,i.e., the damping increases with increasing injectionduration. With upstream injection in the case of bothinstabilities, the time delay for maximum damping ispredicted to increase from 2.6 to 3.2 msec as theinjection duration decreases, while the amount ofdamping increases with increasing injection duration.For the two instabilities, optimal timing for maximumdamping is almost identical for a given injectionlocation. This is because the instability frequency isnearly the same for both instabilities.

Comparing the damping predictions shown in fig.10 with the actual control effectiveness results shown infig. 3 reveals a number of similarities and differences.The main similarity is that for the case I instability withdump plane injection and the case II instability withupstream injection, there is good agreement in theoptimum time delay for maximum damping/control.For the case I instability with dump plane injection themaximum damping is predicted at between 1.2 and 1.6msec whereas the most effective control was observedat 1.0 msec. For the case II instability with upstreaminjection the maximum damping is predicted at between

dump injection

time [msec]

0 1 2 3 4 5 6 7 8 9 10 11

q' [C

O2*

che

milu

min

esce

nce]

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040p.w.=600µsecp.w.=1000µsecp.w.=1200µsec

upstream injection

time [msec]

0 1 2 3 4 5 6 7 8 9 10 11


(a) (b)Figure 8. Flame response measurements for dump plane and upstream injection

Valve opening pulse

phase delay,to

Period, T

0 2 4 6 8 10 120.02

0.01

0

0.01

0.02

0.03

time [msec]

Hea

t rel

ease

[V

]

q’secondary

Figure 9. Flame response for dump plane injection superimposed on the unstable pressure trace


2.6 and 3.2 msec, while the most effective control wasobserved at 2.5 msec. This suggests that the FlameResponse Rayleigh Index provides a reasonably goodestimate of the delay required for optimum control.

The main difference between the Flame ResponseRayleigh Index predictions and the actual controleffectiveness results is the inability of the FlameResponse Rayleigh Index to predict the fact that theinstability was not suppressed at any time delay ineither the case I - upstream injection combination or thecase II - dump plane injection combination. This is notsurprising, however, since the Flame ResponseRayleigh Index only predicts the time delay formaximum damping, and it does not predict whether theamount of damping is sufficient to suppress the

instability. Therefore the Flame Response RayleighIndex could not predict that an injection duration of 600microseconds did not result in enough damping tosuppress the instability in the two cases where controlwas achieved. Even though the Flame ResponseRayleigh Index did not predict the fact that no controlwas achieved in case II with dump plane injection, it isinteresting to note that it did predict the time delay atwhich the modulated secondary fuel injection wouldincrease the pressure fluctuations, i.e., approximately2.8 msec.

The flame structure (fig. 5), Rayleigh Indexdistribution (fig. 6) and secondary fuel distribution (fig.7) results suggest another explanation as to why dumpplane injection is effective at controlling the case I

Figure 10. Flame Response Rayleigh Index versus time delay prediction for case I and case II instabilities with dump plane and upstream injection.

Case II with dump plane injection

time delay (msec)

0 1 2 3 4

Fla

me

resp

onse

Ray

leig

h in

dex

-0.010

-0.008

-0.006

-0.004

-0.002

0.000

0.002

0.004

0.006

0.008

0.010


Case II with upstream injection

time delay (msec)

0 1 2 3 4

Fla

me

resp

onse

Ray

leig

h in

dex

-0.010

-0.008

-0.006

-0.004

-0.002

0.000

0.002

0.004

0.006

0.008

0.010


Case I with dump plane injection

time delay (msec)

0 1 2 3 4

Fla

me

resp

onse

Ray

leig

h in

dex

-0.025

-0.020

-0.015

-0.010

-0.005

0.000

0.005

0.010


Case I with upstream injection

time delay (msec)

0 1 2 3 4

Fla

me

resp

onse

Ray

leig

h in

dex

-0.025

-0.020

-0.015

-0.010

-0.005

0.000

0.005

0.010

0.015


Case II with dump plane injection

time delay (msec)

0 1 2 3 4

Fla

me

resp

onse

Ray

leig

h in

dex

-0.010

-0.008

-0.006

-0.004

-0.002

0.000

0.002

0.004

0.006

0.008

0.010


Case II with upstream injection

time delay (msec)

0 1 2 3 4

Fla

me

resp

onse

Ray

leig

h in

dex

-0.010

-0.008

-0.006

-0.004

-0.002

0.000

0.002

0.004

0.006

0.008

0.010


Case I with dump plane injection

time delay (msec)

0 1 2 3 4

Fla

me

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onse

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leig

h in

dex

-0.025

-0.020

-0.015

-0.010

-0.005

0.000

0.005

0.010


Case I with upstream injection

time delay (msec)

0 1 2 3 4

Fla

me

resp

onse

Ray

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h in

dex

-0.025

-0.020

-0.015

-0.010

-0.005

0.000

0.005

0.010

0.015



instability while upstream injection is not. Figure 5 (a)shows the evolution of the flame during one period ofthe case I instability along with the correspondingpressure oscillation. As discussed previously, in orderfor the secondary fuel to produce damping it must burnwhen the pressure oscillation is near its minimum,which corresponds to images (5), (6) and (7). Theseimages show that the flame is inside the vortex corewhich is positioned almost directly in front of the dumpplane injection holes at this time. Therefore secondaryfuel which is injected at this location and at this timewill penetrate into the vortex and burn almostimmediately, resulting in maximum damping. On theother hand, secondary fuel injected at the upstreaminjection location enters the combustion chamberthrough the annular mixing section and travels aroundthe periphery of the vortex and as a result burns moreslowly resulting in less damping. Another way ofsaying this is the optimum injection location is onewhich most effectively introduces the secondary fuelinto the region of maximum damping (as identified bythe Rayleigh Index distribution) since this is thelocation where combustion is occurring at the time ofminimum pressure.

This same reasoning can be applied to the case IIinstability where minimum pressure corresponds toflame images (4), (5) and (6) in fig. 5 (b). These imagesshow that there are two regions of intense combustionat this time. One is located almost directly downstreamof the annular exit of the mixing section and the other islocated further downstream and closer to the outer wallof the combustor. The first region is perfectly located atthe exit of the annular mixing section for rapid burningof the secondary fuel injected at the upstream injectionlocation, and therefore will produce maximumdamping. On the other hand, secondary fuel injected atthe dump plane location can not readily access either ofthe regions of intense combustion and therefore willburn more slowly and result in less damping. Again themost effective control is achieved when the secondaryfuel is introduced directly into a region of damping asindicated by the Rayleigh Index distribution of theunstable flame in question.

ConclusionsThe effective suppression of unstable

combustion using modulated secondary fuel injectiondepends on the location and timing of the secondaryfuel injection. The flame response function for aparticular secondary injection location and the pressureoscillation for a particular instability can be used tocalculate the Flame Response Rayleigh Index fromwhich the optimum injection timing (relative to thezero-crossing of the pressure oscillation) required formaximum damping can be determined. Althoughoptimum timing of secondary injection is critical it is

not sufficient to ensure suppression of the instability.One must also consider the spatial and temporaldistribution of the injected secondary fuel relative to thespatial and temporal evolution of the unstable flame,since this interaction can have a significant effect on themagnitude of damping that is achieved. The results ofthis study indicate that the optimum injection location isone which most effectively introduces the secondaryfuel into a damping region defined by the RayleighIndex distribution for the instability of interest. Thiswill ensure rapid combustion of the secondary fuel andtherefore maximum damping.

AcknowledgementThis research was conducted with support from the

Office of Naval Research under Contract #N00014096-1-0405. with Dr. Gabriel Roy as the contract monitor.

References1.Hermann, J., Gleis, S. and Vortmeyer, D., CombustionScience and Technology, Vol. 118, pp. 1-25, 1996.2.Yu, K. H., Wilson, K. J. and Schadow, K. C.,Proceedings of the Twenty-seventh Symposium(International) on Combustion, The CombustionInstitute, pp. 2039-2046, 1998.3.Jones, C. M., Lee, J. G. and Santavicca, D. A., Journalof Propulsion and Power, Vol. 15, No. 2, 1999.4.Lee, J. G., Kim, K. and Santavicca, D. A.,Proceedings of the Twenty-eighth Symposium(International) on Combustion, The CombustionInstitute, pp. 739-746, 2000.5.Sattinger, S. S., Neumeier, Y., Nabi, A., Zinn, B. T.,Amos, D. J. and Darling, D. D., Journal of Engineeringfor Gas Turbines and Power, Transactions of theASME, Vol. 122, No. 2, pp. 262-268, 2000.6.Seume, J. R., Vortmeyer, N., Krause, W., Hermann, J.,Hantschk, C. -C., Zangl, P., Gleis, S., Vortmeyer, D.and Orthmann, A., Journal of Engineering for GasTurbines and Power, Transactions of the ASME, V.120, n. 4, pp. 721-726, 1998.7.Cohen, J. M., Rey, N. M., Jacobson, C. A. andAnderson, T. J., Journal of Engineering for GasTurbines and Power, Transactions of the ASME, V.121, n. 2, p. 281- 284, 1999.8.Hibshman, J. R., Cohen, J. M., Banaszuk, A.,Anderson, T. A. and Alholm, H. A., ASME Paper 99-GT-215, 1999.9.Dowling, A. P., AIAA Paper 99-3571, 1999.10.Mohanraj, R., Neumeier, Y. and Zinn, B. T., Journalof Propulsion and Power, Vol. 16, No. 3, pp. 485-491,2000.11.Fleifill, M., Hathout, J. P., Annaswamy, A. M. andGhoniem, A. F., AIAA Paper 2000-0708, 2000.12.Hathout, J. P., Annaswamy, A. M. and Ghoniem, A.F., RTO Symposium on Active Control Technology for


Enhanced Performance Operational Capabilities ofMilitary Aircraft, Land Vehicles and Sea Vehicles,Braunschweig, Germany, May 8-11, 2000.13.Cohen, J. M., Stufflebeam, J. H. and Proscia, W.,ASME Paper 2000-GT-83, 2000.14.Sivasegaram, S. and Whitelaw, J. H., Active Controlof Noise and Vibration, ASME, DSC-38, pp. 69-74,1992.15.Najm, H. N., Knio, O. M., Paul, P. H. and Wyckoff,P. S., Combustion Science and Technology, Vol. 140,pp. 369-403, 1998.16.Samaniego, J. M., Egolfopoulos, F. N. and Bowman,C. T., Combustion Science and Technology, Vol. 109,pp. 183-203, 1995.17.Yip, B. and Samaniego, J. –M., “Direct C2 RadicalImaging in Combustion Instabilities,” CombustionScience and Technology, Vol. 84, pp. 81-89, 1995.18.Venkataraman, K. K., Preston, L. H., Simons, D. W.,Lee, B. J., Lee, J. G. and Santavicca, D. A., Journal ofPropulsion and Power, Vol.15, No. 6, pp. 909-9181999.19.Dasch, C. J., Applied Optics, Vol. 31, No. 8, pp.1146-1152, 1992.20.Shih, W.P., Lee, J.G. and Santavicca, D.A.,Proceedings of the Twenty-sixth Symposium(International) on Combustion, The CombustionInstitute, pp. 2771-2778, 1996.21.Lozano, A., Yip, B. and Hanson, R. K., Experimentsin Fluids, Vol. 13, pp. 369-376, 1992.22.Samaniego, J. –M., Yip, B., Poinsot T. and Candel,S., Combustion and Flame, Vol. 94, pp. 363-380, 1993.

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