Combination of Active Instability Control and Passive Measures to Prevent Combustion Instabilities

Combination of Active Instability Control and Passive Measures

to Prevent Combustion Instabilitiesin a 260MW Heavy Duty Gas Turbine

J. Hermann, A. Orthmann

IfTA, Ingenieurbuero fuer Thermoakustik GmbH,

D-82194 Gröbenzell, Industriestr. 33

S. Hoffmann, P. BerenbrinkSiemens AG Power Generation,D-466 Muelheim, Wiesenstr. 35

Abstract

Reducing the output of NOx pollutants and enhancingefficiency are the two major aims pursued by developersof modern gas turbines. In order to achieve them, leanpremix combustion is preferred, turbine inlet tempera-tures and thus power densities within the combustionchamber system being continuously increased toaugment efficiency. Due to this fact, the tendency ofmodern combustion systems to develop so-called self-excited combustion oscillations keeps increasing.

After briefly discussing the oscillation problemsencountered with the annular combustion chamber of aSiemens type V94.3A stationary gas turbine, particularattention will be paid to suppressing these oscillationsby passive and active means. The passive measurespresented, i.e. extending the burner nozzle, wereintended to detune the combustion system by prolongingthe time lag required by the combustible mixture exitingthe burner outlet to reach the combustion zone.Moreover, to suppress periodic vortex shedding, anotherpossible cause for combustion instabilities, thoseextensions were inclined in a certain angle with respectto the main flow direction. To prevent the in-phase lockof all 24 burners promoting the excitation of anyazimuthal mode, the burners were selected to havedifferent time lags, and were arranged asymmetricallywithin the annular combustion chamber. In addition tothese passive measures, a multi-channel ActiveInstability Control (AIC) system was implemented toachieve further damping. With the AIC systempresented, any burner oscillations occurring aremeasured by p-ressure sensors; their signals areprocessed by means of a multi-channel controller, andthen transmitted to actuators designed to damp downcombustion oscillations. The points of interventionselected to do so were the gas supplies of the pilotflames. In order to achieve optimum response, everysingle one of the 24 burners was fitted with its ownactuator.

In field demonstrations for various type V94.3A gasturbines, the presented measures were successfullytested and active suppression of combustion oscillationsproved to be highly flexible in dealing with variousoscillation problems.

1. Introduction

Self-excited combustion oscillations or instabilities areobserved in various kinds of industry-type combustionor propulsion systems, reaching from domestic heatingdevices to gas turbines and rocket motors. Theoscillations cause substantial pressure fluctuations atdiscrete frequencies which, particularly with systemscharacterised by high power densities, may reach levelsleading to mechanical failure of the combustionchamber, or of upstream or downstream plantcomponents.

Developments targeted at reducing NOx pollution andincreasing efficiency levels for modern stationary gasturbines involve lean burning and increased turbine inlettemperatures and thus power densities within thecombustion chamber. Since these measures tend to bringabout self-excited combustion instabilities, avoidingthese instabilities constitutes one of the main tasks indeveloping modern combustion systems.

2. Rayleigh criterion and time lag

A fundamental element of the driving mechanism ofself-excited combustion oscillations is the excitation ofpressure oscillations by a fluctuating heat release rate ofthe flame. Generally speaking, for the heat release rateoscillations to result in amplification of pressureoscillations, heat addition must occur at or around timesof high pressure. Lord Rayleigh [1] was the first to statethis criterion, now bearing his name, which wasdeveloped further by various other authors, e.g. Putnamand Dennis [2].

To evaluate the Rayleigh criterion, the phase shiftbetween pressure and heat release fluctuations can beused. Phase shifts occur because every heat release fluc-

Paper presented at the RTO AVT Symposium on “Active Control Technology for

Enhanced Performance Operational Capabilities of Military Aircraft, Land Vehicles and Sea Vehicles”,

held in Braunschweig, Germany, 8-11 May 2000, and published in RTO MP-051.

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tuation produces the corresponding pressure fluctuationonly after a certain delay or \"time lag\" (and vice-versa).Time lags in combustion systems consist of variouscomponents τi , e.g. acoustic and convective time lags,as well as time lags attributable to the processes ofmixing and reacting (see e.g. Hermann et al. [3] or, for aslightly different model, Lieuwen et al. [4]). Thesechronological components - and thus the system stability- are generally hard to predict. Accordingly, measures tosuppress oscillations based on knowledge about timelags can usually only be taken when self-excitedoscillations in a combustion system are already present.

3. Feedback mechanisms

For self-excitation of pressure oscillations to occur, it isnot sufficient that the Rayleigh criterion be fulfilled.There must also be a suitable feedback mechanismbetween oscillations of pressure and heat release rate:pressure oscillations caused by fluctuations of the heatrelease rate must interact with other effects involved inthe combustion process that in their turn - directly orindirectly - cause further oscillations of the heat releaserate. This can for instance be due to sound pressurewaves within the combustion chamber propagatingagainst the flow direction. When these waves interactwith the flow at fuel or air feed points or flame-stabilising components (bluff-bodies, wakes, etc),fluctuations of mass flow rate, equivalence ratio orperiodic vortex shedding can occur. Subsequently, thesedisturbances will travel, at convective speeds, into thecombustion zone where they cause correspondingfluctuations of the heat release rate. Other possiblefeedback mechanisms involve variations of the flamefront area and periodic break-up, atomization andvaporization of liquid fuel (see e.g. Putnam [5]).

4. Possibilities of avoiding and/or

suppressing self-excited combustionoscillations

During the design phase of a combustion system,predictions about its tendency to develop self-excitedcombustion oscillations are still only of limited validity.Thus, the development of methods allowing to avoidand/or suppress this type of oscillation during systemtesting is of crucial importance. The means available todo so can be subdivided, in principle, into two groups:passive and active measures.4.1. Passive measures

The term passive measures refers to modifications of acombustion system that, during system operation, will notbe changed any more and/or will require no externalsupply of energy. Typical passive measures are\"detuning\" a system by modifying its burners or theacoustics of its combustion chamber, by increasingacoustic damping via Helmholtz-type resonators, or bydisturbing the propagation of sound waves via baffles(see e.g. Culick [6]). For passive measures to besuccessful, it is normally indispensable to do intensiveresearch on system behaviour. Accordingly, any measuresthus found are often valid and/or effective for a specificsystem only.

4.2. Active measures

Additionally, so-called active measures are aninteresting alternative, the term active referring to anymeans of acting upon the current status of anycombustion system in a targeted, controlled manner.Normally, an external power supply will be required.One example of this kind of active system is \"ActiveInstability Control\" (AIC) of combustion oscillations.AIC uses an actuator to modulate some suitableparameter of a combustion system. Under idealconditions, modulation is performed in a manner to havethe corresponding system variable fluctuate precisely incounter-phase with the fluctuations constituting thecombustion instabilities, thus damping them. To do so,some system parameter characterising the correspondingoscillation is measured, and processed via a controller,in order to generate the control signals for the actuators.For industry-type combustion systems, influencing heatrelease fluctuations by modulating fuel supplies hasproved to be an efficient and practicable means ofintervention. Because of the high volumes of air to bemoved, another type of intervention - via systemacoustics by imposing sound waves in counterphase tothe sound field in the combustion chamber, e.g. byloudspeakers - is suitable only for low-powercombustion systems.

5. Combustion instabilities in Siemens

type Vx4.3A gas turbines and ways ofavoiding them

5.1. Combustion instabilities and eigenmodes

in annular combustion chambers

Siemens type Vx4.3A gas turbines (see Figure 1) featureannular combustion chambers comprising 24 hybridburners distributed peripherally (see Figure 2). Hybridburners make it possible to operate gas turbines oneither gaseous or liquid fuels. The gas-burning modeoffers, in addition to purely diffusion-based or premixoperations, the possibility of combining both types ofoperation. Moreover, so as to stabilise the premixedflame, every burner features a small diffusion-basedpilot-flame.

Using the standard hybrid burner within the annularcombustion chamber of type Vx4.3A gas turbinesresulted in unwanted self-excited combustioninstabilities which limited, above all, the maximumachievable power output of the turbine. Moreover, itwas found that certain part load operating modes weresubject to oscillations. In addition to high soundpressure levels at discrete frequencies, standing soundwaves - typical of combustion instabilities - weremeasured; these are characterised by localised amplitudeminima and maxima, also called nodes and antinodes.Owing to their preferred orientation along thecircumference of annular combustion chambers, thesemodes are designated azimuthal. Figure 6 shows atypical azimuthal mode, corresponding to the firstharmonic, within the annular combustion chamber.

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One of the 24Ring CombustionHybrid BurnersChamberInletChannelCompressorTurbine

ExhaustChannelFigure 1:Half-section drawing of the Vx4.3A series gas turbine.Fuel oil returnFuel gas flow to thepilot burner nozzleFuel gas flow to thediffusion burner nozzleFuel oil inletSteam or waterSteam orwater injectionMain air flowFuel oil to thepremix burner nozzleMain air flowPremixed gas pipesFuel gas flow to thepremix burner nozzleFigure 2:Vx4.3A Hybrid Burner Ring (HBR)

Combustor - One Annular Combustor with 24Burners.5.2. Measures used to suppress combustion

instabilities5.2.1 Passive measures

Considering the fact that the generation of self-excitedcombustion instabilities depends on certain time lags,selectively modifying those time lags is one possiblemeans of suppressing combustion instabilities. The timelag most easily adapted is the convective time lag or thetime required to convey the fuel/air mixture into thecombustion zone. In order to make it possible to adjustthis parameter in a precisely targeted manner, acylindrical extension - called a cylindrical burner outlet(CBO) - was welded onto the burner nozzle (see Figure3). The length of this extension was selected so as toprolong the convective time lag by slightly more thanone quarter of the period of the self-excited oscillation.As already mentioned in section 3, one possible cause forfluctuations of the heat release rate of the flame and thusfor combustion instabilities are periodically generated

Fuel/air mixturefor the premixedoperating modeFuel gas fromdiffusion burnerDiagonal swirlerFuel gas frompilot burnerFuel oil fromdiffusion burnerFlameAxial swirlerCBOFigure 3:The standard Siemens Hybrid Burner (top)and with schematic CBO extension (bottom).vortices, which can be provoked by flow disturbances inthe shear layer. To prevent these flow instabilities, thecylindrical extensions attached to the burner nozzles wereinclined by 10° in respect to the flow axis on two

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Figure 4:Asymmetric arrangement of 8 ABO pairs

along the circumference of the annularcombustion chamber.neighbouring burners. Owing to the angle of inclinationcharacterising these extensions, they are designatedasymmetric burner outlets (ABOs). ABOs cause anuneven shear layer distribution around the burner nozzle,thus reducing the formation of coherent structures, anddisplace the combustion zone downstream of its formerposition, thus increasing the time lag.

Another passive measure taken is the use of severalburner types characterised by differing flame frequencyresponses and installing burners belonging to the sametype asymmetrically, with reference to potentialazimuthal modes: if burners belonging to the same typeare not precisely located, for example on the potentialpressure antinodes of the azimuthal modes to beprevented, they will not be optimally excited tocombustion oscillations by the prevailing acoustics.Figure 4 shows an example for an asymmetric burnerarrangement using ABOs.

5.2.2 Active measures

The AIC system implemented for V94.3A turbinescorresponds, in principle, to the active system developed

for V84.3A prototype tests. However, majorcomponents, such as controller, actuators and certainmethods to implement the system at the turbine, wereredesigned comprehensively and developed into asystem fully optimised for the power generationindustry. Detailed descriptions are to be found in Seumeet al. [7] for the first AIC implementation, and inHoffmann et al. [8] and Hermann et al. [9] for the re-designed version. Figure 5 schematically shows theAIC-set-up for the gas turbine.

Active control was done by modulating the pilot gassupply for the various hybrid burners. The pilot gas waschosen because the pilot flames that stabilise the mainpremix flames, exert a very high degree of influence onthe main flames. Owing to reduced mass flows, controlvia pilot gas flames is substantially easier thanmodulating the main gas flow. To obtain optimumcontrol over the system, every burner was fitted with itsown actuator, a Moog-made rapid direct drive valve(DDV). The amplitude loss of the used 3rd generation ofthe DDV valve is less than 3dB up to a frequency of 420Hz and the valve can be used with a maximum allowedambient temperature of about 120°C.

The success of AIC strongly depends on a sufficiently highmodulation of the heat release rate of the pilot flame andthus, indirectly, on a high modulation of the mass flow ratein the pilot gas system by the actuator. Since the mass flowrate modulation on the other hand is strongly dependent onthe acoustic field in the pilot gas system or rather itsresonant behavior, the pilot gas system was tuned so as toallow for maximum mass flow rate modulation at thefrequencies to be damped. Details about the tuning of a fuelsystem for AIC can be found in Hermann et al. [10] andHantschk et al. [11].

Since any individual burner can excite combustionoscillations, each of the 24 burners has to be controlled.This requires, in addition to 24 actuators, a multi-channel AIC system providing the same number ofcontrol loops. However, the number of sensors and controlunits needed can be reduced by taking advantage of the

Piezo PressureTransducerRing CombustionChamberControl SystemVolumeDirect Drive ValvePilot Gas Main SystemPilot Gas PipeBurnerameFlTurbineCompressorFigure 5:Schematical representation of the AIC setup for the Siemens Model Vx4.3A heavy duty gas turbine.

Ring combustionchamber (cutawayview across theValve 1gas turbineaxis)Sensor 1Control Loop 1Burner-1Valve 13Figure 6:Use of the symmetry of azimuthal modes,

e.g. for the first harmonic. One sensor andone controller provide the input signals fortwo DDVs.symmetry of the excited azimuthal modes. As described bySeume et al. [7] it is possible to use a signal measured at acertain circumferential location of the azimuthal mode - orthe annular combustor - to determine not only the actuatorinput signal for this particular location but also those forseveral other locations. Depending on the possible excitableazimuthal modes in the V94.3A, a total of 12 control loopswere used, with each loop comprising a pressure sensor and2 valves as actuators (see Figure 6).

The multi-channel controller is a self-contained modularindustrial system and fully integrated into the gasturbine control system. In addition, the hard- andsoftware set-up is optimised with respect to shortcommissioning and implementation time scales. Themain control work is done by 6 digital signal processors,each of them handling two control loops. The controlalgorithm used works in the frequency domain andallows, in its latest version in combination with newexpanded hardware outputs, simultaneous processing ofany two oscillation frequencies.

As input signal for the control loops, the sound pressurevalue measured at the burner flange by high temperaturepiezoelectric transducers is used. As shown by Seumeet al. [7], these signals are sufficiently correlated withsound pressure levels prevailing within the combustionchamber. For this purpose, a total of 12 transducers wasinstalled along the circumference of the gas turbine.5.2.3 Combining passive and active measuresGas turbines are often operated over a wide range ofpower levels, ambient temperatures and in differentmodes of operation. This makes it a difficult task toprotect all operating points against potential combustioninstabilities by passive measures. Furthermore,implementing passive measures may bring about, besideany successful suppression of combustion oscillations atformerly unstable operating points, instabilities at pointsformerly quiet.

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In contrast to that, the active control of combustionoscillations is a very flexible measure. At any operatingpoint, AIC can easily be used to suppress unexpectednew instabilities as well as those that cannot beovercome by passive measures.

The successful implementation of the passive and activemeasures described, as well as various combinations ofthese measures for different gas turbines with nominalpower output levels between 233 MW and 267 MW atISO conditions will be described in the followingsection.

6. Results

6.1. Application of passive measures

The first tests, using the passive measures developed,were designed to research several variants of installingasymmetrical burner extensions. 3, 5 and 8 pairs wereused, the 8-pair arrangement being installed both sym-metrically and asymmetrically (see Figure 4) along thecircumference of the annular combustion chambers. Theimpacts of the various arrangements in terms ofcombustion oscillations were verified by running up theturbine to its new stability limits and/or a new level ofmaximum achievable turbine power. As the number ofABO pairs was increased, damping results improved;more particularly, asymmetrical arrangements givenidentical numbers of ABO pairs resulted in significantimprovements (see Figure 7). With the bestarrangement, it was possible to increase maximumturbine power by 7 percentage points. A further increaseof ABO pairs to a total of 9 failed to produce any furtherimprovement.

A further series of experiments was run to test variouscombinations of CBOs installed in a similar, i.e.asymmetric, manner as the ABO arrangements. Just aswith ABO experiments, an increasing number of CBOsresulted in shifting stability limits towards higher levelsof achievable maximum turbine power (see Figure 8).The best result achieved amounted to an increase ofturbine power by 9 percentage points, with 20 CBOs.

112 utp108ut Or]ew%104[o Pevita100leR96eOO,n.OOCiOleABABymsABABABAI a06 6as88+B11111Figure 7:Stability limit at different ABO arrangements.

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112 108putut Ower%]104[oPve iat100lRe96neCilCBOCBOBOeCAI +as0600B1122Figure 8:Stability limit at different CBO arrangements.6.2. Combination of passive and active

measures

The good results achieved by passive measures werefurther improved by using an Active Instability Controlsystem. Furthermore, in situations where implementingpassive measures causes instabilities at points formerlystable – which can occur in some configurations – AICwas successfully applied to overcome these problems.More particularly, short-term stability problems at partload operations were dealt with successfully. In theprocess, AIC once again proved its high level offlexibility with regard to solving oscillation problems ofvarious kinds.

6.2.1 AIC during start-up and part load

operations

Using CBOs with a special burner configurationunexpectedly lead to increased oscillation tendenciesduring the start-up phase. During this phase, operationof the hybrid burners is ensured by means of diffusion-type flames since combustion chamber temperatures arestill too low to stabilise a premix flame. Subsequent tostart-up, and while loads increase up to the switch-overpoint towards mixed operation (combining diffusion andpremix operations), the second and third harmonics ofthe annular combustion chamber were excited. Due tothe strenuous commissioning schedule, there was nomore time left for optimisation of the start-up sequences.Therefore it was decided to use the AIC system in orderto resolve this issue. Even though, in view of its controlvia pilot flames, AIC was designed for premixoperations, additional pilot gas was introduced duringdiffusion operation, while simultaneously the AIC wasactivated. This resulted in very good damping of thecombustion oscillations, thus allowing the system to runup through the start-up and part load phases to mixed-mode operation without any problem. Figure 9 showsthe sound pressure spectrum (the overlapped spectrumof the 12 AIC sensors) with and without AIC at a certainoperating point. By activating AIC for both dominantfrequencies, a damping by 20 dB (second harmonic) and15 dB (third harmonic) was achieved. In order to allowthe simultaneous damping of the second and third

5]B[d0 etud-5ilmp-10A er-15suse-20rP -25undSo-30-350200400600800AIC onAIC offFrequency [Hz]Figure 9:Suppression of two frequency peaks during

part load operation by AIC.

Figure 10:Separate damping of two dominant

eigenmodes of a combustion instability byAIC.harmonics within the annular combustion chamber, theAIC system was improved in terms of its independencein controlling valves located on opposite points of thecombustion chamber. With this improvement thelimitations on damping for certain frequencies describedby Hermann et al. [9] were eliminated.

The individual damping of the two dominanteigenfrequencies of the combustion instability by theAIC system can be seen in Figure 10. It shows thefrequency spectrum of the sound pressure versus time ina kind of contour plot: dark regions signify highpressure amplitudes. The two grey horizontal streaks at145 Hz and 290 Hz belong to the two dominantfrequencies of the prevailing combustion instability. Itcan be seen that by slowly reducing the AIC outputsignal used to suppress the 3rd harmonic this moderesurges step by step (line darkens for 70s < t < 85s).During the next 23 seconds only one of the self-excitedfrequencies - the 2nd harmonic at 145 Hz - is damped byAIC. After switching off the complete control loop at t =108 s also the 2nd harmonic resurges to higheramplitudes: at that time a dark line appears at f = 145 Hz.After another 19 seconds, at t = 127 s, the AIC system isreactivated to damp both frequencies which leads again

to a complete suppression of the combustion instability.This example shows that in the prevailing case the twomodes are excited independently of each other andtherefore every mode must be suppressed separately byAIC to achieve a good damping of the combustioninstability.

6.2.2 Increasing base load operating levelsIn addition to part load operations, AIC demonstrated itshigh levels of flexibility in suppressing oscillations alsofor peak load operations. In combination with passivemeasures, it was thus possible to edge up stability limitsfor most applications by a few percentage points. Forexample, by using AIC in combination with the bestarrangement of ABOs (9 pairs), it was possible to in-crease maximum turbine power by 3%, and incombination with the best CBO arrangement (20 CBOs)by 2%. In the latter case, further load increase was notpossible because the maximum permissible turbine inlettemperature had already been reached. In addition tothese improvements, with the AIC system active, theNOx emissions could be reduced significantly byreducing the mean pilot gas mass flow rate: in premixmode, the pilot flames are the primary source of NOxemissions.

6.2.3 Problems with frequency shifts

Near base-load operation, arising combustionoscillations did not only increase the pressure amplitudebut simultaneously also led to a substantial shift of theoscillation frequency from 155 Hz to about 170 Hz (seeFigure 11). During the tests it was observed thatdamping of these oscillations depends on synchronisedmodification of AIC set-up parameters. The determinedset of parameters allowing optimum AIC performanceindicates that, when this frequency shift occurs, thecombustion zone will simultaneously be shifted towardsthe burner nozzle. To allow efficient compensation ofthe changes in boundary conditions thus produced, theAIC control algorithm was complemented by anappropriate frequency tracking feature. Taking thismeasure substantially improved long-term stability atbase-load levels.

6.2.4 Failure tolerance and long-term AIC useDuring the different tests, valve failures were simulatedin order to test their impact on the overall AICperformance. A maximum of four valves was switchedoff during AIC runs resulting in no noticeabledegradation of the system performance, regardless of thevalve position. Switching off more than four valveswere not tried out due to time constraints.

The Active Instability Control system presented here iscurrently being used for base load, with several V94.3Agas turbines featuring 20-CBO configurations. Two ofthe systems, in operation since January 1999, havedemonstrated outstanding long-term stability. Up to nowthe AIC-Systems have operated for approximately 6,000hours each. In co-operation with the gas turbine controlsystem, the AIC system provides fully automated, stablegas turbine operations over the entire operating range.Moreover, inspection of some of the actuators used

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demonstrated that the wear and tear of the moving partsis negligible.

7. Summary

Self-excited combustion oscillations occurring withinthe annular combustion chamber of the Siemens typeV94.3A stationary gas turbine featuring 24 hybridburners limited, above all, the maximum achievableturbine power output. To avoid or suppress theseoscillations, various passive and active measures weredeveloped and successfully implemented. Stabilityimprovements achieved by every single measure weredemonstrated on various gas turbines.

In a first step, burner nozzles were equipped withcylindrical extensions whose length was adapted toextend the time lag between the flow entering thecombustion chamber and the combustion zone. In orderto avoid fluid-dynamic instabilities and change the timelag of the flames, the cylindrical extensions ofneighbouring burners were inclined towards each otherin an angle of 10° with reference to the main flowdirection. By using an increasing number of burner pairsfeaturing these extensions, it was possible to increaseturbine power in steps by suppressing the excessiveflame oscillations. It was found to be highly effective touse an asymmetrical arrangement for 8 to 10 pairs. Ascompared to the standard configuration, these pairsallowed turbine power to be increased by 7%. Similarresults have been achieved for the not inclinedcylindrical extensions. These extensions were likewiseinstalled asymmetrically within the combustionchamber. The best arrangement totalling 20 modifiedburner nozzles improved the stability limit by 9percentage points, as compared to standard burners.

Frequency Track of the highestSound Pressure Amplitued

175]170zH[165y cn160eu155qer150F1451400:00:00

0:00:05

0:00:10

]-1.0

Sound Pressure Amplitude at the above Frequency[ stinU dezi0.5

lamroN0.00:00:00

0:00:05

0:00:10

Time [h:m:s]

Figure 11:Frequency shift during arising sound

pressure amplitude (without activated AIC-System).

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The results achieved by means of passive measures wereimproved even more by employing an Active InstabilityControl (AIC) system. For the best burner configurationhaving ordinary, i.e. straight, extensions, AIC made itpossible to increase the stability limit by 2%, as well asby 3% for the best configuration having inclined nozzleextensions. Moreover, at base load operations, AICallows a reduction of NOby lowering the pilot gas mass flows necessary forx emissions by more than 60%stable operation. Problems entailed by substantialfrequency shifts whenever combustion instabilitiesoccurred at maximum turbine power were rectified byrapidly and automatically adapting the appropriatecontrol parameters. In addition to stabilising base loadoperations, the AIC system successfully dealt withstability problems occurring under part load conditionsin diffusion operation of the gas turbine. At this pointAIC reduced the two dominant frequency peaks by 20dB (2nd harmonic at 145 Hz) and 15 dB (3rd harmonic at290 Hz). To allow - in contrast to former AIC set-ups -the control of two modes with an even and an odd modenumber, the AIC system was improved in terms of itsindependence in controlling any two modes.

The successful damping of combustion instabilities invarious operating points of the gas turbine demonstratesthe high flexibility of the employed active measures indealing with this problem and allowing stable gasturbine operation over the entire range. The presentedAIC system is currently being used with several V94.3Agas turbines. The field leading installation, implementedin January 1999 in a base loaded machine, has beenoperating for approximately 6,000 hours and continuesto demonstrate its excellent long-term stability.

8. References

1. Rayleigh, Lord J.W.S.: The Explanation of Certain

Acoustical Phenomena. Nature, pp. 319-321, 18.July 1878.

2. Putnam, A.A., Dennis, W.R.: Burner Oscillations of

the Gauze-Tone Type. The Journal of the AcousticSociety of America, Vol. 26, No. 5, pp. 716-725,19.

3. Hermann, J.; Zangl, P.; Gleis, S.; Vortmeyer, D.:

Untersuchung der Anregungsmechanismenselbsterregter Verbrennungsschwingungen aneinem Verbrennungssystem für Flüssigkraftstoff.17. Deutscher Flammentag, Hamburg-Harburg,VDI-Bericht Nr. 1193, S. 251-260, 1995.

4. Lieuwen, H.T.; Torres, H.; Clifford, J.; Zinn, B.T.:

A Mechanism of Combustion Instability in LeanPremixed Gas Turbine Combustors. ASME paper99-GT-3.

5. Putnam, A.A.: Combustion Driven Oscillation in

Industry. Elsiever.

6. Culick, F.E.C.: Combustion Instabilities in Liquid-Fueled Propulsion Systems – An Overview,AGARD CP 450, pp. 1-1 – 1-73, 1988.

7. Seume, J.R.; Vortmeyer, N.; Krause, W., Hermann, J.;

Hantschk, C.-C.; Zangl, P.; Gleis, S.; Vortmeyer, D.;Orthmann, A.: Application of Active CombustionInstability Control to a Heavy Duty Gas Turbine.ASME Paper No. 97-AA-119, 1997

8. Hoffmann, S., Hermann, J. and Orthmann, A.: Active

Flame Instability Control for Heavy Duty GasTurbines. PowerGen Europe 99, Frankfurt,Germany.

9. Hermann, J.; Orthmann, A.; Hoffmann, S.:

Application of Active Combustion Control to aHeavy Duty Gas Turbines. 14th Int. Symposium onAirbreathing Engines, Florence, Italy, 5-10 Sep1999.

10. Hermann, J., Gleis, S., and Vortmeyer, D.: Active

Instability Control (AIC) of Spray Combustors byModulation of the Liquid Fuel Flow Rate, Combust.Sci. and Tech., Vol. 118, pp. 1-25, 1996,.

11. Hantschk, C., Hermann, J., and Vortmeyer, D.:

Active Instability Control with Direct Drive ServoValves in Liquid-Fuelled Combustion Systems, 26.Int. Symp. on Combustion, Naples, 1996.

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