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Twenty-Seventh Symposium (International) on Combustion/The Combustion Institute, 1998/pp. 1849–1856

AN EXPERIMENTAL STUDY OF COMBUSTION DYNAMICS OF A PREMIXEDSWIRL INJECTOR

J. C. BRODA, S. SEO, R. J. SANTORO, G. SHIRHATTIKAR and V. YANGThe Pennsylvania State University

University Park, PA 16802, USA

A comprehensive experimental study has been conducted on combustion instabilities in a premixedswirler injector system at various equivalence ratios, chamber pressures, and inlet air temperatures. Astability map has been determined to indicate the range of operating conditions conducive to the occur-rence of instabilities. The amplitude of instabilities was found to be a strong function of the equivalenceratio, with pressure oscillations as high as 20% of the mean chamber pressure and unsteady velocitiescomparable to the mean flow values observed for equivalence ratios around 0.6. On the other hand, beyondan initial threshold value of inlet air temperature at which instability suddenly initiated, variations in inletair temperature had minimal effect on the strength of instabilities. Measurements of steady and unsteadyflame structures carried out using CH chemiluminescence and photographic imaging techniques indicatethat the onset of instabilities can potentially cause significant alterations in flame structure, sometimeseven causing near extinction of the flame during certain periods of the oscillation cycle. Coupled longi-tudinal oscillations were observed in the combustion chamber and upstream duct. Instability characteristicssuch as frequency, mode shape, and phase obtained through pressure measurements were found to be inexcellent agreement with predictions from a linear acoustic analysis. A strong correlation was found be-tween the heat release and pressure fluctuations near the dump plane, indicating a possible mechanismfor creating and sustaining instabilities.

Introduction

The concept of lean premixed combustion in gasturbines has become widely accepted in recent yearsas an effective means to meet stringent environmen-tal standards on NOx emissions [1]. However, leanpremixed systems by virtue of the lack of intrinsicdamping mechanisms, strong flame holding capabil-ity, and the proximity of lean limits are more suscep-tible to the excitation of combustion instabilities [2].The mechanisms of combustion instabilities havebeen extensively studied in the past, especially inrockets [3,4], industrial burners [5], dump combus-tors in ramjets [6], and pulse combustors [7,8], butlittle work can be found relevant to modern gas tur-bines. Active control has received a lot of attentionrecently [9–11], but work done in that area wasaimed more at the end result of suppressing the in-stabilities than at the fundamental aspects of theproblem. Despite several recent parametric studieson combustion dynamics of swirl injectors [12,13],understanding of the physiochemical mechanismsgoverning instabilities in lean premixed gas turbinesis far from completion. There is a strong need forfundamental investigations into the various under-lying mechanisms responsible for excitation and sus-tenance of flow oscillations in engines. With this ob-jective in mind, a series of experimental studies ofcombustion dynamics of lean premixed swirl injec-tors have been conducted in the present work by

means of nonintrusive optical diagnostic methods,including CH chemiluminescence and photographicimaging techniques. Effects of equivalence ratio andinlet flow conditions with respect to temperatureand pressure were examined systematically, with em-phasis placed on unsteady heat release mechanismsand their interactions with local flow oscillations. Inaddition, a companion acoustic analysis was con-ducted to facilitate data analysis and consequentlyprovided more physical insight into the transient re-sponse of flame dynamics to acoustic disturbances.

Experimental Setup

Figure 1 shows schematically the test facility em-ployed in the present work. Compressed air is deliv-ered to a cylindrical combustor through a premixerat a maximum mass flow rate of 270 g/s at 2.4 MPaor 180 g/s at 3.8 MPa. The inlet air can be heatedto a temperature as high as 870 K at the maximumflow rate, thus providing the capability to simulateactual temperatures at the gas turbine compressorexit. Three generic swirlers with vane angles of 358,458, and 558 relative to the incoming air stream wereused, although the present paper only reports resultsobtained using the 458 case. Each swirler consists ofeight straight vanes attached to a central bluff bodymounted flush with the dump plane. Natural gas is

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Fig. 1. Schematic of experimentalsetup (not to scale—units indicatedin mm).

injected radially from the center body through ten0.75-mm-diameter holes immediately downstreamof the swirler vanes. Thus, conditions inside thecombustor are expected to be mostly premixed, butcold flow experiments have been planned for an ac-curate assessment on the degree of premixing of thefuel with air.

To acoustically isolate the test section from the airsupply line, a choked venturi is installed at the inletentrance. The combustor nozzle is also choked toprevent any downstream disturbances from travelingupstream and to maintain the desired chamber pres-sure. Eight high-frequency pressure transducers po-sitioned at different axial and tangential locations be-tween the two choked points are employed to recordacoustic pressure signals. The combustion chamberis comprised of several independent parts that areheld together by a hydraulically pressurized assem-bly. Each stainless steel section houses one or moretransducers that can be added to or removed fromthe overall chamber. This feature makes it possiblenot only to position the transducers at optimum lo-cations but also to vary the total chamber lengthfrom 140 to 370 mm to study its effect on instabilitybehavior. In the present work, a chamber length of235 mm has been selected for the experiments. Anoptically accessible cylindrical quartz section is usedto visualize the instantaneous flame dynamics. Thequartz cylinder is protected during hot-fire experi-ments by impingement of cooling air pressurized atthe chamber pressure to equalize forces on thequartz cylinder walls.

During each experiment, all important test param-eters were digitized and stored for subsequent anal-ysis. Conditioned CH chemiluminescence imageswere taken using an ICCD camera triggered by thereference pressure signal from one of the transduc-ers. The camera was equipped with a 50-mm focallength lens and a narrow bandpass filter centered at431 nm. Images were obtained by focusing the cam-era on the center plane of the cylindrical quartz sec-tion. Emissions of key combustion species such asNOx, CO2, O2, and CO were carefully monitoredusing gas analyzers. This enabled simultaneous eval-uation of performance parameters relating to low

emissions, high performance, and stable combus-tion.

Acoustic Analysis

A linear acoustic analysis was established to facili-tate data analysis and to help provide more physicalinsight into the observed phenomena. The analysiscovers a domain including both the inlet duct andcombustion chamber. The choked combustor exitnozzle is assumed to be acoustically compact andacts as an acoustically closed boundary. The admit-tance function is (c 1 1)Me/2, with Me being theMach number at the nozzle entrance. However, thesituation with the choked inlet venturi is quite dif-ferent. It basically serves as an acoustic damper andcan effectively dissipate disturbances arising fromdownstream [14]. Consequently, the acoustic field inthe inlet section is predominated by an upstream-running wave.

The entire domain for the acoustic analysis is di-vided into three regions: a constant-area inlet duct,a combustion chamber, and a premixer within whichthe variation of the flow passage area due to the fuelsupply line and swirler vanes is taken into account.The temperature nonuniformity in the combustor isalso considered based on numerical results of a sta-ble flame. The acoustic field in each region is ex-pressed as the superposition of two plane wavespropagating in opposite directions. Matching con-ditions at the interfaces are derived by requiringcontinuities of acoustic pressure and mass flow rate.The procedure eventually leads to a transcendentalequation for oscillation frequencies. A comprehen-sive description of the model is given in Ref. [15].

Results and Discussion

A series of experiments were performed with aswirl vane angle of 458. The test conditions are sum-marized as follows:

COMBUSTION DYNAMICS OF PREMIXED SWIRL INJECTOR 1851

Fig. 2. Effect of equivalence ratio and chamber pressureon amplitude of instabilities.

Fig. 3. Pressure time trace and corresponding powerspectral density showing superposition of 1L and 2Lmodes. Pc 4 0.47 MPa and f 4 0.57.

Chamber Pressure,MPa (psia)

EquivalenceRatio, f

InletVelocity, m/s

ù0.30 (44) 0.50 , f , 0.80 83ù0.48 (70) 0.50 , f , 0.80 83ù0.65 (95) 0.50 , f , 0.75 83

Inlet air temperatures, Tin, between 500 and 700 Kwere chosen for most experiments because no insta-bilities were observed at temperatures below 550 K.Higher values of Tin were occasionally used becausethey helped stabilize the flame near lean blow out(LBO) limits. Experimental conditions were con-fined to a maximum chamber pressure of 0.65 MPa.

Stability Map

Significant effort was First devoted to generate astability map based on measured pressure oscilla-tions and flame intensity signals from a photomulti-plier tube (PMT). The results are summarized in

Fig. 2. The parameters identified to have a majorimpact on the onset of instabilities are inlet air tem-perature, equivalence ratio f, and combustionchamber length Lc. Instabilities take place only if Tinis greater than a threshold value , which lies be-T*intween 650 and 670 K. In addition, f must fall in therange between 0.5 and 0.7 for instability to be in-duced. If those two conditions are not met simulta-neously, the flame remains stable. Inlet air tempera-tures higher than do not seem to alter theT*ininstability strength in terms of the acoustic pressureamplitude. In contrast, oscillations are quite sensi-tive to the equivalence ratio and may reach ampli-tudes as high as 15–20% of the mean chamber pres-sure around f ù 0.6. The instability strength alsodepends on combustor length. Although not shownhere, a companion study of a generic counter-rotat-ing swirl injector showed a drastic increase in oscil-lation amplitude when Lc was increased to 350 mm.

The coupling of oscillations between the chamberand the fuel supply line was examined by chokingthe fuel flow right at the injection location for manyexperiments. Under this condition, only a slight de-crease in chamber pressure fluctuation was ob-served, suggesting that the instabilities encounteredin the present study are intrinsic in nature and notcoupled to the fuel supply line.

The presence of hysteresis effects with respect toinlet temperature and equivalence ratio on instabil-ities was noted under certain conditions. For a fixedequivalence ratio, stable combustion cannot be re-established from an unstable operation unless theinlet temperature is reduced to a level significantlylower than , sometimes as low as 630 K. Likewise,T*inflames often stay unstable even below the equiva-lence-ratio limit at which they initially became un-stable. The mechanisms of hysteresis are currentlyunder investigation.

Oscillation Characteristics

The measured acoustic pressure signals clearly in-dicate the presence of standing, longitudinal oscil-lations in the system. Figure 3 presents a typical timetrace of the acoustic pressure signal and its spectralcontent. Two well-defined harmonics are exhibited,a strong one around 1750 Hz and a much weakerone around 3500 Hz. They correspond to the First(1L) and Second (2L) modes of longitudinal acousticwaves in the combustion chamber. To further iden-tify the acoustic mode structure, pressure measure-ments were conducted at the same axial but differentcircumferential locations. No discernible variationsof acoustic signals in terms of both amplitude andphase were observed in the transverse plane. Theacoustic waves are indeed longitudinal in the presentstudy.

Results from the linear acoustic analysis agreequite well with the measured frequencies, pressure

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Fig. 4. Amplitude and phase profiles of dynamic pres-sure and velocity for 1L mode from acoustic model. Com-parison with experimental points (solid squares).

Fig. 5. Effect of equivalence ratio and chamber pressureon NOx and CO emissions (corrected to 15% O2).

amplitudes, and phases at various locations. Figure4 shows the predicted and measured acoustic pres-sure as well as the predicted velocity distribution inthe entire domain for the First mode of oscillation.Because the linear analysis cannot calculate the ab-solute magnitude of oscillation, the comparison ismade by matching the calculated pressure fluctua-tion at the combustor exit with the measured data.The acoustic mode shape clearly indicates that un-steady heat release provides the energy for drivingstanding oscillations in the combustion chamber.The acoustic wave then propagates upstream

through the premixer, with much of its energy ab-sorbed by the shock wave in the inlet venturi. Thepremixer behaves as a silencer for disturbances orig-inating from the combustor and consequently leadsto a substantial decrease in the wave amplitude. Theirregular distribution of the wave amplitude in theupstream section results from the variation of flowpassage area in that region.

Emissions Data

For a combustion instability study to be reallymeaningful from a practical sense, great care mustalso be given to engine performance and pollution.High engine efficiencies and low emissions are anabsolute requirement. Hence, NO, NO2, CO, CO2,and O2 emissions have been systematically moni-tored in the study. Plots of NOx and CO concentra-tions versus equivalence ratio are particularly im-portant and are presented in Fig. 5. The trendsdisplayed are typical of lean premixed gas turbinecombustors [12,13,16,17]. Especially noteworthy isthe low NOx level in the f 4 0.5–0.6 region. Thisalso indicates that the fuel-injection scheme used inthe present work provides adequate fuel-air premix-ing.

Flame Structure–Instability Mechanisms

Photographic imaging and CH chemilumines-cence techniques were employed to capture un-steady flame dynamics. Figure 6 shows two typicaltime-averaged photo images under stable and unsta-ble operating conditions. The chamber pressure andequivalence ratio were fixed at 0.48 MPa and 0.6,respectively, but the flame became unstable whenthe inlet temperature was increased from 570 to 660K. Figure 7 represents the calculated temperaturecontours and streamline patterns for the stable flamecorresponding to Fig. 6a. The analysis is based oncomplete conservation equations in three dimen-sions and a three-step global reaction mechanism formethane combustion with air [18]. Good agreementwith experimental observation is obtained in termsof flame shape and heat release distribution. Thestreamline plot indicates the existence of three re-circulation zones in the chamber as a result of ad-verse pressure gradients. The central toroidal vortex,consisting of two inner recirculation bubbles in thewake of the center body, is principally caused by thestrong swirl and flow expansion. The corner recir-culation zone results from the sudden enlargementof the combustor configuration. These different fea-tures agree well with other studies of swirling flows[19,20], which reveal that the sizes of the central andcorner recirculation regimes are strongly dependenton the inlet swirl level, equivalence ratio, and com-bustor geometry.

When instabilities occur, the flame becomes much

COMBUSTION DYNAMICS OF PREMIXED SWIRL INJECTOR 1853

Fig. 6. Photographic images of (a) steady and (b) un-steady flame structures.

Fig. 7. Numerical calculations of streamlines and temperature contours of a stable flame at Pc 4 0.48 MPa, f 4

0.60, and Swirl number S 4 0.69.

shorter and penetrates into the corner recirculationzone, as evidenced in Fig. 6b. The unburned com-bustible mixture is rolled into large vortical struc-tures originating from the corners of the center bodyand reacts vigorously once in contact with local hoteddies. Periodic heat release then occurs, providingthe energy required for driving flow oscillations. Thedynamic evolution of shear-layer structure may beattributed to the strong velocity fluctuation at thedump plane. The acoustic analysis suggests that theaxial velocity fluctuation u8 downstream of the swir-ler is about 45 m/s for the present case with p8 about20% of the mean pressure. This value is very signifi-cant compared with the local mean velocity of 82m/s. The impact of u8 on the downstream vorticalflow development and its ensuring influence onflame holding and spreading characteristics are tre-mendous.

The unsteady flame dynamics are further exam-ined by taking chemiluminescence images of CHradicals, which provide a qualitative measure of heatrelease rate. Figure 8 shows the phase-locked imageswith reference to the local acoustic pressure overone cycle of oscillation. Each image is an average of100 instantaneous shots taken at the same phase.The observed phenomenon of combustion oscilla-tion may be visualized as follows. Initially, the pre-mixed combustible mixture is entrained and mixedwith hot gases in the vortical structures downstreamof the center body (i.e., image 1). Heat release thenfollows after a short chemical induction time. Theresultant flow expansion tends to push the flame out-ward and simultaneously blocks the inlet flow at thedump plane due to the local pressure rise (i.e., im-ages 2–4). Moreover, the phase profile of Fig. 4 in-dicates that, at that time, u8 is just coming back fromstrongly negative values. Thus, the absolute flow ve-locity is probably much lower, and the flame tendsto anchor itself closer to the combustor dump. Theensuing decrease in the incoming mass flow accord-ingly retards the heat release and causes the cham-ber pressure to drop. The subsequent pressure drop

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Fig. 8. Phase-resolved CH chemiluminescence images showing the evolution of unsteady heat release during onecycle of local dynamic pressure. Axial location of pressure transducer is indicated on the First image.

then facilitates the delivery of the fresh mixture intothe chamber (i.e., images 6–8). However, a high flowvelocity (u8 close to its maximum) may stretch andweaken the flame (i.e., images 9–11), convecting itdownstream and delaying the chemical reaction to alater time to re-energize flow oscillations (i.e., im-ages 12–1). Then, the entire process repeats.

To further explore the mutual coupling betweencombustion response and acoustic motions, the mea-sured heat release fluctuation needs to be correlatedwith local pressure oscillation. The resultant Ray-leigh’s parameter ^p8q̇8&, defined as the time-aver-aged product of pressure and heat release fluctua-tions, provides a qualitative measure of the extent towhich unsteady heat release drives or suppresses in-stabilities [21,22]. In this regard, the light intensitysignal of each CH chemiluminescence image ofFig. 8 has been multiplied by the corresponding lo-cal pressure oscillation to yield a two-dimensionaldistribution of Rayleigh’s parameter. Figure 9 showsthe result for a typical unstable flame under condi-tions corresponding to those for Fig. 3. As explainedearlier, the large velocity fluctuation downstream ofthe premixer induces strong vortical motions en-training the premixed combustible mixture directlyinto the wake region of the center body. This phe-

nomenon is exactly reflected in the two-dimensionalcontour plots of Rayleigh’s index. Strong interactionsbetween acoustic oscillations and unsteady heat re-lease occur immediately behind the center body, asopposed to the situations under stable operatingconditions where a conical flame anchored at thecenter body spreads out smoothly.

To identify the influence of unsteady heat releaseon longitudinal oscillations, the Rayleigh index is in-tegrated over the entire cross section at each axiallocation, giving the result shown in the graph inFig. 9. The First mode of the acoustic wave plays amuch greater role in driving the instability. It shouldbe cautioned that Rayleigh’s index only provides aqualitative measure of combustion sensitivity and itscontribution to driving instability. The behavior ofeach acoustic mode can be quantified only if all driv-ing and damping mechanisms are taken into ac-count. In fact, although the Rayleigh index shown inFig. 9 suggests that both the First and Second modescan be excited by unsteady heat release, the Secondmode is most likely stable if all damping mechanisms(such as viscous dissipation and nozzle damping) areconsidered. The First mode must be unstable andthe Second stable in order to exercise limit-cycle os-cillations [23].

COMBUSTION DYNAMICS OF PREMIXED SWIRL INJECTOR 1855

Fig. 9. Two-dimensional and axial variation of Rayleigh’sindex. Contribution of 1L and 2L modes.

The sudden onset of the instabilities at a certainthreshold inlet air temperature may be attributedT*into the fact that the flame becomes shorter with alarger spreading angle at a higher inlet temperature.Consequently, more heat release takes place nearthe pressure antinode point of the 1L mode down-stream of the dump plane. This more concentratedenergy-addition process gives rise to a higher Ray-leigh index ^p8q̇8&, which could become high enoughto overcome the inherent damping mechanisms andsuddenly promote strong instabilities. The same ar-gument can also be applied to explain the observa-tion that instabilities are usually more severe for alonger chamber due to the relatively concentrateddistribution of heat release with respect to the modeshapes of acoustic oscillations. This explanation ofthe onset of instabilities is, of course, only qualitative(and somewhat speculative at this point) and cer-tainly requires future experimental and analytical in-vestigations.

Conclusions

An experimental study of combustion instabilityhas been undertaken to characterize a generic gas-turbine swirler injector at operating conditions simi-lar to practical applications. The range of conditionsat which instabilities occur has been recorded usinga stability map, and limits for inlet air temperaturesand equivalence ratios have been obtained. A de-tailed acoustic analysis of the frequencies and mode

shapes of instabilities has been carried out to obtainmore physical insight into the acoustic characteris-tics. The instability mechanism has been argued tostem from the strong interaction between flame dy-namics and flow oscillations inside the combustor,but more work is needed to provide an in-depthphysical understanding of the mechanisms respon-sible for the generation and sustenance of combus-tion instabilities. In particular, detailed analysis ofcollected data and more advanced numerical and an-alytical techniques should help attain that goal.

Acknowledgments

This material was prepared with the support of the U.S.Department of Energy, Federal Energy Technology Cen-ter, through a cooperative agreement with the South Caro-lina Energy Research and Development Center at Clem-son University, Cooperative Agreement NumberDE-FC21-92MC29061.

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