[american institute of aeronautics and astronautics 42nd aiaa aerospace sciences meeting and exhibit...

42nd AIAA Aerospace Sciences and Exhibition 5-8, Jan., 2004, Reno, Nevada

Combustion Instabilities and Control of a Multi-Swirl Atmospheric Combustor

Tongxun Yi† and Ephraim J. Gutmark††

Department of Aerospace Engineering and Engineering Mechanics University of Cincinnati, Cincinnati, OH 45220-0070

Abstract Thermo-acoustic instability and lean blowout (LBO) are investigated experimentally in an atmospheric swirling combustor. Possible factors affecting combustion instability are identified. With less than 1.0% of the combustion air, monochromatic or continuous air forcing of the swirling shear layer and fuel line, can reduce pressure oscillation amplitude by up to 90% to 98.5%; Phase-shift air forcing of the flame can reduce pressure oscillation amplitude by 78%. When pressure pulsations are attenuated, simultaneous reduction of NOx is observed and the flat flame front changes into a conical shape. For LBO, with decreasing equivalence ratio, relatively intense oscillations emerge from smooth turbulent combustions, followed by a more stable and quieter state before blowout. The regions of intense heat release oscillations overlap those of high mean heat release. Depending on the swirling flow field, lifted flame or anchored flame is observed for LBO. LBO limit extends at higher air flow rate. A dump swirler with a lower velocity region near the dump plane substantially extends the LBO limit by 42.8%. High frequency air forcing of the fuel line helps maintaining stable combustion near LBO. 1. Introduction Low emission and wide stability are among the basic requirements of gas turbine combustors. LPP burning is a promising approach for low emissions, especially for a compact combustor, due to the relatively low flame temperature and short resident time. However, LPP combustion is susceptible to dynamic instability and static instability. Dynamic instability refers to the positive coupling between acoustic pressure and heat release rate oscillations, as described by Rayleigh’s criterion [1]. Static instability refers to lean blowout. Random oscillations of pressure and heat release rate are among the typical features of turbulent combustions. Under certain conditions, well organized, self-sustained and internally coupled oscillations of pressure and heat release may occur, i.e., dynamic instability. Time-lag mechanism [2], convective entropy wave [3], and shedding vortices [4][5] are among the ______________________________________

† Graduate Research Assistant †† AIAA Associate Fellow, Chaired Professor and Ohio Eminent Scholar. [email protected]

common mechanisms of dynamic instability Both passive and active control strategies have been developed to stabilize the strong combustion oscillations. Typical passive control strategies include dissipating acoustic energy, reducing the coherence of shedding vortices [10][11], and modifying the combustor geometry. Active control strategy utilizes an actuator to introduce external energy to decouple the positive feedback between pressure and heat release rate oscillations. The actuator may work in a prescribed manner like monochromatic forcing, or work according to a feedback law such as phase-shift, LQG/LTR, LMS and adaptive control laws [12][13][14][15]. Some of the active control strategies also looked at emissions, efficiency and extended flammability [9][16]. Most of the previous works are for bluff-body stabilized combustors or dump combustors. Less work has been reported for swirling combustors, although they have been extensively utilized in industry. Swirling flows and associated breakdown have a profound effect on air-fuel mixing, flame structure, stability, combustion intensity and pollutant formation [6][7]. Paschereit and Gutmark obtained planar or helical unstable modes by changing the exit area of an atmospheric premixed swirling combustor. Using phase-shift acoustic forcing,

1 American Institute of Aeronautics and Astronautics

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

AIAA 2004-635

Copyright © 2004 by Tongxun, Yi and Ephraim, J. Gutmark. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

they achieved pressure attenuation and emission reduction simultaneously. The success of acoustic forcing is attributed to the reduced coherence of vortices, instead of anti-sound effect [9]. Static instability or LBO usually occurs at low equivalence ratios or during rapid transient processes such as aircraft taking off or sudden power reduction. Combustion near LBO is substantially different from the normal combustion, which considerably complicates the modeling, analysis and control. Gutmark extended the LBO limit of a premixed dump combustor by generating small-scale vortices using shear layer forcing. Closed-loop forcing is found to be more effective than open loop forcing [17]. Sturgess described the LBO sequence as intermittency in shear flame, severe intermittency in shear flame, and large-scale axial movement of the flame region, and then lean blowout. He found that LBO limit could be extended by exit blockage [18]. Durbin and Ballal noticed that LBO was improved by increasing the outer swirl intensity provided that the inner swirl is stronger than the outer swirl [23]. Thiruchengode and Nair denote the local flame extinction and reignition preceding LBO as LBO precursors. Based on the detection of LBO precursors using thresholding, statistical analysis or frequency analysis, they extended the LBO limit by modifying the fuel injected to the flame stabilization zone. [19]. In this experimental study, influencing factors of dynamic instability for a multi-swirl atmospheric combustor are identified. Dynamic instability is effectively attenuated by monochromatic or continuous air forcing of the swirling shear layer, fuel line and flame. Dynamic instability, smooth turbulent combustion, relatively intense oscillations, even quieter combustion and LBO are observed sequentially with decreasing equivalence ratio. Near LBO, the flame stabilization region seems to lie within the swirling shear layer instead of the vortex breakdown region. A dump swirl with lower velocity region near the dump plane has an extended LBO limit. 2. Experiment Setup The key components of the combustion rig include a triple annular research swirler (TARS) shown in Fig. 1, a 35 kW electrical air heater, an air conditioning section and a 26’’ long combustion chamber. Two combustion chambers are used. One is a steel chamber with length-diameter ratio 4.5, and the other is a quartz

chamber with length-diameter ratio 6.5. The TARS has three air passages and the fuel injection points are uniformly distributed between the outer and middle swirls. Inlet velocity profile and swirl number can be changed by different swirlers combinations. The combustor is fueled by propane. The data acquisition system includes a Dspace board CP1104 and Dspace software, BNC-2110 DAQ board and labview software, Kistler Piezoelectric dynamic pressure transducer 7061B, Druck PMP 4000 pressure transducer, OH sensitive silicon optical fiber CF1493-42, high resolution (1024x1024 pixel) Roper Scientific super blue ICCD camera with Nikkon 105 mm/F4.5 UV lens with OH optic filter, and IMR3000P gas analyzer. The output voltage of the OH optic fiber is assumed to be proportional to the local heat release rate [22]. The actuator is an automotive fuel injector PB2-1600 from RC Engineering. 3. Characteristics of Dynamic Instability Multiple factors affect dynamic instability. A counter-rotating swirl over a co-rotating swirl, liquid fuel over gaseous fuel, a cylindrical section chamber over of a square section chamber, covered TARS (the inner and intermediate swirlers are blocked), and mixing tube introduce dynamic instability at a lower equivalence ratio. These mechanisms may be associated with the vortex dynamics and air-fuel mixing. A combustion chamber with larger length-diameter ratio is easier to introduce strong oscillations. Sound dissipation level near the dump plane also plays a role. A small hole near the dump plane could postpone unstable combustion to a higher equivalence ratio, and substantially reduce the oscillation amplitude. High power and high air inlet temperature also favor dynamic instability. Fig. 2 shows the effects of air inlet temperature. With inlet air temperature 423 K, unstable combustion occurs at equivalence ratio 0.78. But without heater, the combustion remains stable up to equivalence ratio 0.84. There is no doubt that equivalence ratio affects dynamic instability. For this rig, unstable combustion usually occurs at equivalence ratio between 0.75 and 1.2. Combustion process may experience stable, semi-stable and unstable states with a small increase of equivalence ratio. Hysteresis phenomenon is observed. Dynamic instability can be initiated by increasing the equivalence ratio to a critical point while keeping


the constant air flow rate. But decreasing equivalence ratio from this point will not make the unstable combustion disappear immediately. For example, dynamic instability begins to appear at equivalence ratio 0.78 at

28568Re == νUD , where U and are

the average axial velocity and the exit diameter of the TARS exit respectively, and

D

ν is the kinetic viscosity. But for the dynamic instability to disappear, the equivalence ratio should be below 0.7. It is also noticed that a fast increase of equivalence ratio will trigger unstable combustion at a lower equivalence ratio. Heat capacity of the rig also affects dynamic instability. Strong oscillations may exist just after the start up of the combustor. However, the strong oscillations may disappear after the combustor rig is warmed up. 4. Active Control of Dynamic Instability 4.1 Air Forcing of Swirling Shear Layer Unstable combustion is observed at equivalence ratio 0.78 with power 65 kW. The quartz combustion chamber, covered swirl and 1’’ mixing tube are used. Fig. 3 shows the spectra of the pressure and OH signals. Unstable combustion occurs at 260 Hz. The dominant acoustic mode is a planar quarter wave mode. Fig. 4 shows that, the OH amplitude and its phase relationship with pressure change axially. In fact, the phase does not change azimuthally. The minimum phase lies 3’’ above the dump plane, where the combustion is most intense. It is clear that the local heat release rate oscillations are in phase with the pressure oscillations. Fig.5 shows pressure and the phase-locked spatially averaged OH intensity by the ICCD camera. The average OH intensity is an indicator of global heat release rate. It is clear that, most of the time, pressure oscillation and global heat release oscillation are in phase. So Raleigh’s criterion is satisfied. Filtered pressure is used as the trigger signal. The ICCD camera is working at gate width 100 us, intensifier gain 128, interval 0.16 ms (about 150 phase difference) and gate delay 0~8 ms (roughly two pressure cycles). OH optic filter (bandwidth 10 nm around wavelength 310 nm) is used. Monochromatic air forcing of the swirling shear layer can effectively stabilize pressure oscillations. The control is introduced to the outer swirler at one location from air supply at 90 Psi. The control air flow rate is less than 1% of the combustion air. Fig. 6 shows the pressure and heat release oscillations with 100 Hz air

forcing. Unstable combustion is attenuated within 350 ms. Pressure and heat release rate oscillation amplitudes are reduced by 98.5% (36.5 dB) and 96.6%, respectively. Fig. 7 shows the effects of control gain with 100 Hz air forcing. Large control gain is more effective. However, very large gain above 1 is not necessary. Fig. 8 shows that steady and continuous air forcing is also very effective in pressure stabilization. It has been found that NOx formation is also reduced when pressure oscillations are stabilized. The reduction of pressure oscillations and emissions may be a result of enhanced air-fuel mixing due to the generation of small-scale vortices. These small-scale vortices may grow from the air forcing disturbances via Kelvin-Helmoltz instability mechanism. The improved air-fuel mixing avoids trapping of large fresh fuel packets within the large vortices, or unburned air-fuel mixture within large vortices. So the intense heat release rate oscillations are considerably reduced after the breakup of the large vortices. As a result, less thermal energy will be transferred to the acoustic field. According to Raleigh’s criterion, as long as the heat addition to the acoustic field is smaller than the acoustic energy dissipation across the boundaries, combustion instability will be suppressed. The emissions reduction may be due to more uniform lean combustion as a result of improved mixing. The success of pressure attenuation by steady air forcing may result from the reduced coherence of large vortices within swirling flows due to asymmetric air forcing. Fig. 9 shows a conical shaped flame structure when pressure oscillations are attenuated with 100 Hz air forcing. Without control, the flame is not symmetric, with intense combustion on one side, and the flame front is rather flat and yellow indicating soot formation and reduced combustion efficiency. With 100 Hz forcing, the flame becomes more symmetric, with intense burning within the center region above the dump plane. The mean flame structures are obtained by averaging 128 instantaneous ICCD images (intensifier gain 128). 4.2 Air Forcing of Fuel Line For the steel chamber, unstable combustion is observed at equivalence ratio 0.83 and power of 27 kW. Covered swirl and 1’’ mixing tube are used. The dominant acoustic mode occurs at 225 Hz with amplitude 71 Pa. In this forcing mechanism, the control air and fuel are introduced to the TARS from different passages, and are ejected outside together. The control air


is introduced from air supply at 90 Psi, with flow rate less than 1% of combustion air. Monochromatic air forcing of the fuel line stabilizes combustion oscillations. Fig. 10 shows the stabilization of pressure stabilization with 240 Hz air forcing. Pressure oscillation amplitude is reduced by 92% (23 dB). Fig. 11 shows the effects of air forcing frequency with control gain 1. High frequency air forcing achieves more pressure attenuation. Small gain less than 0.1 is not effective, and very large gain above 1 is not necessary. Steady and continuous air forcing of the fuel line is also effective in stabilizing pressure pulsations, as shown in Fig. 12. Unstable combustion can be stabilized within 80 ms. Emissions, especially NOx, are also reduced. The reduction in pressure oscillations and emissions by fuel line air forcing may be due to the enhanced momentum and penetration of fuel out of the fuel nozzle, which improves air-fuel mixing. The fuel line forcing may also generate small-scale vortices within the swirling shear layer. 4.3 Air Forcing of the Flame The experimental setup and working conditions are the same as fuel line forcing. Baseline NOx concentration is 15 ppm. Monochromatic air forcing of the flame attenuates pressure oscillations, however, it is not so effective as shear layer forcing or fuel line forcing. Most effective forcing frequency lies within 50 to 250 Hz. With control gain 1.0, the maximum reduction of pressure oscillations is 65% at 225 Hz, with 30% NOx reduction. Continuous air forcing is not effective. Better results are achieved by phase-shift air forcing. Fig. 13 shows the phase-shift control diagram. The pressure signal is filtered by a 4th order band-pass butterworth filer (180 to 300 Hz), and then amplified and sent to a switch. The output TTL signal controls the on/off of the fuel injector via a relay. Fig. 14 shows the effects of control phase, with control gain 1. Here, the pressure amplitude and NO concentration are normalized by the baseline values. The most effective control phases for pressure attenuation and NO reduction are opposite. Maximum pressure attenuation is 78% (13.2 dB), achieved at control phase around 1000. Fig. 15 shows the pressure dynamics at control phase 121.50. Pressure pulsation is stabilized within 200 ms. Not surprisingly, large control gain is more effective for pressure attenuation and NO

reduction (Fig. 16). However, control gains above 1 are not necessary. The attenuation of pressure oscillation by phase-shift air forcing may be due to the modified instantaneous flame structure, and thus flame area and instantaneous heat release rate. Under proper control phase, the forcing-induced heat release rate oscillations may considerably counteract the intrinsic heat release rate oscillations. Thus, less chemical heat release will be converted to acoustic energy. If the energy addition to the acoustic field is smaller than acoustic energy dissipation across the boundaries, pressure oscillations level will be reduced. The reduction of NOx formation may be due to the local flame cooling by the external forcing air. Since emission rate is exponentially dependent on flame temperature, the NOx formation is reduced due to reduced flame temperature 5. Characteristics and Control of Static Instability Extending LBO limit helps to maintain low emissions and improve flight safety. Different from dynamic instability, LBO usually occurs at a relatively low equivalence ratio or during rapid transient period. Static instability is not characterized by strong pressure oscillations or heat release oscillations. LBO may be affected by local strain rate, flame curvature, turbulent burning speed, turbulence intensity, boundary conditions and heat loss. Flame may be extinguished at high stretch rates due to the rapid cooling of fresh air-fuel, and may also be extinguished at low stretch rates due to the radiative heat loss. High strain rate may occur when a large vortex intrudes into the flame within swirling flows [20][21]. The flame response to unsteady strain rate is in a large extent determined by the time scales of large turbulent structures, mass transfer, heat transfer and chemical kinetics. Structural changes have been observed for combustion near LBO [18][19]. In this study, LBO is approached by reducing equivalence ratio while keeping Re constant. 26’’ quartz tube and 1’’ mixing tube are used. Mixing tube is not used here. It is found that fast reduction of fuel supply leads to flame blowout at a higher equivalence ratio. At higher Re and higher equivalence ratio (Fig. 17), the flame extends from the swirler exit almost in a cylindrical shape, with a sudden expansion towards the chamber walls about 3’’ above the dump plane, forming a rather flat flame front.


The abrupt flame expansion may be associated with vortex breakdown. The flame exhibits three azimuthal lobes, maybe associated with shear layer dynamics. But near LBO (Fig.17), intense combustion seems to occur within the shear region, and the flame structure is not symmetric. The asymmetric flame structure may be a result of the asymmetric geometry of TARS due to machining tolerance. However, the most likely reason may be the intrinsic three-dimensionality of swirling shear flows and the precessing vortex core. The region of high heat release rate oscillations overlaps that of high mean heat release, as shown in Fig. 18. At lower Re and higher equivalence ratio (Fig. 19), the flame extends from the swirl exit in a conical shape, with the cone tip touching the chamber wall at a much smaller angle. The mean flame structures shown in Fig. 17~ 19 are obtained by averaging 128 instantaneous ICCD images (intensifier gain 250). With decreasing equivalence ratio, combustion field exhibits a sequence of structure changes as indicated by the spectra shown in Fig. 20, where Re . At equivalence ratio 0.75, the low frequency components (below 100 Hz) of the OH output is basically a random signal, which is typical of turbulent combustions. At equivalence ratio 0.58, relatively intense combustion noise emerges, indicating the occurrence of a bifurcation phenomenon. The flame periodically vibrates and tilts to one side. The peak clusters around 17 Hz in the OH spectrum may be associated with the large-scale flame extinction and reignition. However, at equivalence ratio 0.55, the relatively intense combustion noises disappear, resulting in an even quieter state. The spectrum shows reduced amplitude with frequency, a typical chaotic behavior. The may result from of a cascade of successive bifurcations. With a further reduction of equivalence ratio, flame is lifted upwards to the chamber exits and anchors there, and extinguishes shortly. This process is accompanied by excessive CO formation.

41000=

Fig. 21 shows that LBO limit is extended at higher Re. This may be associated with the flame stabilization mechanism for swirling flows. For swirling flows, the flame is stabilized by the recirculating vortex breakdown region at the swirler exit. At high Re, the tangentially velocity is high, and consequently the negative radial pressure gradient is high, which favors the inward motion of hot products to the recirculating vortex breakdown region. Here, the LBO limit is defined as the critical equivalence

ratio at which flame blowout occurs. It is found that high frequency air forcing of the fuel line helps to improve combustion efficiency near LBO. At 16000=eR and equivalence ratio 0.56, the NO and CO concentration is 15 ppm and 55 ppm, respectively. Fig. 22 shows that, high frequency air forcing (above 250 Hz) can effectively reduce CO up to 60%. However, low frequency forcing would exacerbate combustion near LBO. High frequency air forcing extends the LBO limit may be due to the enhanced generation of small-scale vortices within the swirling shear layer. The mean flame images by ICCD camera show that most heat release occurs within the swirling shear layer near LBO. The enhanced generation of small-scale vortices within this region considerably improves air-fuel mixing and turbulent combustion speed. This helps to extend LBO limit. The flame stabilization mechanism seems to play an important role in LBO. Fig. 23 and Fig. 24 show the formation of CO, NOx and exit temperature for the TARS swirl with blocked inner and middle swirl. At , excessive amount of CO and large temperature drop at exit occur at equivalence ratio below 0.42. The flame extinguishes at equivalence ratio 0.32, much lower than LBO limit for the uncovered swirl. Covering the swirl extends LBO may be because of the very low speed region near the dump plane. A considerably amount of fuel are injected into this region through the fuel ports within the middle swirl. This forms the primary flame stabilization region. Even through the global equivalence ratio is considerably low, the local equivalence ratio within this region can be fairly high. This is why anchored flame is observed for the covered swirl near LBO while the flame is lifted off towards to chamber exit for the uncovered swirl.

16000Re =

6. Conclusions Combustion instability is affected by multiple factors, such as combustor geometry, inlet velocity profile, fuel, acoustic boundary condition, combustion air inlet condition and so on. A subtle change of these parameters could considerably affect the occurrence and intensity of combustion oscillations. For certain setups, monochromatic, continuous or phase-shift air forcing of the fuel supply, swirling shear layer and the flame can effectively attenuate pressure pulsations up to 36.6 dB, with simultaneous reduction of NOx. Flame structural changes are observed within swirling combustions flows.


With decreasing equivalence ration at constant Re, dynamic instability, smooth turbulent combustion, intensified combustion oscillation, even quieter combustion, and LBO, are observed sequentially. Higher combustion air flow rate has an extended LBO limit for swirling flows. A swirling flow with a small velocity region near the dump plane can considerably extend LBO, and high frequency air forcing of the fuel line helps to improve combustion efficiency near LBO. Reference: [1] Rayleigh, J.W.S, “The Theory of Sound,” Dover Publications, New York, 1945. [2] Crocoo, L., Grey, J., and Harrje, D.T., “ Theory of Liquid Propellant Rocket Combustion Instability and Its Experiment Validation,” ARS J., vol.30, No.2, 1960, pp.159-168. [3] Chu, B.T., “On the Generation of Pressure Waves at a Plane Flame,” Fourth Int. Symp. on Comb., 1953, pp.603-612. [4] Gutmark, E.J., Parr, T.P., Schadow, K.C., “Visualization of Vortex Dynamics in Flame Combustion, ” Bulletin of the American Physical Society, Vol.31, No.10, pp.1681. [5] Poinsot, T., Trouve, R.F., Veynante, D.P., Candel, S.M. and Esposito, E.J., “ Vortex Driven Acoustically Coupled Combustion Instabilities,” J. Fluid Mech., Vol.177, pp.262-292. [6] Lilley, D.G., “Swirl Flows in Combustion: A Review,”AIAA Journal, vol.15, 1977, No.5. [7] Froud, D.Y., Beale, A.J., O’Ooherty, T., Syred, N., “Studies of Helmoltz Resonance in a Swirl Burner Furnace System,” 26th International Symposium on Combustion, 1996, pp.3355~3362. [8] Sivasegaram, S., and Whitelaw, J.H., “The Influence of Swirl on Oscillations in Ducted Flames,” Combust. Sci. and Tech., Vol.85, 1991, pp.195. [9] Paschereit, C.O., Gutmark, F.J. and Weisenstein,W., “Coherent Structures in Swirling Flows and Their Role in Acoustic Combustion Control, ” Physics of Fluid, Vol.11, No.9, 1999. [10] Schadow, K.C., and Gutmark, E.J., "Combustion Instability Related to Vortex Shedding in Dump Combustors and their Passive Control, " Progress in Energy and Combustion Sciences, Vol.18, 1992, pp.117-132. [11] Gutmark, E.J., and Grinstein, F., "Mixing in Non-Circular Jets, " Annual Review of Fluid Mechanics, Vol. 31, 1999, pp. 239-272.

[12] Schadow, K.C., Gutmark, E.J., and Parr, T.P., "Study of Combustion Dynamics for Passive and Active Control, " Progress in Aeronautics and Astronautics, 1993, pp.258-277. [13] Annaswamy, A.M., O.EI-Rifai, Fleifilj, M., and Ghoniem, A.F., “A Model-Based Self-Tuning Controller for Thermoacoustic Instability,” Combust. Sci. and Tech., Vol.135, 1998, pp.213-240. [14] Riley, A.J., Park, S., Dowling, A.P., Evesque., S., and Annaswamy, A.M., “Adaptive Closed-Loop Control on an Atmospheric Gaseous Lean-Premixed Combustor, ” GT-2003-38418, 2003. [15] Neumeier, Y., and Zinn, B.T., “Active Control of Combustion Instabilities with Real Time Operation of Unstable Combustor Modes,” AIAA Paper 96-0758, 1996. [16] Gutmark, E.J., Parr, T.P., Wilson, K.J., Hanson-Parr, D.M., and Schadow, K.C., “ Use of Chemiluminescence and Neural Networks in Active Combustion Control, ” 23rd Symposium (International) on Combustion, The Combustion Institute, 1990, pp.1101-1106. [17] Gutmark, E.J., Parr, T.P., Wilson, K.J. and etc, “Closed-Loop Control of a Flame and a Dump Combustor,” IEEE Control Systems Magazine, Vol. 13, No.2, 1993, pp.74~78. [18] Sturgess, G.J., and Heneghan, S.P., “Effects of Back-Pressure in a Lean Blowout Research Combustor,” Journal of Engineering for Gas Turbines and Power, Vol.115, 1993, pp.486. [19] Thiruchengode, M., and Nair, S., “An Active Control System for LBO Margin Reduction in Turbine Engines,” AIAA 2003-1008, 2003. [20] Bayliss, A., “From Traveling Waves to Chaos in Combustion,”, SIAM Journal on Applied Mechanics, Vol.54, No.1, 1994, pp.147-174. [21] Ju, Y., “On the Extinction Limit and Flammability Limit of Non-Adiabatic Stretched Methane-Air Premixed Flames,” J. Fluid Mech., Vol.342, 1997, pp.315-334. [22] Habib, N., “On the Adequacy of Certain Experimental Observables as Measurements of Flame Burning Rate,” Combustion and Flame, Vol.113, 1998, pp.312-332. [23] Durbin, M.D., and Ballal, D.R., “Studies of Lean Blowout in a Step Swirl Combustor,” Journal of Engineering for Gas Turbines and Power. Vol.118, 1996.


Figures

Main Fuel

Pilot Fuel

Middle Swirler

Inner Swirler

Outer Swirler

Fig. 1. TARS Structure

Dominant Acoustic Mode (Air Inle T=298 K)

0

50

100

150

200

250

300

350

0.5 0.6 0.7 0.8 0.9

Equivalence Ratio

Peak Amplitude (Pa)Peak Frequency

Dominant Acoustic Mode (Air Inlet T=423 K)

-50

0

50

100

150

200

250

300

350

400

450

0.5 0.6 0.7 0.8 0.9

Equivalence Ratio

Peak Amplitude (Pa)Peak Frequency

Fig. 2 Effects of Air Inlet Temperature

Fig. 3 Spectral Analysis of Pressure and OH Signal


Fig. 4. Dynamics of Pressure and OH Signal

P' and Q'(dot)

-400-300-200-100

0100200300400500

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49

Snapshots

P' and Q'(dot)

Q'(dot)x20P'

P'xQ'(dot)

-2000-1000

0100020003000400050006000

1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49Snapshots

Fig. 5 Pressure Oscillation and Global Heat Release Oscillation (Phase-locked Measurement)

Fig. 6. Dynamics of Pressure and OH Signal for 100 Hz Air Forcing


Effects of Control Gain on Pressure Amplitude

050

100150200250300350

0.001 0.01 0.1 1 10 100

Phase-shift Gain

Pressure Peak (Pa)

Fig. 7. Effects of Control Gain on Pressure Stabilization

Fig. 8. Continuous Air Forcing of Swirling Shear Layer

(a) Baseline (b) 100 Hz Air Forcing

Fig. 9. Mean Flame Structure with and without Control

Fig. 10. 240 Hz Air Forcing of the Fuel Line


Dominant Pressure Amplitude VS. Forcing Frequency

0102030405060

0 50 100 150 200 250 300 350 400 450 500

Frequency (Hz)

Pa

Fig. 11. Effects of Air Forcing Frequency

Fig. 12. Pressure Attenuation With Continuous Air Forcing

Fig. 13. Phase-Shift Control Diagram

P r e s s u r e A t t e n u a t i o n a n d N O R e d u c t i o n . V S . C o n t r o l P h a s e

00 . 10 . 20 . 30 . 40 . 50 . 60 . 70 . 80 . 9

0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0

C o n t r o l P h a s e

P r e s s u r e A t t e n u a t i o n

N O R e d u c t i o n

Fig. 14. Pressure Attenuation and NO Under Different Control Phase


Fig. 15 Pressure Dynamics with Phase-Shift Air Forcing

Pressure Attenuation and NO Level VS. Control G ain

0

0.2

0.4

0.6

0.8

1

1.2

0.1 1 10 100Control Gain

Pressure AttenuationNO Level

Fig. 16 Effects of Phase-Shift Gain

(a) Equivalence Ratio 0.75 (b) Equivalence Ratio 0.55

Fig. 17 Mean Flame Structure at 41440Re =


Fig. 18 RMS of Heat Release Rate Oscillations at 41000Re =



Fig. 19. Mean Flame Structure at 16000Re =

Fig. 20 OH Spectrum at 41440Re =

LBO Limit with Inlet Reynolds Number

0.5

0.51

0.520.530.540.55

0.56

0.57

16000 21000 26000 31000 36000 41000Re

LBO limit

Fig. 21. LBO Limit with Re


C O a n d N O C o n c e n t ra t io n .V S . F o rc in g F re q u e n c y

0

0 . 2

0 . 4

0 . 6

0 . 8

1

1 . 2

1 . 4

1 . 6

1 . 8

0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0

F r e q u e n c y ( H z)

N o rm a lize d C O C o n c e n t ra t io nN o rm a lize d N O C o n c e n t ra t io n

Fig. 22. NO and CO With Monochromatic Air Forcing

0

5 0 0

1 0 0 0

1 5 0 0

2 0 0 0

2 5 0 0

0 .3 0 .3 5 0 .4 0 .4 5 0 .5 0 .5 5

E q u iv a le n c e R a t io

[CO]

- 2

0

2

4

6

8

1 0

1 2

[ C O ] ( p p m ) [ N O ] ( p p m )

Fig. 23. No and CO Near LBO

E x i t T e m p e r a tu r e n e a r L B O

0

2 0 0

4 0 0

6 0 0

8 0 0

1 0 0 0

1 2 0 0

0 .3 0 .3 5 0 .4 0 .4 5 0 .5 0 .5 5

E q u iv a le n c e R a t io

K

Fig. 24. Exit Temperature Near LBO


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