Initiation and suppression of combustion instabilities by active control

Twenty-Second Symposium (International) on Combustion/The Combustion Institute, 1988/pp. 1363-1370

INITIATION A N D S U P P R E S S I O N O F C O M B U S T I O N I N S T A B I L I T I E S BY ACTIVE CONTROL T. POINSOT, D. VEYNANTE, F. BOURIENNE, S. CANDEL, E. ESPOSITO AND J. SURGET* EM2C Laboratory, C.N.R.S., Ecole Centrale des Arts et Manufactures et CNRS 92295 Chatenay-Malabry, FRANCE

An active instability control (AIC) system is used to suppress low-frequency combustion instabilities in a non-premixed multiple-flame eombustor and to study their initiation. The AIC device is based on a feedback control loop: The signal provided by a microphone is filtered, processed and sent to driver units plugged on the air duct. It allows the suppression of all unstable modes of the combustor and also provides a new and powerful method to study the instability initiation. Starting from a controlled flame with AIC, the control system is switched off and the growth of the instability is analysed through high speed Schlieren cinematography and sound pressure and reaction rate measurements. Nonlinear spectral analysis is used to extract spectral information during the oscillation growth from the initiation to the limit cycle. Results reveal that five different phases take place between stable combustion and fully established instability. The source of the first flame movements is the flow excitation by longitudinal acoustic waves. When the flow oscillation amplitude is high enough to allow interactions between flame sheets, combustion becomes periodically enhanced. This coupling leads to a fast instability growth until a limit cycle is reached.

Introduction Combustion instabilities occur in many practical systems such as power plants, jet engine afterburners and rocket engines. Unstable combustion has many undesirable features. It induces large amplitude oscillations of the flow, mechanical vibrations of the combustor and of other components of the system, enhances the heat transfer rates at the combustor walls and in extreme cases leads to the total loss of the system. 1-7 There is no satisfactory general prediction method available for combustion instabilities. This is because the physical mechanisms are diverse and complex. The nonlinear features of the problem are another major difficulty in the development of analytical descriptions. Furthermore, these features manifest themselves in most experiments, and combustion instabilities are usually observed in the fully nonlinear form of a limit cycle. The initiation phase is observed in a small number of confgurations (essentially in solid propellant rocket experiments). This phase, however, is of considerable importance, and its analysis may provide valuable information for the following reasons: 1) The oscillation onset is a key mechanism in combustion instability. *ONERA, 92322 Chatillon

2) Linear models are conveniently derived to describe the initiation phase which corresponds to small amplitude levels. 3) In many circumstances initial oscillations do not lead to limit cycles but to flame extinction, thus preventing simple studies of the phenomenon. While it is dearly important to define the disturbanees leading to instability, the initiation of combustion instabilities has not been studied thoroughly because a reliable experimental method is missing. The initiation phase cannot be controlled and triggered simply. It requires, for example, a sudden modification of the geometrieal configuration like that used by Sterling and Zukoski3 in a study of oscillation onset in a dump eombustor or by Vaneveld et al. 4 in their investigation of the instability initiation in a backward facing step eombustor. Another ingenious device was used in the rocket instability studies conducted by Croeeo and his group. The rocket chamber was artificially stabilized by placing baffles in the cavity. These baffles were rapidly consumed by the intense reaction taking place in the chamber, and the instability growth could be observed after a short period of time. In these examples, the step variations imposed at the eombustor inlet or the baffles placed in the chamber can affect the growth of the instability in an undetermined way. Active control constitutes a more flexible and powerful method in the instability triggering. This

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UNSTEADY FLAMES

method uses external control loops to suppress combustion oscillations. Although the principles of active control are described in the early studies of Tsien8 and Crocco and Cheng, 9 practical demonstration of the concept has only been achieved in the past few years. Modern active control systems are somewhat similar to 'Anti-Sound' (see FfowcsWilliamsl° for a review of this subject) and have been applied to the control of oscillations in Rijke tubes (Hecklll), premixed laminar burners (Lang et al.lz), laboratory reheat channels (Dines, 13 Bloxsidge and Langhorne, 14 Bloxsidge et al. l~) or non-premixed turbulent combustors (Poinsot et al. i6). A major advantage of active control methods is that initiation may be studied by starting from a controlled regime and switching the control loop off. No modification of the flow conditions is required, and the initial instant of the initiation phase is precisely known. The study of Lang et al. ]z presents instability growth history curves in a laminar burner and shows that the results are independent of the control system parameters. The work of Lang was continued by using a high power non-premixed turbulent combustor developed at EM2C laboratory. This combustor exhibits strong combustion instabilities which are described in the following section. The control system and its performance are then briefly reviewed and initiation results are reported and analysed through nonlinear spectral analysis and high speed Schlieren cinematography.

Experimental Configuration and Unstable Mode Without Control

Experimental Configuration: The 250 kW turbulent combustor is sketched in Fig, 1, This combustor is 30 cm long and has a rectangular 10 × 5 cm 2 cross section. The chamber lateral walls are quartz windows allowing the visualization of the flames. The chamber is connected upstream to a long duct of the same cross section. This duct is fitted with instrumentation plugs. Upstream and downstream ends of the combustor are acoustically open. Air is supplied to the combustor at the upstream end of the duct. Propane is injected into the air flow through six narrow rectangular slots, The slots are in recess with respect to three backward facing steps.

Active

control

system

,

/ \

FIG. 1. Schematic diagram of the combustor. obtained from C2 or OH light emission measureinents. 16'24 Both radicals provided equivalent results. Figures 2 and 3 present spectral analysis of pressure and heat release signals without and with active control. Without control, these spectra reveal strong peaks at 230 Hz. This instability is charaeterized by a particularly high acoustic level (1700 Pa) and large movements of the flame front. A Schlieren picture of the flames indicates that the mean structure of the jets is modulated by acoustic oscillation (Fig. 4). In addition to the turbulence of the flow, large scale structures ("puffs") appear as characteristic patterns of the unstable reacting flow and are convected downstream. These structures are formed at the resonant acoustic frequency (230 Hz). This instability mode is the result of the interaction between a resonant duet mode and the S[~'ctralamplitude(dB)

lie.

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Description of an Unstable Regime: The experiments were performed at an air flow rate of 24 g/s and an equivalence ratio of 0.4. For this regime, the combustor exhibits a strong instability corresponding to a longitudinal acoustic mode of the combustion system at a frequency of 230 Hz. In this experiment, the local heat release rate was

60. 50.

CO~'fROt ON

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FIG. 2. Spectra of pressure signal with and without active control

COMBUSTION INSTABILITIES

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FIG. 3. Spectra of C2 radical emission with and without active control

non-steady heat release of the flame. A complete description of a similar mode is given in Poinsot et al. 24 or Sterling and Zukoski. 3

Control of Instability The oscillation is suppressed with an active control device. A microphone located on the test duct detects the acoustic pressure upstream of the chamber. The microphone signal is then filtered, phase-shifted with an analog "bucket-brigade" delay line, amplified and sent to a pair of driver units. These loudspeakers are plugged on the test duct, one facing the other at 20 cm from the microphone. A low-pass filter with a cut-off frequency of 3 kHz is used to suppress Larsen effects. The delay time introduced by the delay line is (2.1 + Td/) ms where Tat is a time chosen by the operator. For the frequency of 230 Hz, the different components of the control device (microphone, filter, amplifier, and loudspeaker) introduce a delay of 1.35 ms. The total delay of the control device is then: Trot =

FIG. 4. Sehlieren picture of the flames without control sion of the harmonics (Fig. 2). The residual noise is principally due to the flow turbulence. At the same time, the peak at 230 Hz in the heat-release signal is reduced by 22 dB (Fig. 3) and a Schlieren picture of the flames shows a standard pattern of turbulent burning jets withouts puffs (Fig. 6). The flame structure with AIC appears to be similar to Amplitude of t h e

3.45 + Tdl (ms)

Figure 5 represents the amplitude of the acoustic pressure with respect to the delay introduced by the control loop. For a sut~ciently high amplifier gain, and for delays between 4.7 and 6.4 ms the pressure level is reduced to 120 Pa. When the delay is greater than 6,4 ms, the acoustic level exceeds the sound pressure level without control. The AIC system then acts as an excitation device and enhances the instability. In this case, the combustion region is set into strong pulsations, and extinction of one of the injectors may occur. For time delays in the range 4.7 to 6.4 ms, spectral analysis of the pressure signal reveals an attenuation of 24 dB of the peak at 230 Hz, with a complete suppres-

acoustic pressure mth respect to delay

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FIG. 5. Amplitude of the acoustic pressure anaplitude versus total delay

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UNSTEADY FLAMES

FIG. 7. Time history of instability

- At time t3 = 0.8 s, both signal amplitudes increase faster and a limit cycle is obtained after time t4 = 1 s. It is worth noting that the limit cycle is obtained when the heat release signal reaches a zero minimum value. The growth of the oscillation amplitude is limited by the fact that the total heat release must remain positive. Therefore, the limit cycle amplitude obtained in Fig. 7 is mainly determined by the saturation of combustion rate oscillations and not by acoustic losses.

FIG. 6. Sehlieren picture of the flames with control that of a stable combustion region. It is also worth noting that AIC greatly increases the stability and operating domains of the combustor.

Initiation of Turbulent C o m b u s t i o n Instabilities

Time History of Instability Initiation: The AIC system will now be used to trigger instability and study the oscillation onset. Figure 7 displays the time evolution of pressure oscillation P and heat release signal Q during instability growth. Before time t = 0, instability is controlled with a gain of 4 and a delay of 5.2 ms. At time t = to = 0.08 s, the AIC system is switched-off and instability begins to grow. The following phenomena are observed: - The instability growth between time to and t~ = 0.4 s is slow indicating that the linear instability gain is only slightly greater than one.

A more accurate description of the different phases pictured in Fig. 7 can be deduced from a spectral analysis of the pressure and heat release signals combined with high-speed cinematography of the flames.

Spectral Analysis and High-Speed Cinematography of Instability Initiation: The spectral content of pressure and heat release signals during instability onset constitutes basic information on the growth of the limit cycle. For example, this information may be used to qualify some of the theoretical models put forward by Culick and his coworkers 6 to describe the process of instability triggering and growth. The main difficulty here is to be able to perform spectral analysis on time varying signal. Procedures for estimating the power spectral density (PSD) of deterministic or stochastic signals are generally based on the fast Fourier transform (FFT) and pose problems of sampling, segmentation and windowing (Oppenheim and Sharer Is ). The statistical stability of such power spectral densities is only achieved by averaging many elementary spectra. Special difficulties are encountered in the analysis of short data sequences such as the initiation of an instability. When the data length is limited, the statistical stability of a stan-

COMBUSTION INSTABILITIES dard spectral analysis may be assured only at the expense of resolution, because this resolution is inversely proportional to the duration of observation of the signal. Therefore the FFT based approach is not well suited to short data records. To overcome this difficulty, one may use recent nonlinear spectral methods (see Kay and Marple 19 for a review and perspective on modem spectral analysis). While this approach is not widely used in fluid mechanics, it is tested by us on velocity signals obtained from laser Doppler velocimetry (Veynante and Candel. 2° zz Results indicate that the maximum entropy spectral estimation, due to Burg and improved by Fougere, is well adapted to very short data sequences. The power spectrum is obtained by maximizing the entropy of the time series under the constraint that the known autocorrelation samples satisfy the Wiener-Khinchin relation (ChenZ3). In this approach, the computation time increases rapidly with the number of samples under analysis and with the order of the digital filter used to model the data. In our case, this computational time is higher by a factor of about 100 than that corresponding to FFT techniques. For each channel (microphone and photomultiplier), 8,000 samples are taken at a frequency of 4,000 Hz. Power spectral densities are computed from 200 samples (obtained during 50 ms). Results are smoothed by averaging six consecutive spectra. This nonlinear spectral analysis is now used along with high-speed Schlieren cinematography (2,000 frames/s) to characterize the instability onset. Figure 8 presents the time variations of the spectral amplitudes of the fundamental oscillation mode (250 Hz) and its harmonic frequencies for the microphone (Fig. 8a) and photomultiplier signals (Fig. 8b). These figures suggest that five phases may be distinguished during the instability onset: 1) an induction phase from to to tl, where the oscillation grows slowly. The fundamental mode appears as soon as the control system is switched-off (time to). No harmonic frequencies are present either in the microphone or in the photomultiplier signals. The Schlieren observation reveals that the two flame sheets issuing from each injector system are flapping in a varicous (breathing) mode. Furthermore, the three injector systems induce oscillations which are in phase, indicating that this initial movement is due to a longitudinal acoustic mode (Fig. 9a). 2) a modulation phase from tl to t2 characterized by strong modulations of the photomultiplier time signal at a frequency close to 60 Hz. This frequency appears in Fig. 8b in the form of a high amplitude of the 190 Hz mode during this period. The high speed cinematography shows that the flame sheets interact only intermittently (roughly each fifth cycle). From instant t2, flame sheets interact at each cycle.

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FIG. 8. Frequency histories of (a) microphone signal (b) photomultiplier signal 3) a precursor phase from tz to t3. When the oscillation amplitude is large enough (instant t2), the varicous mode induces a strong interaction between the pair of flame sheets corresponding to each injector at each oscillation cycle (Fig. 9b). During this phase, the flow itself is submitted to an important structural change. The periodic interactions between flame sheets create small-scale local turbulence and mixing and induce a fast periodic increase of combustion intensity. It is accompanied by the occurrence of the first harmonic frequency (500 Hz) in the photomultiplier signal (Fig. 8b) and a fast increase of the sound fundamental mode intensity (Fig. 8a). 4) a burst phase between t3 to t4 where the amplitude of both signals grows rapidly (Fig. 7). This phase corresponds to the most active part of the instability initiation mechanism: the acoustic mode induces strong interactions between flame sheets and enhances non-steady combustion. This oscillatory heat release acts as a pressure source and feeds energy into the acoustic mode, thereby increasing its amplitude. The instability growth is fast and strongly nonlinear. Figure 8 shows that the harmonic frequencies of both signals increase rapidly. This nonlinear behavior appears also in the high-speed vizualization: the flame sheets issued from the injectors are periodically broken into large lumps propagating downstream (Fig. 9c).

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UNSTEADY FLAMES

B e h a v i o r of t h e t w o flame

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FIG. 9. Sketch of flame front oscillations at different instants: (a) Induction phase (b) Precursor phase (c) Beginning of the burst phase

5) a saturation phase after t4 where the signal amplitudes reach a stabilized level. Only the 250 Hz and 500 Hz modes are detected in the microphone signal (Fig. 8a) while the photomultiplier signal exhibits many other harmonic frequencies. This phase is characterized by the increase of the loss mechanisms and the apparition of a limit cycle. As indicated before, this limit cycle is mainly due to the periodic extinction of the flames behind the injectors. This evidence suggests the following additional remarks: - The five phases described above have been identified in every initiation experiment performed with the AIC system. Only the induction and modulation durations were submitted to variations. - The origin of the first oscillation is acoustic.

During the induction phase, the flame oscillations are generated by the low frequency acoustic mode of the system, and they are slowly amplified. - Fluid mechanical effects appear only in the modulation phase when the flame sheets begin to interact and to modify the turbulent heat release rate. - The real combustion instability begins at the precursor and burst phases when the flame sheets interact at each cycle. The unsteady heat release caused by this periodic interaction couples with the acoustic mode, and the oscillation amplitude increases until nonlinear saturation is achieved.

Conclusion

This article shows that active control may be used to suppress combustion instabilities and study their initiation. The experimentation conducted on a nonpremixed combustor provides new indications on oscillation triggering and initial growth. The analysis uncovers five successive phases of induction, modulation, precursor, burst and saturation. The instability origin is the low-frequency acoustic mode of the system. This mode induces a breathing motion of the flame sheets (induction phase). This movement induces first an intermittent interaction of flame sheets (modulation phase). When interaction begins to take place at each oscillation cycle (precursor phase), the nonsteady heat release rate in the combustor increases rapidly. A resonant coupling between the feedback effect provided by this oscillatory reaction rate and the acoustic field leads to a rapid growth of the flame displacement amplitude and the occurrence of large scale structures in the burst phase. Finally, in the saturation phase a limit cycle is obtained when the reaction rate reaches a zero minimum value during the oscillation cycle. This nonlinear effect is due to the reacting flow processes and not to acoustic waves whose amplitude remains in the linear range. More generally, AIC appears as a powerful investigation tool for combustion instability studies. By allowing studies of the instability onset, AIC will probably help us to understand the intrinsic nature of many combustion oscillations. In this paper the initial flame movements are triggered by a low-frequency acoustic mode, but this oscillation would not grow beyond the modulation phase without a strong fluid mechanical response due to the flame sheet interactions. Acoustic and fluid mechanical effects are both important although not at the same instant. This example suggests that the classical definitions of "acoustical" and "fluid mechanical" instabilities do not describe the complex reality of many combustion instability mechanisms.

COMBUSTION INSTABILITIES

Acknowledgment The high speed Schlieren cinematographic system was provided and operated by M. Pequignot and Donet from ONERA. Their help is gratefully acknowledged.

REFERENCES 1. BARRERE, M. AND WILLIAMS, F. A.: Eleventh

2.

3.

4.

5.

6. 7.

8. 9.

10. 11.

Symposium (International) on Combustion, p. 169, The Combustion Institute, 1968. Cnocco, L.: Tenth Symposium (International) on Combustion, p. 1101, The Combustion Institute, 1965. STERLING, J. D. AND ZUKOSKI, E. E.: Longitudinal Mode Combustion Instabilities in a Dump Combustor. AIAA Paper 87-0220, 1987. VANEVELD, L., HOM, K. AND OPPENHEIM, A. K.: Secondary Effects in Combustion Instabilities Leading to Flashback. AIAA Paper 82-0037, 1982. CROCCO, L., HARRJE, D. T. AND REARDON, F. H.: Transverse Combustion Instability in Liquid Propellant Rocket Motors. ARS Preprint 1491-60, 1960. CULICK, F. E. C. : Comb. Sci. Tech. 3, 1 (1971). ZIKIKOUT, S., CANDEL, S., POINSOT, T., TnOUVE, A. AND ESPOSITO, E.: Twenty First Symposium (International) on C o m b u s t i o n , p1427, T h e Combustion Institute, 1988. TSlEN, H. S.: J. Amer. Rocket Soc. 22, 256 (1952). CROCCO, L. AND CHENG, S. I.: Theory of Combustion Instability in Liquid Rocket. Agardograph No. 8, Butterworths Sci. Pub. London, 1956. FFOWCS-WILLIAMS, J. E.: Proe. Roy. Soc. London 395, 63 (1984). HECKL, M. A.: Active Control of the Noise from a Rijke Tube. Proc. of the IUTAM Symposium, Lyon, Springer-Verlag, 1985.

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12. LANG, W., POINSOT, T. AND CANDEL, S.: Comb. Flame 70, 281 (1987). 13. DINES, P. J.: Active Control of Flame Noise, Ph.D. Thesis, Cambridge University, 1984. 14. BLOXSlDGE, G. J. AND LANGHORNE, P. J.: Active Control of an Acoustically Driven Combustion Instability, Colloque Euromech 213, 1986. 15. BLOXSIDGE, G. J., DOWLING, A. P., HOOPER, N. AND LANGHORNE, P. J.: Active Control of Reheat Buzz, AIAA Paper 87-0433. 16. POINSOT, T. J., BOUnIENNE, F., LANG, W., CANDEE, S. M. AND ESPOSITO, E. J.: Suppression of Combustion Instabilities by Active Control, AIAA Paper 87-1876. Also accepted for publication in the Journal of Propulsion and Power. 17. KELLER, J. O. AND SAITO, T.: Comb. Sci. Tech. 53, 137 (1987). 18. OPPENHEIM, A. V. AND SHAFER, R. W.: Digital Signal Processing, Prentice Hall, Englewood Cliffs, 1975. 19. Kay, S. M. AND MAnPLE, S. L.: Speetrnm Analysis. A Modern Perspective. Proc. IEEE, 69, 11, p. 1380, 1981. 20. VEYNANTE, D. AND CANDEL, S. M.: Application of Nonlinear Spectral Analysis to Laser Doppler Velocimetry, Int. Specialists Meeting on the Use of Computers in Laser Veloeimetry, I.S.L., May 18-20, 1987. 21. VEYNANTE, D. AND CANDEL, S. M.: A Promising Approach in Laser Doppler Veloeimetry Data Processing: Signal Reconstruction and Nonlinear Spectral Analysis, Signal Processing 14, 295300 (1988). 22. VEYNANTE, D. AND CANDEL, S. M.: Application of Nonlinear Spectral Analysis and Signal Reconstruction to Laser Doppler Velocimetry. Exp in Fluids 6, 534-540 (1988). 23. CHEN, C. H.: Nonlinear Maximum Entropy Spectral Analysis Methods for Signal Recognition, Research Studies Press, 1982. 24. POINSOT, T. J., TROUVE, A. C., VEYNANTE, D. P., CANDEL, S. M., ESeOSITO, E. J.: J. Fluid Mech. 177, 265 (1987).

COMMENTS W. Cheng, M.I.T., USA. What do you think determines the induction period of the growth to limit cycle oscillation? Author's Reply. We do not know what determines the induction period of the oscillation growth. This time is not absolutely constant and can change from 30 percent from one case to another. The linear amplification coefficient is clearly responsible for one part of this time. Our experiments indicate a global trend: the oscillation growth is faster when

the controlled regime is far from stable regime limits.

J. w. Daily, Univ. of Colorado, USA. This is a very nice piece of work which contributes a great deal to the understanding of combustion instabilities and their control. However, the amplitude of oscillation is quite low and this fact has a bearing on purely acoustic control of high amplitude insta-

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bility. Many combustors e x p e r i e n c e pressure oscillations as large as 50% of t h e total pressure. U n d e r t h e s e conditions, recovery from the upset condition may be very difficult unless a direct fluid mechanical intervention is used which results in large amplitude velocity changes.

Author's Reply. It is absolutely clear that active control systems cannot work on a fully established oscillation if the amplitude of this oscillation is completely out of reach of a reasonably loud speaker. However, if we start with a table flow with the active control system switched on, and increase the equivalence ratio to obtain an unstable regime, then our own experience shows that very little energy is n e e d e d to control the initial instability growth and that active control remains eft]cient. This m e t h o d would allow initiation studies even in eombustors with large oscillation amplitudes.

A. Ghoniem, M.I.T., USA. What is the effect of the p r e s e n c e of the instability on the total rate of burning? Is t h e r e a c o m p r o m i s e b e t w e e n unstable

combustion with high rate of b u r n i n g and stable combustion with low b u r n i n g rate? Did you consider the possibility that the vortex shedding p h e n o m e n o n b e h i n d the obstacles could be responsible for the o b s e r v e d oscillations?

Author's Reply. The p r e s e n c e of instability generates a higher total rate of burning but also much higher oscillations: the reaction rate varies from zero to twice its mean value. It is not possible to define any compromise unless we give precise values of which instability (and round) level is acceptable. The shear layer vortex s h e d d i n g frequency in this configuration is around 4000 Hz while the instability frequency is 250 Hz. It is possible to identify hydrodynamic vortices on the high s p e e d Schlieren fihn and there are much smaller than the structures corresponding to the instability frequency. No coalescenses of these hydrodynamic vortices can be seen. Therefore, the dynamics of the shear layer alone cannot be responsible for the low frequency oscillation. This does not mean that the shear layer and the whole recirculation zone have no effect on the instability. The low frequency oscillatory behavior of the recirculating flow b e h i n d the injectors might be an important p h e n o m e n o n to take into account.