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Liquid-fueled active instability suppression
Twenty-Seventh Symposium (International) on Combustion/The Combustion Institute, 1998/pp. 2039–2046
LIQUID-FUELED ACTIVE INSTABILITY SUPPRESSION* K. H. YU, K. J. WILSON and K. C. SCHADOW Research & Technology Group Code 4832000 Naval Air Warfare Center China Lake, CA 93555, USA
Active instability suppression using periodic liquid-fuel injection was demonstrated in a dump combustor. The controller fuel, which made up 12%–30% of the total heat release, was pulsed directly into the combustion chamber, and the injection timing was adjusted with respect to the combustor pressure signal. Because the injection timing determined the degree of interaction between pulsed fuel sprays and periodic large-scale flow features, it significantly affected the spatial distribution of fuel droplets inside the combustion chamber. Simple closed-loop control of the pulsed injection timing was applied to two different cases that developed natural instabilities. In the first case, the instability frequency was unchanged at the onset of the closed-loop control, and this fact allowed up to 15 dB reduction in the sound pressure level. A detailed investigation showed that the pressure oscillation amplitude reached the minimum value when the start of the pulsed fuel injection was synchronized with the inlet vortex shedding process. In the second case, the same controller was applied to a higher output combustor, where the injection timing affected not only the oscillation amplitude but also the instability frequency. For the high output case, the controller was able to suppress the oscillations initially, but it could not maintain the suppressed amplitude, resulting in unsteady modulation of the oscillation amplitude and frequency. The intermittent loss of control was linked to the frequency-dependent phase shift, associated with an electronic band-pass filter. The present results open up the possibility of utilizing direct pulsed liquid-fuel injection for active combustion control in propulsion devices, but they also show the limitation of a simple phase-delay approach in completely suppressing the natural oscillations under certain conditions.
Introduction Advanced air-breathing propulsion systems of the future will require some types of combustion control to achieve high performance. Active combustion control is an attractive idea because it can be adapted to various situations, providing high-performance characteristics over a wide range of operating envelope and eliminating costly design changes typically required in passive approaches. The need is critical especially for suppressing spontaneous oscillations that develop in ducted systems featuring a source of heat release. Extensive reviews of active instability suppression techniques are found in Candel [1] and McManus et al. [2]. Zinn & Neumeier [3] also provide an overview of research and developmental needs for practical applications. In this study, we attempt to make the control technique more suitable for practical propulsion systems by using liquid fuel for control and pulsating it directly into the combustor to maximize control efficiency.
*Approved for public release; distribution is unlimited. This work is declared a work of the U.S. government and is not subject to copyright protection in the United States.
Recent advances in active control and liquid-fuelactuator technology have provided an ideal background for extending active combustion control to liquid-fueled combustors. An extensive amount of active control work has been accomplished in the past decade not only to suppress combustion instabilities [4–11], but also to improve combustion efficiency [11–14], to extend flammability limits [11], and to reduce pollutant emission [11,13–18]. Although some studies used liquid fuel, this study is unique in that a liquid fuel is pulsated directly into the combustion chamber and periodic vortex-droplet interaction [19] is utilized as a means to control fuel-droplet dispersion. The fuel-droplet size in this case needs to be kept sufficiently small, since the droplet dispersion is more strongly affected by interaction with flow structures when the particle Stokes number is less than unity [20–24]. In the following, the results of liquid-fueled active instability suppression experiments are presented. Because our emphasis was on the use of liquid fuel for active control, a simple phase-delay circuit was used instead of a more-sophisticated controller using model-based designs [25–27] or an adaptive technique [28]. More specifically, a small amount of liquid fuel was periodically injected at the natural instability frequency, and the injection timing was
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Fig. 1. Fuel-droplet dispersion as a result of interaction with vortex. (a) airflow without fuel injection, (b) in-phase fuel injection, (c) out-of-phase fuel injection. The arrows in (b) and (c) indicate the timing at which the pulsed fuel injection is on.
such interaction. According to the Rayleigh criterion [30,31], the ability to control the phase of local heat release fluctuations is essential for effective active instability suppression. Experimental Set-up
Fig. 2. Model ramjet dump combustor with direct liquid-fuel injection for control.
systematically adjusted in an effort to control both temporal and spatial fuel distributions, which will ultimately affect the local heat release rate. Fig. 1 is an example of such an interaction, which shows the intensity contours obtained from phase-averaged planar Mie-scattering images of fuel-droplet dispersion [19]. Fuel-droplet dispersion was significantly affected depending on the injection timing. A recent numerical study [29] also indicated that the local heat release rate could be controlled as a result of
Experiments were performed in a 102-mm diameter axisymmetric dump combustor with adjustable inlet and nozzle dimensions. First, flow velocity and combustor and inlet dimensions were systematically modified to find naturally unstable operating conditions for which the liquid-fueled active instability suppression technique can be tested. Fig. 2 shows the schematics of the rig and the combustor dimensions for two particular cases that resulted in self-sustained oscillations. Air was supplied from a high-pressure tank through a long inlet pipe having the inside diameter of 41.3 mm. A sonic nozzle, which was placed in the upstream end of the inlet pipe, was used to meter the flow rate and at the same time provided acoustic isolation. Fuels were supplied at two different locations along the inlet. First, gaseous ethylene (C2H4) was injected steadily into the inlet pipe through a choked orifice at 16 inlet diameters upstream of the dump plane. The premixed inlet flow was then augmented with periodic
LIQUID-FUELED ACTIVE INSTABILITY SUPPRESSION
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Detailed Case Study (70 kW Combustor)
Fig. 3. Liquid-fuel actuation system and a simple phasedelay closed-loop control.
liquid-fuel injection at the dump plane for fast-response active control. Liquid ethanol (C2H6O) or heptane (C7H16) was used as the controller fuel in the experiments. In contrast to the upstream ethylene flow, which was held steady in time, liquid-fuel flow was pulsated into the combustor at the instability frequency and with 50% duty cycle. Fig. 3 shows the liquid-fuel injection system and the active control circuit that was used to control the injection scheduling. A Kistler pressure transducer, mounted at one inlet diameter downstream of the dump plane, was used to detect the oscillations in combustor pressure. Then, with the combustor pressure signal as reference, the phase shift for the injection cycle was digitally controlled using a Wavetek Variable Phase Synthesizer. The liquid fuel was injected through four fuel actuators that were spaced 908 apart along the circumference of the inlet at the dump plane. Each actuator system consisted of a Bosch Jettronics injector and a swirl-based fog atomizer with 300 lm exit diameter. The initial injection angle was fixed at 458 with respect to the air flow direction. Table 1 lists three different combustor conditions that resulted in instabilities. The inlet flow was highly turbulent with the Reynolds number, ReD, ranging between 7.7 2 104 and 2.5 2 105.
First, the demonstration experiments were performed under conditions of relatively low combustor output. For this case, the average pressure in the combustor was only 2% higher than the ambient pressure at the nozzle exit. Nevertheless, naturally occurring combustion oscillations were observed under certain operating conditions. The pressure oscillations occurred at or near 35 Hz and were particularly intense in the vicinity of the lean-mixture flammability limit. The combustor pressure oscillations were characterized as a function of the overall equivalence ratio (f) and the controller-fuel injection frequency. Fig. 4 shows the shaded contour plot of the peak spectral amplitude that was measured from the pressure spectrum taken at each condition. The oscillation amplitude was high when the injection frequency was between 32 and 38 Hz. The oscillation frequency often shifted toward the injection frequency, but it was not always identical to the injection frequency. To determine the origin of the instability frequency, acoustic analysis was performed that revealed that both the quarter-wave mode of the inlet and the Helmholtz mode of the combustor-inlet system occurred at 35 Hz. The phase and amplitude of 35-Hz oscillations were measured at different axial locations, and the results are presented in Fig. 5. Toward the upstream, the amplitude of pressure oscillations increased slightly and the phase of 35-Hz oscillations trailed that near the dump plane. Although this trend is somewhat similar to that observed in a duct with longitudinal waves, the result is not consistent with pure quarter-wave mode nor does it match that of the Helmholtz mode oscillations. This indicates that the present instability is the result of more complex interaction between the two acoustic modes interacting with combustion heat release. The closed-loop control, which was shown in Fig. 3, was applied to suppress the amplitude of the oscillations. To assist the phase lock, the pressure signal was filtered between 25 and 40 Hz using a Butterworth band-pass filter. Fig. 6 shows the changes
TABLE 1 Average flow conditions during unstable combustor operation Flow Rate (g/sec) Case 1A 1B 2
Unstable Conditions
m ˙ air
m ˙ C2H4
m ˙ C2H6O
m ˙ C7H16
f
P¯comb/Pexit
f (Hz)
!( p8)2/P¯ comb
45 45 146 5 2
1.0 1.3 5.1
0.75 0.75 —
— — 0.73
0.47 0.58 0.59
1.02 N/A 1.59
34 35 98
0.008 0.005 0.092
*Uncertainties in the measured quantities are the same as the last digit except as noted.
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Fig. 4. Maximum spectral amplitude of combustor pressure oscillations at various conditions. (1012 psi)
Fig. 6. Pressure oscillation spectral amplitude as a function of injection timing. The two straight lines show the amplitude levels for uncontrolled cases.
Fig. 5. Relative amplitude and phase of local pressure oscillations in the combustor and the inlet.
in the spectral amplitude of the dominant frequency as the phase of the input signal to the electronic control unit (ECU) was digitally shifted by various amounts. In addition to the controlled phase shift, the rest of the electronic circuit including the filter imposed an additional phase delay. The actual phase delay with respect to the pressure signal is denoted on the top abscissa. The oscillation amplitude was sensitive to the injection timing, allowing a 12-dB and 15-dB reduction in sound pressure level for Cases 1A and 1B, respectively. A comparison of these two cases show that the phase relation was not affected by small changes in operating conditions. The maximum amplitudes were very close to the natural oscillation amplitudes without the closedloop control, which are shown as horizontal lines. Also shown in the figure are two arrows indicating the proper fuel injection timings relative to the vortex shedding process that led to the maximum and
Fig. 7. Comparison of uncontrolled and controlled pressure oscillations. (a) pressure-time trace for Case 1A, and (b) pressure spectra for Case 1B.
minimum oscillation amplitudes. The physical timings were deduced by combining the results from the inlet pressure and velocity measurements with the cold flow visualization. The results indicate that the oscillation amplitude reached the maximum level when the fuel-on cycle followed each vortex shedding by a quarter period and the minimum level when it was synchronized with the vortex shedding process.
LIQUID-FUELED ACTIVE INSTABILITY SUPPRESSION
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Fig. 8. Luminosity intensity contours superimposed on natural light photographs of (a) uncontrolled and (b) controlled flames for Case 1B conditions.
Figure 7a shows the transient behavior of the combustor pressure as the proper phase delay was applied at time t 4 0, and Fig. 7b shows the comparison of the pressure spectra. The high-amplitude oscillations were quickly brought under control, and all of the harmonics as well as the fundamental were effectively suppressed. Under control, the rms pressure amplitude was maintained well below 0.5% of the combustor pressure. It should also be noted that oscillations at a very small level are still needed to maintain the phase-lock and vortex-synchronized fuel injection. Figure 8 shows short-time-exposed photographs of the flames under high- and low-amplitude pressure oscillations. The luminosity intensity contours were superimposed on the images to increase the contrast. As the pressure oscillation amplitude was reduced, the flames were observed further downstream from the dump plane. This suggests a possible compromise of combustion efficiency, associated with instability suppression in this case. However, as will be shown in the next section, this behavior depends on the particular system characteristics, and it does not always occur in other configurations.
Higher Output Combustor (270 kW) To test the present controller under more severe operating conditions, the combustor dimensions were modified, and higher mass flux conditions were explored for instabilities (Case 2). Because the pulsed-spray characteristics associated with the fuel injection system would be critical in the present control mechanism, the same actuator configuration that worked well in the detailed study was utilized again in the high-output combustor. Also, to increase the potential for oscillating heat release, heptane was used as the controller fuel, because it has 66% higher
Fig. 9. System characterization for Case 2 conditions.
combustion enthalpy than ethanol. Nevertheless, the relative contribution on the total heat release by the controller fuel was reduced from more than 25% in Case 1 to about 12% in Case 2. Another naturally occurring instability was observed around 100 Hz. The respective combustor and flow conditions were shown in Fig. 2 and Table 1. In a similar manner as before, the system behavior was characterized under a closed-loop control operation with the band-pass filter set between 80 and 120 Hz. However, unlike in the previous case, the main peak frequency shifted with the phase delay. The responses of the oscillation amplitude and frequency are shown in Fig. 9. Fitting the sine curve through the data, it can be seen that the two curves are roughly 708 out of phase. Because the change in instability frequency for this case was related to the
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Fig. 10. Comparison of uncontrolled and controlled pressure oscillations for Case 2. (a) pressure-time trace, and (b) pressure spectra.
Fig. 11. Measured phase-shift characteristics of the controller.
change in combustor temperature, this result suggests that the amount of instability suppression can be optimized with respect to combustion efficiency. For instance, the ECU phase shift of 2108 resulted in highest frequency but relatively low amplitude. Figure 10 shows the pressure-time trace and the comparison of spectra. The rms amplitude of pressure oscillations was significantly reduced with the closed-loop control, but the amount of suppression was much smaller in comparison with the previous case. Also, the examination of the transient behavior revealed a more serious problem in this case. At the onset of the control, the oscillation amplitude was suppressed within a few cycles. Immediately after
Fig. 12. Transient system response to onset of control at time t 4 0. (a) measured amplitude and (b) apparent frequency of pressure oscillations, and (c) fuel injection phase based on the controller frequency response.
the suppression, however, the amplitude started to grow back, reaching almost the uncontrolled level, before the control was established once again. This behavior was repeated intermittently, rendering concerns on the practical usefulness. The investigation on this behavior revealed a limitation of the simple fixed-phase-type controller, particularly associated with the nonstationary instability frequency. Although a band-pass filter was needed to obtain the phase lock, the filter would introduce an electronic phase shift that is not constant within the frequency range of interest. As a consequence, an additional frequency-dependent phase shift would be added to the preassigned phase delay. Thus, if the oscillation frequencies were drifting, they could cause significant changes in phase shift and the eventual loss of control. Fig. 11 shows the measured phase shift associated with the Butterworth band-pass filter used in the present setting. In the frequency range covering the band-pass width, the phase shift could change by as much as 51808. Figure 12 is used to explain the intermittent loss of control in the present case. Fig. 12a shows the transient response of the measured pressure shortly before and after the onset of the control. In Fig. 12b, the apparent frequency of the oscillations was deduced as a function of time by measuring the zero crossing. Two sets of data are plotted because every other zero crossing corresponds roughly to one period of oscillation. The curve fit coincides with the average of the two. Fig. 12c shows the resulting phase shift associated with the frequency change in Fig. 12b. At about 40 ms after the control was turned on, the oscillation amplitude reached the minimum value. At the same time, the apparent frequency of
LIQUID-FUELED ACTIVE INSTABILITY SUPPRESSION
the oscillation was lowered by almost 20 Hz, about a half of the band-pass filter width. As a result, the overall phase delay shifted by 1808, making the new phase more suitable for pressure amplification than suppression. As the oscillation amplitude increased, the frequency returned to the original level, and once again the phase setting shifted into the suppression mode.
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phase setting with the observed frequency would be useful. Acknowledgments This work was supported by the Office of Naval Research with Dr. Gabriel Roy as the scientific officer.
REFERENCES
Summary and Concluding Remarks In an effort to make active control more practical for dump combustors, active instability suppression experiments were carried out using a periodic liquidfuel injection system, which was closed-loop controlled with respect to the flow oscillations. The control was applied to suppress self-sustained pressure oscillations in a 102-mm axisymmetric dump combustor. Whereas the control used a simple fixed phase delay that had been studied previously, the direct liquid-fuel injection and the use of vortexdroplet interaction make this study unique. The control system was applied for two different combustor conditions with mixed results. They are listed in the following: 1. The pulsed fuel injection system, which was designed to utilize timing-dependent interaction between fuel droplets and temporal flow features, was effective in suppressing pressure oscillations. In a controlled demonstration experiment, up to a 15-dB reduction in sound pressure level was achieved with proper phasing of fuel injection and pressure. 2. The fuel injection whose timing was synchronized with the inlet vortex shedding led to the suppression of pressure amplitude in the present case. On the contrary, the fuel injection timing that was delayed a quarter cycle after the vortex shedding resulted in highest amplitude oscillations. 3. Depending on the system identification, there could be some compromises between instability suppression and combustion efficiency with the control activation. Potential efficiency benefit with the forcing amplitude is well known [12], so this result is not unique to active control and would be applicable to passively suppressed combustion instabilities as well. However, it emphasizes the flexibility associated with the active control technique, because the overall performance can be optimized depending on the relative importance of each characteristic. 4. A simple fixed-phase-type controller may not be effective in a combustor where the oscillation frequencies drift significantly with the control. The main problem is the frequency-dependent phase shift associated with the frequency filter. For such a case, an adaptive controller that can change the
1. Candel, S. M., in Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1992, pp. 1277–1296. 2. McManus, K. R., Poinsot, T., and Candel, S., Prog. Energy Combust. Sci., 19:1–29 (1993). 3. Zinn, B. T. and Neumeier, Y., 35th Aerospace Sciences Meeting, AIAA-97-0461, 1997. 4. Sivasegaram, S., Tsai, R-F., and Whitelaw, J. H., Combust. Sci. Technol. 105:67 (1995). 5. Schadow, K. C., Gutmark, E., and Wilson, K. J., Combust. Sci. Technol. 81:285–300 (1992). 6. Gulati, A. and Mani, R., J. Propul. Power 8(5):1109– 1115 (1992). 7. Langhorne, P. J., Dowling, A. P., and Hooper, N., J. Propul. Power 6:324–333 (1990). 8. Poinsot, T., Bourienne, F., Candel, S., and Esposito, E., J. Propul. Power 5:14–20 (1989). 9. Bloxsidge, G. J., Dowling, A. P., Hooper, N., and Langhorne, P. J., AIAA J. 26:783–790 (1988). 10. Lang, W., Poinsot, T., and Candel, S., Combust. Flame 70:281–289 (1987). 11. McManus, K. R., Vandsburger, U., and Bowman, C. T., Combust. Flame 82:75–92 (1990). 12. Yu, K., Trouve, A., and Candel, S., 29th Aerospace Sciences Meeting, AIAA-91-0367, 1991. 13. Gutmark, E., Parr, T. P., Hanson-Parr, D. M., and Schadow, K. C., in Twenty-Third Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1990, pp. 1101–1106. 14. Brouwer, J., Ault, B. A., Bobrow, J. E., and Samuelsen, G. S., in Twenty-Third Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1990, pp. 1087–1092. 15. Gutmark, E., Parr, T. P., and Hanson-Parr, D. M., CEAS/AIAA 16th Aeroacoustics Conference, 1995. 16. Parr, T. P., Gutmark, E. J., Wilson, K. J., Yu, K., Smith, R. A., Hanson-Parr, D. M., and Schadow, K. C., in Twenty-Sixth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1996, pp. 2471–2477. 17. Keller, J. O. and Hongo, I., Combust. Flame, 80:219– 237 (1990). 18. Sivasegaram, S. and Whitelaw, J. H., in Proc. Ninth ONR Propulsion Meeting, 1995, pp. 272–285. 19. Yu, K. H., Parr, T. P., Wilson, K. J., Schadow, K. C., and Gutmark, E. J., in Twenty-Sixth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1996, pp. 2843–2850.
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20. Lazaro, B. J. and Lasheras, J. C., Phys. Fluids A 1:1035–1044 (1989). 21. Longmire, E. K. and Eaton, J. K., J. Fluid Mech. 236:217–257 (1992). 22. Glawe, D. D. and Samimy, M., J. Propul. Power 9(1):83–89 (1993). 23. Chung, J. N. and Troutt, T. R., J. Fluid Mech. 186:199– 222 (1988). 24. Chang, E. and Kaliasanath, K., AIAA J. 34(6):1160– 1166 (1996). 25. Yang, V., Sinha, A., and Fung, Y.-T., J. Propul. Power 8(1):66–73 (1992).
26. Fung, Y.-T. and Yang, V., J. Propul. Power 8(6):1282– 1289 (1992). 27. Annaswamy, A. M., Fleifil, M., Rumsey, J. W., Hathout, J. P., and Ghoniem, A. F., MIT report 9705, Cambridge, MA, 1997. 28. Billoud, G., Galland, M. A., Huynh Huu, C., and Candel, S., Combust. Sci. and Technol. 81:257–283 (1992). 29. Chang, E. and Kailasanath, K., 35th Aerospace Sciences Meeting, AIAA-97–0126, 1997. 30. Lord Rayleigh, in The Theory of Sound, Dover, Mineola, NY, 1945, p. 227. 31. Putnam, A., Combustion Driven Oscillations in Industry, Elsevier, New York, 1971.
LIQUID-FUELED ACTIVE INSTABILITY SUPPRESSION* K. H. YU, K. J. WILSON and K. C. SCHADOW Research & Technology Group Code 4832000 Naval Air Warfare Center China Lake, CA 93555, USA
Active instability suppression using periodic liquid-fuel injection was demonstrated in a dump combustor. The controller fuel, which made up 12%–30% of the total heat release, was pulsed directly into the combustion chamber, and the injection timing was adjusted with respect to the combustor pressure signal. Because the injection timing determined the degree of interaction between pulsed fuel sprays and periodic large-scale flow features, it significantly affected the spatial distribution of fuel droplets inside the combustion chamber. Simple closed-loop control of the pulsed injection timing was applied to two different cases that developed natural instabilities. In the first case, the instability frequency was unchanged at the onset of the closed-loop control, and this fact allowed up to 15 dB reduction in the sound pressure level. A detailed investigation showed that the pressure oscillation amplitude reached the minimum value when the start of the pulsed fuel injection was synchronized with the inlet vortex shedding process. In the second case, the same controller was applied to a higher output combustor, where the injection timing affected not only the oscillation amplitude but also the instability frequency. For the high output case, the controller was able to suppress the oscillations initially, but it could not maintain the suppressed amplitude, resulting in unsteady modulation of the oscillation amplitude and frequency. The intermittent loss of control was linked to the frequency-dependent phase shift, associated with an electronic band-pass filter. The present results open up the possibility of utilizing direct pulsed liquid-fuel injection for active combustion control in propulsion devices, but they also show the limitation of a simple phase-delay approach in completely suppressing the natural oscillations under certain conditions.
Introduction Advanced air-breathing propulsion systems of the future will require some types of combustion control to achieve high performance. Active combustion control is an attractive idea because it can be adapted to various situations, providing high-performance characteristics over a wide range of operating envelope and eliminating costly design changes typically required in passive approaches. The need is critical especially for suppressing spontaneous oscillations that develop in ducted systems featuring a source of heat release. Extensive reviews of active instability suppression techniques are found in Candel [1] and McManus et al. [2]. Zinn & Neumeier [3] also provide an overview of research and developmental needs for practical applications. In this study, we attempt to make the control technique more suitable for practical propulsion systems by using liquid fuel for control and pulsating it directly into the combustor to maximize control efficiency.
*Approved for public release; distribution is unlimited. This work is declared a work of the U.S. government and is not subject to copyright protection in the United States.
Recent advances in active control and liquid-fuelactuator technology have provided an ideal background for extending active combustion control to liquid-fueled combustors. An extensive amount of active control work has been accomplished in the past decade not only to suppress combustion instabilities [4–11], but also to improve combustion efficiency [11–14], to extend flammability limits [11], and to reduce pollutant emission [11,13–18]. Although some studies used liquid fuel, this study is unique in that a liquid fuel is pulsated directly into the combustion chamber and periodic vortex-droplet interaction [19] is utilized as a means to control fuel-droplet dispersion. The fuel-droplet size in this case needs to be kept sufficiently small, since the droplet dispersion is more strongly affected by interaction with flow structures when the particle Stokes number is less than unity [20–24]. In the following, the results of liquid-fueled active instability suppression experiments are presented. Because our emphasis was on the use of liquid fuel for active control, a simple phase-delay circuit was used instead of a more-sophisticated controller using model-based designs [25–27] or an adaptive technique [28]. More specifically, a small amount of liquid fuel was periodically injected at the natural instability frequency, and the injection timing was
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DROPLET AND SPRAY COMBUSTION
Fig. 1. Fuel-droplet dispersion as a result of interaction with vortex. (a) airflow without fuel injection, (b) in-phase fuel injection, (c) out-of-phase fuel injection. The arrows in (b) and (c) indicate the timing at which the pulsed fuel injection is on.
such interaction. According to the Rayleigh criterion [30,31], the ability to control the phase of local heat release fluctuations is essential for effective active instability suppression. Experimental Set-up
Fig. 2. Model ramjet dump combustor with direct liquid-fuel injection for control.
systematically adjusted in an effort to control both temporal and spatial fuel distributions, which will ultimately affect the local heat release rate. Fig. 1 is an example of such an interaction, which shows the intensity contours obtained from phase-averaged planar Mie-scattering images of fuel-droplet dispersion [19]. Fuel-droplet dispersion was significantly affected depending on the injection timing. A recent numerical study [29] also indicated that the local heat release rate could be controlled as a result of
Experiments were performed in a 102-mm diameter axisymmetric dump combustor with adjustable inlet and nozzle dimensions. First, flow velocity and combustor and inlet dimensions were systematically modified to find naturally unstable operating conditions for which the liquid-fueled active instability suppression technique can be tested. Fig. 2 shows the schematics of the rig and the combustor dimensions for two particular cases that resulted in self-sustained oscillations. Air was supplied from a high-pressure tank through a long inlet pipe having the inside diameter of 41.3 mm. A sonic nozzle, which was placed in the upstream end of the inlet pipe, was used to meter the flow rate and at the same time provided acoustic isolation. Fuels were supplied at two different locations along the inlet. First, gaseous ethylene (C2H4) was injected steadily into the inlet pipe through a choked orifice at 16 inlet diameters upstream of the dump plane. The premixed inlet flow was then augmented with periodic
LIQUID-FUELED ACTIVE INSTABILITY SUPPRESSION
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Detailed Case Study (70 kW Combustor)
Fig. 3. Liquid-fuel actuation system and a simple phasedelay closed-loop control.
liquid-fuel injection at the dump plane for fast-response active control. Liquid ethanol (C2H6O) or heptane (C7H16) was used as the controller fuel in the experiments. In contrast to the upstream ethylene flow, which was held steady in time, liquid-fuel flow was pulsated into the combustor at the instability frequency and with 50% duty cycle. Fig. 3 shows the liquid-fuel injection system and the active control circuit that was used to control the injection scheduling. A Kistler pressure transducer, mounted at one inlet diameter downstream of the dump plane, was used to detect the oscillations in combustor pressure. Then, with the combustor pressure signal as reference, the phase shift for the injection cycle was digitally controlled using a Wavetek Variable Phase Synthesizer. The liquid fuel was injected through four fuel actuators that were spaced 908 apart along the circumference of the inlet at the dump plane. Each actuator system consisted of a Bosch Jettronics injector and a swirl-based fog atomizer with 300 lm exit diameter. The initial injection angle was fixed at 458 with respect to the air flow direction. Table 1 lists three different combustor conditions that resulted in instabilities. The inlet flow was highly turbulent with the Reynolds number, ReD, ranging between 7.7 2 104 and 2.5 2 105.
First, the demonstration experiments were performed under conditions of relatively low combustor output. For this case, the average pressure in the combustor was only 2% higher than the ambient pressure at the nozzle exit. Nevertheless, naturally occurring combustion oscillations were observed under certain operating conditions. The pressure oscillations occurred at or near 35 Hz and were particularly intense in the vicinity of the lean-mixture flammability limit. The combustor pressure oscillations were characterized as a function of the overall equivalence ratio (f) and the controller-fuel injection frequency. Fig. 4 shows the shaded contour plot of the peak spectral amplitude that was measured from the pressure spectrum taken at each condition. The oscillation amplitude was high when the injection frequency was between 32 and 38 Hz. The oscillation frequency often shifted toward the injection frequency, but it was not always identical to the injection frequency. To determine the origin of the instability frequency, acoustic analysis was performed that revealed that both the quarter-wave mode of the inlet and the Helmholtz mode of the combustor-inlet system occurred at 35 Hz. The phase and amplitude of 35-Hz oscillations were measured at different axial locations, and the results are presented in Fig. 5. Toward the upstream, the amplitude of pressure oscillations increased slightly and the phase of 35-Hz oscillations trailed that near the dump plane. Although this trend is somewhat similar to that observed in a duct with longitudinal waves, the result is not consistent with pure quarter-wave mode nor does it match that of the Helmholtz mode oscillations. This indicates that the present instability is the result of more complex interaction between the two acoustic modes interacting with combustion heat release. The closed-loop control, which was shown in Fig. 3, was applied to suppress the amplitude of the oscillations. To assist the phase lock, the pressure signal was filtered between 25 and 40 Hz using a Butterworth band-pass filter. Fig. 6 shows the changes
TABLE 1 Average flow conditions during unstable combustor operation Flow Rate (g/sec) Case 1A 1B 2
Unstable Conditions
m ˙ air
m ˙ C2H4
m ˙ C2H6O
m ˙ C7H16
f
P¯comb/Pexit
f (Hz)
!( p8)2/P¯ comb
45 45 146 5 2
1.0 1.3 5.1
0.75 0.75 —
— — 0.73
0.47 0.58 0.59
1.02 N/A 1.59
34 35 98
0.008 0.005 0.092
*Uncertainties in the measured quantities are the same as the last digit except as noted.
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Fig. 4. Maximum spectral amplitude of combustor pressure oscillations at various conditions. (1012 psi)
Fig. 6. Pressure oscillation spectral amplitude as a function of injection timing. The two straight lines show the amplitude levels for uncontrolled cases.
Fig. 5. Relative amplitude and phase of local pressure oscillations in the combustor and the inlet.
in the spectral amplitude of the dominant frequency as the phase of the input signal to the electronic control unit (ECU) was digitally shifted by various amounts. In addition to the controlled phase shift, the rest of the electronic circuit including the filter imposed an additional phase delay. The actual phase delay with respect to the pressure signal is denoted on the top abscissa. The oscillation amplitude was sensitive to the injection timing, allowing a 12-dB and 15-dB reduction in sound pressure level for Cases 1A and 1B, respectively. A comparison of these two cases show that the phase relation was not affected by small changes in operating conditions. The maximum amplitudes were very close to the natural oscillation amplitudes without the closedloop control, which are shown as horizontal lines. Also shown in the figure are two arrows indicating the proper fuel injection timings relative to the vortex shedding process that led to the maximum and
Fig. 7. Comparison of uncontrolled and controlled pressure oscillations. (a) pressure-time trace for Case 1A, and (b) pressure spectra for Case 1B.
minimum oscillation amplitudes. The physical timings were deduced by combining the results from the inlet pressure and velocity measurements with the cold flow visualization. The results indicate that the oscillation amplitude reached the maximum level when the fuel-on cycle followed each vortex shedding by a quarter period and the minimum level when it was synchronized with the vortex shedding process.
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Fig. 8. Luminosity intensity contours superimposed on natural light photographs of (a) uncontrolled and (b) controlled flames for Case 1B conditions.
Figure 7a shows the transient behavior of the combustor pressure as the proper phase delay was applied at time t 4 0, and Fig. 7b shows the comparison of the pressure spectra. The high-amplitude oscillations were quickly brought under control, and all of the harmonics as well as the fundamental were effectively suppressed. Under control, the rms pressure amplitude was maintained well below 0.5% of the combustor pressure. It should also be noted that oscillations at a very small level are still needed to maintain the phase-lock and vortex-synchronized fuel injection. Figure 8 shows short-time-exposed photographs of the flames under high- and low-amplitude pressure oscillations. The luminosity intensity contours were superimposed on the images to increase the contrast. As the pressure oscillation amplitude was reduced, the flames were observed further downstream from the dump plane. This suggests a possible compromise of combustion efficiency, associated with instability suppression in this case. However, as will be shown in the next section, this behavior depends on the particular system characteristics, and it does not always occur in other configurations.
Higher Output Combustor (270 kW) To test the present controller under more severe operating conditions, the combustor dimensions were modified, and higher mass flux conditions were explored for instabilities (Case 2). Because the pulsed-spray characteristics associated with the fuel injection system would be critical in the present control mechanism, the same actuator configuration that worked well in the detailed study was utilized again in the high-output combustor. Also, to increase the potential for oscillating heat release, heptane was used as the controller fuel, because it has 66% higher
Fig. 9. System characterization for Case 2 conditions.
combustion enthalpy than ethanol. Nevertheless, the relative contribution on the total heat release by the controller fuel was reduced from more than 25% in Case 1 to about 12% in Case 2. Another naturally occurring instability was observed around 100 Hz. The respective combustor and flow conditions were shown in Fig. 2 and Table 1. In a similar manner as before, the system behavior was characterized under a closed-loop control operation with the band-pass filter set between 80 and 120 Hz. However, unlike in the previous case, the main peak frequency shifted with the phase delay. The responses of the oscillation amplitude and frequency are shown in Fig. 9. Fitting the sine curve through the data, it can be seen that the two curves are roughly 708 out of phase. Because the change in instability frequency for this case was related to the
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Fig. 10. Comparison of uncontrolled and controlled pressure oscillations for Case 2. (a) pressure-time trace, and (b) pressure spectra.
Fig. 11. Measured phase-shift characteristics of the controller.
change in combustor temperature, this result suggests that the amount of instability suppression can be optimized with respect to combustion efficiency. For instance, the ECU phase shift of 2108 resulted in highest frequency but relatively low amplitude. Figure 10 shows the pressure-time trace and the comparison of spectra. The rms amplitude of pressure oscillations was significantly reduced with the closed-loop control, but the amount of suppression was much smaller in comparison with the previous case. Also, the examination of the transient behavior revealed a more serious problem in this case. At the onset of the control, the oscillation amplitude was suppressed within a few cycles. Immediately after
Fig. 12. Transient system response to onset of control at time t 4 0. (a) measured amplitude and (b) apparent frequency of pressure oscillations, and (c) fuel injection phase based on the controller frequency response.
the suppression, however, the amplitude started to grow back, reaching almost the uncontrolled level, before the control was established once again. This behavior was repeated intermittently, rendering concerns on the practical usefulness. The investigation on this behavior revealed a limitation of the simple fixed-phase-type controller, particularly associated with the nonstationary instability frequency. Although a band-pass filter was needed to obtain the phase lock, the filter would introduce an electronic phase shift that is not constant within the frequency range of interest. As a consequence, an additional frequency-dependent phase shift would be added to the preassigned phase delay. Thus, if the oscillation frequencies were drifting, they could cause significant changes in phase shift and the eventual loss of control. Fig. 11 shows the measured phase shift associated with the Butterworth band-pass filter used in the present setting. In the frequency range covering the band-pass width, the phase shift could change by as much as 51808. Figure 12 is used to explain the intermittent loss of control in the present case. Fig. 12a shows the transient response of the measured pressure shortly before and after the onset of the control. In Fig. 12b, the apparent frequency of the oscillations was deduced as a function of time by measuring the zero crossing. Two sets of data are plotted because every other zero crossing corresponds roughly to one period of oscillation. The curve fit coincides with the average of the two. Fig. 12c shows the resulting phase shift associated with the frequency change in Fig. 12b. At about 40 ms after the control was turned on, the oscillation amplitude reached the minimum value. At the same time, the apparent frequency of
LIQUID-FUELED ACTIVE INSTABILITY SUPPRESSION
the oscillation was lowered by almost 20 Hz, about a half of the band-pass filter width. As a result, the overall phase delay shifted by 1808, making the new phase more suitable for pressure amplification than suppression. As the oscillation amplitude increased, the frequency returned to the original level, and once again the phase setting shifted into the suppression mode.
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phase setting with the observed frequency would be useful. Acknowledgments This work was supported by the Office of Naval Research with Dr. Gabriel Roy as the scientific officer.
REFERENCES
Summary and Concluding Remarks In an effort to make active control more practical for dump combustors, active instability suppression experiments were carried out using a periodic liquidfuel injection system, which was closed-loop controlled with respect to the flow oscillations. The control was applied to suppress self-sustained pressure oscillations in a 102-mm axisymmetric dump combustor. Whereas the control used a simple fixed phase delay that had been studied previously, the direct liquid-fuel injection and the use of vortexdroplet interaction make this study unique. The control system was applied for two different combustor conditions with mixed results. They are listed in the following: 1. The pulsed fuel injection system, which was designed to utilize timing-dependent interaction between fuel droplets and temporal flow features, was effective in suppressing pressure oscillations. In a controlled demonstration experiment, up to a 15-dB reduction in sound pressure level was achieved with proper phasing of fuel injection and pressure. 2. The fuel injection whose timing was synchronized with the inlet vortex shedding led to the suppression of pressure amplitude in the present case. On the contrary, the fuel injection timing that was delayed a quarter cycle after the vortex shedding resulted in highest amplitude oscillations. 3. Depending on the system identification, there could be some compromises between instability suppression and combustion efficiency with the control activation. Potential efficiency benefit with the forcing amplitude is well known [12], so this result is not unique to active control and would be applicable to passively suppressed combustion instabilities as well. However, it emphasizes the flexibility associated with the active control technique, because the overall performance can be optimized depending on the relative importance of each characteristic. 4. A simple fixed-phase-type controller may not be effective in a combustor where the oscillation frequencies drift significantly with the control. The main problem is the frequency-dependent phase shift associated with the frequency filter. For such a case, an adaptive controller that can change the
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