1 American Institute of Aeronautics and Astronautics

DYNAMICS OF COMBUSTION OSCILLATIONS WITH AND WITHOUT PHASE-DELAY CONTROL

J. H. Uhm, S. Acharya

Louisiana State University Mechanical Engineering Department

Baton Rouge, LA 70803

Abstract Detailed measurements of the pressure and heat release fields are presented in a swirl-stabilized combustor with the goal of identifying the basic mechanisms driving the unsteady heat release, and to explore the effect of feedback control on the heat release. Measurements show that the pressure and heat release signatures are characterized by both the dominant acoustic instability mode at 235 Hz and a low frequency mode at 10 Hz. The low frequency mode is observed to be related to the bulk mode of the air-delivery chamber, and is seen to contribute to a decorrelation between the pressure and heat release signals. Phase-locked CH images clearly reveal the heat release dynamics contributing to the instability. These dynamics are strongest in the annular mixing layer regions, and is therefore believed to be related to dynamics of the swirl-induced toroidal structures. Feedback control with both acoustic and fuel modulation are seen to be effective.

1. Introduction

Large-amplitude combustion oscillations, driven by the longitudinal acoustic mode of the combustor, lead to performance degradation, hazardous noise level and structural damage or system failure1. Problems with combustion oscillations are particularly acute in lean premixed systems used for power generation. Although lean combustion can give rise to very low concentrations of NOx, their practical applications are restricted by the possibility of incomplete combustion, large-amplitude combustion oscillations, and susceptibility to blow-off and flashback2. Therefore, feedback control has been suggested as a means for providing robust operation of combustors operating close to the lean limit. In this paper, combustion oscillations in a non-premixed swirl-stabilized spray combustor operating close to the lean limit has been studied, and detailed measurements of the heat release (chemiluminescence and CH imaging) and pressure have been made in order to identify the basic mechanisms.

Combustion oscillations have been studied by many investigators, who have considered either the acoustic or the bulk mode oscillations. Flame modulation at low frequencies have been observed by a number of investigators (e.g., Bradley et al3 and Gutmark et al4) close to the lean and rich limits in a swirl burner. Flame detachment and reattachment to the flame holder at a low frequency of 8 Hz was shown in Gutmark et al4. In addition, a number of researchers have shown that combustion oscillations of acoustic or bulk-mode (at frequencies that are typically around 300 and 40 Hz, respectively), are modulated by a low frequency mode. Example of such studies include: Cohen et al5 in a lean premixed single-nozzle combustor and at realistic engine operating conditions, Poinsot et al.6 in non-premixed multiple-flame combustor, Isella et al.7 in premixed dump combustor, Langhorne8 in ethylene/air premixed flame stabilized in the wake of a conical gutter, and De Zilwa et al.9 in ducted round sudden-expansions combustor with different sizes of the upstream cavities. In these studies, the primary emphasis is on active control directed at suppressing the dominant mode oscillations, and little attention has been paid to the low frequency mode.

Recently the mechanism of combustion oscillations associated with the operation of lean-premixed combustion close to the extinction limit was investigated by De Zilwa et al.10,11 and Uhm12. These studies described in detail the flame movement at a frequency much lower than the acoustic mode frequency. The oscillations observed close to the extinction limits were observed to be related to the local extinction of the flame due to high strain rates close to the expansion plane followed by re-light at downstream locations where the strain rate was smaller. These oscillations were greatly amplified when the duct exit was constricted with a nozzle, due to the coupling between the bulk mode and the low frequency oscillations. Combustion oscillations, close to stoichiometry, with acoustic or bulk modes modulated by low frequency were also investigated by Uhm (2001), but efforts directed at active control were not successful.

41st Aerospace Sciences Meeting and Exhibit6-9 January 2003, Reno, Nevada

AIAA 2003-339

Copyright © 2003 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

2 American Institute of Aeronautics and Astronautics

Recent efforts in developing active control strategies for suppressing combustion oscillations have been primarily based on premixed fuel and air in laboratory scale combustors, **,**,**. The present experiments are performed in a swirl-stabilized spray combustor comprising of inner and annular airflow with swirl, and liquid fuel spray through eight circumferentially located nozzles. The main purpose of this paper is to identify the mechanism of the combustion oscillations dominated by an acoustic mode with a superimposed low frequency bulk mode, and to explore active control strategies using air and fuel modulation schemes.

2 Experimental Setup The experiments were carried out in a multi-fuel-feed combustor operating at nearly 150kW heat release. The combustor configuration, shown in figure 1, consists of two concentric pipes that serve as settling chambers for the coaxial jets. Swirl vanes with 45 degree angle are placed in the inner and outer air streams at the exit of the settling chambers that are accelerated through concentric large area ratio nozzles. Each settling chamber can be acoustically forced by an array of eight 75-Watt loudspeakers (Sanming 75-A) driven by four audio amplifiers (Radio Shack MPA-250) and mounted at equal polar angles about the circumference of the chambers. The air supply lines piped from a reservoir pressurized to 150 psi are used to deliver the inner and annular air. The airflow rates are measured using Dwyer VFC series rotameters. Provision for the placement of eight liquid fuel atomizers (Arizona Mist with 0.012” orifices) equally spaced between the coaxial jets is provided. Pressure-fed nozzles provide the atomization of the liquid spray. A fuel nozzle coupled to an automotive fuel injector is located at the geometric center of the inner air stream. Independent fuel supply lines are provided to the central injector and a pair of four unmodulated circumferential fuel nozzles. Ethanol is used as the liquid fuel and is pressurized to a maximum of 250 psi in three fuel tanks by high-pressure nitrogen.

The combustor is equipped with a square shell with 7.63 inch sides and windows that are either stainless steel or quartz for optical access. A high sensitivity, water-cooled pressure transducer (Kistler 6061B) is mounted at a normalized axial distance at 2.36 inch from the expansion plane to measure pressure oscillation and to provide the feedback signal for control. Light emission recorded at the CH radical wavelength using a photodiode (Melles Griot) with a 430 nm centered-optical filter was taken as a measure of the heat release fluctuations from the flame. These signals were then processed in real time using a digital signal processor (DS1103, DSPACE, 333 MHz Motorola power PC) to be used in active control. The spatial and temporal variations in the heat release were visualized using a Princeton Instruments PI-MAX

512x512 ICCD camera with a UV lens (Electrophysics) and a bandpass filter (DIOP) at 430nm for CH chemiluminesence imaging. The images could be triggered with respect to the pressure oscillations in the combustor to observe the fluctuations in heat release at different instances of the oscillation cycle.

primary and secondary fuellines

inlet section

combustor section

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fuel modulation nozzlesecondary fuel nozzleprimary fuel nozzle

Top View of Combustor

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Figure 1: Coaxial Jet Spray Combustor

3. Results and Discussions Pressure characteristics of combustion oscillations: The trace of the pressure signal at two locations (2.36 inch upstream and downstream of the dump plane) is shown in Figure 2 for flow conditions corresponding to 60 SCFM of air and 4 GPH of fuel. At both locations, a dominant frequency of 235 Hz (see pressure spectra in Figure 4) modulated by a low frequency of 12 Hz is observed. The low frequency modulation is attributed to the acoustics of the settling chamber, and as will be discussed later, plays a role in the effectiveness of the control strategy.

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Figure 3 shows the frequency and magnitude of the large-amplitude combustion oscillations (greater than 1.5kPa) as a function of the annular air flow rate for three different values of the inner stream flow rate. The oscillations are dominated by a quarter-wave mode frequency in the combustor section. The dominant frequency ranged from 227 to 235 Hz and is almost invariant with the flow rate or equivalence ratio. The pressure oscillation amplitude increases with the annular airflow rate except for the case of the inner airflow rate of 21 CFM which shows stable combustion oscillations over a certain range of annular flow rates (75-105 CFM). In this range, the decrease in oscillations at 21CFM primary air flow rate is evident in Figs 3b and 4b, and is presumably associated with changes in flow dynamics at the higher primary air flow rates. As the annular air flow rates are increased (points a, b and c in Fig. 3b and Fig. 4a), there is a decrease in the spectral bandwidth associated with the instability, and a decrease in the magnitude of the secondary peak (Fig. 4a). In Fig. 5a, it can be seen that a low frequency mode is superimposed on the dominant instability, but this low frequency mode can be significantly influenced by the primary air flow rate (Fig. 5b). The low frequency appears to be associated with the bulk mode of the primary air delivery chamber; the bulk mode pressure oscillations produce velocity fluctuations in the primary air stream, and the magnitude of the fluctuations are reduced with an increase in the primary air flow rates.

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Flame behaviors on combustion oscillations: For phase-resolved CH images, a discrete peak in the oscillations are required to avoid the ambiguity caused by the low frequency oscillations in the flame movement. Hence, specific flow conditions were chosen for the cycle-resolved measurements. Figure 6 shows the pressure signal for one cycle of the oscillations, its spectrum with a dominant frequency of 235 Hz, and CH images synchronized on the pressure signal. The heat release may be assumed to be represented by the measured gray level of CH chemiluminescence intensity (shown as a color scale in the figure).

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Figure 6: Pressure signal and spectrum, heat release distribution of CH radical intensity synchronized on the pressure signal of instability cycle (235 Hz) in the condition primary and secondary fuel supply. Inner and annular airflow rates: 16 and 44 CFM, respectively. Primary and secondary fuel flow rate: 2.0 gph Exposure time of CCD camera: 0.1 ms. Figure 6a shows intense heat release in the region where fuel and air mix, and combustion takes place. The high-heat release region is seen to be roughly symmetrical with a butterfly shape. The reaction zone starts to extend toward the expansion plane in figure 6b, and then expands in all directions and attaches to the flame holder in figure 6c. The thickest reaction zones with the lowest peak intensities are observed during the low-pressure instances of the oscillation cycle (Figs 6c-6f). As the pressure recovers (Figs 6g-6i), the flame again lifts off the flame holder and becomes more compact and intense.

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Figure 7: Pressure signal and spectrum (top row), and heat release distribution of CH radical intensity synchronized with the pressure signal of instability cycle (213 Hz). Secondary fuel supply of 2.5 gph. Primary and secondary air: 11 and 55 CFM

In order to provide clearer CH-images, not influenced by the three-dimensional corner flows in the square combustor, only the fuel injectors adjacent to the flat combustor walls were activated. Thus, the primary mixing and combustion is expected to be primarily due to the fuel injected near the flat side walls which are not influenced significantly by the three-dimensional corner flows. Figure 7 shows the CH images phase-locked with respect to the pressure signal. The region of maximum heat release is seen to located downstream of the expansion plane, and in the annular mixing-layer region between the primary and secondary air streams. The dynamics of this high-heat release region appears to be in phase with the pressure oscillations, with high CH levels and a compact lifted flame at peak pressures (Figure 7a) and low CH levels with a broader attached flame at minimum pressures (Figure 7f). Thus with the changes in the pressure levels, the flame attaches or lifts-off at the dump plane, and this flame movement introduces dynamics (heat release variation) close to the

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5 American Institute of Aeronautics and Astronautics

dump plane. Therefore, the primary heat release dynamics of interest which contributes to the instability are associated with the high heat release regions in phase with the pressure.

In order to explain the heat release dynamics shown in Figs. 3a-3i, an exploded view of Figs. 3a and 3b is also shown, with postulated flow structures indicated. It is postulated that with the high degree of swirl present, both the inner and annular air streams lead to a vortex-core structure with a strong central recirculation and a weaker outer recirculation. These vortical flow structures have inherent dynamics or transverse movements that lead to the heat release variations. These axial and transverse movements can be seen by comparing the locations of high CH-values in figures. 3a-f. In figures 3a-c (high pressures) the high intensity regions are well defined, and appear to correlate with the swirl-induced vortical structures. In Figs. 3d-3f, as pressure levels decrease, the CH values reduce in the central recirculating region. Beyond the location of pressure minimum (figures 7g-i), evidence of flow structures are not as evident with reduced thickness of the reaction zone (identified as high CH-regions) and a decrease of CH values in these regions.

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Figure8: Phase relationship between pressure and heat release during oscillations cycle and Rayleigh index at the same flow condition as that in figure 7.

Figure 8 shows the plot of the Rayleigh index

(integral of the product of the pressure and heat release fluctuations). These plots indicate that the pressure and

heat release are out-of-phase with the pressure close to the dump plane (at location marked “b”) where negative Rayleigh index is obtained, and in-phase at location marked “a” where positive Rayleigh index is obtained. Instability arises from the regions of positive Rayleigh index, and for control, heat release variations in these regions must be altered.

Low frequency mode in combustion oscillations: The existence of a low frequency mode superimposed on the primary instability has been reported by De Zilva et al.9 for premixed gaseous systems. As noted earlier, the present pressure and CH-photo-diode measurements also reveal low frequency oscillations at around 12 Hz. This low frequency mode is due to the cavity between the acoustically-closed upstream inlet end and the constricted exit nozzle of the air-delivery inlet that leads to a bulk mode oscillation in the flame.

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Figure 9: (a) Pressure and CH signals (Black and red lines, respectively) (b) Variation of the maximum peak in the pressure and CH-emission in circle symbol of (a)

The existence of these oscillations can be clearly seen in figures 2 and 5a and in figures 9 and 10 where the time-variations in the peak pressure and CH levels are plotted. The low frequency (12 Hz) flame movement makes the control of the combustion oscillations difficult due to changes in the heat release distribution from one cycle of thermo-acoustic instability (at 235 Hz) to another. It is evident in figure

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6 American Institute of Aeronautics and Astronautics

9 that there is a time lag (of the order of 2-5 milliseconds) between the peak in the pressure oscillation and the peak in the CH level. The variability in the time lag is presumably associated with the cycle to cycle variations in the flame movement.

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(c) (d) Figure 10: Time traces of maximum and minimum peaks of pressures and corresponding CH signals, and of those signal at points, presumably maximum peak of pressure. Typical trigger location for CH imaging is denoted by (a) through (d). Black & red lines: maximum peak and minimum peak of pressure &CH signals. Blue & green lines: pressure &CH values around trigger point. Square and cycle solid symbols: trigger points Inner and annular airflow rates: 16 and 44 CFM. Primary and secondary fuel flow rate: 2.0 gph.

Figure 10 shows the time traces of the pressure and

CH chemiluminescence signals along the locus of the peak pressure and CH values. Also shown are the corresponding pixel-averaged CH values and the CH images. Note that the peak pressures are modulated by the low frequency of 12 Hz, and therefore the time traces show variations in the peak values of both pressure and CH. As in figure 9, it is of specific interest to correlate CH values with the locations of maximum

pressure. In view of this, the CH imaging was triggered at locations indicated by (a) through (d) in figure 10 (shown by red symbols). It is evident from figure 10 that the maximum in the peak pressure does not always correspond to the maximum in the CH value. At location (a) and (b) there is a close correspondence between the maximum pressure and the maximum heat release (CH image shows large pixel intensity values), while at location (c) where the pressure peak has a local maximum, CH intensity values are relatively lower. Thus, for control purposes, in order to introduce secondary fuel that will produce heat release oscillations out of phase with the baseline or driving heat release dynamics, the pressure signal should not be used as the control input. A better measure of the control input is the heat release signal itself. Acoustic and Fuel-modulation Control: In this section, we report the results of active control using both the acoustic modulation and fuel modulation strategies. Acoustic modulation alters the flow dynamics and mixing, while fuel modulation directly controls the timing of the secondary fuel injection. With both strategies, the heat release dynamics is effected, and therefore has the potential of controlling instabilities. For the air modulation speakers on both the primary and secondary air streams are actuated with feedback control. For the fuel modulation, control fuel (8% of the total fuel) is injected through the centrally located injector, so that, the fuel reaches the regions of positive Rayleigh index shown in figure 8. In the baseline case, the control fuel injector is also activated in order to maintain the same equivalence ratio as in the control cases, but the actuation frequency is 350Hz which is distinct from the instability frequency of 235Hz (Fig. 11). Thus, for the baseline, modulation of the control fuel injector does not interfere with the instability. When control is activated, the actuation frequency is then linked to the feedback signal at the instability frequency.

Figure 11a shows the spectra of pressure and heat release for the baseline case. The figure shows a dominant frequency of 235 Hz and its harmonic of 470 Hz. The fuel modulation frequency of 350 Hz is also seen in the spectra of the heat release.

Figure 12 shows the results of phase delay control and the reduction and amplification of the amplitude in the oscillations using 2 loudspeakers opposite each other for the annular air stream, and the results of phase delay control using an automotive fuel injector. As can be seen in Fig. 12, different phase angles lead to different levels in suppression of the oscillations. The optimum phase angles are different for the two actuators since they influence the fuel-air mixing differently. The base line for acoustic forcing is around 8 % less than that for fuel modulations since acoustic control was difficult to achieve at higher power levels.

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This was related to the control authority exerted by the speakers.

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Figure 12: Phase delay control of secondary air acoustic forcing and fuel modulations. Annular airflow rate: 44 CFM. Inner airflow rate: 15 and 16 CFM for air forcing and fuel modulations, respectively. Primary and secondary fuel flow rates 1.7 and 2.1 gph for the former and both 2.0 gph for the latter.

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(c) Figure 13: Time averaged heat release distribution of CH radical intensity without and with control using loudspeakers. Primary and secondary airflow rate: 15 and 44 CFM, respectively. Primary and secondary fuel Flow rate: 1.7 and 2.1 gph, respectively. Exposure time: 4.3 ms With control (a) reduction at delay time of 0.5 ms, (b) amplification at delay time of 3.5 ms (c) without control.

Time averaged CH images with and without acoustic modulations are shown in figure 13. Stable combustion, associated with the suppression of the oscillations in Fig. 13a, shows a slightly lifted flame with no reaction zone just above the flame holder, and thinner and smaller reaction zone than those associated with the baseline large-amplitude oscillations. The reaction zones, with control, presumably do not have sufficient thermal energy to excite the pressure and because they are away from the flame holder and the pressure anti-node. For the baseline, large-amplitude oscillations in figure 13c, the CH images show a wider heat release distribution and thicker reaction zone (relative to that observed for stable combustion) and the flame is anchored on the flame holder. The larger regions of heat release coupled with higher heat release levels near the flame holder or pressure anti-node lead to the enhancement of the thermo-acoustic coupling. For conditions, where control enhances the instability, a thicker intense heat release distribution is again observed relative to the stable case.

In order to determine the effect of the amplitude of control authority with fuel modulation, the amount of control fuel was systematically varied in a series of

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experiments. Figure 14 shows that an increase in the amount of control fuel from 7.3% to 10% led to a slight increase in the dominant frequency and a substantial decrease in the amplitude of the peak instability (from nearly 2kPa to 0.5 kPa). This substantial decrease in the pressure oscillation with only a 2.7% increase in the amount of control fuel indicates the potential benefit of using a proportional actuator for active control.

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(d) (e) Figure 14: (a) Dominant frequency and rms pressure fluctuations using fuel modulations at phase angle of 1.5 ms with different fuel modulation rates, (b)-(e) time averaged CH radical intensities synchronized at 1.5 ms time delayed on the maximum pressure signal and different fuel modulation rates/total fuel 7.3, 8.2, 9.1, and 10 %, respectively. Primary and secondary airflow rate: 16 and 44 CFM, respectively. Primary and secondary fuel Flow rate: around 2 gph. Exposure time: 4.3 ms.

The time-averaged CH images shown in Fig. 14b-

14d indicate that with increasing levels of control (or lower pressure oscillations), the CH levels close to the pressure anti-node (flame holder) correspondingly reduce (compare Figs. 14b and 14e). This is consistent with the earlier observations comparing the controlled and baseline CH images (Fig. 13).

4. Concluding Remarks The characteristics of large-amplitude combustion oscillations in a swirl-stabilized spray combustor are investigated through measurements of wall pressure,

CH chemiluminescence, and CH images. The dominant frequency of the oscillations varied marginally with the flow conditions and ranged from 225 to 235 Hz with oscillation rms pressure amplitudes greater than 1.5 kPa.

Phase-locked CH images revealed that the heat release dynamics driving the instability was associated with the region of highest heat release. As pressure levels decreased during an instability cycle, the peak CH levels decreased, but the flame grew in size and attached to the flame holder. Thus, the flame is observed to be compact, intense and lifted at instances of high pressure, while the flame is less intense, attached, and wider at instances of low pressure. A low frequency mode of 10 Hz was observed to be superimposed on the dominant instability mode. The low frequency mode is due to the bulk-mode oscillation of the air-delivery chamber, and results in the axial movement of the flame. The amplitude of the low frequency mode was relatively low compared to that of the dominant frequency, but it did contribute to a decorrelation of the peaks in the pressure and CH-signals. This decorrelation, in turn, makes feedback control less effective. It is speculated that the low frequency oscillations are associated with the local extinction of the flame followed by re-stabilization downstream.

Active control was shown to be successful with both acoustic modulation of the air stream and modulation of a centrally-located fuel stream. The amount of control fuel used is shown to have a significant effect with a 2.7% increase in the control fuel leading to a threefold reduction in the pressure oscillations. This observation indicates that proportional drive actuators driven by a model-based controller (with variable gain) can lead to improvements in the controller effectiveness.

5. Acknowledgements

This work was carried out with financial support

from the Propulsion program of the US Office of Naval Research under contract N00014-97-1-0957. Help from Mr. Y. S Goh is acknowledged.

6. References

1. Austin, J. A., Tilson, J. R. and McKenzie, I. R. I. (2000) Combustion oscillation in a staged LPP combustor. Presented at the RTO AVT Symposium on Active Control Technology for Enhanced Performance Operation Capabilities of Military Aircraft, Land Vehicles and Sea Vehicles, Braunschweig. 2. Keller, J. O., Vaneveld, L., Korschelt, D., Hubbard, G. L., Ghoniem, A. F., Daily, J. W. and Oppenheim, A. K. (1981) Mechanism of instabilities in turbulent combustion leading to flashback. AIAA Journal, Vol. 20, pp. 254-262.

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