Characteristics and control of combustion instabilities in a swirl-stabilized spray combustor

Active control of combustion instabilities in a swirl-stabilized spray combustor was investigated. The combustor was characterized by a thermoacoustic instability at approximately 200 Hz. Both open- and closed-loop control tests were conducted. A feedback phase-delay control was implemented using a high-speed microprocessor. The phase-delay control clearly indicates the potential for suppressing the thermoacoustic instabilities in swirl-stabilized combustion. Two flow conditions, one corresponding to high NOx and another corresponding to high-pressure oscillations, were selected for feedback-loop control studies. Best control was achieved with 60-90 deg phase lag, and for the high-pressure oscillation baseline case, the peak amplitude of the pressure oscillations decrease by a factor of three. It was observed that control of pressure oscillations could be achieved without adversely effecting or increasing the NOx emissions. In fact, NOx emissions decrease with control. Velocity measurements were performed for both the uncontrolled and controlled conditions using a two-component laser Doppler velocimeter system. With control, the rms values of the velocity fluctuations decrease significantly (over 50% in certain regions), and this reduction is observed in both the coherent and random components of the velocity fluctuations. Control also leads to an increase in the strength of the central recirculation region, which promotes flame stability.