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Acoustic control of combustion instabilities and emissions in a gas-turbine combustor

Unstable thermoacoustic modes were investigated and controlled in an experimental low-emission swirl stabilized combustor, in which the acoustic boundary conditions were modified to obtain combustion instability. The swirl stabilized combustor was operated in premixed mode as well as with a diffusion type flame. A closed loop active control system was employed to suppress the thermoacoustic pressure oscillations and to reduce NOx emissions. Microphone and OH emission detection sensors mere utilized to monitor the combustion process and provide input to the control system. An acoustic actuation was utilized to modulate the air flow and thus affecting the mixing process and the combustion. Suppression levels of up to 5 dB in the pressure oscillations and a concomitant reduction of NOx emissions were obtained using an acoustic power of less than 0.002% of the combustor power. It was shown that the control system reduced the coherence of the vortical structures which gave rise to the thermoacoustic instability.