Low-bandwidth open-loop control of combustion instability

A new strategy for open-loop control of combustion oscillations using a high-momentum air jet modulated at low frequencies is described. The oscillations in the swirl-stabilized spray combustor of interest are dominated by a longitudinal acoustic mode (235 Hz) with a low-frequency (13 Hz) bulk mode (of the upstream cavity) superimposed. An effective strategy for control is demonstrated to be achieved through employment of a new concept that uses low-bandwidth, open-loop modulation of a high-momentum air jet that penetrates directly into the region of strong combustion dynamics. It is shown that with low-frequency modulation (1-7 Hz) of the high-momentum air jet, the pressure oscillations can be reduced significantly (by a factor of nearly 10). Square-wave modulation is demonstrated to be considerably more effective than sine-wave modulation. These results are extremely promising as high-bandwidth actuation is not required for effective control.