Suppression of Combustion Noise and Combustion Oscillation by Thermo-Acoustic Active Control Using Secondary Flame

Abstract

A feedback combustion control system based on the active noise control (ANC) principle was investigated with an experimental lean premixed dump combustor. In the control system, secondary diffusion flames were used as the sound source to attenuate pressure fluctuation in the combustion system. Piezo-valves with fast response were used to modulate the flow rate of the fuel for the secondary flames and to generate controlled sound by heat release oscillation. We investigated two kinds of controllers, that is, a time delay type controller and a model-based type one. In the experiment of the former, the dominant resonant mode of the combustion noise was suppressed more than 10dB in maximum. Clear periodicity of the suppression effect according to delay time indicates that the suppression is due to the acoustic interaction between the main flame and the secondary flames. In the model-based controller, robust control algorithms (mixed H2/H∞) were used. In addition, the gain-scheduled control was implemented to tune in to the shift of the dominant mode of the combustion noise, which resulted from variation of equivalence ratio. This control system effectively suppressed the dominant mode by more than 25dB without increase in other modes.