Open-loop control of thermoacoustic instabilities by the external acoustic forcing at different frequencies

Using external off-resonance forcing synchronized with the self-excited oscillation system is a primary method for the open-loop control of thermoacoustic instability. Current studies were mainly focused on the control of the acoustic pressure oscillation of the system, neglecting the control of flame heat release rate (HRR) perturbation. This paper studies the control effects of external forcing at different frequencies on both limit cycle oscillations and quasiperiodic oscillations. Results indicate that the external disturbance forces the flame to oscillate at a frequency that deviates from the resonant mode of the combustion chamber, thus suppressing the thermoacoustic coupling in the system and reducing the acoustic pressure. When the external frequency is low, the HRR perturbation will be enhanced. However, during synchronization with high-frequency external forcing, due to the low-pass filtering features of the flame describing function (FDF), the overall HRR perturbation exhibits reduced amplitudes.