Effect of entropy waves generated by different flame models on the thermoacoustic instability

Thermoacoustic instability can be an important problem for aero-engine and gas turbine combustion chambers, especially for lean premixed combustors designed for low NOx emissions. This instability is caused by the interaction of acoustic waves with unstable heat release. Entropy waves generated in the combustion chamber also produce acoustic waves during acceleration, known as indirect combustion noise or entropy noise, which are reflected upstream to affect the thermoacoustic instability. In this paper, the effect of entropy waves generated by different flame models on the thermoacoustic instability is investigated, and the main controlling factors and coupling mechanisms of entropy thermoacoustic modes are studied, in which both flames and entropy waves break the original acoustic field and introduce new modes. It is found that entropy waves change the modal frequency of ITA, the choked outlet and the flame plane may form similar boundary conditions with the same time delay for ITA modes. In addition, the filter flame decreases the frequency of entropy thermoacoustic modes and ITA modes.