Effects of acoustic liner on thermoacoustic instabilities in a premixed swirl combustor

Acoustic liner is generally adopted to damp noise in aero-engines and gas turbines. This paper presents a systematic study about the effects of an acoustic liner on thermoacoustic instabilities in a laboratory scale burner. A premixed swirl combustion system is constructed and a specially designed single layer acoustic liner is located at downstream of the flame zone. A low order network model is built to recognize the thermoacoustic modes of the combustion system. The Transfer Element Method (TEM) method is applied to analyze the absorption of the acoustic liner with bias flow. Experiments under the boundaries of the rigid wall and the acoustic liner without and with tunable bias flow are carried out, respectively. Furthermore, the temperature of the bias flow is adjusted to evaluate its effects on combustion oscillations. The result of low order modeling shows that the low-frequency mode around 116 Hz is the Helmholtz mode of the upstream plenum, while the higher frequency mode near 300 Hz is the quarter wave mode of the combustion chamber. The experimental result shows that the instability of the Helmholtz mode can be triggered and further enhanced with the increase of the bias flow Mach number. Meanwhile, the instability of the 1/4 wave mode is completely suppressed. The Helmholtz mode that is triggered by the bias flow can be attenuated by raising the temperature of the bias flow, while no substantial changes are observed in the quarter wave mode.