Experimental investigation of passive control of self-excited azimuthal thermoacoustic instabilities in a turbulent premixed annular combustor with microjet air injection

In this work we systematically examined the effect of implementing air microinjection in different flame characterization zones to suppress azimuthal thermoacoustic instabilities in a premixed annular combustor. Our experimental results showed that air injection in the downstream zone (DZ) and the outer recirculation zone (ORZ) had minimal impact on the amplitudes of thermoacoustic instability. However, injecting air into the flame brush merging zone (MZ) and the inner recirculation zone (IRZ) can lead to a significant reduction of such instability amplitudes, especially the azimuthal thermoacoustic coupling was nearly entirely suppressed in the IRZ case. Further investigations revealed that air injection in the MZ and IRZ notably altered local flame characteristics. A common feature was the significant weakening of the chemical reaction intensity in IRZ. Additionally, the thermoacoustic instability was found to be transitioned from stable limit-cycle oscillations to intermittent oscillations. In the MZ case, intermittent thermoacoustic oscillations were characterized by periodic bursts, while in the IRZ case, irregular mode switching was experimentally observed. Despite this variability, long-term continuous statistical investigation indicated that the combustion system preferred mode in both cases was a slow spinning standing mode with bulk swirl modulation. Flame dynamics investigations were conducted to identify vortex disturbances within the shear layer as the primary driver of azimuthal instability. When air was injected into the IRZ, these disturbances were effectively disrupted, thus dampening the thermoacoustic instability significantly. This finding was confirmed by performing Rayleigh index analysis. Finally, the long-term continuous transient testing and operations involving the switching on and off of such air microinjection in the IRZ were investigated. Our investigations confirmed that properly injecting air in IRZ was a rapid-response and highly effective passive control approach for effectively mitigating azimuthal thermoacoustic instabilities in the annular swirling combustor.