Suppression of thermoacoustic instability in a swirl-stabilized combustor by inducing blockage in the inlet flow stream

Abstract

Swirl flows are often used for flame stabilization in gas turbine combustors. However, when these combustors are operated at lean fuel/air ratios, they are prone to thermoacoustic instability. In this study, we experimentally investigate the effect of distribution of the blockage in the inlet flow on the transition of the combustion dynamics from combustion noise to thermoacoustic instability. We acquire unsteady pressure fluctuations and heat release rate fields (CH∗ chemiluminescence) by capturing the flame images to investigate this transition in the thermoacoustic system. We utilize a turbulence generator with two different configurations to modify the inlet flow dynamics to achieve passive control of thermoacoustic instability. To that end, using flow restrictors, we induce blockage in the inlet flow upstream of the swirler, perpendicular to the bulk flow direction. We observe that in one case, there is a reduction in the amplitude of the periodic oscillations while in the other case, there is a suppression of thermoacoustic instability. In the former case, the blockage in the inlet flow stream is more distributed, while in the latter, the same degree of blockage is clustered into one region of the inlet flow stream. The field of the local instantaneous acoustic energy production (p ′ (t) q ′ (x, y, t)) shows the presence of coherence during the occurrence of thermoacoustic instability for the experiments without any blockage. This emerging coherence is disrupted with the inclusion of the blockage. In the case with clustered blockage, the coherence is suppressed significantly, while for the case with the distributed blockage, it is reduced slightly. Analysis of spatial variance, performed on the local instantaneous acoustic energy production indicates the disruption of the coherent spatial structures with the flow restrictors, which subsequently suppresses thermoacoustic instability.