Attenuation mechanism of thermoacoustic instability using a secondary gas injection

Thermoacoustic instability is a self-excited phenomenon characterized by large-amplitude acoustic pressure fluctuations arising from the coupling among system acoustics, unsteady heat release rate, and vorticity fields. The onset of thermoacoustic instability causes catastrophic damage to the combustor. The behavior of large-scale vortex structures in the shear layer region between the inner and outer recirculation zones plays a critical role in driving thermoacoustic instability. In this study, we experimentally attempt to suppress thermoacoustic instability by injecting secondary gas into the shear layer, including the investigation on the dynamics of the shear layer region and the coupling during the attenuation process. The frequency of large-scale vortices in the shear layer decreases at a high flow rate of secondary gas injection. The coupling between the heat release rate and vorticity fields weakens rapidly during the attenuation state close to well-suppressed thermoacoustic instability, which is clearly shown by complex network analysis.