Experimental Investigation of Coherent Structures in a Generic Dual Swirl Burner Under Broadband Noise Excitation

Swirling flows in gas turbine (GT) combustors, used for flame stabilization, are prone to precessing vortex core (PVC), a self-excited global hydrodynamic instability associated with vortex breakdown. This instability leads to large-scale coherent structures and significant flame fluctuations, potentially causing thermoacoustic instability. While previous studies have explored PVC excitation and suppression under single-frequency acoustic excitation, GT combustors exhibit inherent noise dynamics that vary with operating conditions and combustor designs. It is thus essential to examine how these coherent structures interact with inherent combustor noise. This study experimentally investigates the flow features and coherent structures of an unconfined counter-rotating dual swirl burner. The aim is to understand the interaction between PVC and broadband noise excitation, to develop strategies for mitigating thermoacoustic instability. Schlieren image velocimetry captures mean flow characteristics, while proper orthogonal decomposition analysis identifies dominant coherent structures. The study examines two excitation types: (i) frequencies lower than natural PVC frequency, and (ii) broadband noise excitation at varied amplitudes. Preliminary results show natural flow includes single and double helical PVC modes at St = 0.53. Low-frequency acoustic actuation (St = 0.46) suppresses PVC, while broadband noise excites both PVC modes.