FLOW - ACOUSTIC - FLAME INTERACTION DURING THE TRANSITION TO THERMOACOUSTIC INSTABILITY IN A TURBULENT LIFTED JET FLAME COMBUSTOR

Transition to thermoacoustic instability from combustion noise via intermittency occurs in a turbulent lifted jet flame combustor when the position of burner inside a confinement is changed. The role of the coupling of flame, flow and duct acoustics leading to transition from combustion noise to thermoacoustic instability is captured with the simultaneous measurement of acoustic pressure, CH* chemiluminescence images of the flame and Mie scattering images of the unburnt jet of fuel/air. Combustion noise which corresponds to the stable operation of the combustor is associated with the incoherent fluctuations in the flow and aperiodic irregular oscillations in unsteady pressure. In contrast, full blown thermoacoustic instability is associated with self-organized coherent vortex structures in the flow and ordered regular fluctuations in the acoustic pressure. Between irregularity (combustion noise) and order (thermoacoustic instability), the intermediate state 'intermittency' have characteristics of both combustion noise and thermoacoustic instability. Intermittency is manifested as seemingly random appearance of bursts of high-amplitude periodic oscillations amidst regions of low-amplitude aperiodic fluctuations in the acoustic pressure. During intermittency, in the low-amplitude regime, we observe incoherent, seemingly random, fluctuations in the flow. However, in the high-amplitude regime during intermittency, we observe coherent vortices in the flow. As we change the control parameter towards the condition of thermoacoustic instability, self-organized coherent vortices exist longer in time. Finally self-sustained highamplitude pressure oscillations and self-organized coherent vortex structures are observed at the occurrence of full-blown thermoacoustic instability.