Flame dynamics during intermittency in a turbulent combustor

Abstract

On the reduction of equivalence ratio from stoichiometry to lean conditions, turbulent combustors exhibit intermittency before and after the occurrence of thermoacoustic instability. In this paper, we compare the dynamic characteristics of this intermittency prior to and post thermoacoustic instability. The analysis of the unsteady pressure fluctuations within the combustor suggests that the intermittency prior to and post thermoacoustic instability are of type II. We also compare the flame dynamics during either state using high speed Mie scattering images acquired simultaneously with unsteady pressure. We observe that during both regimes of intermittency, the flame switches between two distinct patterns of oscillations; one where the flame oscillates in an aperiodic manner due to the inherent turbulent fluctuations, and another where the flame exhibits periodic roll-up as a consequence of the periodic vortex shedding at the dump plane. Nevertheless, it is observed that the flame dynamics during intermittency prior to and post the occurrence of thermoacoustic instability are different. During intermittency before the occurrence of thermoacoustic instability, the flame along the inner shear layer is stabilized by the stagnation point flow behind the bluff body. In contrast, during intermittency after the occurrence of thermoacoustic instability, the flame along the inner shear layer is stabilized by the recirculation zone created by the bluff body.