Synchronization behaviour during the dynamical transition in swirl-stabilized combustor: Temporal and spatiotemporal analysis

In this paper, an analysis of the coupled behaviour between the acoustic pressure and the heat release rate fluctuations in a swirl-stabilized combustor during the transition of the system dynamics from combustion noise to thermoacoustic instability via intermittency is performed. Both temporal and spatiotemporal analysis of these coupled oscillations are carried out using the framework of synchronization theory. The results of the synchronization transition observed in the swirl-stabilized combustor are compared and contrasted with those recently reported by Pawar et al. (J. Fluid Mech., vol. 827, 2017, pp. 664-693) and Mondal et al. (J. Fluid Mech., vol. 811, 2017, pp. 659-681) in a bluff-body stabilized combustor. Further, a rigorous analysis of the synchronization transition of these oscillations in the frequency domain is presented, thus shedding light on the onset of phase and frequency lock-on in systems with a turbulent reactive flow. In addition, the spatiotemporal dynamics of the flame-acoustic interaction during the two states of thermoacoustic instability, that is, weakly and strongly correlated periodic oscillations observed in the system are compared. Despite the global synchrony and the acoustic driving being higher during the state of strongly correlated periodic oscillations as compared to the state of weakly correlated periodic oscillations, an increased amount of spatial incoherence in the reaction rate field is observed during this state. In short, our study reasserts a unified synchronization transition of coupled oscillations in these turbulent combustors, wherein the mechanisms behind the onset of thermoacoustic instabilities are apparently different.