Abstract: S. A. Pawar, S. Mondal, N. B. George, R. I. Sujith (2019)

The coupled behavior of the acoustic pressure and the heat release rate fluctuations is studied in a swirl-stabilized combustor during the transition of system dynamics from combustion noise to thermoacoustic instability via intermittency. The framework of synchronization is used to analyze the temporal as well as the spatiotemporal behavior of these oscillations. The results of the synchronization transition observed in this combustor are further compared and contrasted with those recently reported by Pawar et al. ("Thermoacoustic Instability as Mutual Synchronization Between the Acoustic Field of the Confinement and Turbulent Reactive Flow," Journal of Fluid Mechanics, Vol. 827, Sept. 2017, pp. 664-693) and Mondal et al. ("Onset of Thermoacoustic Instability in Turbulent Combustors: An Emergence of Synchronized Periodicity Through Formation of Chimera-like States," Journal of Fluid Mechanics, Vol. 811, Jan. 2017, pp. 659-681) in a bluff-body stabilized combustor. A rigorous analysis of the synchronization transition of these oscillations in the frequency domain is also presented. The temporal analysis reveals that the synchronization states and the synchronization transition to thermoacoustic instability of the coupled acoustic pressure and heat release rate fluctuations are similar for both swirl-stabilized and bluff-body stabilized combustors. On the other hand, the spatiotemporal analysis of both the combustors during the state of strongly correlated periodic oscillations shows that, despite the acoustic power production being higher during this state, an increased amount of spatial incoherence is visible in the instantaneous and phase-conditioned phasor field of the reaction zone of the swirl-stabilized combustor as compared to that of the bluff-body stabilized combustor. In short, the present study unifies the synchronization transition of coupled oscillations in these turbulent combustors, wherein the mechanisms behind the onset of thermoacoustic instabilities are apparently different.

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Temporal and Spatiotemporal Analyses of Synchronization Transition in a Swirl-Stabilized Combustor