Mitigation of limit cycle oscillations in a turbulent thermoacoustic system via delayed acoustic self-feedback

Abstract

In this paper, we report the first observation of complete mitigation of thermoacoustic instability in a bluff-body stabilized turbulent combustor through the method of self-coupling. Self-coupling is achieved by coupling the acoustic field of the combustor to itself through a coupling tube. We characterize the effects of such acoustic self-feedback on the thermoacoustic instability of the system by varying the length and diameter of the coupling tube. We observe that the amplitude and the dominant frequency of the acoustic pressure fluctuations gradually decrease as the length of the coupling tube is increased. A complete suppression of thermoacoustic instability is observed when the coupling tube length is nearly 1.5 times the combustor length. Meanwhile, as we approach the suppression of thermoacoustic instability, the dynamical behavior of acoustic pressure changes from the state of limit cycle oscillations to low amplitude aperiodic oscillations via intermittency. We also study the coupling between the acoustic field and the unsteady flame dynamics for different conditions of self-coupling in the combustor. As the combustor approaches the state of complete suppression, the temporal synchrony between the acoustic pressure and the global heat release rate signals changes from the state of synchronized periodicity to desynchronized aperiodicity through intermittent synchronization. From the spatiotemporal analysis of the combustor flow field, we find complete disruption of the coherent spatial structures of acoustic energy production observed during the state of thermoacoustic instability when the combustor is self-coupled with a tube of optimized size. Thus, we anticipate self-coupling to be a viable option to mitigate high amplitude thermoacoustic oscillations in turbulent combustion systems present in gas turbines and rocket engines.