Control of growth of local heat release rate fluctuations to suppress thermoacoustic instability

Abstract: This experimental study investigates the dynamical transition from stable operation to thermoacoustic instability in a turbulent bluff-body stabilized dump combustor. We conduct experiments to characterize the dynamical transition utilizing acoustic pressure and local heat release rate fluctuations. We observe the transition to thermoacoustic instability for these experiments as we decrease the equivalence ratio towards a fuel-lean setting. More importantly, we observe significant growth of local heat release rate fluctuations near the bluff-body well before the appearance of large-scale spatial or temporal patterns during the occurrence of thermoacoustic instability. By strategically positioning slots (perforations) on the bluff-body, we ensure the reduction of the growth of local heat release rate fluctuations at the stagnation zone near the bluff-body at operating conditions far away from the onset of thermoacoustic instability. This reduction in the local heat release rate fluctuations ensures that the transition to thermoacoustic instability is avoided. We find that modified configurations of the bluff-body that do not quench these local heat release rate fluctuations at the stagnation zone results in the transition to thermoacoustic instability. We also reveal that an effective suppression strategy based on the growth of local heat release rate fluctuations requires an optimization of the area ratio of the slots for a given bluff-body position. Further, the suppression strategy also depends on the spatial distribution of perforations on the bluff-body. Notably, an inappropriate distribution of the slots which does not quench the local heat release rate fluctuations at the stagnation zone may even result in a dramatic increase in amplitudes of pressure oscillations.