  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.
