We present a novel and an efficient way to mitigate oscillatory instability in turbulent reactive flows. First, we construct weighted spatial correlation networks from the velocity field obtained from high-speed particle image velocimetry. Using network measures, we identify the optimal location for implementing passive control strategies. By injecting micro-jets at this optimal location, we are able to reduce the amplitude of the pressure oscillations to a value comparable to what is observed during the state of stable operation. This approach opens up new avenues to control oscillatory instabilities in turbulent flows.

We use complex network theory to investigate the dynamical transition from stable operation to thermoacoustic instability via intermittency in a turbulent combustor with a bluff body stabilized flame. A spatial network is constructed, representing each of these three dynamical regimes of combustor operation, based on the correlation between time series of local velocity obtained from particle image velocimetry. Network centrality measures enable us to identify critical regions of the flow field during combustion noise, intermittency, and thermoacoustic instability. We find that during combustion noise, the bluff body wake turns out to be the critical region that determines the dynamics of the combustor. As the turbulent combustor transitions to thermoacoustic instability, during intermittency, the wake of the bluff body loses its significance in determining the flow dynamics and the region on top of the bluff body emerges as the most critical region in determining the flow dynamics during thermoacoustic instability. The knowledge about this critical region of the reactive flow field can help us devise optimal control strategies to evade thermoacoustic instability.

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Tackling the problem of emissions is at the forefront of scientific research today. While industrial engines designed to operate in stable regimes produce emissions, attempts to operate them at "greener" conditions often fail due to thermoacoustic instability. During thermoacoustic instability, hazardous high amplitude periodic oscillations lead to failure of these engines in power plants, aircrafts and rockets. Yet, identifying the onset of thermoacoustic instability remains elusive due to spatial variability and the continuous evolution of spatiotemporal patterns in the reacting flow field. Here, we show experimental evidence of early manifestation of the onset of thermoacoustic instability at certain zones. Our findings allow us to identify a critical threshold that enables us to distinguish stable operating regimes from hazardous operations. This opens new perspectives for predicting the onset of thermoacoustic instability and could be a step forward to "greener" operations. The developed methodology is applicable for other systems exhibiting phase transitions.

Experiments are performed in a partially-premixed bluff-body stabilized turbulent combustor by varying the mean flow velocity. Simultaneous measurements obtained for unsteady pressure, velocity and heat release rate are used to investigate the dynamic regimes of intermittency (10.1 m/s) and thermoacoustic instability (12.3 m/s). Using wavelet analysis, we show that during intermittency, modulation of heat release rate occurring at the acoustic frequency fa by the heat release rate occurring at the hydrodynamic frequency fh results in epochs of heat release rate fluctuations where the heat release is phase locked with the acoustic pressure. We also show that the flame position during intermittency and thermoacoustic instability are essentially dictated by saddle point dynamics in the dump plane and the leading edge of the bluff-body.