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Chaos, 28(6), 063125 (2018) DOI:10.1063/1.5028159
On the emergence of critical regions at the onset of thermoacoustic instability in a turbulent combustor
V. R. Unni, A. Krishnan, R. Manikandan, N. B. George, R. I. Sujith, N. Marwan, J. KurthsWe 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.
Emergence of order from chaos is a common sight in nature. Synchronous flashing of fireflies, Mexican wave in a football stadium, triggering of riots, collective behaviour of a school of fish or a swarm of birds, emergence of consciousness from the interplay of millions of neurons, and the evolution of life are some of the examples seen in nature. Formation of convection cells, pattern formation in the BelousovâZhabotinsky reaction, and the emergence of coherent vortices in a turbulent flow are examples of order emerging from disorder in fluid systems. An important fluid dynamic system exhibiting the emergence of order from disorder is a combustor, which houses a confined turbulent reactive flow. During normal operation, the reactive flow field exhibits incoherent turbulent fluctuations. However, under certain operational conditions, the flow field reorganizes, and a spatially ordered periodic behavior emerges. During this dynamic regime known as thermoacoustic instability, the acoustic field inside the combustor exhibits dangerous large amplitude oscillations. In this paper, using complex spatial networks, we characterize the spatial dynamics of the combustor during the stable operation (chaotic oscillations), the thermoacoustic instability (limit cycle oscillations), and the transition regime from stable operation to thermoacoustic instability known as intermittency. Further, using network measures, we identify the critical regions of the reactive flow field that influences the dynamics of the reactive flow field during thermoacoustic instability.