On the emergence of large clusters of acoustic power sources at the onset of thermoacoustic instability in a turbulent combustor

Abstract

In turbulent combustors, the transition from stable combustion (i.e. combustion noise) to thermoacoustic instability occurs via intermittency. During stable combustion, the acoustic power production happens in a spatially incoherent manner. In contrast, during thermoacoustic instability, the acoustic power production happens in a spatially coherent manner. In the present study, we investigate the spatiotemporal dynamics of acoustic power sources during the intermittency route to thermoacoustic instability using complex network theory. To that end, we perform simultaneous acoustic pressure measurement, high-speed chemiluminescence imaging and particle image velocimetry in a backward-facing step combustor with a bluff body stabilized flame at different equivalence ratios. We examine the spatiotemporal dynamics of acoustic power sources by constructing time-varying spatial networks during the different dynamical states of combustor operation. We show that as the turbulent combustor transits from combustion noise to thermoacoustic instability via intermittency, small fragments of acoustic power sources, observed during combustion noise, nucleate, coalesce and grow in size to form large clusters at the onset of thermoacoustic instability. This nucleation, coalescence and growth of small clusters of acoustic power sources occurs during the growth of pressure oscillations during intermittency. In contrast, during the decay of pressure oscillations during intermittency, these large clusters of acoustic power sources disintegrate into small ones. We use network measures such as the link density, the number of components and the size of the largest component to quantify the spatiotemporal dynamics of acoustic power sources as the turbulent combustor transits from combustion noise to thermoacoustic instability via intermittency.

Suppression of thermoacoustic instability by targeting the hubs of the turbulent networks in a bluff body stabilized combustor

“…The formation of these large-scale vortical structures leads to the coherent production of acoustic power sources over large clusters (Krishnan et al. 2019 b ), as observed in figure 8( c ). A large vortex is also shed from the tip of the bluff body (figure 8 b , at time instant ).…”

“…The large-scale vortical structure then convects towards the bluff body (figure 6b at time instants B and C). Recently, Krishnan et al (2019b) have shown that the formation of large-scale vortical structure leads to the conglomeration of acoustic power sources in the flow field. Hence, we observe a coherent acoustic power production during the formation of the vortical structure (figure 6c at time instants A and B).…”

“…Krishnan et al (2019a) used a weighted correlation network analysis to identify an optimal location in the flow field to implement a passive control strategy to mitigate thermoacoustic instability. Recently, we investigated the spatiotemporal dynamics of acoustic power sources by constructing time-varying spatial networks during the different dynamical states of combustor operation (Krishnan et al 2019b). We uncovered that as the turbulent combustor transitions from combustion noise to thermoacoustic instability via intermittency, small fragments of acoustic power sources, observed during combustion noise, nucleate, coalesce and grow in size to form large clusters at the onset of thermoacoustic instability.…”

“…We simultaneously acquire the acoustic pressure measurement, high-speed flame images and high-speed PIV for the current study. (Reproduced with permission from Krishnan et al (2019b)).…”

Identifying vortical network connectors for turbulent flow modification

“…2017; Krishnan et al. 2019 b ), wall turbulence (Iacobello, Scarsoglio & Ridolfi 2018 b ), mixing in turbulent channel flow (Iacobello et al. 2018 a , 2019 a , b ) and isotropic magnetohydrodynamic turbulence (Gürcan 2018).…”