Onset of thermoacoustic instability in turbulent combustors: an emergence of synchronized periodicity through formation of chimera-like states

Abstract

Thermoacoustic systems with a turbulent reactive flow, prevalent in the fields of power and propulsion, are highly susceptible to oscillatory instabilities. Recent studies showed that such systems transition from combustion noise to thermoacoustic instability through a dynamical state known as intermittency, where bursts of large-amplitude periodic oscillations appear in a near-random fashion in between regions of low-amplitude aperiodic fluctuations. However, as these analyses were in the temporal domain, this transition remains still unexplored spatiotemporally. Here, we present the spatiotemporal dynamics during the transition from combustion noise to limit cycle oscillations in a turbulent bluff-body stabilized combustor. To that end, we acquire the pressure oscillations and the field of heat release rate oscillations through high-speed chemiluminescence ($CH^{\ast }$) images of the reaction zone. With a view to get an insight into this complex dynamics, we compute the instantaneous phases between acoustic pressure and local heat release rate oscillations. We observe that the aperiodic oscillations during combustion noise are phase asynchronous, while the large-amplitude periodic oscillations seen during thermoacoustic instability are phase synchronous. We find something interesting during intermittency: patches of synchronized periodic oscillations and desynchronized aperiodic oscillations coexist in the reaction zone. In other words, the emergence of order from disorder happens through a dynamical state wherein regions of order and disorder coexist, resembling a chimera state. Generally, mutually coupled chaotic oscillators synchronize but retain their dynamical nature; the same is true for coupled periodic oscillators. In contrast, during intermittency, we find that patches of desynchronized aperiodic oscillations turn into patches of synchronized periodic oscillations and vice versa. Therefore, the dynamics of local heat release rate oscillations change from aperiodic to periodic as they synchronize intermittently. The temporal variations in global synchrony, estimated through the Kuramoto order parameter, echoes the breathing nature of a chimera state.

Pattern formation during transition from combustion noise to thermoacoustic instability via intermittency

“…Combustion noise exhibits irregular temporal Tony et al 2015) and spatio-temporal dynamics (Kheirkhah et al 2016;Mondal et al 2017). In our study, during the occurrence of combustion noise, we observe disordered spatiotemporal dynamics.…”

“…One can observe a region upstream of the bluff body with a cluster of similar values of p Q . This clustering phenomenon observed in the spatial dynamics hints at self-organization and the emergence of localized coherent spatial structures amidst incoherent regions (grainy regions) (Mondal et al 2017). In other words, the local interactions at the small scale in the field of p Q , enabled by the favourable local flame speeds, result in coherence at the large scale.…”

“…A recent study by Kirthy et al (2016) on a backward-facing step combustor reported that spatial amplification of velocity disturbances in a globally stable but locally convectively unstable flow could result in thermoacoustic instability. Further, Mondal, Unni & Sujith (2017) recognized that during the occurrence of thermoacoustic instability, the instantaneous relative phases between the acoustic pressure oscillations and the local heat release rate fluctuations exhibit spatial synchrony. During the occurrence of intermittency, regions of synchrony exist amidst regions of asynchrony, resembling a chimera state.…”

Investigation of flame behavior and dynamics prior to lean blowout in a combustor with varying mixedness of reactants for the early detection of lean blowout

“…However, earlier study has shown that phase difference between two signals calculated through HT closely matches with that calculated through cross-correlation even for a set of broadband signals. 42 In other words, we can expect a gradual shift between the dynamics of phase difference evaluated through HT and cross-correlation, as one moves gradually from narrow band to broadband signal. The signals used in the present study are mostly broadband in nature.…”

Suppression of thermoacoustic instability in a swirl-stabilized combustor by inducing blockage in the inlet flow stream

“…obtained from CH * chemiluminescence of the flame showed that during the occurrence of intermittency, regions of coherent behaviour emerge amidst incoherent regions. 15 From the perspective of dynamical systems theory, previous studies have shown that thermoacoustic systems undergo transitions from a stable operation to limit cycle oscillations through bifurcations. For instance, Lieuwen, 16 reported supercritical and subcritical…”