We investigate the route to self-excited thermoacoustic instability in a laminar flow multiple flame matrix burner. With an increase in the equivalence ratio, the thermoacoustic system that is initially quiet (stable operation) transitions to limit cycle oscillations through two distinct dynamical states, namely, bursting oscillations and mixed mode oscillations. The acoustic pressure oscillations transition from quiescence to large amplitudes during bursting oscillations. Such high amplitude bursting oscillations that occur well ahead of the onset of limit cycle oscillations can potentially cause structural damage. The thermoacoustic system exhibits hysteresis. The transition to limit cycle oscillations is replicated in a phenomenological model containing slow-fast time scales. © 2019 Author(s).

R. I. Sujith

Department of Aerospace Engineering

Fulltext

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IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

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IIT Madras

Bursting and mixed mode oscillations during the transition to limit cycle oscillations in a matrix burner.

Chaos

In this paper, an analysis of the coupled behaviour between the acoustic pressure and the heat release rate fluctuations in a swirl-stabilized combustor during the transition of the system dynamics from combustion noise to thermoacoustic instability via intermittency is performed. Both temporal and spatiotemporal analysis of these coupled oscillations are carried out using the framework of synchronization theory. The results of the synchronization transition observed in the swirl-stabilized combustor are compared and contrasted with those recently reported by Pawar et al. (J. Fluid Mech., vol. 827, 2017, pp. 664-693) and Mondal et al. (J. Fluid Mech., vol. 811, 2017, pp. 659-681) in a bluff-body stabilized combustor. Further, a rigorous analysis of the synchronization transition of these oscillations in the frequency domain is presented, thus shedding light on the onset of phase and frequency lock-on in systems with a turbulent reactive flow. In addition, the spatiotemporal dynamics of the flame-acoustic interaction during the two states of thermoacoustic instability, that is, weakly and strongly correlated periodic oscillations observed in the system are compared. Despite the global synchrony and the acoustic driving being higher during the state of strongly correlated periodic oscillations as compared to the state of weakly correlated periodic oscillations, an increased amount of spatial incoherence in the reaction rate field is observed during this state. In short, our study reasserts a unified synchronization transition of coupled oscillations in these turbulent combustors, wherein the mechanisms behind the onset of thermoacoustic instabilities are apparently different. © 2018, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.

AIAA Aerospace Sciences Meeting, 2018

Synchronization behaviour during the dynamical transition in swirl-stabilized combustor: Temporal and spatiotemporal analysis

The dynamic characteristics of the intermittency exhibited by turbulent combustors using LPG as fuel before and after the occurrence of thermoacoustic instability were compared. The unsteady pressure fluctuations analysis suggested that the intermittency before and after thermoacoustic instability are of type II. The flame dynamics during either state were also compared through a high speed Mie scattering images acquired simultaneously with unsteady pressure. During both regimes of intermittency the flame switches between an oscillation where the flame oscillates in a periodic manner due to the inherent turbulent fluctuations and an oscillation where the flame exhibits periodic roll-up as a consequence of the periodic vortex shedding at the dump plane. The flame dynamics during intermittency before and after thermoacoustic instability varied. The flame along the inner shear layer was stabilized by the stagnation point flow behind the bluff body during intermittency before the occurrence of thermoacoustic instability while the flame along the inner shear layer was stabilized by the recirculation zone created by the bluff body during intermittency after the occurrence of thermoacoustic instability.

Proceedings of the Combustion Institute

Flame dynamics during intermittency in a turbulent combustor

In this paper, we report on the existence of the phenomenon of change of criticality in a horizontal Rijke tube, a prototypical thermoacoustic system. In the experiments, the phenomenon is shown to occur as the criticality of the Hopf bifurcation changes with varying air flow rates in the system. The dynamics of a nonlinear system exhibiting Hopf bifurcation can be described using a Stuart-Landau equation (SLE) in the vicinity of the bifurcation point. The criticality of Hopf bifurcations can be determined by the Landau constant of the Stuart-Landau equation, which represents the effect of nonlinearities in the system. We propose an SLE to model the bifurcations seen in the horizontal Rijke tube. We identify a rescaled version of Strouhal number as the Landau constant, which determines the criticality of the bifurcation in the present study.

Change of criticality in a prototypical thermoacoustic system

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 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. © 2016 Cambridge University Press.

Journal of Fluid Mechanics

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

Thermoacoustic instability is the result of a positive coupling between the acoustic field in the duct and the heat release rate fluctuations from the flame. Recently, in several turbulent combustors, it has been observed that the onset of thermoacoustic instability is preceded by intermittent oscillations, which consist of bursts of periodic oscillations amidst regions of aperiodic oscillations. Quantitative analysis of the intermittency route to thermoacoustic instability has been performed hitherto using the pressure oscillations alone. We perform experiments on a laboratory-scale bluff-body-stabilized turbulent combustor with a backward-facing step at the inlet to obtain simultaneous data of acoustic pressure and heat release rate fluctuations. With this, we show that the onset of thermoacoustic instability is a phenomenon of mutual synchronization between the acoustic pressure and the heat release rate signals, thus emphasizing the importance of the coupling between these non-identical oscillators. We demonstrate that the stable operation corresponds to desynchronized aperiodic oscillations, which, with an increase in the mean velocity of the flow, transition to synchronized periodic oscillations. In between these states, there exists a state of intermittent phase synchronized oscillations, wherein the two oscillators are synchronized during the periodic epochs and desynchronized during the aperiodic epochs of their oscillations. Furthermore, we discover two different types of limit cycle oscillations in our system. We notice a significant increase in the linear correlation between the acoustic pressure and the heat release rate oscillations during the transition from a lower-amplitude limit cycle to a higher-amplitude limit cycle. Further, we present a phenomenological model that qualitatively captures all of the dynamical states of synchronization observed in the experiment. Our analysis shows that the times at which vortices that are shed from the inlet step reach the bluff body play a dominant role in determining the behaviour of the limit cycle oscillations. © 2017 Cambridge University Press.

Journal	Data powered by TypesetChaos
Publisher	Data powered by TypesetAmerican Institute of Physics Inc.
ISSN	10541500
Open Access	No