The dynamics of combustion-driven thermoacoustic oscillations for a ducted laminar premixed flame has been investigated in lean equivalence ratio conditions. Combustion instability appears in the system as acoustic pressure and flame surface oscillations following a Hopf bifurcation. Further change in the control parameter leads to subsequent bifurcations, causing a rich dynamical behavior such as quasi-periodic and intermittent burst oscillations to appear in the system. During the burst oscillation phase, the system dynamics resembles the flame blowout phenomenon, which is of interest in practical combustion applications. © 2012 The Japan Society of Fluid Mechanics and IOP Publishing Ltd.

R. I. Sujith

Department of Aerospace Engineering

ACOUSTIC PRESSURES

BURST OSCILLATION

Combustion instabilities

control parameters

Dynamical behaviors

Equivalence ratios

FLAME SURFACE

LAMINAR PREMIXED FLAMES

LEAN BLOW OUT

PRACTICAL COMBUSTION

Quasi-periodic

System dynamics

THERMOACOUSTIC OSCILLATIONS

BLOWOUTS

Hopf bifurcation

PLASMA OSCILLATIONS

combustion

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IIT Madras

Investigating the dynamics of combustion-driven oscillations leading to lean blowout

Fluid Dynamics Research

The occurrence of thermoacoustic instability has been a major concern in the combustors used in power plants and propulsive systems such as gas turbine engines, rocket motors. A positive feedback between the inherent processes such as the acoustic field and the unsteady heat release rate of the combustor can result in the occurrence of large-amplitude, self-sustained pressure oscillations. Prior to the state of thermoacoustic instability, intermittent oscillations are observed in turbulent combustors. Such intermittent oscillations are characterized by an apparently random appearance of bursts of large-amplitude periodic oscillations interspersed between epochs of low-amplitude aperiodic oscillations. In most of the earlier studies, the pressure oscillations alone have been analyzed to explore the dynamical transition to thermoacoustic instability. The present chapter focuses on the instantaneous interaction between the acoustic field and the unsteady heat release rate observed during such a transition in a bluff-body-stabilized turbulent combustor. The instantaneous interaction of these oscillations will be discussed using the concepts of synchronization theory. First, we give a brief introduction to the synchronization theory so as to summarize the concepts of locking of phase and frequency of the oscillations. Then, the temporal and spatiotemporal aspects of the interaction will be presented in detail. We find that, during stable operation, aperiodic oscillations of the pressure and the heat release rate remain desynchronized, whereas synchronized periodic oscillations are noticed during the occurrence of thermoacoustic instability. Such a transition happens through intermittent phase-synchronized oscillations, wherein synchronization and desynchronization of the oscillators are observed during the periodic and the aperiodic epochs of the intermittent oscillations, respectively. Further, the spatiotemporal analysis reveals a very interesting pattern in the reaction zone. Phase asynchrony among the local heat release rate oscillators is observed during the stable operation, while they become phase-synchronized during the onset of thermoacoustic instability. Interestingly, the state of intermittent oscillations corresponds to a simultaneous existence of synchronized periodic and desynchronized aperiodic patterns in the reaction zone. Such a coexistence of synchrony and asynchrony in the reactive flow field mimics a chimera state. © 2018, Springer Nature Singapore Pte Ltd.

Green Energy and Technology

Synchronization transition in a thermoacoustic system: Temporal and spatiotemporal analyses

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 of Fluid Mechanics

Thermoacoustic instability as mutual synchronization between the acoustic field of the confinement and turbulent reactive flow

Thermoacoustic instability and lean blowout are the major challenges faced when a gas turbine combustor is operated under fuel lean conditions. The dynamics of thermoacoustic system is the result of complex nonlinear interactions between the subsystems-turbulent reactive flow and the acoustic field of the combustor. In order to study the transitions between the dynamical regimes in such a complex system, the time series corresponding to one of the dynamic variables is transformed to an ɛ-recurrence network. The topology of the recurrence network resembles the structure of the attractor representing the dynamics of the system. The transitions in the thermoacoustic system are then captured as the variation in the topological characteristics of the network. We show the presence of power law degree distribution in the recurrence networks constructed from time series acquired during the occurrence of combustion noise and during the low amplitude aperiodic oscillations prior to lean blowout. We also show the absence of power law degree distribution in the recurrence networks constructed from time series acquired during the occurrence of thermoacoustic instability and during the occurrence of intermittency. We demonstrate that the measures derived from recurrence network can be used as tools to capture the transitions in the turbulent combustor and also as early warning measures for predicting impending thermoacoustic instability and blowout. © 2017 Author(s).

Fulltext

Chaos

Recurrence networks to study dynamical transitions in a turbulent combustor

Practical applications involving combustion suffer from serious issues such as combustion instabilities and sudden loss of flame: flame flashback and blowout. These phenomena are related to dynamical changes in the combustion system. Here, we summarize our recent studies on the application of recurrence-based methods to identify such dynamical transitions as well as for the characterization of combustion dynamics in laboratory combustors. © Springer International Publishing Switzerland 2016.

Springer Proceedings in Physics

Recurrence plots for the analysis of combustion dynamics

Analysis of thermoacoustic instabilities were dominated by modal (eigenvalue) analysis for many decades. Recent progress in nonmodal stability analysis allows us to study the problem from a different perspective, by quantitatively describing the short-term behavior of disturbances. The short-term evolution has a bearing on subcritical transition to instability, known popularly as triggering instability in thermoacoustic parlance. We provide a review of the recent developments in the context of triggering instability. A tutorial for nonmodal stability analysis is provided. The applicability of the tools from nonmodal stability analysis are demonstrated with the help of a simple model of a Rjike tube. The article closes with a brief description of how to characterize bifurcations in thermoacoustic systems. © The Author(s) 2016.

International Journal of Spray and Combustion Dynamics

Non-normality and nonlinearity in thermoacoustic instabilities

Bifurcation analysis is performed on experimental data obtained from a simple setup comprising ducted laminar premixed conical flames in order to investigate the features of nonlinear thermoacoustic oscillations. It is observed that as the bifurcation parameter is varied, the system undergoes a series of bifurcations leading to characteristically different nonlinear oscillations. Through the application of nonlinear time series analysis to pressure and flame (CH chemiluminescence) intensity time traces, these oscillations are characterized as periodic, aperiodic, or chaotic oscillations, and subsequently the nature of the obtained bifurcations is explained based on dynamical systems theory. Nonlinear interaction between the flames and the acoustic modes of the duct is clearly reflected in the high speed flame images acquired simultaneously with pressure time series. © 2012 by ASME.

Journal of Engineering for Gas Turbines and Power

Bifurcations of self-excited ducted laminar premixed flames

Nonlinear Dynamics and Chaos

Bifurcation Analysis of Thermoacoustic Instability in a Horizontal Rijke Tube

Physical Review E

Dynamic properties of unstable motion of swirling premixed flames generated by a change in gravitational orientation

Progress in Energy and Combustion Science

Lean blowoff of bluff body stabilized flames: Scaling and dynamics

Journal	Fluid Dynamics Research
ISSN	01695983
Open Access	No