Complex thermoacoustic oscillations are observed experimentally in a simple laboratory combustor that burns lean premixed fuel-air mixture, as a result of nonlinear interaction between the acoustic field and the combustion processes. The application of nonlinear time series analysis, particularly techniques based on phase space reconstruction from acquired pressure data, reveals rich dynamical behavior and the existence of several complex states. A route to chaos for thermoacoustic instability is established experimentally for the first time. We show that, as the location of the heat source is gradually varied, self-excited periodic thermoacoustic oscillations undergo transition to chaos via the Ruelle-Takens scenario. © 2012 American Institute of Physics.

R. I. Sujith

Department of Aerospace Engineering

Fulltext

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IIT Madras

Route to chaos for combustion instability in ducted laminar premixed flames

Chaos

We identify mechanisms through which open-loop control of thermoacoustic instability is achieved in a laminar combustor and characterize them using synchronization theory. The thermoacoustic system comprises two nonlinearly coupled damped harmonic oscillators - acoustic and unsteady heat release rate (HRR) field - each possessing different eigenfrequencies. The frequency of the preferred mode of HRR oscillations is less than the third acoustic eigenfrequency where thermoacoustic instability develops. We systematically subject the limit-cycle oscillations to an external harmonic forcing at different frequencies and amplitudes. We observe that forcing at a frequency near the preferred mode of the HRR oscillator leads to a greater than 90 % decrease in the amplitude of the limit-cycle oscillations through the phenomenon of asynchronous quenching. Concurrently, there is a resonant amplification in the amplitude of HRR oscillations. Further, we show that the flame dynamics plays a key role in controlling the frequency at which quenching is observed. Most importantly, we show that forcing can cause asynchronous quenching either by imposing out-of-phase relation between pressure and HRR oscillations or by inducing period-2 dynamics in pressure oscillations while period-1 in HRR oscillations, thereby causing phase drifting between the two subsystems. In each of the two cases, acoustic driving is very low and hence thermoacoustic instability is suppressed. We show that the characteristics of forced synchronization of the pressure and HRR oscillations are significantly different. Thus, we find that the simultaneous characterization of the two subsystems is necessary to quantify completely the nonlinear response of the forced thermoacoustic system. © 2019 Cambridge University Press.

Journal of Fluid Mechanics

On the mechanism of open-loop control of thermoacoustic instability in a laminar premixed combustor

In this paper, we investigate the coupled behvior of the acoustic field in the confinement and the unsteady flame dynamics in a laboratory scale spray combustor. We study this interaction during the intermittency route to thermoacoustic instability when the location of the flame is varied inside the combustor. As the flame location is changed, the synchronization properties of the coupled acoustic pressure and heat release rate signals change from desynchronized aperiodicity (combustion noise) to phase synchronized periodicity (thermoacoustic instability) through intermittent phase synchronization (intermittency). We also characterize the collective interaction between the multiple flamelets anchored at the flame holder and the acoustic field in the system, during different dynamical states observed in the combustor operation. When the signals are desynchronized, we notice that the flamelets exhibit a steady combustion without the exhibition of a prominent feedback with the acoustic field. In a state of intermittent phase synchronization, we observe the existence of a short-term coupling between the heat release rate and the acoustic field. We notice that the onset of collective synchronization in the oscillations of multiple flamelets and the acoustic field leads to the simultaneous emergence of periodicity in the global dynamics of the system. This collective periodicity in both the subsystems causes enhancement of oscillations during epochs of amplitude growth in the intermittency signal. On the contrary, the weakening of the coupling induces suppression of periodic oscillations during epochs of amplitude decay in the intermittency signal. During phase synchronization, we notice a sustained synchronized movement of all flamelets with the periodicity of the acoustic cycle in the system. © 2018 The Combustion Institute.

Proceedings of the Combustion Institute

Phase synchronization and collective interaction of multiple flamelets in a laboratory scale spray combustor

An experimental study on a turbulent, swirl-stabilized backward facing step combustor is conducted to understand the spatiotemporal dynamics during the transition from combustion noise to thermoacoustic instability. By using a turbulence generator, we investigate the change in the spatiotemporal dynamics during this transition for added turbulence intensities. High-speed CH* images of the flame (representative of the field of local heat release rate fluctuations ((Formula presented.) (x,y,t))) and simultaneous unsteady pressure fluctuations (p'(t)) are acquired for different equivalence ratios. In the study, without the turbulence generator, as the equivalence ratio is reduced from near stoichiometric values, we observe an emergence of coherence in the spatial dynamics during the occurrence of intermittency, enroute to thermoacoustic instability. As the turbulence intensity is increased using the turbulence generator, we find that there is an advanced onset of thermoacoustic instability. Spatial statistics and the instantaneous fields of (Formula presented.) show that during the transition from combustion noise to thermoacoustic instability, the emergence of coherent spatial structures in the instantaneous fields of (Formula presented.) for the experiments with higher turbulence intensities is advanced. However, as the equivalence ratio is reduced further, we notice that higher turbulence intensities result in the reduction of the strength of the pressure oscillations during the state of thermoacoustic instability. We find that, at these low equivalence ratios, there is a decrease in the coherence due to the dispersal of (Formula presented.), which explains the reduction in the strength of the pressure oscillations. © The Author(s) 2017.

International Journal of Spray and Combustion Dynamics

Spatiotemporal dynamics during the transition to thermoacoustic instability: Effect of varying turbulence intensities

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 a result of the positive feedback between the acoustic pressure and the unsteady heat release rate fluctuations in a combustor. We apply the framework of the synchronization theory to study the coupled behavior of these oscillations during the transition to thermoacoustic instability in a turbulent bluff-body stabilized gas-fired combustor. Furthermore, we characterize this complex behavior using recurrence plots and recurrence networks. We mainly found that the correlation of probability of recurrence (C P R), the joint probability of recurrence (J P R), the determinism (D E T), and the recurrence rate (R R) of the joint recurrence matrix aid in detecting the synchronization transitions in this thermoacoustic system. We noticed that C P R and D E T can uncover the occurrence of phase synchronization state, whereas J P R and R R can be used as indices to identify the occurrence of generalized synchronization (GS) state in the system. We applied measures derived from joint and cross recurrence networks and observed that the joint recurrence network measures, transitivity ratio, and joint transitivity are useful to detect GS. Furthermore, we use the directional property of the network measure, namely, cross transitivity to analyze the type of coupling existing between the acoustic field (p ′) and the heat release rate (q ′) fluctuations. We discover a possible asymmetric bidirectional coupling between q ′ and p ′, wherein q ′ is observed to exert a stronger influence on p ′ than vice versa. © 2018 Author(s).

Coupled interaction between unsteady flame dynamics and acoustic field in a turbulent combustor

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