Recently, there has been a growing interest in understanding and characterising intermittent burst oscillations that presage the onset of combustion instability. We construct a deterministic model to capture this intermittency route to instability in a bluff-body stabilised combustor by coupling the equations governing vortex shedding and the acoustic wave propagation in a confinement. A feedback mechanism is developed wherein the sound generated due to unsteady combustion affects the vortex shedding. This feedback leads to a variation in the time of impingement of the vortices with the bluff body causing the system to exhibit chaos, intermittency, and limit cycle oscillations. Experimental validation of the model is provided using various precursor measures that quantify the observed intermittent states. © 2016 Informa UK Limited, trading as Taylor & Francis Group.

R. I. Sujith

Department of Aerospace Engineering

Akshay T. Seshadri

Acoustic wave propagation

Chaos theory

combustion

Dynamical systems

Feedback

stability

Vortex shedding

Combustion instabilities

Deterministic modeling

Experimental validations

Feedback mechanisms

Intermittency

INTERMITTENCY ROUTE

Limit Cycle Oscillation (LCO)

UNSTEADY COMBUSTION

Vortex flow

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IIT Madras

A reduced-order deterministic model describing an intermittency route to combustion instability

Combustion Theory and Modelling

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. © 2019 Cambridge University Press.

Fulltext

Journal of Fluid Mechanics

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

Swirl flows are often used for flame stabilization in gas turbine combustors. However, when these combustors are operated at lean fuel/air ratios, they are prone to thermoacoustic instability. In this study, we experimentally investigate the effect of distribution of the blockage in the inlet flow on the transition of the combustion dynamics from combustion noise to thermoacoustic instability. We acquire unsteady pressure fluctuations and heat release rate fields (CH∗ chemiluminescence) by capturing the flame images to investigate this transition in the thermoacoustic system. We utilize a turbulence generator with two different configurations to modify the inlet flow dynamics to achieve passive control of thermoacoustic instability. To that end, using flow restrictors, we induce blockage in the inlet flow upstream of the swirler, perpendicular to the bulk flow direction. We observe that in one case, there is a reduction in the amplitude of the periodic oscillations while in the other case, there is a suppression of thermoacoustic instability. In the former case, the blockage in the inlet flow stream is more distributed, while in the latter, the same degree of blockage is clustered into one region of the inlet flow stream. The field of the local instantaneous acoustic energy production (p ′ (t) q ′ (x, y, t)) shows the presence of coherence during the occurrence of thermoacoustic instability for the experiments without any blockage. This emerging coherence is disrupted with the inclusion of the blockage. In the case with clustered blockage, the coherence is suppressed significantly, while for the case with the distributed blockage, it is reduced slightly. Analysis of spatial variance, performed on the local instantaneous acoustic energy production indicates the disruption of the coherent spatial structures with the flow restrictors, which subsequently suppresses thermoacoustic instability. © 2018, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.

AIAA Aerospace Sciences Meeting, 2018

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

Thermoacoustic instability is a plaguing problem encountered in many combustion systems. The large amplitude acoustic oscillations, a noted aspect of this instability, can have a detrimental effect on the performance of the system, and sometimes even cause lasting damage to the system components. The aim of this study is to estimate the amplitude of the limit cycle oscillations observed during thermoacoustic instability corresponding to the axial acoustic modes using pressure measurements acquired during stable combustion. Firstly, an equation is derived which describes the slow-varying amplitude of oscillations in certain reduced-order models of combustion systems involving vortex shedding. Subsequently, a procedure is detailed, wherein this equation is used in conjunction with the measured pressure time series and some information about the system to predict the instability amplitude. The estimation capability of this technique is then tested using acoustic pressure data obtained from laboratory scale bluff-body and swirl stabilized combustors. It is observed that the estimated amplitudes are in good agreement with actual values. © 2018, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.

Predicting the amplitude of limit cycle oscillations in thermoacoustic systems with vortex shedding

Thermoacoustic instability is a plaguing problem encountered in many combustion systems. The large-amplitude acoustic oscillations, which are a noted aspect of this instability, can have a detrimental effect on the performance of the system; and they sometimes even cause lasting damage to the system components. The aim of this study is to estimate the amplitude of the limit-cycle oscillations observed during thermoacoustic instability corresponding to the axial acoustic modes using pressure measurements acquired during stable combustion. First, an equation is derived that describes the slow-varying amplitude of oscillations in certain reduced-order models of combustion systems involving vortex shedding. Subsequently, a procedure is detailed, wherein this equation is used in conjunction with the measured pressure time series and some information about the system to predict the instability amplitude. The estimation capability of this technique is then tested using acoustic pressure data obtained from laboratory-scale bluff-body and swirl-stabilized combustors. It is observed that the estimated amplitudes are in good agreement with actual values. Copyright © 2018 by the American Institute of Aeronautics and Astronautics, Inc.

AIAA Journal

Predicting the amplitude of limit-cycle oscillations in thermoacoustic systems with vortex shedding

Gas turbine engines are prone to the phenomenon of thermoacoustic instability, which is highly detrimental to their components. Recently, in turbulent combustors, it was observed that the transition to thermoacoustic instability occurs through an intermediate state, known as intermittency, where the system exhibits epochs of ordered behaviour, randomly appearing amidst disordered dynamics. We investigate the onset of intermittency and the ensuing self-organization in the reactive flow field, which, under certain conditions, could result in the transition to thermoacoustic instability. We characterize this transition from a state of disordered and incoherent dynamics to a state of ordered and coherent dynamics as pattern formation in the turbulent combustor, utilizing high-speed flame images representing the distribution of the local heat release rate fluctuations, flow field measurements (two-dimensional particle image velocimetry), unsteady pressure and global heat release rate signals. Separately, through planar Mie scattering images using oil droplets, the collective behaviour of small scale vortices interacting and resulting in the emergence of large scale coherent structures is illustrated. We show the emergence of spatial patterns using statistical tools used to study transitions in other pattern forming systems. In this paper, we propose that the intertwined and highly intricate interactions between the wide spatio-temporal scales in the flame, the flow and the acoustics are through pattern formation. © 2018 Cambridge University Press.

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

In combustors, the transition from low-amplitude, aperiodic fluctuations termed combustion noise to large-amplitude, periodic oscillations termed combustion instability is presaged by an intermediate regime in flow conditions characterized by bursts of intermittent, high-amplitude, periodic oscillations that appear in a near-random fashion amidst aperiodic fluctuations. In this study, we show that, a reduced-order model from first principles that incorporates the hydrodynamic-acoustic coupling can capture these intermittent burst oscillations and the subsequent flow-acoustic lock-in observed in combustors. The physical mechanism that leads to intermittency in pressure fluctuations is described using the model. The paper concludes by illustrating through the model, ideas that use intermittency in the signal as an early warning signal - a precursor - to an impending combustion instability. © 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Proceedings of the Combustion Institute

A reduced-order model for the onset of combustion instability: Physical mechanisms for intermittency and precursors

The dynamic transition from combustion noise to combustion instability was investigated experimentally in two laboratory-scale turbulent combustors (namely, swirl-stabilized and bluff-body-stabilized backward-facing-step combustors) by systematically varying the flow Reynolds number. We observe that the onset of combustion-driven oscillations is always presaged by intermittent bursts of high-amplitude periodic oscillations that appear in a near-random fashion amidst regions of aperiodic low-amplitude fluctuations. These excursions to periodic oscillations last longer in time as operating conditions approach instability and finally the system transitions completely into periodic oscillations. A continuous measure to quantify this bifurcation in dynamics can be obtained by defining an order parameter as the probability of the signal amplitude exceeding a predefined threshold. A hysteresis zone was observed in the bluff-body-stabilized configuration that was absent in the swirl-stabilized configuration. The recurrence properties of the dynamics of intermittent burst oscillations were quantified using recurrence plots and the distribution of the aperiodic phases was examined. From the statistics of these aperiodic phases, robust early-warning signals of an impending combustion instability may be obtained. © 2014 Cambridge University Press.

Intermittency route to thermoacoustic instability in turbulent combustors

The role of non-normality and nonlinearity in thermoacoustic interaction in a Rijke tube is investigated in this paper. The heat release rate of the heating element is modeled by a modified form of King's law. This fluctuating heat release from the heating element is treated as a compact source in the one-dimensional linear model of the acoustic field. The temporal evolution of the acoustic perturbations is studied using the Galerkin technique. It is shown that any thermoacoustic system is non-normal. Non-normality can cause algebraic growth of oscillations for a short time even though the eigenvectors of the system could be decaying exponentially with time. This feature of non-normality combined with the effect of nonlinearity causes the occurrence of triggering, i.e., the thermoacoustic oscillations decay for some initial conditions whereas they grow for some other initial conditions. If a system is non-normal, then there can be large amplification of oscillations even if the excited frequency is far from the natural frequency of the system. The dependence of transient growth on time lag and heater positions is studied. Such amplifications (pseudoresonance) can be studied using pseudospectra, as eigenvalues alone are not sufficient to predict the behavior of the system. The geometry of pseudospectra can be used to obtain upper and lower bounds on the growth factor, which provide both necessary and sufficient conditions for the stability of a thermoacoustic system. © 2008 American Institute of Physics.

Physics of Fluids

Thermoacoustic instability in a Rijke tube: Non-normality and nonlinearity

Combustion Science and Technology

Velocity and Flame Wrinkling Characteristics of a Transversely Forced, Bluff-Body Stabilized Flame, Part I: Experiments and Data Analysis

Velocity and Flame Wrinkling Characteristics of a Transversely Forced, Bluff-Body Stabilized Flame, Part II: Flame Response Modeling and Comparison with Measurements

Journal	Data powered by TypesetCombustion Theory and Modelling
Publisher	Data powered by TypesetTaylor and Francis Ltd.
ISSN	13647830
Open Access	No