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.

R. I. Sujith

Department of Aerospace Engineering

Combustors

Dynamical systems

Oscillators (mechanical)

stability

Turbulence

Combustion instabilities

INTERMEDIATE REGIMES

Intermittency

PERIODIC OSCILLATION

Physical mechanism

Precursors

Pressure fluctuation

Reduced order models

combustion

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IIT Madras

Proceedings of the Combustion Institute

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

Experiments were performed in a partially premixed bluff-body stabilized combustor in the regimes of combustion noise, intermittency, and thermoacoustic instability. Simultaneous measurements of unsteady pressure fluctuations and flow-field using time-resolved two-component particle image velocimetry reveal dominant dynamics at 141.9 Hz which is responsible for thermoacoustic instability. In the intermittent regime that presages thermoacoustic instability, there are two distinct frequencies: a low-frequency component at 30.7 Hz dominant in the velocity spectra (hydrodynamic mode) and a higher frequency component at 176.4 Hz dominant in the pressure spectra (acoustic mode). Examining the phase relationship between the two modes in the intermittent regime using a variant of the Dynamic Mode Decomposition (DMD) confirms that the appearance of bursts of periodic pressure oscillations coincide with the time instants when the hydrodynamic and the acoustic modes are phase synchronized. To identify the flow structure dynamics observed only during sound production, we compute ridges in the fields of backward-time finite time Lyapunov exponents. The roll up of shear layers from the dump plane and the leading edge of the bluff body and subsequent impingement on combustor walls are identified as the dominant features of the flow during thermoacoustic instability as well as during the bursting stage of intermittency. We show convincingly that these identified dynamics correspond to the acoustic mode using DMD filtered flow fields comprising only of the acoustic mode. © 2019 Author(s).

Fulltext

Physics of Fluids

Lagrangian analysis of intermittent sound sources in the flow-field of a bluff-body stabilized combustor

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.

AIAA Aerospace Sciences Meeting, 2018

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

Wind tunnel experiments carried out on a pitch-plunge aeroelastic system in the presence of fluctuating flows reveal that flutter instability is presaged by a regime of intermittency. It is observed that as the flow speed gradually increases towards the flutter speed, there appears intermittent bursts of periodic oscillations which become more frequent as the wind speed increases and eventually the dynamics transition into fully developed limit cycle oscillations, marking the onset of flutter. The signature from these intermittent oscillations are exploited to develop measures that forewarn a transition to flutter and can serve as precursors. This study investigates a suite of measures that are obtained directly from the time history of measurements and are hence model independent. The dependence of these precursors on the size of the measured data set and the time required for their computation is investigated. These measures can be useful in structural health monitoring of aeroelastic structures. © 2018 Elsevier Ltd

Journal of Sound and Vibration

Investigations on precursor measures for aeroelastic flutter

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

Combustion noise has been traditionally thought of as stochastic fluctuations present in the background of the dynamics in combustors amongst the flow, heat release and the chamber acoustics. Through a series of determinism tests, we show that these aperiodic fluctuations are in fact chaotic of moderately high dimensions (d0 8-10). These chaotic fluctuations then transition to high amplitude combustion instability when the operating conditions are varied towards leaner equivalence ratios. Precursors to such a transition from chaos to dynamics dominated by periodic oscillations are of interest to designers and operators of combustors in estimating the boundaries of operability. We introduce a test for chaos, known as 0-1 test for chaos in the literature, as a measure of the proximity of the combustor to an impending instability. The measure is robust and shows a smooth transition for variation in flow conditions towards instability enabling thresholds to be set for operational boundaries.

International Journal of Spray and Combustion Dynamics

Loss of chaos in combustion noise as a precursor of impending combustion instability

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.

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

About the Zero Mach Number Assumption in the Calculation of Thermoacoustic Instabilities

Proceedings of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences

A new test for chaos in deterministic systems

Combustion dynamics and control: Progress and challenges

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

Journal	Data powered by TypesetProceedings of the Combustion Institute
Publisher	Data powered by TypesetElsevier Ltd
ISSN	15407489
Open Access	No