We experimentally investigate the effect of noise on the stability of a thermoacoustic system operating in a bistable region. The thermoacoustic system chosen for investigation is a ducted non-premixed flame. The system is excited by random fuel flow rate fluctuations which affects the system dynamics as parametric noise. Under the influence of noise, the system undergoes transition from stable to oscillatory state. In particular, transition is observed even when the amplitude of the noise is significantly less than the triggering amplitude of the corresponding deterministic system. While triggering from noise of low amplitudes, phase portraits reveal that the system evolves transiently towards an unstable periodic orbit, before eventually growing to a stable periodic orbit. A stochastic stability map is constructed to represent the probability of the system to undergo transition. It is also observed that the amplitude of the oscillatory state is affected by the noise level. © 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

R. I. Sujith

Department of Aerospace Engineering

NOISE INDUCED TRIGGERING

NON-PREMIXED FLAME

NONLINEAR TIME-SERIES ANALYSIS

Stochastic stability

UNSTABLE ATTRACTORS

Chaos theory

Stochastic systems

Thermoacoustics

time series analysis

System stability

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

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

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Proceedings of the Combustion Institute

This paper reports the effect of inlet flow turbulence intensity on the combustion instability characteristics in a backward facing step combustor. The inlet turbulence intensity is varied by a turbulence generator. Unsteady pressure measurements and OH∗chemiluminescence images are recorded over a wide range of operating conditions at different inlet turbulence intensities. The study shows an early onset of instability at low turbulence level, i.e., higher turbulence postpones the onset of instability to higher Reynolds number Re and/or higher equivalence ratio Φ. The early onset of instability in the Re and Φ parameter spaces is due to the change in system parameters such as flame speed and size of the recirculation zone downstream of the step at different turbulence levels. Further, the onset is characterized as subcritical bifurcation. At low Re, the hysteresis zone width is small for low turbulence levels and it is large at higher turbulence levels; and at higher Re, the hysteresis width remains constant at all turbulence levels. Investigation of instability characteristics reveals that there are momentary slippages from limit cycle orbit into brief silent regimes in an intermittent manner. The frequency of occurrence of the momentary silent regimes increases with reduction in turbulence, indicating that higher turbulence helps in maintaining the system in a stable limit cycle orbit. High-speed chemiluminescence imaging reveals the necessity of the vortex rollup in the recirculation zone to grow up to the top wall by dilatation from the heat release for the onset of instability. Considerations of the effect of turbulence on both the flame speed and the recirculation zone size together explain all the observed bifurcation trends. These results suggest that inlet flow turbulence should not just be considered as background noise. The turbulence effects on both the flame and flow should be considered in predicting the instability characteristics. © 2018 Elsevier Ltd.

Effect of inlet flow turbulence on the combustion instability in a premixed backward-facing step 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.

Fulltext

International Journal of Spray and Combustion Dynamics

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

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

Flame-acoustic interactions are witnessed in the context of combustion instability in gas turbine combustors and other propulsion devices, such as rockets, besides confined combustion systems in general, such as furnaces and heaters. The confinement causes acoustic standing wave modes that interact with the flame to cause fluctuations in all quantities to grow in amplitude. This review categorizes the different canonical flame-holding geometries that mostly involve flow recirculation zones for flame stabilization, which are inherently unstable and feed into the flame-acoustic interaction cycle. The receptivity of the nonreacting shear layer to prevalent acoustic forcing in terms of development of coherent structures and instability of different hydrodynamic modes in the recirculation are detailed. The case of reacting flow instabilities involves several mechanisms of flame-acoustic coupling, such as vortex combustion; vortex-wall interactions; vortex-vortex interactions; flame area fluctuations and equivalence ratio, vorticity, and entropy fluctuations; bifurcations of the combustor dynamics; and its Strouhal scaling. In some instances, flame blowout dynamics as well as flashback are coupled with the prevalent acoustic oscillations. Some of these aspects have been well diagnosed in the recent literature, notably using planar laser-induced fluorescence for flame marking and particle image velocimetry to characterize the flow in a time-resolved manner. Reduced order models based on empirically determined flame transfer functions and predictive tools for onset of combustion instability based on time series analysis, either from a nonlinear dynamical viewpoint or a data-driven approach, are in current vogue. © 2017 Taylor & Francis.

Combustion Science and Technology

Dynamics and Diagnostics of Flame-Acoustic Interactions

The aeroelastic response of a NACA 0012 airfoil in the flow regimes prior to flutter is investigated in a wind tunnel. We observe intermittent bursts of periodic oscillations in the pitch and plunge response, that appear in an irregular manner from a background of relatively lower amplitude aperiodic fluctuations. As the flow speed is increased, the intermittent bursts last longer in time until eventually transitioning to a fully developed periodic response, indicating the onset of flutter. The repeating patterns in the measured response are visualized using recurrence plots. We show that statistics of the recurrence states extracted from these plots can be used to develop model-free precursors that forewarn an impending transition to flutter, well before its onset. © 2015 Elsevier Ltd.

Journal of Fluids and Structures

Precursors to flutter instability by an intermittency route: A model free approach

Experimental investigation of noise induced triggering in thermoacoustic systems

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

The role of non-normality and nonlinearity in flame-acoustic interaction in a ducted diffusion flame is investigated in this paper. The infinite rate chemistry model is employed to study unsteady diffusion flames in a Burke-Schumann type geometry. It has been observed that even in this simplified case, the combustion response to perturbations of velocity is non-normal and nonlinear. This flame model is then coupled with a linear model of the duct acoustic field to study the temporal evolution of acoustic perturbations. The one-dimensional acoustic field is simulated in the time domain using the Galerkin technique, treating the fluctuating heat release from the combustion zone as a compact acoustic source. It is shown that the coupled combustion-acoustic system is non-normal and nonlinear. Further, calculations showed the occurrence of triggering; i.e. the thermoacoustic oscillations decay for some initial conditions whereas they grow for some other initial conditions. It is shown that triggering occurs because of the combined effect of non-normality and nonlinearity. For such a non-normal system, resonance or 'pseudoresonance' may occur at frequencies far from its natural frequencies. Non-normal systems can be studied using pseudospectra, as eigenvalues alone are not sufficient to predict the behaviour of the system. Further, both necessary and sufficient conditions for the stability of a thermoacoustic system are presented in this paper. © 2008 Cambridge University Press.

Journal of Fluid Mechanics

Non-normality and nonlinearity in combustion-acoustic interaction in diffusion flames

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.

Journal	Proceedings of the Combustion Institute
ISSN	15407489
Open Access	No