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.

R. I. Sujith

Department of Aerospace Engineering

Gireeshkumaran Thampi

combustion

Combustors

Oscillators (mechanical)

Reynolds number

Backward facing step

Bluff-body stabilized

Combustion instabilities

Intermittency

PERIODIC OSCILLATION

Thermoacoustic instability

Turbulent reacting flows

wave-turbulence interaction

stability

Instability

Turbulence

Turbulent flow

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IIT Madras

Journal of Fluid Mechanics

Liquid rockets are prone to large amplitude oscillations, commonly referred to as thermoacoustic instability. This phenomenon causes unavoidable developmental setbacks and poses a stern challenge to accomplish the mission objectives. Thermoacoustic instability arises due to the nonlinear interaction between the acoustic and the reactive flow subsystems in the combustion chamber. In this paper, we adopt tools from dynamical systems and complex systems theory to understand the dynamical transitions from a state of stable operation to thermoacoustic instability in a self-excited model multielement liquid rocket combustor based on an oxidizer rich staged combustion cycle. We observe that this transition to thermoacoustic instability occurs through a sequence of bursts of large amplitude periodic oscillations. Furthermore, we show that the acoustic pressure oscillations in the combustor pertain to different dynamical states. In contrast to a simple limit cycle oscillation, we show that the system dynamics switches between period-3 and period-4 oscillations during the state of thermoacoustic instability. We show several measures based on recurrence quantification analysis and multifractal theory, which can diagnose the dynamical transitions occurring in the system. We find that these measures are more robust than the existing measures in distinguishing the dynamical state of a rocket engine. Furthermore, these measures can be used to validate models and computational fluid dynamics simulations, aiming to characterize the performance and stability of rockets. © 2019 Author(s).

Fulltext

Chaos

Dynamical systems approach to study thermoacoustic transitions in a liquid rocket 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

We study the dependence of the flame sheet response of an open V-flame when subjected to simultaneous but independently controlled – (1) narrowband, coherent disturbances, (2) mean flow velocity, and (3) turbulent broadband disturbances. We re-examine the data presented in Humphrey et al. (2018). Flame edge and flow field were obtained from Mie scattering images and PIV vector fields, respectively. We determine the flame sheet response, which allows us to understand the effects of kinematic restoration on the local and global flame dynamics. We find some highly nonlinear behaviors in the flame sheet response, which have not been reported previously. In particular, we observe (1) oscillations in the rise and peak region of the flame response, and (2) early onset of the nonlinear coupling between narrowband and broadband disturbances, resulting in oscillatory decay immediately downstream of the flame holder region. The spatial oscillations seen in the flame response arise only when unalike disturbances such as vortical and flame wrinkle disturbances have a comparable wavelength and, thus, interfere. We then find that higher nominal velocity and turbulent intensities increase asymmetry in the flame response, and subsequently affect the local and global heat release response (HRR). We analytically calculate HRR from flame sheet response. We find that there is a monotonic increase in the response with increasing flame length for lower nominal velocities, possibly due to higher flame symmetry. Moreover, the HRR calculated from flame sheet response captures the effect of kinematic restoration very well. Notably, there is a decrease in HRR with increasing turbulence intensity due to enhanced smoothing from kinematic restoration at higher turbulence levels. We further find that the flame response dependence on harmonic forcing manifest in the low-pass filter characteristics of the global HRR response. © 2019 The Combustion Institute

Combustion and Flame

Nonlinear flame response dependencies of a V-flame subjected to harmonic forcing and turbulence

A model problem comprising two subsytems (acoustic and hydrodynamic), both admitting unsteady flows, has been studied using numerical simulations. Simulations capture all key features observed in an experiment with a similar configuration, and reveal the differences in flow field evolution that manifest as an intermittent prelude to instability in this nonlinear system, coupling acoustic and hydrodynamic phenomena. The system consists of a uniform flow that enters a long duct and leaves through an orifice into the atmosphere. The duct supports acoustic modes, while the shear layer at the boundary of the jet emerging from the orifice can develop into a train of vortices. Over some range of inflow velocities that support a stable operation, duct acoustic modes and shear layer breakup result in aperiodic, small amplitude pressure fluctuations. As the flow rate is increased, fluctuations become essentially periodic and have much larger amplitudes. This transition from a quiescent to a (so-called) whistling state occurs over a range of flow rates and is characterized by intermittent, moderately large, aperiodic fluctuations. As in the experiment, the aperiodic time series from the quiescent state is found to be multifractal; multifractality is then lost as the flow transitions to whistling. Simulation flow fields reveal differences in the development of vortices in the bounding shear layer, and near the orifice walls, that give rise to the large changes in pressure fluctuations, as well as the intermittent behavior that precedes whistling. © The Author(s) 2019.

International Journal of Aeroacoustics

A numerical study of an acoustic–hydrodynamic system exhibiting an intermittent prelude to instability

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.

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

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.

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

In this paper, we show how the phenomenon of intermittency observed in systems with turbulent flow-sound interaction is related to the formation of homoclinic orbits in the phase space. Such orbits that emerge via the intersection of the stable and unstable manifold of an equilibrium configuration result from interactions that happen at multiple spatial/temporal scales associated with turbulent convection and wave propagation. Through a quantification of the time spent by the dynamics in the aperiodic states using recurrence plots, we show how the presence of homoclinic orbits in the dynamics may be convincingly demonstrated, which is often not possible through a visual inspection of the phase space of the attractor. © 2013 AIP Publishing LLC.

Identifying homoclinic orbits in the dynamics of intermittent signals through recurrence quantification

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 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.

Fluid Dynamics Research

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

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.

Journal	Data powered by TypesetJournal of Fluid Mechanics
Publisher	Data powered by TypesetCambridge University Press
ISSN	00221120
Open Access	No