The dynamic transitions preceding lean blowout were investigated experimentally in a laboratory scale turbulent combustor by systematically varying the flow Reynolds number (Re). Previous studies on combustor dynamics have shown that the onset of large-amplitude, combustion-driven oscillations is, at times, presaged by intermittent bursts of high-amplitude periodic pressure pulsations. These intermittent bursts appear in a near random fashion amidst regions of aperiodic low-amplitude fluctuations, provided the underlying flow-field is turbulent. In the present study, we show that intermittent burst oscillations are also observed in combustors close to the lean blowout limit. We show that such intermittent oscillations emerge through the establishment of homoclinic orbits in the phase space of pressure oscillations. The formation of such orbits points to the complex nature of the interaction between the hydrodynamics and acoustic subsystems, which operate over a range of different time scales. High-speed flame images reveal that the intermittent states observed prior to lean blowout correspond to aperiodic detachment of the flame from the bluff-body lip. These findings are consistent with other reports of possibly intermittent states in the literature. Copyright © 2015 Taylor & Francis Group, LLC.

R. I. Sujith

Department of Aerospace Engineering

BLOWOUTS

combustion

Dynamics

Phase space methods

Reynolds number

Thermoacoustics

ACOUSTIC SUBSYSTEMS

Different time scale

DYNAMIC TRANSITIONS

Intermittency

LEAN BLOW OUT

LEAN-BLOWOUT LIMITS

PRESSURE OSCILLATION

Turbulent combustion

Combustors

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

Intermittency as a Transition State in Combustor Dynamics: An Explanation for Flame Dynamics Near Lean Blowout

Combustion Science and Technology

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

We investigate the route to self-excited thermoacoustic instability in a laminar flow multiple flame matrix burner. With an increase in the equivalence ratio, the thermoacoustic system that is initially quiet (stable operation) transitions to limit cycle oscillations through two distinct dynamical states, namely, bursting oscillations and mixed mode oscillations. The acoustic pressure oscillations transition from quiescence to large amplitudes during bursting oscillations. Such high amplitude bursting oscillations that occur well ahead of the onset of limit cycle oscillations can potentially cause structural damage. The thermoacoustic system exhibits hysteresis. The transition to limit cycle oscillations is replicated in a phenomenological model containing slow-fast time scales. © 2019 Author(s).

Chaos

Bursting and mixed mode oscillations during the transition to limit cycle oscillations in a matrix burner

The dynamic characteristics of the intermittency exhibited by turbulent combustors using LPG as fuel before and after the occurrence of thermoacoustic instability were compared. The unsteady pressure fluctuations analysis suggested that the intermittency before and after thermoacoustic instability are of type II. The flame dynamics during either state were also compared through a high speed Mie scattering images acquired simultaneously with unsteady pressure. During both regimes of intermittency the flame switches between an oscillation where the flame oscillates in a periodic manner due to the inherent turbulent fluctuations and an oscillation where the flame exhibits periodic roll-up as a consequence of the periodic vortex shedding at the dump plane. The flame dynamics during intermittency before and after thermoacoustic instability varied. The flame along the inner shear layer was stabilized by the stagnation point flow behind the bluff body during intermittency before the occurrence of thermoacoustic instability while the flame along the inner shear layer was stabilized by the recirculation zone created by the bluff body during intermittency after the occurrence of thermoacoustic instability.

Proceedings of the Combustion Institute

Flame dynamics during intermittency 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

The transition in dynamics from low-amplitude, aperiodic, combustion noise to high-amplitude, periodic, combustion instability in confined, combustion environments was studied experimentally in a laboratory-scale combustor with two different flameholding devices in a turbulent flow field. We show that the low-amplitude, irregular pressure fluctuations acquired during stable regimes, termed 'combustion noise', display scale invariance and have a multifractal signature that disappears at the onset of combustion instability. Traditional analysis often treats combustion noise and combustion instability as acoustic problems wherein the irregular fluctuations observed in experiments are often considered as a stochastic background to the dynamics. We demonstrate that the irregular fluctuations contain useful information of prognostic value by defining representative measures such as Hurst exponents that can act as early warning signals to impending instability in fielded combustors. © 2014 Cambridge University Press.

Journal of Fluid Mechanics

Multifractality in combustion noise: Predicting an impending combustion instability

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

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.

Journal	Data powered by TypesetCombustion Science and Technology
Publisher	Data powered by TypesetTaylor and Francis Inc.
ISSN	00102202
Open Access	No