An experimental investigation of the bistable region of instability in a thermoacoustic system comprising of ducted, premixed laminar flames has been performed. The stability diagram of the system is obtained and the bistable region for a range of flame locations at different fuel-air mixture equivalence ratios is identified. Subsequently, threshold amplitudes for triggering instability in the system using sinusoidal acoustic forcing, introduced externally, is obtained. It is observed that depending on how close the system is to the Hopf point and the nature of oscillations at the Hopf point, the triggered oscillations can exhibit different dynamical behavior. Copyright © 2011 by ASME.

R. I. Sujith

Department of Aerospace Engineering

BISTABLES

Dynamical behaviors

Equivalence ratios

Experimental investigations

FLAME LOCATION

Fuel-air mixtures

Hopf point

Premixed flame

PREMIXED LAMINAR FLAMES

Stability diagram

SUBCRITICAL INSTABILITY

THERMOACOUSTIC SYSTEMS

THRESHOLD AMPLITUDE

Exhibitions

Thermoacoustics

Turbines

stability

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IIT Madras

Investigation of Subcritical Instability in Ducted Premixed Flames

Proceedings of the ASME Turbo Expo

Nonlinear self-excited thermoacoustic oscillations appear in systems involving confined combustion in the form of coupled acoustic pressure oscillations and unsteady heat release rate. In this paper, we investigate the nonlinear transition undergone by thermoacoustic oscillations to flame blowout via intermittency, in response to variation in the location of the combustion source with respect to the acoustic field of the confinement. A ducted laminar premixed conical flame, stabilized on a circular jet exit with a fully developed exit velocity profile, was investigated. Transition to limit cycle oscillations from a non-oscillatory state was observed to occur via a subcritical Hopf bifurcation. Limit cycle oscillations underwent a further bifurcation to quasi-periodic oscillations characterized by the repeated formation of elongated necks in the flame that pinch off as pockets of unburned fuel-air mixture. The quasi-periodic state loses stability, resulting in an intermittent state identified as type II through recurrence analysis of phase space trajectories reconstructed from the acoustic pressure time trace. In this state, the flame undergoes repeated lift-off and reattachment. Instantaneous flame images suggest that the intermittent flame behaviour is influenced by jet flow dynamics. © 2012 Cambridge University Press.

Journal of Fluid Mechanics

Nonlinear self-excited thermoacoustic oscillations: Intermittency and flame blowout

Thermoacoustic oscillations occur due to the interaction between the acoustic field in the combustor and the unsteady heat release rate. This interaction gives rise to a wide variety of dynamical states. In our experiments on a ducted laminar premixed flame, a prototypical thermoacoustic system, we observed presence of type-II intermittency in addition to limit cycle oscillation and quasi-periodic oscillation. Each dynamical state of the system are investigated based on the analysis of the reconstructed phase space and recurrence plots. The intermittent state resulted from the bifurcation of a quasi-periodic state and further, was followed by flame blowout. The dynamical behavior of the flame during intermittency is also discussed through flame imaging. Copyright © 2012 by L. Kabiraj and R. I. Sujith.

50th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition

Bifurcation analysis and observation of intermittency in combustion-driven thermoacoustic oscillations

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.

Fulltext

Chaos

Route to chaos for combustion instability in 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.

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.

Physics of Fluids

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

Nonlinear Dynamics and Chaos

Triggering in the horizontal Rijke tube: non-normality, transient growth and bypass transition

Investigation of subcritical instability in ducted premixed flames