Flame dynamics and unsteady heat release rate of self-excited azimuthal modes in an annular combustor

In gas turbines, thermoacoustic oscillations grow if moments of high fluctuating heat release rate coincide with moments of high acoustic pressure. The phase between the heat release rate and the acoustic pressure depends strongly on the flame behaviour (specifically the time delay) and on the acoustic period. This makes the growth rate of thermoacoustic oscillations exceedingly sensitive to small changes in the acoustic boundary conditions, geometry changes, and the flame time delay. In this paper, adjoint-based sensitivity analysis is applied to a thermoacoustic network model of an annular combustor. This reveals how each eigenvalue is affected by every parameter of the system. This information is combined with an optimization algorithm in order to stabilize all thermoacoustic modes of the combustor by making only small changes to the geometry. The final configuration has a larger plenum area, a smaller premix duct area and a larger combustion chamber volume. All changes are less than 6% of the original values. The technique is readily scalable to more complex models and geometries and the inclusion of further constraints, such that the combustion chamber itself should not change. This demonstrates why adjoint-based sensitivity analysis and optimization could become an indispensible tool for the design of thermoacoustically-stable combustors.

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Self-Sustained Instabilities in an Annular Combustor Coupled by Azimuthal and Longitudinal Acoustic Modes

Volume 1B: Combustion, Fuels and Emissions ◽

10.1115/gt2013-95010 ◽

2013 ◽

Cited By ~ 19

Author(s):

Jean-Francois Bourgouin ◽

Daniel Durox ◽

Jonas P. Moeck ◽

Thierry Schuller ◽

Sébastien Candel

Keyword(s):

Heat Release ◽

Heat Release Rate ◽

Release Rate ◽

High Speed ◽

High Amplitude ◽

Acoustic Mode ◽

Azimuthal Mode ◽

Longitudinal Acoustic ◽

Acoustic Modes ◽

Annular Combustor

Annular combustors may give rise to various types of combustion instabilities. Some of the resulting oscillations coupled by transverse acoustic modes are commonly observed in practice and their suppression or reduction is an important issue which needs to be considered. The present study is carried out in a system comprising an annular plenum feeding 16 swirling injectors confined by two cylindrical quartz tubes opened to the atmosphere. Calculations based on a Helmholtz solver provide a suitable estimate of frequencies observed experimentally and reveal the modal structure corresponding to the longitudinal and transverse oscillations. High speed images obtained under reactive conditions are then processed to extract the structure of heat release rate perturbations and match this structure with that of the coupling acoustic mode. It is found that the transverse instability is coupled by a first azimuthal mode which is characterized by a time varying spin ratio. This index gives the respective levels of rotating components in the azimuthal mode. Another instability arising at a lower frequency is coupled by a longitudinal acoustic mode giving rise to high-amplitude oscillations in heat release rate in which most of the flames (but not all) are synchronized and in phase with the pressure perturbation.

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Lagrangian Analysis of Flame Dynamics in the Flow Field of a Bluff Body-Stabilized Combustor

Journal of Engineering for Gas Turbines and Power ◽

10.1115/1.4044873 ◽

2019 ◽

Vol 142 (1) ◽

Author(s):

C. P. Premchand ◽

Nitin B. George ◽

Manikandan Raghunathan ◽

Vishnu R. Unni ◽

R. I. Sujith ◽

...

Keyword(s):

Heat Release ◽

Heat Release Rate ◽

Release Rate ◽

Bluff Body ◽

Leading Edge ◽

Mean Flow ◽

Lagrangian Analysis ◽

Acoustic Frequency ◽

Thermoacoustic Instability ◽

Flame Dynamics

Abstract Experiments are performed in a partially premixed bluff body-stabilized turbulent combustor by varying the mean flow velocity. Simultaneous measurements obtained for unsteady pressure, velocity, and heat release rate are used to investigate the dynamic regimes of intermittency (10.1 m/s) and thermoacoustic instability (12.3 m/s). Using wavelet analysis, we show that during intermittency, modulation of heat release rate occurring at the acoustic frequency fa by the heat release rate occurring at the hydrodynamic frequency fh results in epochs of heat release rate fluctuations where the heat release rate is phase locked with the acoustic pressure. We also show that the flame position during intermittency and thermoacoustic instability are essentially dictated by saddle point dynamics in the dump plane and the leading edge of the bluff body.

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Flame Dynamics Intermittency in the Bistable Region Near a Subcritical Hopf Bifurcation

Journal of Engineering for Gas Turbines and Power ◽

10.1115/1.4038326 ◽

2018 ◽

Vol 140 (6) ◽

Cited By ~ 11

Author(s):

D. Ebi ◽

A. Denisov ◽

G. Bonciolini ◽

E. Boujo ◽

N. Noiray

Keyword(s):

Heat Release ◽

Heat Release Rate ◽

Release Rate ◽

Acoustic Pressure ◽

Oscillation Amplitude ◽

Acoustic Damping ◽

Intermittent Behavior ◽

Flame Dynamics ◽

Bistable Region ◽

Subcritical Hopf Bifurcation

We report experimental evidence of thermoacoustic bistability in a lab-scale turbulent combustor over a well-defined range of fuel–air equivalence ratios. Pressure oscillations are characterized by an intermittent behavior with “bursts,” i.e., sudden jumps between low and high amplitudes occurring at random time instants. The corresponding probability density functions (PDFs) of the acoustic pressure signal show clearly separated maxima when the burner is operated in the bistable region. The gain and phase between acoustic pressure and heat release rate fluctuations are evaluated at the modal frequency from simultaneously recorded flame chemiluminescence and acoustic pressure. The representation of the corresponding statistics is new and particularly informative. It shows that the system is characterized, in average, by a nearly constant gain and by a drift of the phase as function of the oscillation amplitude. This finding may suggest that the bistability does not result from an amplitude-dependent balance between flame gain and acoustic damping, but rather from the nonconstant phase difference between the acoustic pressure and the coherent fluctuations of heat release rate.

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Investigation of Flame Dynamics in a V - Flame Combustor During Combustion Instability

ASME 2014 Gas Turbine India Conference ◽

10.1115/gtindia2014-8345 ◽

2014 ◽

Author(s):

R. Vishnu ◽

R. I. Sujith ◽

Preeti Aghalayam

Keyword(s):

Heat Release ◽

Heat Release Rate ◽

Release Rate ◽

Large Amplitude ◽

Gas Turbines ◽

High Speed ◽

Combustion Instability ◽

Flame Dynamics ◽

The Stability ◽

Flame Behavior

Propulsion systems such as gas turbines are susceptible to combustion instability, when operated at lean equivalence ratio [1]. During combustion instability, there is a nonlinear interaction between combustion and acoustics leading to large amplitude acoustic oscillations. These large amplitude oscillations are detrimental to the stability of the combustor and can cause damages to the structural integrity of the combustor, flame flash back or blow off. The main source of nonlinearity is in the heat release rate caused due to the velocity perturbations at the flame holder [2]. The heat release rate fluctuations are due to the variation in the flame surface area. Hence there is a need to understand the flame dynamics that contributes to the heat release rate fluctuations. The present study aims in understanding the stability of a V - flame combustor by varying the flame location inside an acoustic resonator. By varying the flame location the instability regimes of the combustor are identified. At the flame locations where the system exhibits combustion instability, acoustic pressure oscillations are acquired simultaneously with high speed images of the flame front fluctuations so that a correlation can be made between them. Tools from dynamical systems theory are applied to the pressure signal to quantify different dynamical states of the system during combustion instability. Moreover the flame dynamics at each dynamical state are investigated. It is observed that combustion instability is characterized by interesting dynamical states such as frequency locked state, quasi-periodic oscillations, period 3 oscillations and chaotic oscillations. High speed imaging of the flame reveals different interesting patterns of flame behavior during combustion instability. Flame wrinkling, roll up of flame elements, separation as islands of the flame elements and mutual annihilation of flame elements were some of the interesting flame behavior observed. This study helps in understanding the role of nonlinear heat release rate mechanism in establishing different dynamical states during combustion instability.

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