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Research Papers: Gas Turbines: Combustion, Fuels, and Emissions

Impact of Shear Flow Instabilities on the Magnitude and Saturation of the Flame Response

[+] Author and Article Information
Steffen Terhaar

Chair of Fluid Dynamics
Hermann-Föttinger-Institut,
Technische Universität Berlin,
Müller-Breslau-Str. 8,
Berlin 10623, Germany
e-mail: steffen.terhaar@tu-berlin.de

Bernhard Ćosić, Christian Oliver Paschereit

Chair of Fluid Dynamics
Hermann-Föttinger-Institut,
Technische Universität Berlin,
Müller-Breslau-Str. 8,
Berlin 10623, Germany

Kilian Oberleithner

Laboratory for Turbulence Research in Aerospace
and Combustion,
Monash University,
Clayton, VIC 3800, Australia

Contributed by the Combustion and Fuels Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received December 18, 2013; final manuscript received January 13, 2014; published online February 18, 2014. Editor: David Wisler.

J. Eng. Gas Turbines Power 136(7), 071502 (Feb 18, 2014) (9 pages) Paper No: GTP-13-1454; doi: 10.1115/1.4026530 History: Received December 18, 2013; Revised January 13, 2014

Amplitude-dependent flame transfer functions, also denoted as flame describing functions, are valuable tools for the prediction of limit-cycle amplitudes of thermoacoustic instabilities. However, the effects that govern the transfer function magnitude at low and high amplitudes are not yet fully understood. It is shown in the present work that the flame response at perfectly premixed conditions is strongly influenced by the growth rate of vortical structures in the shear layers. An experimental study in a generic swirl-stabilized combustor was conducted in order to measure the amplitude-dependent flame transfer function and the corresponding flow fields subjected to acoustic forcing. The applied measurement techniques included the multi-microphone-method, high-speed OH*-chemiluminescence measurements, and high-speed particle image velocimetry. The flame response and the corresponding flow fields are assessed for three different swirl numbers at 196 Hz forcing frequency. The results show that forcing leads to significant changes in the time-averaged reacting flow fields and flame shapes. A triple decomposition is applied to the time-resolved data, which reveals that coherent velocity fluctuations at the forcing frequency are amplified considerably stronger in the shear layers at low forcing amplitudes than at high amplitudes, which is an indicator for a nonlinear saturation process. The strongest saturation is found for the lowest swirl number, where the forcing additionally detached the flame. For the highest swirl number, the saturation of the vortex amplitude is weaker. Overall, the amplitude-dependent vortex amplification resembles the characteristics of the flame response very well. An application of a linear stability analysis to the time-averaged flow fields at increasing forcing amplitudes yields the decreasing growth rates of shear flow instabilities at the forcing frequency. It therefore successfully predicts a saturation at high forcing amplitudes and demonstrates that the mean flow field and its modifications are of utmost importance for the growth of vortices in the shear layers. Moreover, the results clearly show that the amplification of vortices in the shear layers is an important driver for heat release fluctuations and their saturation.

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Figures

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Fig. 1

Mechanisms that are involved in the transfer function of velocity fluctuations at the combustor inlet to the flame response. Investigated mechanisms are shaded gray.

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Fig. 2

Preliminary investigation of the influence of the tangential velocities (S = 0.8) on the eigenvalues of the axisymmetric (m = 0) mode

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Fig. 3

Preliminary investigation of the influence of the Reynolds number on the eigenvalue α of the axisymmetric m = 0 mode at x/Dh = 0.25. The filled marker indicates the selected value of Re = 5000.

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Fig. 4

Sketch of the combustor test rig and the experimental setup

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Fig. 5

Flame response for acoustic forcing at f = 196 Hz for three swirl numbers

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Fig. 6

Time-averaged flow field superimposed on the normalized OH*-chemiluminescence distribution at increasing forcing amplitudes. Solid lines indicate zero axial velocity. First row: S = 0.6. Second row: S = 0.8. Third row: S = 1. Fourth row: S = 0.8 nonreacting.

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Fig. 7

Phase-averaged flow fields and flame positions at four equidistant phase angles (S = 0.8, u'/u0 = 0.3). First row: streamlines of the coherent velocity superimposed on the normalized coherent through-plane vorticity. Second row: normalized phase-averaged OH*-chemiluminescence distribution and superimposed phase-averaged velocity vectors. Solid lines indicate zero phase-averaged axial velocity.

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Fig. 8

Streamwise development of the coherent fluctuation intensity K(x) at S = 0.8 for increasing forcing amplitudes

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Fig. 9

Maximum coherent fluctuation intensity Kmax at increasing forcing amplitudes

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Fig. 10

Comparison of the measured to calculated coherent radial velocity distribution at S = 0.8 and u'/u0 = 0.1

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Fig. 11

First row: amplitude ratio A of the m = 0 mode from the stability analysis. Second row: modified amplitude ratio A' from the triple decomposition.

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