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Gas Turbines: Combustion, Fuels, and Emissions

Open-Loop Control of Combustion Instabilities and the Role of the Flame Response to Two-Frequency Forcing

[+] Author and Article Information
Bernhard Ćosić1

Chair of Fluid Dynamics, Hermann-Föttinger-Institut,  Technische Universität Berlin, Müller-Breslau-Strasse 8, D-10623 Berlin, GermanyBernhard.Cosic@tu-berlin.de

Bernhard C. Bobusch, Jonas P. Moeck, Christian Oliver Paschereit

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

1

Corresponding author.

J. Eng. Gas Turbines Power 134(6), 061502 (Apr 12, 2012) (8 pages) doi:10.1115/1.4005986 History: Received August 18, 2011; Revised August 19, 2011; Published April 09, 2012; Online April 12, 2012

Controlling combustion instabilities by means of open-loop forcing at non-resonant frequencies is attractive because neither a dynamic sensor signal nor a signal processor is required. On the other hand, since the mechanism by which this type of control suppresses an unstable thermoacoustic mode is inherently nonlinear, a comprehensive explanation for success (or failure) of open-loop control has not been found. The present work contributes to the understanding of this process in that it interprets open-loop forcing at non-resonant frequencies in terms of the flame’s nonlinear response to a superposition of two approximately sinusoidal input signals. For a saturation-type nonlinearity, the fundamental gain at one frequency may be decreased by increasing the amplitude of a secondary frequency component in the input signal. This effect is first illustrated on the basis of an elementary model problem. In addition, an experimental investigation is conducted at an atmospheric combustor test-rig to corroborate the proposed explanation. Open-loop acoustic and fuel-flow forcing at various frequencies and amplitudes is applied at unstable operating conditions that exhibit high-amplitude limit-cycle oscillations. The effectiveness of specific forcing parameters in suppressing self-excited oscillations is correlated with flame response measurements that include a secondary forcing frequency. The results demonstrate that a reduction in the fundamental harmonic gain at the instability frequency through the additional forcing at a non-resonant frequency is one possible indicator of successful open-loop control. Since this mechanism is independent of the system acoustics, an assessment of favorable forcing parameters, which stabilize thermoacoustic oscillations, may be based solely on an investigation of burner and flame.

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Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Block diagram for the model system given by Eq. 1. L and N represent the linear acoustic dynamics and the nonlinear feedback from the heat release, respectively.

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Figure 2

Test-rig overview

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Figure 3

Cut through the burner passage

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Figure 4

Sectional view of the swirl generator

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Figure 5

Effect of open-loop loudspeaker forcing frequency on heat release and pressure oscillations associated with the instability. Left axis: natural mode’s heat release oscillation amplitude (∘), right axis: rms pressure (□); results are normalized to the baseline case without forcing.

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Figure 6

Combined acoustic response of trombone and loudspeakers in terms of normalized acoustic velocity fluctuation versus frequency

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Figure 7

Dependence of natural mode’s heat release oscillation amplitude at the instability frequency on loudspeaker forcing amplitude for different forcing frequencies. Results are normalized to the baseline case without forcing.

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Figure 8

Effect of open-loop FRV forcing frequency on heat release and pressure oscillations associated with the instability. Left axis: natural mode’s heat release oscillation amplitude (∘), right axis: reduction of SPL (□). Results are normalized to the baseline case without forcing.

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Figure 9

Top: heat release spectrum with loudspeaker forcing; bottom: heat release spectrum with FRV forcing. Results are normalized to the baseline case without forcing.

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Figure 10

Top: pressure spectrum with loudspeaker forcing; Bottom: pressure spectrum with FRV forcing. Results are normalized to the baseline case without forcing.

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Figure 11

Dependence of the flame response gain at 155 Hz on the secondary forcing amplitude for different secondary forcing frequencies. Results are normalized to the baseline case without secondary forcing.

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Figure 12

Influence of additional forcing amplitude on the phase relation of velocity and heat release fluctuations at 155 Hz

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Figure 13

Reduction of linear gain at 170 Hz for three fuel modulation frequencies. Increasing amplitude at 170 Hz and constant modulation amplitude of 30% of the total fuel flow.

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Figure 14

Acoustic energy gain/loss balance and influence of open-loop forcing

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