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Gas Turbines: Combustion, Fuels, and Emissions

Acoustic Response of a Helmholtz Resonator Exposed to Hot-Gas Penetration and High Amplitude Oscillations

[+] Author and Article Information
Bernhard Ćosić1

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

Thoralf G. Reichel, Christian Oliver Paschereit

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

1

Address all correspondence to this author.

J. Eng. Gas Turbines Power 134(10), 101503 (Aug 22, 2012) (9 pages) doi:10.1115/1.4007024 History: Received June 18, 2012; Revised June 20, 2012; Published August 22, 2012; Online August 22, 2012

Helmholtz resonators are often used in the gas turbine industry for the damping of thermoacoustic instabilities. To prevent thermal destruction, these devices are usually cooled by a purging flow. Since the acoustic velocity inside the neck of the resonator becomes very high already at moderate pressure oscillation levels, hot-gas penetration cannot always be fully avoided. This study extends a well-known nonlinear impedance model to include the influence of hot-gas intrusion into the Helmholtz resonator neck. A time-dependent but spatially averaged density function of the volume flow in the neck is developed. The steady component of this density function is implemented into the nonlinear impedance model to account for the effect of hot-gas intrusion. The proposed model predicts a significant shift in the resonance frequency of the damper towards higher frequencies, depending on the amplitude of the acoustic velocity in the neck and the temperature of the penetrating hot gas. Subsequently, the model is verified by the experimental investigation of two resonance frequencies (86 Hz and 128 Hz) for two hot gas temperatures (1470 K and 570 K) and various pressure oscillation amplitudes. The multimicrophone method, in combination with a microphone flush-mounted in the resonator volume, is used to determine the impedance of the Helmholtz damper. Additionally, a movable ultra-thin thermocouple was used to determine the degree of hot-gas penetration and the change of the mean temperature at various axial positions in the neck. A very good agreement between the model and the experimental data is obtained for all levels of pressure amplitudes and of hot-gas penetration depths. The mean air temperatures in the neck were accurately predicted too.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Worst case ratio of resonance frequencies with and without hot-gas penetration with respect to the ratio of temperatures between combustion chamber and resonator volume

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Figure 2

Assumed behavior of the air velocity (top) and density (bottom) together with averaged density (ρ¯n), density of the plenum (ρv), density of the combustion chamber (ρcc), and the integration limits of Eq. 12

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Figure 3

Iterative solution procedure

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Figure 4

Test rig overview

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Figure 5

Sketch of the Helmholtz resonator setup

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Figure 6

Acoustic velocity in the neck (top) and damping performance in terms of the real part of the admittance (bottom) for different purging flow velocities for a pressure amplitude of p̂=840 Pa

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Figure 7

Damping performance in terms of the real part of the admittance for the 86 Hz configuration for various pressure amplitudes and a hot gas temperature in the microphone tube of ≈573 K. Symbols indicate interpolated measurement points and solid lines represent the results of the model.

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Figure 8

Damping performance in terms of the real part of the admittance for the 128 Hz configuration for various pressure amplitudes and a hot gas temperature in the microphone tube of ≈1373 K. Symbols indicate interpolated measurement points and solid lines represent the results of the model.

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Figure 9

Temperature profile in the neck for the 86 Hz configuration for various pressure amplitudes forcing at 88 Hz and a hot gas temperature in the microphone tube of ≈573 K. Symbols indicate interpolated measurement points.

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Figure 10

Temperature profile in the neck for the 128 Hz configuration for various pressure amplitudes forcing at 121 Hz and a hot gas temperature in the microphone tube of ≈1373 K. Symbols indicate interpolated measurement points.

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Figure 11

Comparison of measured damping ability with and without hot-gas penetration at two different forcing levels for two mean flow temperatures

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