Optimising the acoustic damping of multiple Helmholtz resonators attached to a thin annular duct

Abstract Acoustic pressure pulsations can be problematic in industrial pipelines, especially when the excitation frequency matches an acoustic resonance frequency of the pipeline. The objective of this paper is to investigate the effectiveness of Helmholtz resonators (HRs) in multiple arrangements on the attenuation of acoustic pressure pulsations in piping systems. In a resonant pipeline (i.e., an acoustic standing wave scenario), maximal attenuation is achieved when the HR is inserted at the acoustic pressure antinode. The insertion loss (IL) in an off-resonant system is found to be relatively consistent, unless there is a coupling between the HR and the downstream end termination in which case there is a decrease in attenuation. Multiple, small-volume HRs in various configurations can achieve the same level of damping as that of a single HR with the same total volume. Moreover, it is shown that the use of multiple HRs placed at strategic spacing intervals along the length of a pipeline can yield significant acoustic damping, without the need for characterizing the acoustic waves in the pipeline system. An axial spacing of a quarter wavelength of the frequency of interest between multiple HRs is shown to increase the peak attenuation, which is indicative of a favorable coupling between HRs. The effect of flow velocity and its directionality with respect to the sound source is also investigated. The results presented in this paper provide practical techniques that can be used for the implementation of HR in pipeline systems.

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Combustion-Driven Oscillation Mitigation in Large Industrial Combustors

Volume 13: Vibration, Acoustics and Wave Propagation ◽

10.1115/imece2015-53339 ◽

2015 ◽

Cited By ~ 1

Author(s):

Reza Kashani

Keyword(s):

Finite Element ◽

Element Model ◽

Industrial Applications ◽

Steam Boiler ◽

Element Analysis ◽

Acoustic Damping ◽

Thermoacoustic Instability ◽

Helmholtz Resonators ◽

Candidate Solution ◽

Quarter Wave

Thermoacoustic instability has been reported in various combustion systems including industrial combustors (heaters, boilers, etc.), propulsion systems (rockets, afterburners) and gas turbine combustors. Thermoacoustic instability can be addressed by adding acoustic damping to the combustion environment. In larger industrial applications, tonal acoustic damping solutions such as quarter wave tubes and/or Helmholtz resonators or even active acoustic damping solutions can be used to quiet this instability. This paper discusses the numerical investigation of mitigating the 30 Hz thermoacoustic instability in a fire-tube 600 horsepower steam boiler. By conducting finite element analysis of the acoustics of the combustion chamber and all the fire-tubes, the 30 Hz mode sympathizing with the heat release dynamics of the chamber is identified. Subsequently, a number of tuned acoustic absorption solutions are experimented with, numerically (on the finite element model of the boiler’s acoustics), and the most suitable (in terms of effectiveness and ease of implementation) candidate solution is identified. Currently, work is underway to implement this solution on the boiler.

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Modelling of Circumferential Modal Coupling Due to Helmholtz Resonators

Volume 2: Turbo Expo 2003 ◽

10.1115/gt2003-38168 ◽

2003 ◽

Cited By ~ 23

Author(s):

Simon R. Stow ◽

Ann P. Dowling

Keyword(s):

Acoustic Waves ◽

Gas Turbines ◽

Helmholtz Resonator ◽

Operating Conditions ◽

Acoustic Energy ◽

Radial Dependence ◽

Helmholtz Resonators ◽

Annular Duct ◽

Linear Waves ◽

Modal Coupling

Lean premixed prevaporised (LPP) combustion can reduce NOx emissions from gas turbines, but often leads to combustion instability. Acoustic waves produce fluctuations in heat release, for instance by perturbing the fuel-air ratio or flame shape. These heat fluctuations will in turn generate more acoustic waves and in some situations self-sustained oscillations can result. A linear model for thermoacoustic oscillations in LPP combustors is described. A thin annular geometry is assumed and so circumferential modes are included but radial dependence is ignored. The formulation is in terms of a network of modules such as straight ducts and area changes. At certain operating conditions, the flow is predicted to be unstable, with linear waves growing in amplitude. Helmholtz resonators can be used to absorb acoustic energy and, when carefully designed and installed at appropriate locations, can stabilise the flow. Helmholtz resonators are included in the model. Connecting a Helmholtz resonator to an annular duct destroys the axisymmetry of the geometry. This results in coupling of the circumferential modes which must be calculated. The model is used to investigate the best arrangement of resonators around the circumference of an annular duct to achieve maximum damping of a circumferential oscillation.

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