Perforated liners consist of sheet metal perforated with multiple holes with diameters of magnitude in the order of millimeters and regular spacing, backed by an air cavity in front of a rigid wall. This type of liner is very effective at absorbing sound and is used in many applications. At the resonance frequency, the liner shifts the phase of the incident wave by 180° thus providing damping through wave cancellation.

The perforations in the liner convert acoustic energy into flow energy through vortex shedding at the rims of the liner apertures.

Applied to gas turbine combustors they can attenuate thermoacoustic instabilities and as such significantly improve the reliability of the gas turbine with an additional benefit to the emissions. The Siemens SGT-100 to 400 engines exploit this technology in their DLE combustion system in a configuration of two concentric liners separated by an air cavity with the rear liner acting as the rigid wall in the conventional setting.

In this paper the evaluation of double perforated liners in the absorption of normal-incident plane acoustic waves in an impedance tube and in a gas turbine combustor environment is investigated.

A one-dimensional impedance model that embodies the electro-acoustic analogy was used to predict the absorption characteristics of the double perforated liner. The model was validated by comparing the predictions with experimental data obtained from the impedance tube, with excellent agreement. With the confidence in the equations of the model in predicting the acoustic behavior, the model was then applied to predict the damping performance under realistic gas turbine combustor operating conditions. The prediction also shows two distinct peaks in the absorption characteristics of a double-liner.

Geometric parameters such as hole diameters & thicknesses of the two liners, gap between the liners and the overall pressure drop across the liners have been considered for the predictions. A parametric study of these parameters carried out using the ISIGHT software with design investigation tools identified the order of importance of the parameters considered for sound absorption.

The work reported in this paper has successfully validated an impedance model in the prediction of double perforated liners in the absorption of normal-incident plane acoustic waves. Based on the parametric study carried out design guidelines are given for designing a double perforated liner for maximum absorption of normal incident acoustic waves.

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