Impedance of SDOF Perforated Liners at High Temperatures

Abstract

Perforates are widely used to attenuate sound waves propagating inside a duct or enclosure. They are often found in a high temperature environment; e.g. car exhaust mufflers, exhaust ducts of aircraft jet engines, and inside combustion chambers of gas turbines. Several studies have been conducted to develop impedance models, including the effect of different configurations and flow conditions. Nevertheless, little has been published on the effect of high temperature and temperature gradients across the liner on the impedance. In this paper, the impedance of different orifices was measured at different temperatures and incident sound pressure levels. The results show that the effect of temperature gradients has to be considered during measurements and modeling. It was also found that the temperature effect on the impedance can be predicted quite well by changing the fluid properties (density, viscosity and speed of sound). If a thin sheet of porous material is added to the perforate top sheet, which is not uncommon in practice, this combination may exhibit additional high temperature effects caused for instance by structural effects in the porous material.