Acoustic performance of dissipative and hybrid silencers in ducts with large transverse dimensions

Abstract

Numerical models will be developed for the prediction of silencer transmission loss under the operating conditions present in gas turbine exhausts. In these systems the large diameter ducts and high operating temperatures produce a challenging acoustic environment due to the unverified behaviour of fibrous materials at high temperatures and the existence of complex sound fields. To understand the behaviour of fibrous materials at high temperatures their bulk acoustic properties are measured using a modified impedance tube which can heat material samples up to a temperature of 500 C. It will be demonstrated that the high temperature material properties can be extrapolated from room temperature measurements given knowledge of the temperature dependant flow resistivity. Finite element numerical models using point collocation and mode matching techniques to predict the transmission loss of silencers are developed and successfully validated. Dissipative silencer designs with various cross-sectional designs are explored numerically and experimentally according to common industry standards. It is demonstrated that transmission loss may be optimised by the arrangement of the fibrous material across the cross-section. The accurate numerical models allow for effe cient silencers to be designed reducing silencer size and cost. A new hybrid silencer is presented combining dissipative and reactive elements with the aim of increasing the low frequency attenuation of large silencers while maintaining an effective broadband spectrum. Measurements and predictions show this innovative design to be successfull. Application of the hybrid silencer allows for more flexible noise control solutions when design is limited by low frequency noise.