Sound Generation in the Outlet Section of Gas Turbine Combustion Chambers

Abstract

Indirect combustion noise is investigated experimentally and numerically. This noise is generated in the outlet nozzle of combustion chambers if the entropy of the medium is nonuniform, which is the case in the exhaust of combustors. The contribution to the total noise emission of aeroengine combustors is not known. A test rig for the experimental investigation of this noise emission in the presence of swirl is first described. The indirect noise is generated in an exchangeable convergent‐divergent nozzle at the exit of the combustor. The noise radiation is studied in a circular exhaust pipe with probe microphones using a radial mode analysis of the microphone signals. First results of the measured sound fields are reported. The experimental situation will be studied numerically with a 4 th order accurate CAA‐method, which is first validated with theoretical results of the literature for the cases of a compact nozzle or diuser and incoming entropy and sound waves in a one‐dimensional mean flow. The agreement with the results of sound generation due to incoming entropy waves and the sound reflection and transmission for incoming sound waves is very good. The method is then applied to the more realistic cases of non‐compact nozzles and it is found that the amplitudes of the generated waves are substantially smaller in comparison to the one‐dimensional theory. The sound generation of real cases like a swirling hot‐spot and entropy waves in a one‐dimensional flow through a convergent‐divergent nozzle as well as plain entropy waves in the swirl flow of the experimental setup are finally studied and the noise emission is computed.