Ferrous Fibre Network Materials for Jet Noise Reduction in Aeroengines. Part I: Acoustic Effects.

This paper describes an investigation into the acoustic effects of introducing tubular modules, made of highly porous ferrous materials, into the exhaust region of a small turbojet aeroengine. The modules are composed of sintered stainless steel fibre networks, with void contents in the approximate range of 80-95%. Some of the modules incorporated radial gradients of void content, produced by bonding together thin layers of different density. Measurements of acoustic intensity, as a function of frequency, have been made with various module geometries and locations, and data have also been acquired relating to engine thrust, engine speed and fuel consumption. It is shown that significant reductions in noise level (up to ~10 dB) can be achieved, at approximately constant thrust and fuel consumption, by the introduction of modules of this type. Such effects can arise both via modification to the gas flow field (and hence to noise generation characteristics) and by attenuation of acoustic waves within the modules. Reductions in noise creation will tend to be specific to the gas flow field within the engine concerned, but acoustic attenuation is expected to be a more general effect. In the second of this pair of papers, an investigation is presented into the thermo-mechanical stability of these fibre network materials in this environment