GB791112A - Improvements in or relating to jet engine silencing apparatus - Google Patents

Abstract

791,112. Exhaust-silencers. CEMENTATION (MUFFELITE), Ltd. March 24, 1955 [April 1, 1954, July 26, 1954], Nos. 9594/54 and 21787/54. Class 7(2) Apparatus for reducing the noise caused by hot gases emitted from a jet engine includes a spreader device for attenuating the low frequency noise of the gases and having an inlet end for association with the outlet of the jet tube 3 to receive the gases and a passageway of substantially constant cross-sectional area which is so shaped and dimensioned that the periphery of the inlet end of the device is less than the periphery of the outlet end thereof so that there is no substantial build-up of back pressure through the device. A conventional silencing means is so arranged in relation to the device that the high-frequency noise of the gases is reduced therein. As shown in Fig. 1, the device which is located in a test house installation 8, comprises an inner member 1, generally cone-shaped and having a surface that is curved between the apex and base thereof, surrounded by a member 2 which follows the shape of the member 1. The member 2 continues beyond the apex of the member 1 and has a belled end greater in diameter than the jet tube 3. The relative positions of the members is maintained by radially spaced struts 5 of aerofoil cross-section, and a cone 6 or frusto-cone 7 is fitted to the base of the member 1. Air flowing through the clearance between the member 2 and tube 3 is induced into the stream of gases; alternatively, the cone 6, which serves to minimize turbulence in the gases emerging from the outlet of the device, is dispensed with and air vents contained in the struts 5 lead to a central hollow in the member 1. The peripheral extent of the gas stream is increased by corrugating or serrating a marginal part of the outlet of the device, or tooth-like projections are provided at the outlet. In modifications, (a) the device is used in combination with a muffler secured on a wheeled cradle and for use in silencing the engine of an aircraft on the ground and at an airport, the frusto-cone 7 having a small apex angle and being covered with soundabsorbing material, and the muffler comprising a cylindrical shell radially spaced from and enclosing the frusto-cone 7 and covered on the inside surface thereof with sound-absorbingmaterial; the mouth of the shell is belled and fits loosely around the outlet of the device so that more air is entrained at the clearance thus formed, (b) the device comprises a bell-mouthed portion 14, Fig. 3, of circular cross-section which gradually changes to a square crosssection at the point 16, and an outlet portion 15 which changes from a square section to a vertical, rectangular crosssection, or elongated elliptical cross-section, the device being designed as a "constant area duct" or a "constant pressure duct"; a wedged-shaped shell 17 diverging in a horizontal plane and lined with soundabsorbing material is shaped for location adjacent the outlet of the portion 15 and has an outlet of square-cross-section; alternatively, the shell 17 is of elliptical shape. and (c) the tube 3, Fig. 7, is tightly coupled to the member 2 and a pump 19 forces water through a series of circular headers 21 in the member 1 and from apertures 24 whereby the water is sprayed into the gas stream. Specifications 766,985 and 766,986 are referred to.