GB985561A - Means for abating the noise made by combustion engines - Google Patents

Abstract

985,561. Jet propulsion nozzles. R. W. RUMBLE. March 27, 1961 [March 25, 1960; June 17, 1960; Oct. 31, 1960], No. 11164/61. Heading F1J. A combustion engine comprises a conduit for the flow of exhaust gases in the form of a jet stream and comprises an outlet nozzle, a casing disposed around the nozzle and having a portion extending downstream of the nozzle, this portion having an inwardly-directed flange having an aperture of the same diameter as the nozzle, means being provided to reduce the pressure in the space formed between the jet stream and the casing so as to reduce the noise level. The invention is described with reference to gas turbine jet engines, the propulsion nozzle 10 of such an engine being shown in Fig. 1 and the jet stream at 12. Disposed around the jet stream 12 is a casing 18 which defines with the jet stream an annular space 20 which is connected to a source of low pressure by ducts 24. A wall 22 is disposed at the outlet end of the casing 18 extending to the outer periphery of the jet stream. The source of low pressure applied to the space 10 may be produced by means of fan blades 30 mounted at the tips of the turbine rotor blades 26 as indicated in Fig. 4, the fan discharging into an annualar duct 32 which extends along the entire length of the engine, the fan discharge passing into the intake of the main engine compressor 36. A further series of fan blades 40 may be disposed within the space 32, the blades being mounted at the tips of a row of compressor blades. In Fig. 5 centrifugal type fan blades 80 are mounted at the tips of the turbine blades 26, the fan blades being enclosed between covers 81, 82 and taking their suction from the annular spaces 20 and 32. In the embodiments described the jet stream may be divided into a number of paths by suitable vanes, the spaces downstream of the vanes being regions of reduced pressure. In Figs. 6 and 7 a series of hollow pyramid-shaped baffles 52 is disposed entirely around the periphery of the nozzle and also within the path of the jet stream, the open ends 54 of the pyramids facing downstream so that the flow of gases tends to create regions of low pressure within the pyramids thereby acting to attenuate the noise produced by the jet stream. The nozzle 50 and diffuser tube 60 are disposed within a casing structure 18, 22 as previously described. In Fig. 8, the jet nozzle comprises a convergent portion 65 and a divergent portion 66, there being an open gap between the two portions, also an outer casing 64 therearound. The flow of gases through the convergent-divergent nozzle causes a suction effect at the throat which produces a region of low pressure within the surrounding jacket space. A series of open pyramid-shaped baffles 72 and aerofoil section vanes 70 are disposed at the throat of the nozzle, the flow of gases creating regions of low pressure within the baffles.