Patent GB-920329-A (GB)
Title: Improvements relating to silencers for jet-propulsion engines
Inventor: STEPAN AUGUST
IPC: F02K1/38
Filed: 1959-12-18
Abstract: 920,329. Jet propulsion nozzles. FAIREY AVIATION Ltd. May 14, 1961 [Dec. 18, 1959], No. 43172/59. Class 110 (3). A silencer for a jet propulsion engine of the type which comprises an open-ended tubular shell having a number of generally longitudinally extending tubes let into its wall, each tube having at its upstream end an inlet aperture outside the shell and at its downstream end a discharge aperture within the wall of the shell, the tubes being distributed around the axis of the shell with their downstream ends arranged in a ring within the downstream end of the shell so as to define between them a discharge nozzle whose mouth has a central aperture of reduced cross-section for the discharge of the jet gases and in which the interior of the silencer is lined with a fluted liner spaced from the flanks of the tubes and from the shell but conforming generally to the interior shape of the silincer, inlet ports being provided for the delivery of a stream of cooling gas into the space between the outer surface of the liner and the adjacent inner surfaces of the tube and shell at the upstream end of the space, has baffle means at the downstream end of the space to cause the reversal of the direction of axial flow of the cooling gas after it has reached the downstream end of the space before it is discharged into the stream of jet gases upstream of the downstream end of the liner. The invention is described with reference to a silencer described in Specification 920,328 forming part of a jet motor mounted at the tip of a helicopter rotor blade. The silencer 10, Fig. 1, comprises a tubular shell 11 having seven longitudinally extending openended tubes 12 leading from apertures 15 in the shell so that atmospheric air may flow through the tubes and mix with the jet of hot gases issuing from the central circular orifice 18, the radially extending channels 19 and small outer orifices 20. The orifice 18 is arranged to be between one-tenth and one-twentieth of the total discharge area of the silencer. The inwardly facing surfaces 22, Fig. 3, of the tubes 12 are enclosed by a liner 23, which has substantially the same configuration as the interior of the silencer, and forms a channel 25 for cooling air, which passes rearwardly to the corrugated flutes 29 where it is deflected by the baffle plates 30 into the upstream direction through the channels 31 to exit ports 32 where it is discharged into the jet stream. A row of small holes 34 are drilled in each baffle plate 30 to allow a small proportion of the cooling air to escape from the liner 23 and pass into a stagnant zone downstream of the end of the liner and so prevent local overheating. The three tubes 12 nearest the helicopter rotor blade are to some extent shielded from atmospheric cooling. In this case, the liner 23 is extended to cover completely the inwardly directed surfaces 22 of the tubes. In a modification the baffle plates form an extension of the inner surface of the liner and holes are provided in them for the passage of the cooling air. In another modification, the cooling air passes through an annular gap or series of gaps into the jet gases. The gap or gaps are formed between the baffle plate and an extension of the inner wall of the liner.