GB783003A - Apparatus for reducing the noise of a fluid stream - Google Patents

Abstract

783,003. Exhaust-silencers. POWER JETS (RESEARCH & DEVELOPMENT), Ltd. March 21, 1956 [March 31, 1955], No. 9390/55. Class 7(2) Silencing apparatus for a fluid stream discharged from a nozzle or exhaust pipe comprises an open-ended, divergent duct 1, the smaller end of which constitutes an inlet 5a for the stream, and a number of axially spaced, apertured walls 9, 10 and 11 extending transversely across the duct. The spacing of the upstream wall 9 from the inlet and the spacing of each subsequent wall from the wall immediately upstream thereof is so chosen in relation to the pressure drop in the stream created by that wall, the configuration of the duct, and the velocity of the stream, that the velocity of the stream is reduced while separation of the boundary layer from the duct wall is substantially prevented and the noise emitted by the stream is reduced. As shown in Fig. 2, the duct 1 which is mounted on a wheeled trolley comprises four, annular, tapered wall sections 5-8 of refractory material, the upstream section 5 including the inlet 5a constituting a tapered socket for reception of the jet nozzle 3 of an aircraft jetpropulsion engine, the walls 9-11 being clamped between those sections. The latter are surrounded by sound-absorbing material 13 and enclosed in a casing 12. The angle of divergence of the duct is 45 degrees. To prevent a rise in back pressure in the jet-pipe of the engine the flow area through the wall 9 is at least equal to that through the inlet 5a, and the duct is provided with projections 14 giving rise to trailing vortices in the stream and which have the effect of further reducng the tendency for boundary layer separation. In a modification, the divergence of the duct first increases and then decreases, the downstream part of the duct being untapered, and the duct is formed by a wall 21, Fig. 3, and a casing 22 between which is sound-absorbing material 23; the walls are of honeycomb construction formed by alternate flat and corrugated strips of an heat-resistant alloy, and behind a section 21a of the duct wall is an annular gallery 30 connected by a pipe 31 to a suction pump 32 so that boundary layer separation is prevented. In further modifications, (a) the inlet 5a of the duct is provided with three ribs with which the jet nozzle 3 engages whereby air is drawn through the gap thus formed and into the duct to cool the walls thereof, (b) the duct has a progressively increasing divergence, or is curved so that the stream of gases is discharged upwardly, or varies in cross-section from a circular inlet to a narrow fishtail outlet. The apparatus may be mounted on the exhaust pipe of an aircraft, in-line, piston-type engine.