Propagation of Sound in a Water Spray Muffler

Abstract

In many jet engine testing facilities, water sprays are installed in the exhaust piping to cool the exhaust gases to temperatures which will not damage the walls of the pipe. The water sprays appreciably alter the character of the sound propagation through the piping. The present paper discusses the dependence of the attenuation of sound on such characteristic parameters of the water-spray muffler as its geometry, the amount of air circulated, the quantity of water sprayed, and the average sizes of the water droplets in the air stream. The attenuation of sound in the muffler is found to be due mainly to viscous losses in the boundary layer between water droplets and air. The theory discussed demonstrates that in order to achieve maximum sound attenuation through such a muffler there are optimum conditions as to quantity of water used and the method of its injection into the air stream. The relative velocity of the air stream to that of the injected water must lie within a critical range in order to fix the average size of the water droplets.