Technique for reducing acoustic radiation in turbomachinery

Abstract

Apparatus for reducing acoustic radiation which occurs during the operation of turbomachinery includes a shroud having a generally cylindrical inner surface and a coaxial rotor having a plurality of blades extending generally radially outwardly at equally spaced circumferential locations. The inner surface of the shroud is circumferentially contoured such that the tip clearance between each of said blades and the inner surface is caused to vary in a periodic manner upon rotation of said rotor. This creates a new periodic unsteady pressure field which is substantially equal to, and out of phase with, an existing periodic pressure field resulting from nonuniform inflow into the blades and results in the radiation from the turbomachinery of reduced blade rate frequency tones. The inner surface of the shroud may have a circumferential sinusoidal contour whose periodicity is an integral multiple of the number of the blades. A plane of the tip end of each blade, or attack plane, is preferably angularly disposed relative to a plane perpendicular to the axis of the shroud and the contoured inner surface includes a plurality of parallel grooves generally aligned with the attack plane of each of the blades. Additionally, the shroud may be indexed about its longitudinal axis until an optimized reduction of blade rate frequency tones has been achieved. Furthermore, the inner surface of the shroud may be defined as a Fourier series enabling a simultaneous reduction of a plurality of harmonics of the blade rate frequency tones.