Reduction of interaction tones from axial flow fans by suitable design of rotor configuration

Abstract

An expression is derived to give details of the acoustic power radiated by a circular array of point dipoles situated in free air. The phase relationships existing between the sources are contrived to be of the type generated by the interaction of axial flow fan aerofoil rows, and thus a simple model for the acoustic output of a fan due to the fluctuating forces on the stator vanes is obtained. The effects of small errors in source (vane) location are investigated, and hence a realistic estimate of the behaviour of such phased arrays derived. It is shown that the large reduction in discrete tone radiation anticipated through a suitable choice of blade and vane numbers is critically dependent on exact cascade geometry, and hence additional methods must be used if tonal annoyance is to be eliminated. The removal of discrete tones from fan noise by the uneven circumferential distribution of the rotor blades is then re-examined in the light of this work, and some suggestions made as to how this technique might be most conveniently and effectively employed. The work is illustrated by theoretical and experimental results for a small axial flow fan.