Basic fan noise generation mechanisms and innovative aerodynamic design techniques for their reduction

Abstract

For low‐speed fans, noise radiation results from fluctuating aerodynamic forces on the blades. These may be characterized by their origin as due to vortex shedding or inflow turbulence ingestion. Both of these force and noise mechanisms are nominally broadband in frequency. However, a specialization of the latter considering a standing pattern of inflow velocity distribution yields the familiar discrete frequency spectrum of harmonics of the blade passage rate. Beneficial changes in noise levels and spectral distributions are described and demonstrated for changes in fan blade form such as thickness distribution and sweep. Blade edge sweep is shown to be particularly useful in reducing noise from both aerodynamic phenomena and, with careful design, effective in reducing discrete frequency noise as well. Examples are presented of automotive type fans designed for low noise using these techniques. Not only is noise reduced, but because of the sophisticated aerodynamic considerations required on loading and load distributions, the efficiency is markedly improved.