Abstract
The various possible mechanisms of noise generation in axial flow fans are described. Quantitative expressions have been derived for the sources of broad band noise by dimensional arguments, from which orders of magnitude estimates have been made. These have been found to give fair agreement with observed levels of broad band noise from an isolated plate under different flow conditions, and from a model fan. It is concluded that broad band noise in fans arises from vortex shedding at the blade trailing edges under normal conditions, but that any large-scale turbulence in the flow can increase the noise significantly. Discrete noise from multi-stage fans has been found to arise from aerodynamic interaction between fixed and moving blades. The amplitude of this noise is strongly dependent on axial spacing between rows, and it appears that stator area may also be a significant parameter.
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