Abstract
Noise and performance tests were conducted on a low tip-speed, half-stage axial flow fan such as is used in automotive applications. The fan has a 356 mm diameter, and eight equally-spaced, variable-pitch blades. The tests were conducted in a free-field environment. Various degrees of rotational noise due to inflow distortion were produced by installing circular rods upstream from the fan. The fan back pressure and speed were varied during the tests. The tests determined the general characteristics of rotational and non-rotational noise as functions of flow coefficient and fan speed. Rotational noise predominates at higher flow coefficients and consists principally of tones related to the blade passage frequency. It is caused by inflow distortion and turbulence. Non-rotational noise, on the other hand, is broad band in character and predominates at lower flow coefficients. It is caused by an interaction of the blade with the tip clearance vortex. The minimum-noise flow coefficient generally occurs between these two regimes and its value is shown to be dependent on the relative levels of the two noise mechanisms. The design changes required to reduce the noise in each regime are found to exhibit opposing trends.
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