Potential for reducing fan noise by housing design

Abstract

Noise is caused by unsteady flow on or near solid surfaces. Reducing the noise generated by fans requires smooth steady internal flow, particularly across the blades and near the cutoff. The housing appears to be the critical design element as it forces a pressure field on the rotor. Data show strong tangential pressure gradients do exist in fan housings. These undesirable gradients produce a continually changing load or pressure rise on the blades. Near the cutoff a load, much above the design point value, is imposed on the blades. As a blade moves from the cutoff to the housing discharge the lead decreases continuously to below the design point value. Each blade, experiencing a cyclic lead, likely goes from pressure surface stall at the cutoff to suction surface stall at the discharge opening. Design point flow occurs only for a short interval between these extremes. A redesigned housing using a radial diffuser upstream of the cutoff and suitably sized collector will reduce the tangential pressure gradient. Measurements at TTU have shown noise reduction of 8 to 12 dB in experimental fans which eliminate housing pressure gradients.