The effects of tip clearance on the noise of low pressure axial and mixed flow fans

Results are presented of a systematic experimental investigation of the effects of tip clearance on both noise and performance of four commercially representative fans (three low pressure axial flow and one mixed flow) in a circular duct, both with and without coincidence of the fan and duct axes (fan/duct eccentricity). It is shown that with eccentricity significant pure tone noise is generated due to blade tip/duct wall interaction, with a plane wave mode component. Reducing tip clearance both improves performance and reduces noise, not only at the maximum efficiency operating point but also in an appreciable low flow rate operating region. With tip clearances small enough to optimize performance and minimize noise fan/duct eccentricity must be kept to a very small value, implying strict precautions in fan and duct manufacture, installation and service conditions.