Experimental noise reduction (aeroacoustical enhancement) of a large diameter axial flow cooling fan through a reduction in blade tip clearance

Aerodynamic and aeroacoustic performance experiments were carried out on four- and eight bladed, 1.542 m diameter, axial flow cooling fans, with constant solidity and hub-to-tip ratio. Tests were conducted in an ISO5801, Type A Fan Test facility. The tip gap (TG) was reduced from 4 mm (0.26% fan diameter) to 2 mm (0.13% fan diameter), to 0 mm, for both fan configurations. The noise profile of each fan configuration at the same TG over the whole volumetric flow rate spectrum was compared to each other. The 4 mm (0.26%) TG is used as a baseline to measure the nett increase or decrease in sound levels. Noise emissions decreased as the TG was reduced. It is discovered that the four bladed fan configuration had lower noise emissions than the eight bladed fan configuration at all blade tip clearances at design flow rate. It is concluded that reducing the TG and number of blades, at constant solidity, reduces sound emissions. The 0 mm TG for the four bladed fan produced the greatest reduction in noise emissions. An increase in fan total-to-static performance is observed when reducing the TG for both fan configurations.