Noise Reduction by Controlling Tip Vortex in a Propeller Fan

Two types of shroud for propeller fans were introduced to reduce the fan noise by controlling the tip vortex of a fan rotor. Three-dimensional vortical flow structures and velocity fluctuation near the fan rotor tip have been investigated by experimental analyses using laser Doppler velocimetry (LDV). The tip vortex formed near the midchord of the rotor tip develops in the tangential direction. This tangential structure of the tip vortex induces an inward radial flow near the leading edge of the rotor tip and a reverse flow between the rotor tip and the shroud, causing a large blockage effect on the through flow (main flow) in the rotor. High velocity fluctuations are observed in the interference region between the tip vortex and the through flow. The shroud designed so as to diminish the reverse flow between the rotor tip and the shroud can decrease the blockage effect due to the tip vortex, which decelerates the through flow in the rotor, thus leading to the reduction in the fan noise.