Characteristics of three-dimensional flow field and velocity fluctuation near the rotor tip in an axial flow fan (relation to the noise generation)

Three-dimensional vortical flow structures and velocity fluctuation near the rotor tip in an axial flow fan have been investigated by experimental analysis using a rotating hot wire probe and a numerical simulation. Both the experimental and the numerical analysis have been performed in the relative frame of reference rotating with the rotor. It is found that a tip leakage vortex is observed in the blade passage, which has a major role near the rotor tip. The tip leakage vortex formed close to the leading edge of the blade tip on suction side grows in the streamwise direction, and forms a local recirculation region resulting from a vortex breakdown inside the blade passage. It causes significant changes in the nature of the tip leakage vortex: large expansion of the vortex and large total pressure loss. High velocity fluctuation is observed in the interference region between the tip leakage vortex and the main flow. Near the casing wall, a discrete frequency is formed between tip leakage vortex center and rotor trailing edge.