The flow field in the tip region of an axial ventilation fan is investigated with a PIV (Particle Image Velocimeter) system at the design condition. Characteristics of a ventilation fan are an extreme low-pressure difference and a large tip clearance with a low rotating speed. Flow fields with three different tip clearances are surveyed on three different circumferential planes, respectively. The phase-locked average method is used to investigate the generation and the development of a tip leakage vortex. The result from PIV system is compared with that from a PDA (Particle Dynamics Anemometer) system. Both data are in good agreement. The structure of the tip leakage vortex for the rotor is illustrated. The characteristic of a leakage vortex is described in both velocity vectors and vortical contours. It is found that the tip leakage flow for a low speed and a low pressure ventilation fan also has a chance to roll up into a discrete vortex at three different tip clearances, which is similar to high speed and high-pressure compressors and turbines. When the tip clearance increases, the scope and the location variation for the tip leakage vortex increase. Finally, the trajectories of the tip leakage vortex by the experimental measurement are compared with predictions from the existing models for high speed and high-pressure compressors and turbines. A good agreement is obtained.

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