In this study, the performance of a contra-rotating axial fan is investigated through experimental measurements and numerical simulations. The fan performance is analyzed under six configurations with varying tip clearance values. Tip clearance for the first and second rotors is defined as a percentage of blade height for the following cases: 4%-4%, 4%-2%, 2%-4%, 2%-2%, 1%-1%, and zero clearance.

Examination of the flow structure in the blade tip region reveals that as tip clearance increases, the tip leakage flow intensifies and the tip leakage vortex strengthens. The tip vortex significantly alters the static pressure field on the suction surface, creating regions of pressure drop. With larger tip clearance, the minimum static pressure on the suction surface shifts further downstream, and total pressure decreases along the leakage path.

Results indicate that under constant-pressure conditions, reducing tip clearance increases the flow rate through the fan. At the constant-flow operating point, fans with smaller tip clearance generate higher outlet pressure. Furthermore, completely eliminating tip clearance improves the fan's stable operating range by up to 14.4%.

Interaction effects of tip clearance between rotors are also examined. Increasing tip clearance in a given rotor row enhances leakage flow within the same row. Variation of the second rotor’s clearance has little effect on the first rotor, whereas increasing the first rotor’s clearance also raises leakage flow in the second rotor.