The three-dimensional Reynolds-averaged Navier-Stokes equations and the standard k-ε turbulent model were adopted to simulate numerically the flow field and performance of a supersonic expander with different tip clearance heights. The results show that tip clearance height significantly impact on the local flow characteristics in the three-dimensional flow passage and the overall performances of the supersonic expander. With the increasing of the tip clearance height

the highest relative Mach number of airflow decreases

and the scope of high speed reduces. The leakage vortex strengthens

scale largens

transverse and radial movement significantly

and the leakage loss increased

but the loss of shock waves and interaction of shock and boundary layer reduced. Expansion ratio of supersonic expander firstly increased and then decreased

and the isentropic adiabatic efficiency reduced gradually. The main source of loss in the three-dimensional flow passage of supersonic expander with clearance is the friction loss of low energy fluid near the suction surface and bottom wall and the leakage loss. The scope of clearance height should be selected between 0.9%h0 and 1.5%h0.