Abstract: Abstract: The centrifugal pumps with semi-open impellers have been widely applied in petrol chemical industry, aerospace industry, power energy, etc. The tip clearance flow is a very complex turbulent flow in which the vortex, secondary flow, and recirculation flow are existent simultaneously, and it plays an important role for internal flow characteristics and external performances. In recent years, lots of numerical and experimental studies have been carried out to investigate the tip clearance flow and improve the performance of the semi-open impeller centrifugal pump. In this paper, numerical simulation and performance test are carried out on the centrifugal pump with a semi-open impeller. Influences of tip clearance on performance and the details of flow in tip clearance layer are studied at four different flow rate conditions. In the numerical simulation, the PRO/E software is used to generate computational geometries. The compatible preprocessor, GAMBIT is used to generate meshes. The commercial CFD software Fluent 6.3 is used to solve the N-S equations. The SIMPLEC algorithm is used to couple pressure and velocity. Second order upwind discretizations have been used for convection terms and the central difference schemes are used for diffusion terms. Three-dimensional viscous incompressible flow field simulations have been performed using multiple reference frame coordinates. The impeller and inducer region is taken as a rotating reference frame with a constant angular speed of 1 450 r/min, and the other surfaces are defined as stationary reference frame. And the boundary conditions are velocity condition at inlet and outflow at outlet of the pump. Also, no-slip wall conditions have been used for the rest surfaces of the pump. Also, the experimental studies are carried out on the tested centrifugal pump with tip clearances of 1.0 mm, and the results are compared with the numerical calculations. Seal gaskets are used to adjust the tip clearance. Tungsten wires (the diameter is 3 mm) are placed in three positions of each blade (the blade tops near the leading edge, trailing edge and middle edge). Once impeller and casing are installed, and bolts are tightened, tungsten wires will be squeezed. Finally, a vernier caliper is used to measure the thicknesses of the squeezed tungsten wires. The tip clearance Δc is 0.8 mm without any seal gasket and tip clearances are adjusted by increasing the seal gasket (thickness of 0.1 mm, brass gasket). After increasing the seal gasket, the tip clearances will be measured. The results show that there exists strong leakage vortex in the impeller inlet, impeller passage and impeller outlet. Due to the different pressures between suction surface and press surface of blades, the fluid pass through the tip clearance forming a lower pressure area, which leads to the leakage vortex. There exist large vortexes in the tip clearance layer at small flow rate condition. With the increasing of flow rate, vortexes become smaller and the flow state becomes more stable. The vortexes almost disappear at large flow rate condition, whereas the flow speed becomes much higher.