Three-dimensional vortical flow structure and separeted flow topology near the casing wall in a mixed flow pump impeller with high specific speed have been investigated by a Reynolds-averaged Navier-Stokes (RANS) flow simulation. The simulation shows that a tip leakage vortex formed close to the leading edge of the impeller tip suction side grows in the tangential direction even at the design operating condition, thus impinging on the adjacent blade leading edge. This interaction between the leakage vortex and the blade pressure surface causes a vortex breakdown and an expansion of the leakage vortex near the pressure surface. The tip leakage flow causes the casing wall boundary layer to separate along the leakage vortex. As a result of the breakdown of the leakage vortex, the separation region due to the tip leakage flow is forced out into the upstream ofthe blade leading edge. On the casing wall in the impeller, a extreamly large separarion region is formed not only by the tip leakage flow, but by effects of high stagger and low solidity.
