The measured pump pressure discharge characteristic for a high specific speed radial pump (ω s = 1.7) reveals distinct discontinuities in part load operation. These pressure discontinuities occur at different threshold volume fluxes when increasing or decreasing the pump discharge and make up a hysteresis loop. The pump impeller characteristic was evaluated experimentally and numerically by taking the difference between the integrated impeller outlet and impeller inlet total pressure. The experimental and numerical characteristics agree well including the volume flux location and magnitude of the pressure discontinuities in the hysteresis loop. For volume fluxes within the hysteresis loop two stable well converged flows were calculated numerically. The numerical calculations were made on coarse and fine grids using commercially available software with and without the impeller clearance leakage flow. Further experimental and numerical comparisons are made at the impeller inlet/outlet with emphasis on the changing flow field in the hysteresis loop flow regime and its coupling to the onset of reverse flow zones. This combined application of numerical and experimental tools provides insight for the hysteresis flow field of a pump impeller characteristic.
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