The behavior of the flow in a vertical semi-axial mixed flow pump has been analyzed by numerical flow simulations of the entire stage, and the results have been compared to test data. As the flow is expected to be unsteady at part load in such a pump, the steady-state simulations were complemented with unsteady flow simulations of the entire machine at one part load operating point. Pressure measurements at different locations in the casing of the pump provided valuable data for the validation of the calculated pressure head. This paper shows that the pump characteristic can be quite accurately predicted from full load to part load by modern numerical tools. Simulations of the unsteady flow, which use much more computer resources, are also feasible in an industrial environment and yield detailed information about the flow patterns and pressure fluctuations in the pump.

The measured pump pressure discharge characteristic for a high specific speed radial pump (ωs = 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.