The flow instabilities in a pump-turbine-stage with variable guide vanes were investigated by means of pressure measurements, flow visualization and numerical simulations over the whole operating range in the pump mode. For the experiments adedicated test rig was designed, which allowed detailed pressure measurements and flow visualizations at the impeller inlet and in the vaned diffuser. A 3D Navier-Stokes code with stage capability was used for the numerical part of the work. The geometryof the test stand was carefully discretized with a computational grid in order to compare the experimental data directly with the simulations. Several types of instabilities were observed, and tests with different guide vane angles showed that these arelargely independent. Numerical simulations of the whole pump-turbine stage (impeller and vaned diffuser) and the impeller alone could reproduce most of these instabilities.
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