Off-Design Behaviour of a Radial Flow Hydraulic Turbine

In this paper, the behaviour of a pump-turbine, in turbine mode, was investigated at part load both experimentally and numerically, in the attempt to achieve a more comprehensive knowledge of the complex phenomena that occur in such conditions. In particular, the purpose of the work was to highlight the onset and progress of flow recirculations that take place inside the vane passages and interact with the blades of the rotor. The experimental results showed that recirculation phenomena occur in two zones: the first close to the leading edge of the runner, the second downstream of the runner trailing edge. The appearance of backward flows is linked with a net exchange of torque which reduces the one transmitted by the fluid to the rotor and has a significant effect on the characteristic curves of the machine. The numerical results confirmed only partially the ones obtained experimentally: the simulations detected the presence of an extended area of recirculated flow downstream of the runner, even if at a flow rate lower than the one obtained by experiments. The recirculation upstream of the runner was detected by the fluid dynamic code more clearly when the conveyor vanes were inserted. Finally, the numerical results confirm the deviation of the internal characteristic from the line of the theoretical one due to the presence of recirculations.