High head pump-turbine: Pumping mode numerical simulations with a cavitation model for off-design conditions

Abstract

Flexibility and energy storage are one of the main challenges of the energy industry at the present time. Pumped Storage Power Plants (PSP), using reversible pump-turbines, are among the most cost-efficient solutions to answer these needs. To provide a rapid adjustment to the electricity grid, pump-turbines are subject of quick switching between pumping and generating modes and to extended operation under off-design conditions. In particular, at part load, instabilities in pump characteristics can occur. It can lead to unsteadiness and even to a shift of the operating point with significant modification of discharge and drop of efficiency. This unstable area is often exposed to the cavitation phenomenon, which can lead to vibrations, loss of performance and sometimes erosion. The paper focuses on the numerical analysis of the pumping mode regime, especially on the part load off-design instabilities, observed as a saddle shaped pump-turbine head curve and the presence and development of the cavitation in the part load area. The investigations were made on the reduce-scaled model of a high head pump-turbine design. Numerical calculations were performed using commercial code with implemented barotropic cavitation model. Some of the numerical results were compared to the experimental data. Flow analysis was stressed on the cavitation influence on the flow behavior and the performance of the machine. The analysis was made for various flow rates and a wide range of NPSH values. The importance of specific parts of the numerical domain for obtained results was investigated and evaluated.