Unsteady numerical analysis of the rotating stall in pump turbine geometry

Main challenges in energy sector nowadays are storing and recovering of a large amount of energy in a short time. Pumped Storage Power Plants (PSP), using reversible pump turbines are among the most cost-efficient solution to answer these needs. To provide a rapid adjustment to the electrical grid, pump-turbines are subjects of quick switching between pumping and generating modes and to extended operation under off-design conditions. To maintain the stability of the grid, the continuous operating area of reversible pump-turbines must be free of hydraulic instabilities. One of the main sources of pumping mode instabilities is the presence of the rotating stall that occurs at the part load. It can be observed as periodic occurrence and decay of recirculation zones in the distributor regions. Consequently, the machine can be exposed to uncontrollable shift between the operating points with the significant discharge modification and the drop of the efficiency. The phenomenon is very complex, three-dimensional and demanding for the investigation. The paper presents cost-efficient numerical methodology that enables the accurate prediction and analysis of the rotating stall. The investigations were made on a reduced-scaled high head pump-turbine design. Unsteady numerical calculations were performed using code FINE/Turbo (TM) and URANS equations. Local flow study was done to describe in details the governing mechanisms of the rotating stall. The analyses enable the investigations of the rotating stall frequencies, the number of stalled cells and the intensity of the rotating stall. Moreover, the unsteady calculations give very good prediction of the pump-turbine performance for both, stable and unstable operating regions. Numerical results show very good qualitative and quantitative agreement with the available experimental data.