Cavitation effects on pressure fluctuation in pump-turbine hump region

The hump characteristic is one of unique instability issues of pump-turbines, which limits the stable and sable range of pumped storage power units. Recent studies show that the hump characteristic has related with cavitation. Hence, in this study, large eddy simulations with cavitation effects were performed to reveal the mechanism of cavitation on pressure fluctuation in a pump-turbine hump region. Numerical results were validated using pressure fluctuation experiments. The results showed that the occurrence of cavitation leads to characteristic frequencies of 1.344 f(n) (f(n) is runner rational frequency) in the draft tube and 2.88 f(n) in the runner. Detailed analyses revealed that the characteristic frequency of 1.344 f(n) originated from the periodic oscillation of backflow at the draft tube outlet, while the characteristic frequency of 2.88 f(n) was caused by the periodic occurrence and collapse of cavitation on the suction surfaces of the blades. In addition, the occurrence of cavitation increased the amplitude of the pressure fluctuation by the frequency of 0.144 f(n) in the stay and guide vanes, which was caused by the periodic rotation of three low-pressure regions formed at the runner outlet and the three low-speed blocking vortices in the stay vanes.