Numerical analysis on the modal characteristics of a pumped storage unit runner in cavitating flow

Abstract

Cavitation is an important issue in pumped storage unit as it can compromise the safe and stable operation of the unit. The present work aims revealing the dynamic behavior of a pump-turbine runner in cavitating flow from a perspective of fluid-structure interaction. A procedure is developed to map the cavities from computation fluid dynamics simulation to finite element model. The calculated flow rate and head agree well with experimental results. A nonlinear relationship between the sonic speed in cavity and vapor volume fraction is applied in the finite element model. The effect of the sonic speed and fluid density on the dynamic behavior of the runner, and their coupled effect are investigated, respectively. Then the modal characteristics of the runner in cavitating flow are analyzed by solving the first 8000 modes, and a harmonic analysis is carried out to determine the main global mode. The results show that the sonic speed (20 m/s–100 m/s) could have a greater effect than the added mass for some modes. Finally, it is shown that the complexity and number of nodal diameter modes greatly increases in cavitating flow, with a lot of local modes which are similar to global nodal diameter mode. This work provides a method for evaluating the dynamic behavior of the pump-turbine runner in cavitating flow and could contribute to the safe operation of the pump-turbine unit.