Cavitation mechanism in turbine runaway process of a pump-turbine

To demonstrate the influence of cavitation on pressure fluctuation in a pump-turbine, one- and three-dimensional coupling numerical simulations were conducted on a typical runaway process that occurs when the turbine rejects the load. Transient flow in the pipelines was simulated using a one-dimensional single-phase flow model. The unsteady three-dimensional turbulence flow in the pump-turbine was calculated using the single- and two-phase flow models. The numerical method was validated with available experimental data. The maximum simulation errors of the rotational speed and fluctuating pressure were 3 and 10%, respectively. Local backflow vortices leading to the cavitation were identified at the high-pressure runner inlet. The cavitation interacted with the vortices during the runaway process when the turbine rejected the load. These interactions increased the existing duration and action range of the backflow vortices at the runner inlet. Under the excitation of the local cavitation backflow vortices and the shock pressure induced by the instantaneous cavity collapse, the amplitudes of the pressure and hydraulic thrust fluctuations using the two-phase flow model increased significantly to approximately thrice that obtained using the single-phase flow model. While using the single-phase flow model, severe fluctuations occurred near the no-load condition. The occurrence instants of severe fluctuations while using the two-phase flow model were delayed by a specific duration compared to the no-load condition; this is attributed to the duration of cavitation development from the inception of cavitation to the maximal cavity volume. This finding is valuable for the accurate prediction and optimization of pressure fluctuations in the turbine runaway process.