Hydraulic loss analysis in a pump-turbine with special emphasis on local rigid vortex and shear

Since the pump as turbine is increasingly employed in energy storage, improving efficiency in both pump and turbine modes is required for economic benefits. This study aims to analyze vortex flow characteristics and vortex control methods in both modes to reduce hydraulic loss. In this paper, a delayed detached eddy simulation was applied in a low specific speed pump-turbine. Based on the entropy production analysis and vorticity binary decomposition in the local vortices, the results show that the local shear is the leading cause of hydraulic loss instead of the existence of vortices. The average wake loss can be 1.6 times higher than the loss in jet regions in pump mode, but there is little difference in the distribution of shear and vortices in the wake flow in turbine mode. The local loss caused by the rotor-stator interaction with a tongue effect at blade passing frequency is up to threefold over the loss without a tongue effect in both modes. Reducing shear and ratio of shear to rigid vorticity of the local vortices via modification in the volute tongue angle to suppress the tongue effect can be an effective way to decrease hydraulic loss in both modes