Vortex characteristics and energy loss analysis of a centrifugal pump as turbine based on velocity triangles

Centrifugal pump as turbine (PAT), an economically feasible and energy-saving technology, is widely used in energy-intensive industrial operations. To reveal the energy loss of the PAT at different working conditions, the entropy production theory and vorticity transport equation were applied to study a forward-curved blade PAT by experimental and numerical simulation techniques. Impeller inlet and outlet velocity triangles are used to analyze the relationship between fluid and energy conversion. The findings demonstrate that the vortex transport equation and the entropy production theory can mutually confirm each other. In the low-flow rate condition, the energy loss mainly comes from the large velocity gradient and shear force. There exists a separate vortex on the blade pressure surface resulting in flow channel blockage. In the high-flow rate condition, there exists an impact vortex on the suction surface of the blade, which is steady and difficult to spread. A negative velocity circulation quantity generated by the absolute velocity separates the flow, which is in the opposite direction of the main flow at the center of the draft tube. This study helps better understand the energy conversion mechanism of PAT, which has reference value for the efficient and economical operation of the PAT.