Theoretical, experimental, and numerical study of special impeller used in turbine mode of centrifugal pump as turbine

The pump-as-turbine (PAT) technique is limited by its undesirable performance and narrow range of high efficiency due to the mismatch between the conventional backward-curve centrifugal impellers and the turbine running condition. This study aims to significantly improve the performance of PAT by designing a special impeller with forward-curve blades for the turbine working condition, and investigating the method for determining the blade inlet and outlet angles that play important roles in the energy conversion. The performance tests of PATs with the designed special impeller and an original backswept blade impeller were performed. The improved performance after replacing the original impeller with the special impeller was demonstrated by numerically investigating the PATs of specific speed of 18.1 with different impellers by use of a verified computational fluid dynamics technique. The suitability of the proposed methods to other cases was verified by designing and experimenting on two other special impellers used in two other centrifugal pumps as turbines with various specific speeds. Compared with the original backward-curve impellers, the experimental maximum efficiencies of special forward-curve impellers of three various specific speeds are more significantly increased and the flow-efficiency curve is flatter, suggesting the obvious superiority of the forward-curve impeller in PAT. The experimental flow rate of best efficiency point (BEP) is very close to the theoretically designed flow rate, indicating that the theoretical relationship expression between the blade inlet angle and design flow rate provides an effective prediction method of BEPs' flow rate in PAT's turbine operating mode with special impeller. A low-cost and convenient way to significantly improve the efficiency of PAT in turbine mode is provided in this study.