Experimental, Numerical, and Theoretical Research on Impeller Diameter Influencing Centrifugal Pump-as-Turbine

One of the limitations of using the pump-as-turbine (PAT) technique is its relatively narrow high-efficiency operating range. When the system pressure head or flow rate decreases, trimming the impeller or replacing the impeller with one of a smaller size is an easy and convenient approach. Therefore, research on the impeller diameter in terms of its influence on PAT is useful. To perform research on how the impeller diameter influences PAT, experimental research was first performed on a single-stage centrifugal PAT with three impellers of different diameters. Experimental results show that PAT flow versus head curve is lessened; its flow versus efficiency and flow versus power curves increased after the best-efficiency point in accordance with increasing impeller diameter. The PAT flow rate, required pressure head, generated shaft power, and efficiency at the best-efficiency point increased in accordance with increasing impeller diameter. Numerical simulation and analysis of the PAT with the three impellers were performed using a verified computational fluid dynamics (CFD) technique. Hydraulic loss and flow field distribution analyses show that the total hydraulic loss within PAT and the hydraulic loss within the radial gap between the rotating impeller and stationery volute decreased in accordance with increasing impeller diameter. Theoretical analysis indicates the PAT theoretical head increased and its required pressure head decreased in accordance with increasing impeller diameter. Theoretical formulas used to predict PAT performances as the impeller diameter changed were verified. Possible reasons for the deviations between the performances predicted by theoretical formulas and test results are explored.