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The modifications of impeller may show diverse impact on centrifugal pump operating in pump and turbine modes.To clarify this problem,the hydraulic performance of a low specific speed centrifugal pump operating in both modes was firstly obtained by CFD method and verified by experiment.Then,based on the single-factor design method,a series of calculations have been conducted to identify the effects of impeller geometry parameters on the hydraulic performance in different modes.The variations of head,shaft power and hydraulic efficiency curves with different impeller parameters were explored.It is found that compared with turbine,the pump shows a more obvious variation of head.The outlet angle has positive impact both on the head consumed by pump or generated by turbine.The change of turbine shaft power is apparently smaller than that of pump for different impeller geometry parameters.Only the outlet width somewhat changes the turbine shaft power.The hydraulic efficiency in both modes shows different variation under different impeller geometric parameters,while the hydraulic efficiency of both modes is reduced with the outlet angle increasing.Meanwhile,the response amount of hydraulic efficiency caused by certain change of impeller parameters was estimated by sensitivity analysis method.It is found that only the appropriate blade number and outlet width can improve the hydraulic performance both in pump and turbine modes.Eventually,the hydraulic loss,skin friction loss and theoretical analysis were performed to explore the reason of hydraulic performance variation due to different impeller parameters.The change of slip factor,impeller inlet area,impeller outlet area or hydraulic loss results in the change of hydraulic performance in both modes.The results can be useful for hydraulic performance improvement for both pump and turbine modes through impeller geometry modification.