Influence of axial-flow pump operating condition on the pre-swirl at impeller chamber inlet

The flow pattern at the inlet of impeller chamber of axial-flow pump has a great influence on the operation of large-scale low-head pumping station.In this work,numerical and experimental methods were used to study this flow for safe,stable and efficient operation,and the conclusions were summarized as follow.(1) The flow pattern is determined by operating condition.(2) Under normal condition,the flow direction is basically along the axial direction of the inlet and no pre-swirl flow exists.(3) Under low flow rate,a pre-swirl flow in the impeller rotating direction appears in the outer ring region of the inlet section,while the inner ring flow keeps in the axial direction toward the impeller.The flow in the middle ring switches between the two regimes,pre-swirl flow and axial flow.(4) This pre-swirl of low flow rate is an indicator of special internal features caused by a second backflow inside the pump.These features and the second backflow are often indicated by a saddle shape on the performance curve.