This paper presents an updated report of certain causes and effects of recirculation at the inlet of a rotodynamic impeller. It demonstrates how certain geometrical features of the inlet parts of the blades are affecting the pattern of the inlet-recirculation, and how this pattern is affected by the flow-rate. It demonstrates the effects of recirculation on the steepness of the QH-curve at reduced flow-rates, and the occasionally observed hysteresis within a certain range of partial flow-rates. Also, it discusses the manner in which recirculation is producing very intense instabilities at reduced NPSH, in axial-flow impellers and inducers. The conclusions arrived at are supported by results of test, reported in different papers. In addition of the above, it lists some of the uses of our present knowledge of the causes and effects of inlet-recirculation, which have been successfully applied in practice.

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