PIV measurement of internal flow characteristics of very low specific speed semi-open impeller

Detailed particle-image velocimetry (PIV) measurements of flow fields inside semi-open impellers have been performed to understand better the internal flow patterns that are responsible for the unique performance of these centrifugal pumps operated in the range of very low specific speed. Two impellers, one equipped with six radial blades (impeller A) and the other with four conventional backward-swept blades (impeller B), are tested in a centrifugal pump designed to be operated at a non-dimensional specific speed of ns=0.24. Complex flow patterns captured by PIV are discussed in conjunction with the overall pump performance measured separately. It is revealed that impeller A achieves higher effective head than impeller B even though the flow patterns in impeller A are more complex, exhibiting secondary flows and reverse flows in the impeller passage. It is shown that both the localized strong outward flow at the pressure side of each blade outlet and the strong outward through-flow along the suction side of each blade are responsible for the better head performance of impeller A.