A Study on the Internal Flow Characteristics of Very Low Specific-Speed Centrifugal Semi-Open Impeller

In the range of very low specific speed (n_s≦100 [m, m^3/min, rpm]), performance of a centrifugal pump is much different from that of a normal n_s centrifugal pump and efficiency of the pump drops rapidly with a decrease of n_s. The purpose of this study is to make clear the internal flow characteristics and to obtain a basic knowledge of the pump performance. In order to examine the reason of unstable performance characteristics of a very low n_s centrifugal pump, the internal flow of a semi-open impeller is measured by PIV. The results show that large passage vortex and strong reverse flow at the outlet of impeller are formed at partial flow rate, and these vortex and reverse flow reduce a tangential velocity at the outlet of impeller and cause the performance instability. Slip factor of the semi-open impeller is higher than factor given by Wiesner.