On Improvement of Characteristic Instability and Internal Flow in Mixed Flow Pumps

Abstract

The difference of pump characteristics between two kinds of mixed flow pumps with low specific speed of 350 (min-1, m3/min, m), which have the same impeller and the different diffuser vanes, is presented in the present paper. It was confirmed from the previous study that a diffuser rotating stall (DRS) occurs in the original type of mixed flow pump at about 65% flow rate of best efficiency point (BEP) and there is an abrupt drop of the total head characteristic. The relationship between pump characteristic instability and internal flow is investigated in detail by using a dynamic PIV measurement system (DPIV) and a commercial CFD code. As a result, the cause of characteristic instability is supposed for the original type as follows. The flow on the vaned diffuser hub-side becomes unstable due to adverse pressure gradient and strong backflow occurs at partial flow rate. Then it impinges against downstream flow from the impeller and the secondary flow from hub- to casing-sides occurs. This secondary flow blocks the downstream flow from the impeller and the inlet flow angle at the leading edge of adjacent diffuser vane decreases. Therefore, the flow separates on the suction surface of the adjacent diffuser vane inlet and a strong vortex is generated. After that, it develops and becomes a stall core. Next, the modified type of pump, where only diffuser vanes are modified, is tested. As a result, the flow rate, at which characteristic instability occurs, is shifted to lower one and the pump operating range becomes widened. It is clarified upon above considerations that the secondary flow has been restricted and diffuser performance has been improved in comparison with the original type.