Generation mechanism and control methods of secondary flows in the impeller of axial flow pumps

Abstract

The secondary flow in the impeller of an axial flow pump is an important factor affecting the safe and stable operation of the unit. However, there is still a lack of systematic research on the generation mechanism of secondary flow and corresponding control strategies in axial flow pumps. To better understand the secondary flow characteristics in the axial flow pump, based on the momentum equation of relative motion, the basic distribution characteristics of the potential rothalpy gradient (PRG, or the reduced static pressure gradient) in the impeller of an axial flow pump were systematically analyzed. Two typical secondary flows were found, namely, trailing-edge hub-shroud type secondary flow at the blade outlet hub side and leading-edge hub-shroud type secondary flow at the blade inlet shroud side. The generation of these secondary flows is directly related to the effect of natural adverse PRG. A new blade design method is proposed. The essential idea of this method is to give the blade loading strategy based on grasping the macro-flow characteristics and control PRG characteristics by adjusting the real blade loading δp (i.e., the static pressure difference between the blade pressure and suction surfaces) and, thereby, control the above-mentioned secondary flows. The application of an axial flow pump showed that the blades designed based on this method can effectively control these secondary flows and reduce pressure fluctuations. The average decrease in pressure fluctuation on the blade inlet shroud side and the outlet hub side is 17.79% and 20.03%, respectively.