Study On Saddle Region Of Axial Flow Pump Based On Motion Characteristics Of Fluid Particles

Axial flow pump is a high specific speed vane pump,which has the characteristics of large flow and low head.It was widely used in South to North Water Transfer,water supply and drainage engineering,farmland irrigation and power plant circulating water engineering.In the practical use of axial-flow pump,there is a saddle area with unstable operation under the condition of small flow.At this time,the impeller blades produce vortex and flow separation,the hydraulic performance drops sharply,and the vibration and noise of the pump increase obviously,which seriously affects the operation stability of axial-flow pump.Therefore,it is of great theoretical and economic significance to explore the internal flow mechanism of axial-flow pump in small flow operation and to understand the root cause of flow disorder of axial-flow pump in saddle area so as to eliminate the unstable saddle area of axial-flow pump operation.In this paper,through the method of theoretical analysis,numerical simulation and experimental verification,the characteristics of fluid particle curve motion were explored,and the mechanism of flow separation caused by fluid particle curve motion was revealed.On this basis,the flow state and flow separation characteristics in the impeller of axial flow pump were analyzed,and the flow separation control mechanism of fluid particle curve motion characteristics was combined to optimize the blade.It improved the unstable state of the axial flow pump under the condition of small flow.The main research work of this paper is as follows:(1)The motion characteristics of particle curve were studied,revealed that the centripetal force was the necessary condition to maintain the motion of particle curve,the source of the centripetal force required by the fluid particle was studied,and the importance of the balance of centrifugal force and centripetal force was proved;(2)Based on the motion characteristics of the fluid particle curve,combined with the motion characteristics of the fluid particle curve,the typical flow separation phenomenon: the flow around the cylinder was taken as the research object,and the essential law was revealed again;the universal law of the flow separation was revealed;and then the new universal mechanism of the control flow separation method was revealed: take targeted methods to maintain the balance of centripetal force and centrifugal force;(3)Based on the flow separation mechanism of fluid particle curve motion characteristics,the current saddle mechanism of axial flow pump was revealed: when the flow rate is small,the fluid flows to the pressure surface,at this time,the suction surface can not provide enough centripetal force to the fluid particle,and the fluid particle is very easy to lose the balance of centripetal force and centrifugal force,thus serious flow separation phenomenon occurs on the suction surface,resulting in saddle area of axial flow pump.The external characteristic experiment and numerical simulation of the axial flow pump model were carried out to analyze the internal flow field of the model pump under different working conditions.By changing the angle of the blade inlet,it was found that the saddle area of the axial flow pump was gradually improved,or even completely eliminated,which preliminarily verified the correctness of the saddle area mechanism of the axial flow pump;(4)In order to improve the flow state of the saddle area of the axial flow pump,the saddle area mechanism of the axial flow pump was revealed based on the flow separation mechanism of the motion characteristics of the fluid particle curve.Taking the concave convex direction of the blade as the optimization variable,two kinds of modified pumps with straight blade bone line and concave blade bone line were designed.Taking the efficiency of axial-flow pump as the objective function and the head as the constraint variable,using the method of numerical simulation,the modified pump with the best optimization effect was selected,and the flow state of the optimized axial-flow pump was studied.It is preliminarily verified that the change of the concave convex direction of blade bone line had a significant improvement on the saddle area of axial-flow pump.The innovation of this paper can be summarized as follows: From the point of view of the curve motion of fluid particle,not only the change of velocity,but also the change of velocity direction of the curve motion should be fully considered,and the characteristics of the curve motion of fluid particle were studied;From the point of view of the balance of centrifugal force and centripetal force,the essential law of flow around the cylinder was studied,and the universal law of flow separation and the new universal mechanism of controlling flow separation were revealed;Based on the flow separation and control method of fluid particle curve motion characteristics,the saddle area mechanism of axial flow pump was revealed,and then the blades of axial flow pump were optimized to improve the saddle area of axial flow pump.