Optimization Analysis Of Flow Passage Reconstruction Of A Vertical Axial Flow Pump System

In large and medium pump stations,especially low lift pump stations,vertical axial flow pump systems are widely used.The inlet passage,the impeller,the guide vane and the outlet passage constitute the vertical flow pump system.For a particular impeller and guide vane,the performance of the pump system is primarily affected by the inlet passage and outlet passage.The inlet passage severs as a connection part between the front pool and impeller of the pump station,it can effectively adjust the flow state of the inlet of impeller and can ensure the hydraulic conditions of the inlet of the impeller.According to the characteristics of the flow,the elbow inlet passage can be divided into three parts:the linear constriction section,the elbow section and the vertical adjustment section.The elbow section is the key part of the elbow inlet passage.The pump system has a large drift angel of the inlet passage and the flow velocity uniformity of the outlet section of the passage.It increases the hydraulic loss of passage and reduces the performance of the pump system.The outlet passage connects the guide vane and the discharge basin to ensure the water flowing out from the guide vane flow smoothly into the pool.The kinetic energy is recovered to the maximum extent without the occurrence of the de-watering and vortex.In the vertical axial pump system,there is a large range of de watering zone inside the straight-tube outlet passage,which increased the hydraulic loss of the passage.While the hydraulic loss of the outlet passage accounts for the largest ratio of the total loss of the pump system,so it effects obviously the performance of the pump system.This paper optimizes the inlet passage and the outlet passage.Through the numerical simulation of the pump system,the uniformity of the flow velocity and the average angel of the weighted velocity at the outlet section of the inlet passage,the hydraulic loss of inlet passage and outlet passage,the typical section vorticity of the outlet passage,and the performance of the pump system are obtained to verify the optimization effect.The outer elbow section of the original inlet passage intersects with the linear constriction section,which increases the difficulty of water flow steering and results the large vortex at the typical longitudinal section of the elbow section.The elbow inlet passage is optimized to tangent the outer profile of the elbow section and the linear constriction section,correspondingly,the cross section area of each elbow section is optimized.It makes the vortex of the typical longitudinal section of the elbow section of the original elbow passage is significantly reduced and the weighted average angel of the outlet section of inlet passage is 0.4° higher and the axial flow velocity uniformity is 1%higher,the hydraulic loss is also reduced by 5mm.The flow velocity of the upper part near the inlet of the original sachem is large,and the flow velocity of the lower part is small,so that the water flow forms a large scale dewatering zone at the bottom of the passage diffusing section.Two optimization schemes are designed for this purpose,the optimization plan 1 is a longitudinal shrinkage scheme,which compressed the space of de-watering,the optimization plan 2 is a lateral contraction scheme,which changes the diffusion method and optimizes for secondary diffusion.And the optimal parameters of the optimization scheme 1 passage and optimization scheme 2 passage are determined,that is,for optimization 1 outlet passage,the distance between the vertical contraction apex and the midpoint of the diffusion is L=300mm,and the height of the vertex from the bottom of the flow passage is ht=400mm;for optimization 2 outlet passage,the lateral contraction point position Bp=300mm,the lateral contraction first stage diffusion angel is 6°.The optimization effect of optimization 1 outlet passage and optimization 2 outlet passage is similar.The low flow velocity region at the bottom of the optimization 1 outlet passage and optimization 2 outlet passage is significantly smaller than the low flow velocity region at the bottom of the original scheme outlet passage.The optimization 1 outlet passage and optimization 2 outlet passage not only suppresses the development of the de-watering zone in the height direction,but also reduces the vorticity of the de-watering zone,and the vorticity changed more gently.Therefore,the hydraulic loss of the outlet passage is significantly reduced and the performance of the pump system is significantly improved.Under the condition of small flow rate,the performance of the optimization 1 outlet passage and optimization 2 outlet passage of pump system is slightly better than that of the original scheme pump system,however,under the condition of large flow rate,the performance of the optimization 1 outlet passage and optimization 2 outlet passage of pump system is obviously better than that of the original scheme pump system,the efficiency can be increased by 3.7 percentage points and the head can be increased by 0.1m.