Study On Cavitation Control Of Axial Flow Pump By Obstacles

Cavitation is a complex physical phenomenon unique to liquids,and cavitation is often generated by low local pressure in the operation of hydraulic machinery,resulting in vibration,noise,cavitation,etc.,which seriously affects the normal operation of machinery,so it is important to study the means of cavitation suppression.In this paper,the axial flow pump with a specific speed of 700 and a model number of 350ZQ-70 is selected as the object of study,its external characteristics and internal flow characteristics are analyzed,and cavitation suppression is explored by arranging obstacles as the main means,and the obstacles are divided into spanwise and streamwise.Firstly,we analyzed the position of the spanwise obstacles,and on this basis,I proposed to arrange the flow-direction obstacles on the back of the vanes.On the basis of verifying that this structure can improve the cavitation resistance of the axial flow pump,I analyzed the influence of the spanwise position and height parameters of the flow-direction obstacles on the cavitation inhibition.The following is the main research and results of this paper.(1)Numerical simulation was used to study the hydraulic performance and cavitation characteristics of the axial flow pump,and the reliability of the numerical simulation calculation results was verified by the hydraulic performance experiments of the axial flow pump.The cavitation characteristics of the axial flow pump at non-designed flow rate were analyzed,and it was found that the head of the axial flow pump operating under high flow rate conditions broke early,and the range of stable operation of the axial flow pump under low flow rate conditions was expanded and the head break was lagged.According to the cavitation characteristic curve the cavitation process was divided into three stages:incipient,developing and intense.And the transient calculation of cavitation under the design flow rate found that:under the initial phase of cavitation,the back of the vane inlet existed less attached bubble,which did not affect the performance of the pump;in the cavitation development stage,the head rose and then fell,attached bubble cavitation developed into sheet cavitation,at this time the bubble mostly stable attached to the back of the vane,forming a relatively smooth area,reducing the flow loss,resulting in a slight increase in pump head;in the intense cavitation stage,the vacuole large pieces off,blocking the flow passage,reducing the effective flow area in the flow passage,seriously affect the normal operation of the pump,blade load is reduced,gradually losing the power capacity of the pump,and greatly reducing the head.(2)The spanwise obstacle arrangement position on the cavitation suppression effect plays an important role,in the back of the blade uniformly selected 9 chordal position analysis on the impact of cavitation,found that the spanwise obstacle arrangement in the back section of the effect is better than the front section,the arrangement position is too close to the front will cause the pump head break in advance,the cavitation has a negative impact.By arranging the spanwise obstacle can make the arrangement position to the vane inlet flow line distribution more symmetrical,reduce the bias flow and backflow,at the same time the obstacle blocked the fluid flow in the boundary layer near the wall surface,increase the pressure,inhibit the development of cavitation.The best effect is achieved when it is arranged at 0.6l_x,which reduced the volume of the vacuole by 65.87%during cavitation development,30.88%during violent cavitation,and increased the fracture head by 12.19%.The next best effect is achieved when it is arranged at 0.8l_x.On this basis,several successive spanwise obstacles were arranged in the range of 0.6～0.8l_x,and the effect of multiple spreading obstacles on the internal flow of the impeller was analyzed.The best results were found when cavitation developed to the arrangement position,the arrangement of multiple obstacles on the pressure and a single spreading obstacles consistent,are narrowing the low pressure area of the back of the blade inlet,in front of the obstacles and the rear of the local high pressure area and low pressure area,where the first obstacle on the pressure of the greatest impact,the resulting high pressure area and low pressure area is more obvious,due to the superposition of multiple obstacles on the fluid blocking effect,so that The closer the obstacle is to the blade outlet,the smaller the high pressure and low pressure areas are,which reduced the volume of the vacuole by 75.65%during cavitation development and 28.49%during violent cavitation,and increased the fracture head from 3.63 m to 4.134 m,resulting in an increase of 13.88%.Comparing the effect of cavitation of a single spanwise obstacle,it is found that the arrangement of multiple spanwise obstacles has better suppression effect on cavitation than a single spanwise obstacle.(3)On the basis of the finding that the spanwise obstacle was arranged in the middle and rear part of the back of the blade to suppress the cavitation,this paper proposes a way to arrange the flow direction obstacle in the middle and rear part of the back of the blade,considering to restrain the fluid flow in the spanwise direction to suppress the cavitation.Through numerical simulation,it is found that the arrangement of flow direction obstacle effectively blocks the deflected flow,weakens part of the return flow,sorts out the turbulent flow field,improves the flow state,optimizes the complex flow structure,and plays a more obvious inhibiting effect on the development of cavitation.The model after the deployment of flow obstacle obviously reduced the range of low-pressure area on different circumferential cross-sections on the back of the blade,increased the high-pressure area on the back of the blade,limited the expansion of the low-pressure area to the outlet position,reduced the inverse pressure gradient,caused the relative high pressure on the inlet side,changed the shape of cavitation,and inhibited cavitation.The best results were obtained when cavitation developed,reducing the volume of the vacuole by 72.5%and by 34.4%for violent cavitation,and increasing the fracture head from 3.63 m to 4 m,an increase of 10.19%.(4)Further study on the spanwise position and height parameters of the flow direction obstacle,it is found that the obstacle at different spanwise positions during the initial and development of cavitation can block the bias flow and return flow from the hub,made the flow distribution more uniform and reduced the energy dissipation,among which the flow direction obstacle arranged on the middle section have the best effect,which can affect both the hub side and the rim side,and the obstacles arranged on the rim side are more effective when the cavitation is intense.The obstacle on the rim side is less effective and the flow distribution is more turbulent when the cavitation is severe.With the increase of the obstacle height parameter,the local high-pressure area produced by the leading edge of the obstacle increases,but the low-pressure area on the side and top of the obstacle also expanded.The height parameter had less effect on the volume of the cavitation bubble and the cavitation performance,where the height of 3 mm increases the pump efficiency by 3.118%in the intense cavitation stage.