| In recent years,with the development of frequency conversion technology and the increasing attention to energy conservation and emission reduction,more and more axial-flow pumps turn to use frequency conversion motor drive.Speed regulation means that the working condition changes.However,there is few engineering case of large axial-flow pump using speed regulation technology.Its hydraulic characteristics,especially the unstable status at small flow conditions and cavitation and flow separation at large flow condition,greatly restrict the speed regulation range of axial-flow pump by frequency conversion technology.Therefore,study the internal flow characteristics of axial-flow pump under unstable working conditions,put forward improvement methods and broaden the application range of axial-flow pump are of theoretical and practical significance.The improvement method is sought by analyzing the internal flow characteristics of axial-flow pump under off design conditions.Through the combination of PIV and numerical simulation,the internal flow characteristics of two axial flow pumps with specific speed of 700 and 1000 are analyzed.Through the combination of numerical simulation and test rig experiment,the head and efficiency of two kinds of axial flow pumps at different flow rates are studied,and the relationship between external characteristics and internal flow characteristics of the pumps under off design conditions is revealed.Then this dissertation puts forward the flow characteristic improvement schemes from adding the inlet and outlet structures of the impeller,in which the inlet structure adopts the scheme of double-inlet.The impeller outlet is connected with the inlet of the guide vane,and the outlet flow of the impeller actually runs into the guide vane body.This dissertation names the modified outlet structure as double-layer flow passage of guide vane body.Both structures are geometrically represented by adding a cylindrical division baffle,which is located in front of and behind the impeller respectively.The structure has specific size requirements.Both of them can regulate the internal flow of axial-flow pump under off design working conditions,improve the performance of the pump and expand the flow range of safe running.Finally,the effectiveness of the outlet structure is verified by real pump experiment.The results achieved are as follows:1.Through the comparative analysis of the internal flow characteristics of axial-flow pump,the double-inlet structure is proposed.It can effectively restrain the vortex formed by the tip leakage under small flow conditions,effectively improve the flow field in front of the impeller,and increase velocity of the main stream so as to improve the head performance.It reduces the positive slope value of the H-Q curve.Under the design condition and large flow conditions,the flow direction within the double-inlet gap is same as the main stream,and the head is equivalent to the pump without double-inlet.Two key geometric parameters are selected as variables to compare its pump external characteristics,and the influence of geometric variables on the performance of axial-flow pump is studied.Base on the H-Q curves generated by simulation,the results of other geometry combinations are predicted by using support vector machine(SVM)algorithm.The results show that the distance between the end of the double inlet and the impeller leading edge should be no greater than 2.5%of the impeller diameter,and the gap between the inner cylinder and the inlet pipe should be no less than 1.75%of the impeller diameter.2.Based on the analysis of the velocity triangle on the impeller blade outlet edge and the flow separation conditions,a new double-layer flow passage structure behind the impeller is proposed.Different from the double-inlet in front of the impeller,the double-layer flow passage is behind the impeller and located near the hub side.The distance from the end of the inner cylinder to the impeller blade outlet edge should be less than 2.5%of the impeller diameter,the gap between the inner cylinder and the guide vane hub shall not exceed 50%of the flow channel width,and the distance which the inner cylinder extends into the guide vane body shall not exceed 1/4 of the length of the guide vane.The flow field analysis and entropy generation analysis show that the structure can effectively inhibit the backflow after the impeller and reduce the area and intensity of vortices,so as to improve the flow quality after the impeller and increase the head under small flow conditions.In case of large flow conditions,the flow direction within the gap is same as the main stream.The cylindrical baffle plays a guiding role in the guide vane channel,reduces the vortex influence area,and improves the flow velocity.The velocity distribution in the main flow channel becomes more reasonable,and the influence between different layers at downstream is reduced.The structure shows effect to inhibit the boundary layer flow separation of the diffuser behind the guide vane,and the overall result is that the head and the efficiency are significantly improved.In this dissertation,the gap coefficient is used as the structural parameter of the double-layer flow passage,and the changes of external characteristics of axial-flow pump when the gap coefficient sets at 1/3,1/4 and 1/5 respectively are studied.The gap coefficient 1/3 shows the best head lifting effect,and the head improvement rate at the highest efficiency point is 8.75%.3.The double-inlet structure shows improvement effect on both axial-flow pumps with two different specific speeds.Considering that the inlet shape also affects the pump performance,in addition to the gap size,the shape of the double-inlet is also compared and studied for the model with specific speed 1000.It is found that the double-inlet with both horn shape and cylinder shape can improve the head.The effect of horn shape is better than cylinder shape under small flow conditions.However,the head difference between the two shapes is less than 3%.The cavitation performance of the axial-flow pump with double-inlet is also studied,and the results of H-NPSH_a curve show that the double-inlet can improve the cavitation performance of the pump. |
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