| Deep well submersible pumps are an important part of the irrigation system’s first hub and water supply pipe network.Its performance directly affects the efficiency and stability of agricultural irrigation projects,and becomes the primary link of water-saving irrigation technology research and development.At present,the efficiency of submersible pump units used in agricultural irrigation is generally not high,and most of the units can only operate more efficiently at the design point.The efficiency at non-design points is low,and the overall adaptability of the pump is poor.As one of the components that affect the performance of the water pump,the diffuser is designed to improve the performance of the submersible pump by studying the working mechanism and optimal design of the diffuser.It has not only important theoretical and practical significance,but also a new idea for the exploration of agricultural water-saving technology by mastering the movement of the liquid inside the diffuser and clarifying the coupling effect of parameters and parameters on the performance of the submersible pump.This thesis is supported by National Natural Science Foundation of China(Grant No.51179116)and Shanxi Provincial Natural Science Foundation of China(Grant No.201601D102045).Combined with the production of enterprises,the submersible pump diffuser of the250QJ125-16/1model is taken as the research object,and the influence of the diffuser structure parameters on the performance of the submersible pump is studyed by using theoretical analysis,computer simulation and experimental research method.The diffuser models with different structural parameters are set up,and the characteristics of the internal flow field are studied for exploring the influence of the diffuser structure parameters on the submersible pump performance.At the same time,under the coupling effect of the diffuser parameters,the performance of the submersible pump is analyzed.The findings are as follows:(1)The size of the diffuser structure must be taken into account the effective engagement with the front impeller outlet and the rear duct outlet or the lower impeller inlet;With an increase in the inlet setting angle and inlet width,the angle between the characteristic curve of the diffuser and the coordinate axis of the flow will be smaller.At the same time,the optimum operating point of a submersible pump offsets to a larger flow rate.According to the characteristics of the diffuser and impeller,the optimum operating point of the submersible pump with different structural parameters diffuser and the same impeller can be determined by calculation and graphic method.(2)NURBS(Non-Uniform Rational B-Splines)method was used to control solid surface for submersible pump impeller,diffuser,and other hydraulic components.Models of blade surface with greater distortion were built by the control point data of the inverse calculation of the suction surface.It can better control the degree of anastomosis between the blade surface and the solid,and further meet the requirements of the engineering calculation.The quality of grid partition is the key to the success of simulation calculation.Among them,the selection of grid number and solution variable convergence accuracy must be reasonable to ensure that it not only meets the actual engineering requirements,but also occupies less resource.(3)Using digital modeling capabilities of Auto CAD,meshing technology of Gambit,and numerical calculations of Fluent,the predicted performance of the pump can be obtained.The relative error between the predicted head of the pump and the design head is 5.37%.The accuracy of the error meets the requirements.It shows that the reliability of the calculation results obtained by simulation using Fluent is relatively high.This method is completely applicable to guide the structure design of the diffuser and predict the performance of the submersible pump.(4)According to the display results of the water flow velocity distribution and pressure distribution inside the diffuser of prototype submersible pump,there are obvious vortex and low pressure areas at the intersection of the blade and the hub and the hub surface near the blade inlet.The change of the diffuser structural parameters at the inlet can easily induce and expand the backflow of the water and the generation of vortices,which has a significant influence on the flow field distribution.Along the direction of the diffuser central axis,the average cross-sectional flow rate gradually decreases,while the axial speed from the inlet to the outlet does not change significantly.Along the axis direction of the diffuser,the mean total pressure of different sections shows a decreasing distribution,which gradually decreased from185.47 kPa to 173.85 kPa(from the entrance to the exit).There is energy loss in the entire flow process.For the flow rate distribution and the total pressure change curve,by comparing the smoothness and the change rate of the slope at any two points on the curve,it is possible to determine where the larger energy loss occurs and then correct the structural parameters.Combining with the structural parameters of the diffuser,it is preliminarily judged that the structural parameters,such as the inlet setting angle of the diffuser blade,the inlet width,the axial length of the blade,the number of the blades etc,have a greater influence on the performance of the submersible pump.According to this,it can provide reference for the improvement of the diffuser structure and parameter optimization.(5)With an increase in the inlet setting angle of diffuser blade,the optimum operating point of a submersible pump offsets to a larger flow rate.This is consistent with the results of previous theoretical analysis.When the inlet setting angle of diffuser blade is approximately27 ~o–30~o,the submersible pump efficiency is relatively high,and it reaches more than 75%,and the flow rate is approximately 95–125 m~3/h.When the inlet setting angle of diffuser blade is less than 27~o or more than 30~o,the submersible pump efficiency is relatively low.The inefficient operating areas should be avoided as much as possible.With the different inlet width of the diffuser blades,the operating conditions corresponding to the highest efficiency point of the submersible pump are also different.With an increase in the inlet width,the optimum operating point of a submersible pump offsets to a larger flow rate.This is consistent with the results of previous theoretical analysis.When the inlet width is between 40mm and55mm,the circumferential speed of the outlet section of the diffuser approaches zero,and the velocity loop of the impeller outlet is basically eliminated,and the efficiency reaches more than 75%.Among them,under the original design conditions,the pump efficiency is the highest when the inlet width is 50mm,which means that the diffuser with 50mm inlet width can better match the design impeller.After increasing the axial length of the diffuser,the length of the liquid flow path is increased,the distortion of the diffuser blade is reduced,and the fluid flow state can be made more stable.For the prototype submersible pump,the optimal interval for the axial length of the diffuser is 80–100mm.The head and efficiency of the submersible pump increase with the increase of the axial length in this interval,and the axial length is corrected to some extent.With the increase of the number of diffuser blades,the head and efficiency of the submersible pump are increasing.For the prototype submersible pump,the number of diffuser blades with good hydraulic performance is 7–10.The number of blades must be selected in combination with the production process.It is not allowed to increase the number of diffuser blades at random by merely increasing the hydraulic efficiency of the pump without considering the ease and cost of the blade shape during casting.The selection of the number of diffuser blades needs to be combined with the pump performance and production costs in order to achieve the highest overall efficiency.The above obtained threshold values of structural parameters,such as inlet setting angle,inlet width,blade axial length and blade number,can meet the higher efficiency of the submersible pump.And it provides an effective reference for guiding the structural parameters design of the diffuser under different working conditions.(6)For the head and efficiency of submersible pump,the effect of diffuser structure comes from two aspects,one is the direct effect of the structural parameters of the diffuser,and the other is the indirect effect of structural parameters through other parameters.The comprehensive effect is not a simple addition or subtraction of direct and indirect effects.It is necessary to calculate the determinant values of each parameter and compare them.For the head of pump,the order of influence from large to small is the inlet setting angle,the axial length,the inlet width and the blade number.For the efficiency of pump,the order of influence from large to small is the axial length,the inlet setting angle,the inlet width and the blade number.For the prototype impeller,when the efficiency of submersible pump is the most highest,the diffuser structural parameters are the following:the inlet setting angle of the blade is 30°,the inlet width is 45mm,the axial length is 100mm,and the number of blades is8.Those parameters are located at the best threshold interval of each parameter.At this time,the efficiency of the submersible pump reaches 83.5%,corresponding to the head is 18.9m.The combination of this parameter further optimizes the performance of the prototype submersible pump.The impaction and effect of the diffuser parameters and coupling relationship on the performance of submersible pump are different.As far as design is concerned,it should be considered not only the order of structural parameters impacting pump performance,but also the beneficial aspects for non-key parameters improving performance.On the basis of taking into account the production process and production costs,the performance of the submersible pump is optimized. |
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