| There has been an increasing demand for bidirectional axial flow pumps that can balance the hydraulic performance in both forward and reverse modes due to the wide application of irrigation and drainage pump stations in coastal areas.The special shape of the bidirectional axial flow impeller however makes it difficult to directly apply the design experience of the unidirectional impeller.This paper initially established an automatic optimum design platform for bidirectional axial flow pump based on NACA 65 airfoil function.An axial flow pump with asymmetrical bidirectional impeller was designed and the pump performance was validated by external characteristic test and pressure fluctuation measurement.Subsequently,the asymmetric bidirectional impeller was adapted as the original design,and the forward and reverse hydraulic performance of the bidirectional axial flow pump was quickly adjusted by modifying the camber angle on each wing section.According to the unsteady calculation results,the internal turbulence intensity,distribution of entropy generation and characteristics of blade hydraulic moment under different camber angle distribution were compared and analyzed.Finally,the influence of blade root clearance radius on the hydraulic performance of bidirectional axial flow pump was predicted by numerical simulation,which can provide references for design of blade root clearance in bidirectional axial flow pump.The specific research contents are as follows:1.The research status of axial flow pumps in the fields of optimization design,impeller clearance flow,unsteady hydrodynamic characteristics and entropy hydraulic loss were summarized.The combination optimization method of bidirectional axial pump impeller and guide vane based on a multilayer neural network and NACA 65 airfoil theory was proposed.Based on the Workbench platform,Matlab code drive,and NACA airfoil function,the three-dimensional modeling of the impeller and guide vane,structural meshing,and numerical simulation settings were automatically proceeded in batches.Taking a one-way axial flow pump as the original scheme,the hydraulic efficiency and head of the axial flow pump at the forward and reverse design flow rates was set as the optimization goal.The multiobjective genetic algorithm was used to solve the optimal solution of the approximate model based on the multilayer neural network,and the design scheme of the bidirectional axial flow pump that could meet the engineering requirements in both forward and reverse hydraulic performance was obtained.2.A test bench for measuring the performance characteristics and pressure pulsation of the bidirectional axial flow pump was built,and the hydraulic performance parameters of the two-way axial flow pump under forward and reverse operating conditions and the dynamic pressure signal at the positions of different axial monitoring points(outlet of straight pipe,inlet of guide vane,and inlet of curved pipe)were measured.The pressure signal was analyzed in both time and frequency domain based on power spectrum function and statistical theory.Finally,after adding a “crossed” baffle in the straight pipe flow path near the entrance of the impeller,the performance characteristics of the axial flow pump under forward operating condition was measured again.The results show that:(1)The relative error between the test measured value and the numerical simulation value meets the requirements,and the optimized design results have reached the engineering requirements.(2)The pressure pulsation intensity and main frequency amplitude at the three monitoring points showed an upward trend with increasing head.Affected by the interaction of impeller and guide vane,the main frequency of pressure pulsation at the inlet of the guide vane is the blade passing frequency,and under the conditions of high head and design head,the main frequency amplitude at this position is the highest.(3)Comparing and analyzing the performance characteristic parameters under forward operating condition with and without the baffle,it can be seen that the inlet channel baffle can effectively restrain the generation of the impeller pre-rotation and improve the operational stability of the axial flow pump near the shut-off point.3.A design method for quickly adjusting the forward and reverse hydraulic performance of bidirectional impeller was proposed.Based on the above optimized impeller scheme(asymmetric bidirectional impeller),two axial flow pump impeller meeting different engineering requirements were designed(unidirectional impeller,symmetric bidirectional impeller)by changing airfoil camber angle in the NACA 65 airfoil function.The unsteady numerical calculation of the axial flow pump with three impeller schemes was carried out,and the influence of different impeller airfoils on the internal turbulence intensity in axial flow pump was studied.The results show that:(1)the forward design flow and forward design efficiency increased with increasing airfoil camber angle,but the reverse design flow and reverse design efficiency decreased with increasing airfoil camber angle.(2)Under forward design flow condition,the pressure difference between the upper and lower blade surface and the low speed area inside impeller decreased with increasing airfoil camber angle,while the pressure fluctuation intensity on blade surface increased with the increasing airfoil camber angle.(3)Under reverse design flow condition,the pressure difference between the upper and lower blade surface,and the low speed area inside impeller increased with increasing airfoil camber angle,while the pressure fluctuation intensity on blade and the velocity fluctuation intensity inside impeller decreased with the increasing airfoil camber angle.4.Entropy production theory was applied to the unsteady calculation results of the above three impeller schemes and the internal hydraulic loss distribution was studied.The results revealed that:(1)turbulent dissipation dominated in entropy production loss,while the direct dissipation in guide vane,straight pipe and elbow channel was negligible.(2)Under forward operation condition,the turbulent dissipation inside the impeller was higher than that of other hydraulic components.Initially it is observed as a steady decreasing trend and then maintaining a stable trend with flow rate increasing.Under reverse operation condition,the turbulent dissipation inside impeller and straight pipe was the highest and decreased with flow rate increasing.(3)Under forward design flow condition,the turbulent dissipation rate at leading edge of blade decreased with airfoil camber angle increasing,but under reverse design flow condition,the turbulent dissipation rate at leading edge of blade increased with airfoil camber angle increasing.5.Based on the unsteady calculation results,the characteristic of blade hydraulic moment under different airfoil scheme were obtained and the influence of blade angle(-2°,0°,+2°)on blade hydraulic moment were analyzed.(1)The blade hydraulic moment decreased with the flow rate increasing,but it increased with blade angle increasing.(2)Under forward design flow condition,the blade hydraulic moment fluctuated periodically in the 10 th impeller rotation cycle,and the main frequency of hydraulic moment fluctuation was 5 times of shaft frequency.The blade hydraulic moment decreased with airfoil camber angle increasing.(3)Under reverse design flow condition,the blade hydraulic moment fluctuated unsteadily in the 10 th impeller rotation cycle,and the main frequency of hydraulic moment fluctuation was the shaft frequency.The blade hydraulic moment and the main frequency amplitude increased with airfoil camber angle increasing.6.Five kinds of blade root gaps with different radial radius were designed for the impeller of bidirectional axial flow pump.Numerical simulations were employed for the axial flow pumps under 5 blade root clearance schemes,and the effect of the blade root gap radius on the performance characteristics of the axial flow pump and the internal flow field of the impeller was analyzed.The results indicate that: under large flow rate condition,the hydraulic performance of the axial flow pump was most significantly affected by the blade root gap radius.When the blade root gap was large,the fluid near the impeller hub would generate a large amount of circumferential velocity component,which mixed with the main flow causing hydraulic loss.In addition,the axial flow velocity and circulation near the hub at impeller outlet would be significantly reduced due to the blade root gap. |
PDF Full Text Request