Research On Pump Cavity Fluid Flow Mechanism And Axial Force Control Method For Centrifugal Pump

Axial force is one of the important factors that restrict the development of centrifugal pump to high speed,good stability and long service life.Nowadays,the research on the liquid flow mechanism of pump cavity is imperfect,and the prediction accuracy of centrifugal pump axial force under different geometric parameters and performance parameters is poor.Synchronously,the traditional axial force balance method reduces the axial force,which is commonly accompanied by the decrease of head and efficiency.In view of this,this study takes IS80-50-315 centrifugal pump as the research object,and adopts the methods of theoretical analysis,experimental research and numerical simulation to study the pressure distribution and leakage characteristics of pump cavity with different geometric parameters and performance parameters.The procedure is as follows:(1)To quantitatively analyze the coupling regulation effect of the clearance of the rear seal ring b2 and the diameter of the balance hole d on the pump cavity pressure and the balance hole leakage,the experimental study on the pump cavity pressure distribution and leakage characteristics is conducted under the matched conditions of different b2 and d.The results demonstrate that when b2<0.6 mm,b2 plays a major role in regulating the rear pump cavity pressure;and when b2≥0.6mm,d plays a major role in regulating the rear pump cavity pressure.The relationship curve between the pressure p5 and the specific area km of balance cavity is similar to the inverse proportional function image,that is,the increase of d can relieve the pressure of the balance cavity liquid,while the increase of b2 can pressurize the balance cavity liquid.When km≤0.73,the equivalent resistance coefficient ζb of balance hole is larger than the resistance coefficient ζm of seal ring,and d plays an important role in regulating the balance hole leakage;when km>0.73,ζm is larger than ζb,b2 plays an important role in regulating the balance hole leakage.Especially,when km>2.23,ζb is far smaller than ζm,and d’s regulating effect on the balance hole leakage can be almost ignored.(2)The mathematical model of pump cavity pressure distribution and leakage is established by three methods,which contributes a lot to the improvement of the prediction accuracy of pump cavity pressure distribution and leakage.The formula for calculating the rotation coefficient of the rear pump cavity has been corrected,and the radius-length ratio of the balance hole and undetermined rotation coefficient ξ were introduced into the formula.ξ at different km has been calibrated according to a large amount of test data,which effectively solved the problem of inaccurate estimation of the rotation coefficient of the rear pump cavity k2.An integrated mathematical model of pump cavity pressure distribution and seal ring leakage considering local head loss is established.In order to improve the prediction accuracy of the model,the local head loss coefficient at the inlet of pump cavity is treated according to the sudden contraction of cross section,and the local head loss coefficient at the inlet of seal ring gap is taken as 0.1,and the local head loss coefficient at the outlet of seal ring gap is taken as 1.0.This model effectively solves the problem of accurately predicting the leakage of front seal ring gap when the flow coefficient of front seal ring is unknown.The prediction accuracy of the three mathematical models is verified by the same set of measured data.The results indicate that the mathematical model based on potential head modification factor and rotation coefficients has the highest prediction accuracy of balance cavity pressure,with the maximum relative error of-3.9%,and the mathematical model based on pump cavity pressure loss coefficient has the highest prediction accuracy of rear pump cavity pressure,with the maximum relative error of 3.7%.(3)Using numerical simulation method,the uneven distribution of the velocity field in the pump cavity along the circumferential direction is studied,and the transient evolution law of the leakage flow in the rear pump cavity channel is analyzed.The results show that the velocity distribution in the boundary layer and the core area of the pump cavity is not axisymmetric,and the larger the radius of the pump cavity is,the smaller the dimensionless circumferential velocity is,and the more obvious the non-uniformity of its circumferential distribution is.The blade position and sectional area of volute are the key factors affecting the liquid flow direction between the pump cavity and volute.For a certain position at the inlet of the pump cavity,the blade far away from this position will lead to the gradually increased radial external flow intensity between the volute and the pump cavity,while the blade close to this position will lead to the gradually increased radial internal flow intensity.The liquid flow direction between the pump cavity and the volute is the mainly radial internal flow when the circumferential angle α of the volute is at 90° and 180°,while it is mainly radial external flow when α is at 270° and 0°.(4)For the purpose of addressing the vital issue that trimming rear shroud of impeller balanced axial force will reduce pump head and efficiency,a novel method to balance axial force by compensating rear shroud of impeller cut was proposed,namely,compensation trimming method,revealing the mechanism of compensating trimming method to balance axial force and recover pump head and efficiency.The experimental results demonstrate,based on the head,efficiency and axial force of the prototype impeller at the design flow rate,when the relative trimming rate of the impeller rear shroud is 3.81%,7.62%and 11.43%,the maximum drops of the pump head,efficiency and axial force are 9.93%,6.18%and 22.3%respectively.Nonetheless,after compensating the trimming amount of the impeller rear shroud plate,the maximum drops of the pump head,efficiency and axial force are 4.18%,2.7%and 83.1%respectively.The numerical analysis results sketch that the decoupling effect of the compensation ring is remarkable,which can significantly reduce the blocking effect of the tip clearance leakage vortex and the volute passage vortex induced by trimming the impeller rear shroud plate.The clearance passage vortex between the compensation ring and the impeller rear shroud plate restricts the medium exchange between the rear pump cavity and the tip clearance,further reducing the pressure of the rear pump cavity.