Study On Flow And Excitation Characteristics Of Asymmetric Impeller Of Centrifugal Pump Under Whirl Effect

Centrifugal pump is an important energy conversion equipment and fluid transport tool,which is widely used in municipal,water conservancy,navigation,aviation and other fields.Stability is the primary condition for centrifugal pump to work normally.The vibration fault in the process of centrifugal pump operation has been one of the main factors affecting its working stability.The main factors leading to the vibration of centrifugal pump are:shafting excitation,electromagnetic excitation and fluid excitation.At present,most of the research is about the response of single vibration excitation factor to the vibration of centrifugal pump,but there are few researches on the vibration of centrifugal pump induced by two factor excitation and multi factor excitation.Therefore,this paper mainly studies the effect of two factors,namely,the shaft excitation caused by the asymmetric impeller and the fluid excitation caused by the uneven circumferential pressure between the impeller and the volute,on the internal flow mechanism of the centrifugal pump and the influence of the rotor dynamic characteristic coefficient.This research is supported by the National Natural Science Foundation of China（no.51879120）,the main contents are as follows:1.In this paper,taking a centrifugal pump with specific speed of 123 as the research object,three asymmetric impeller structures（scheme 2₄,3₃,4₂）are designed according to the original symmetrical impeller model.The simulation calculation and analysis of each scheme are carried out by given vortex parameters.The k-ε turbulence model is used to calculate the centrifugal pump in different conditions,and the simulation results are compared with the experimental results to verify whether the numerical simulation method is effective and the calculation results are reliable.2.The impeller whirl effect is simplified reasonably.Based on the multi region dynamic sliding method,the steady-state numerical simulation of asymmetric impeller structure under different flow rate,eccentricity and whirl frequency is carried out by using ANSYS software.The results show that:compared with the small eccentricity,the energy dissipation in the clearance between impeller and volute is more serious in the original model under the large eccentricity.When the asymmetric impeller structure is selected,not only the energy dissipation in the clearance is less affected by the eccentricity,but also the pressure distribution in the asymmetric impeller structure is more uniform than that in the symmetric impeller structure,The reason is that the asymmetric impeller structure reduces the fluid extrusion at the impeller inlet and the velocity difference in the passage,which reduces the influence of eccentricity on energy dissipation under the whirl effect.The internal pressure gradient of scheme 4₂ pump is the smallest,which indicates that the asymmetric impeller structure can alleviate the influence of eccentricity on energy dissipation at the clearance under the whirl effect,The pressure distribution in the pump is the most uniform in scheme 4₂.3.Unsteady numerical calculation of centrifugal pumps with different impeller structures under whirl effect.A number of monitoring points were set at the clearance between impeller and volute,and the pressure fluctuation in the whirling region was observed under the combined action of fluid excitation and shafting excitation by time domain diagram.It is found that the forward whirling frequency has the greatest influence on the pressure fluctuation at the maximum clearance,and the number of"crest trough"structure decreases with the increase of whirling frequency,but the influence of whirling frequency ratio on the pressure fluctuation at other locations of the clearance is not obvious;Eccentricity and circumferential position of clearance have a great influence on the pressure fluctuation,while the amplitude of pressure fluctuation at each monitoring point of 4₂ scheme is relatively small under different flow and eccentricity,and the average pressure difference between the maximum clearance and the minimum clearance is the smallest,so the circumferential pressure uniformity is the best,and the impeller rotor is not prone to whirl instability,The impeller structure of scheme 4₂ should be selected.4.Based on the quadratic function relationship between fluid exciting force and whirl frequency ratio,the dynamic characteristic coefficient of impeller rotor is solved and the primary and secondary degree of its influencing factors is analyzed.It is found that the greater the mass of impeller water is,the greater the direct additional mass is,and the stiffness coefficient and damping coefficient decrease with the increase of eccentricity.Through the orthogonal test analysis,it is found that the eddy eccentricity has the most obvious effect on the stiffness and damping coefficient,and the flow rate has the greatest effect on the direct added mass;Under the condition of small flow rate,the influence of impeller structure on the stiffness coefficient is greater than that of flow rate,which provides a way to improve the stability of impeller rotor under whirl effect.