Analysis Of Two-phase Flow And Pressure Fluctuation During Start-up Of Self-priming Pump For Drainage

As a special kind of centrifugal pump,self-priming centrifugal pump can be divided into external mixing type,internal mixing type,water ring type and jet type according to different working principles.Four kinds of pumps are widely used in the daily production activities of agricultural irrigation,municipal drainage,petrochemical and food industries.From start to normal operation,the self-priming pump includes start-up process and normal water supply process.The start-up process mainly includes multi-stage complex two-phase flow,such as fluid suction,gas-liquid mixing,gas-liquid separation and fluid reflux.Since the two-phase mixed fluid flows rapidly and changes violently in the starting process of the self-priming pump,the self-priming mechanism of the self-priming pump can not only effectively improve the self-priming performance of the pump,but also help to reduce energy consumption.In this paper,the self-priming process of the self-priming pump and the law of fluid flow in the normal water delivery stage are studied in depth,and the instantaneous flow of the internal fluid is observed by the method of experiment and numerical simulation.At the same time,by cutting the outer diameter of the impeller,the influence of the clearance between the volute and the impeller on the internal flow characteristics in the process of self-priming is analyzed.By monitoring the instantaneous changes of gas content,flow rate and pressure at the important cross-section,interface and inlet and outlet of the pump in the process of self-priming,further clarify the change rule of two phases in the process of self-priming.This research is supported by the National Natural Science Foundation of China(NO.51879120).The main work is as follows:1.A SD4-100 external mixing self-priming pump is selected as the research object.Through the external characteristic test of the self-priming pump,the hydraulic performance parameters of the self-priming pump under different flow rates are obtained and the performance curve is drawn.In order to verify whether the design of the pump meets the requirements of engineering application and whether the results of numerical simulation are reliable,the steady numerical simulation results of the self-priming pump under the same working condition are compared;2.Select the 1/2 thickness section of impeller as the reference surface,analyze the speed distribution,pressure distribution,turbulent kinetic energy,etc.of the self-priming pump,and analyze the flow rule of the fluid in the pump during the normal working stage.The results show that there is a large energy loss at the impeller and volute,and there is ajet wake phenomenon.The magnitude and range of turbulent kinetic energy decrease with the increase of flow rate,and the distribution of vorticity changes with the increase of flow rate.It is mainly distributed in the inlet channel and volute at small flow rate,and in the gas-liquid separation chamber and outlet extension section at large flow rate;3.Using CFX17.1 simulation software,the two-phase unsteady numerical simulation of self-priming stage of self-priming pump is carried out.Monitoring points and monitoring surfaces shall be set at the interface of main flow passage components,volute section and inlet and outlet surfaces to monitor the changes of gas flow and speed,liquid flow and speed,pressure and gas content during the self-priming process.The two-phase distribution,velocity distribution,pressure distribution,streamline distribution and gas holdup variation in the self-priming process are analyzed.The results show that the suction and exhaust of the self-priming pump are mainly concentrated in the early and middle stage of the start-up process,and the gas content of the impeller inlet and the pump outlet are more than 60% and 75%,respectively.There is a layer of obvious gas-liquid mixed zone at the impeller outlet,which leads to the flow disorder from the impeller outlet to the large area inside the volute,reduces the power capacity of the impeller and affects the normal exhaust of the pump.There are many vortices of different sizes in the volute and gas-liquid separation chamber.This is due to the speed difference of the mixed fluid in the volute and the obvious separation of gas-liquid flow at the pump outlet.More vortices will be formed in the gas-liquid separation chamber when the two-phase mixed fluid flows back and remixes with the fluid in the gas-liquid separation chamber.According to the change curve of gas phase flow at the pump outlet and the mass of gas phase in the pump body at the initial state,the more accurate self-priming time is calculated.Finally,by cutting the outer diameter of the impeller to change the clearance between the impeller and the volute,the effect of different clearance on the flow in the self-priming process was observed.The results show that with the increase of the clearance between impeller and volute,the gas phase gathering in the clearance is more obvious,which makes the fluid flow characteristics change,and the exhaust capacity of self-priming pump slightly decreases;4.The pressure fluctuation of two-phase flow in self-priming stage of self-priming pump is simulated.Several monitoring points are set at the impeller inlet,impeller middle passage,impeller outlet,volute section and volute outlet respectively.The research method of "whole before part" is adopted,and the law of pressure fluctuation in this process is analyzed by combining the intuitive time domain diagram and frequency domain diagram.At the same time,the two-phase flow law and the influence of gas content on the performance of the pump were studied.