Research On Transient Internal Flow Characteristics Of Centrifugal Pump As Turbine During Start-up Process

Centrifugal pumps as turbines(PAT)have been widely used to recover residual pressure energy,and they have the advantages of simple structure and easy maintenance.PAT development is of great significance for energy conservation and emission reduction as well as sustainable green development.With the more and more extensive application of PAT in the petrochemical industry,seawater desalination,wastewater treatment and other engineering fields,start-up operation becomes frequent,and transient operating conditions such as valve opening and closing occur from time to time,which is easy to produce transient hydraulic impact.In severe cases,drastic vibration and noise are generated,which affects the safe and stable operation of PAT.However,the current research on the internal flow characteristics of PAT has been concentrated on steady-state conditions,so the research on its transient processes must be emphasized in the future.Therefore,this paper studies the transient internal flow characteristics of the PAT start-up process under pure liquid phase and gas-liquid two-phase conditions.The main contents and conclusions are as follows:(1)Considering the interaction effect between the PAT and the consume pump during the start-up process,the change of the load torque was loaded according to the flow characteristics of the regulating valve.According to the rotation equation,the UDF program was written for calculating the rotational speed change with time during the start-up process.The CFD numerical simulation scheme of the PAT fast start-up and slow start-up process was proposed,and then the transient internal flow characteristics of the PAT start-up process were calculated.The PAT system bench of test and control was built to test the performance of steady-state,start-up process and vibration.The test results and modeling results were compared to prove that the numerical simulation scheme was feasible and accurate.(2)The PAT fast start-up process was numerically simulated under pure liquid phase.The evolution mechanism of the internal flow field,the distribution characteristics of blade load,the changing course of radial force and axial force,and the timefrequency characteristics of pressure fluctuation were discussed.The results show that,with the increase of rotational speed,the low-pressure region,the vortex intensity,the turbulent kinetic energy,and the entropy generation in the impeller and the outlet pipe gradually decrease.The radial force and the axial force reach the maximum at the initial stage of start-up.The amplitude of blade load and the distribution trajectory of the radial force vector gradually decrease with the increase of rotational speed.In general,the pressure fluctuation amplitude and main frequency amplitude in the volute decrease with increasing rotational speed.The maximum fluctuation amplitude and the main frequency amplitude of the radial position at the initial stage of start-up increase with the decrease of radial size.In addition to the front end of the tongue,the maximum fluctuation amplitude and the main frequency amplitude of the circumferential position at the initial stage of start-up gradually increase along the fluid flow direction from the volute inlet to the tongue area.During the start-up process,the number of pressure fluctuations in each impeller rotation period is consistent with the blade number.After the rotational speed tends to be stable,the main frequency of pressure fluctuation is about 6 times the impeller rotation frequency.(3)The PAT slow start-up process was numerically simulated under pure liquid phase.The difference between the slow start-up process and the fast start-up process was compared,and an analysis of the influence of start-up time on the transient internal flow characteristics of the start-up process was performed.The results show that,compared with the fast start-up,the instantaneous pressure head is lower,the velocity distribution in the volute is more uniform,the turbulent kinetic energy and the entropy generation in the impeller and the outlet pipe are smaller at the initial stage of slow start-up.The amplitude of blade load,the radial force vector distribution trajectory,the radial force and the axial force,and their main frequency amplitude are smaller at the initial stage of slow start-up.In addition to the inlet region of the volute,the timeaverage pressure,the pressure fluctuation amplitude,and the main frequency amplitude in the volute are smaller at the initial stage of slow start-up.A longer start-up time makes the internal flow fully developed,which is conducive to improving the velocity distribution characteristics in the volute,reducing the energy loss in the impeller and the outlet pipe,and improving the hydraulic stability of the start-up process.(4)The research scheme of slow start-up process was selected.Based on the Fluent mixture model,the start-up process of PAT was simulated under gas-liquid twophase flow.An analysis of the influence of the inlet gas volume fraction on the transient internal flow characteristics of the start-up process was carried out.The results show that,compared to the pure liquid phase start-up process,the instantaneous pressure head is lower,the turbulent kinetic energy,and the entropy generation in the impeller and the outlet pipe are larger under the gas-liquid two-phase at the same time.During the initial start-up,strong vortices are generated in the impeller where the gas volume fraction is high.As the rotational speed increases,the gas volume fraction in the impeller gradually decreases.When the rotational speed tends to be steady,the gas volume fraction on the pressure surface of the blade is higher than that on the suction surface of the blade.The greater the inlet gas volume fraction,the greater the amplitude of the blade load at the initial start-up stage,the smaller the radial force vector distribution trajectory,and the larger the main frequency amplitude of the axial force in the same rotation period.Except for the volute inlet and the tongue area,the pressure fluctuation amplitude and main frequency amplitude in the volute gradually increase with increasing inlet gas volume fraction at the initial start-up stage.During the same rotation period,the time-average pressure gradually decreases with increasing inlet gas volume fraction.