Research On Internal Flow And Dynamics Characteristics Of Low Specific Speed Multistage Centrifugal Pump

The low specific speed multistage centrifugal pump has the characteristics of low flow rate and high lift.It is widely used in industry and agriculture.It is always difficult to study the low specific speed multistage centrifugal pump with the unsteady internal flow,many flow passage components and multi-clearance.At the same time,the high speed of the rotor cause excessive vibration,the rotor system with unbalanced mass and by the role of fluid exciting force will produce directional deflection which affects the stability of the operation.Therefore,it is necessary to ensure the internal flow stability of the low specific speed multistage centrifugal pump,reduce the vibration of rotor system and enhance the security of operation.For the internal flow and dynamic characteristics of the low specific speed multistage centrifugal pump,the main work of the paper and the main conclusions are as follow:(1)The three-dimensional model of the water body of the low speed multistage centrifugal pump was established.The grid model which has the appropriate grid number was divided by using the ICEM and verified.The steady simulation of the internal flow of the multistage centrifugal pump was performed based on CFX.Analysis of the pressure and velocity distribution of the section in the pump body and the flow line of the impeller and the positive guide vane were done.It is found that leakage flow affects the flow of impeller passage,and there is a small amount of fluid exchange between the impeller and the guide vane operating clearance.The head and efficiency obtained by numerical simulation have less error with the experimental results.(2)Four different types of clearance channels were constructed to study the pressure fluctuation in impellers,guide vanes and clearance channels.The time step and total time of transient calculation were determined after verifying the error.It shows that the dynamic and static interference is the main cause of pressure fluctuation in low specific speed multistage pump.The frequency domain distribution of pressure fluctuation in impeller channel is mainly affected by the number of guide vanes,the frequency domain distribution of pressure fluctuation in guide vane channel is mainly affected by the number of impeller blades,and the frequency domain distribution of pressure fluctuation in clearance channel is mainly affected by the number of guide vanes.The pressure fluctuation in the multistage centrifugal pump is influenced by the wear-ring and inter-stage seal.(3)The radial and axial forces of the impeller are studied based on the transient numerical simulation.The results show that the radial force of the impeller varies in cycles under different flow conditions.The maximum value of the radial force of the impeller is the minimum under the design flow condition,and the maximum value of the radial force is larger as the deviation from the designed flow condition.The radial force of the primary impeller is less than that of the secondary impeller.The radial force has constant quantity and pulsating quantity.The radial force of the first impeller is mainly pulsating quantity,while the radial force of the second impeller is mainly constant quantity.With the increase of flow rate,the axial force of the first stage impeller decreases and the fluctuation decreases.(4)The calculation of critical speed and modal analysis of the rotor system were carried out by using the SAMCEF rotor dynamics software.It is found that there is not large difference between the critical speed of the one-dimensional beam and the three-dimensional solid model.With the increase of the order,the vortex frequency of the rotor increases accordingly.The first four modes are bending deformations.There is no resonance when the rotor works on the designed speed.The influence of bearing support stiffness,gyro moment and the sealing force on critical speed was analyzed.It is found that increasing support stiffness and considering gyro moment and the sealing force will increase the forward eddy frequency of rotor.(5)The transient response analysis of the rotor system was carried out to study the influence of the unbalanced mass and the different starting time on the rotor system in the dry state.It is found that the radial displacement of the second stage impeller which is far away from the bearing position is the largest.When considering the influence of fluid excitation force on the transient response of the rotor system,the unbalanced mass has little effect on the transient response in the wet state,the fluid excitation force has a great influence.The rotor system has the smallest vibration under the designed flow condition and the largest under the small flow condition.