Calculation And Analysis On Critical Rotational Speeds Of The Centrifugal Pump Rotors

Nowadays,the power and speed of the pump rotor is higher and higher,especially in the multistage centrifugal pump rotors.To avoid resonance when the operating speed approaches the critical one and ensure the stable running of the multistage centrifugal pump rotors,the critical rotational speeds must be calculated accurately in "dry" and "wet" state respectively in the design course.The common algorithms to get the critical rotational speeds of the rotors are FEM(Finite Element Method) and Transfer Matrix Method.Here,the FEM is employed in the calculation.In this paper,the main model is a modified multistage centrifugal water pump. The three-dimensional flow through the whole flow passage model of the multistage pump is simulated using CFD technology.The influence of the grid number on the simulation results is studied by comparison.The predicted performance of the multistage pump is obtained through the numerical results.Some important flow phenomena are got.The velocity and pressure distributions within the impellers, guide vanes,et al.are analyzed so that the later calculation on the critical speed can be continued.To the critical rotational speeds of the rotor on complete immersion,it is necessary to considering the fluid-solid coupling reaction.The entity model of the multistage centrifugal pump rotor is built.Then the static and modal analysis are finished,so that the influence of the fluid-solid coupling reaction and spin softening on the critical rotational speeds calculation can be discussed based on the model. Furthermore,the first order critical rotational speed of the rotor with rigid support is got separately using Dunkerley Method and FEM by comparison.Also,the influence of the bearing span,the number of the impeller and supporting stiffness on the natural frequency of the rotor is analyzed respectively using the finite element model consisted of beam,mass and spring.In addition,the "wet" critical rotational speed of the rotor is calculated.The results show that:1) The numerical simulation results agree well with the experimental ones,and the flow states in the multistage centrifugal pump can be revealed correctly.It proved that the simulation method which is adopted in this paper is feasible.2) Generally,the rotor's critical rotational speeds are descresed by the fluid-solid coupling reaction and spin softening.3) The supporting stiffness has great influence on the rotor's critical rotational speed.The preconditions of the calculation are not only simplifying the supporting system accurately but also determining the stiffness and damping matrix reasonably.4) There are great differences between the "dry" critical rotational speeds and the "wet" ones for the multistage pump rotor.In summary,the calculation of the "wet" critical rotational speeds in multistage centrifugal pump rotors is a complicated process.We must take the knowledge which refers to the classic rotor dynamics,hydrodynamic lubrication theory and seal dynamics into account synthetically.The research in this paper will provide evidence for the structural checking of the multistage centrifugal pumps.