Study On The Shields Clearance Flow And Its Rotordynamic Effects For Large Canned Motor Reactor Coolant Pump

Canned motor reactor coolant pump(canned motor RCP)is one of the key parts of the reactor coolant system(RCS)in the AP series pressurized water reactor system.Canned motor RCP should guarantee long term and continuous operation in RCS.The continuous stable operation of canned motor RCP is extremely important to the safety of nuclear reactor system.In order to achieve the safety requirement,accurate analysis and evaluation of the rotordynamic characteristics of canned motor RCP in its design phase is mandatory.Canned motor RCP is designed to be a vertical pump system.The whole rotor is immersed in the liquid reactor coolant.It is the fluid in between the clearance of the stator and rotor shields(or metal cans)that dramatically affects the rotordynamics of canned motor RCP.Besides,accurate rotordynamic analysis is the basis of bearing design and service time assessment.A systematic study on the clearance flow in canned motor RCP is needed to accurately predict the rotordynamic characteristics of the pump.The current study takes the clearance flow inside the space of the long narrow gap between the stator and rotor shields in canned motor RCP as the primary subject.The study includes 1)the rotordynamic coefficient analysis of clearance flow with three dimensional high level turbulence and 2)the rotordynamics of canned motor RCP.The equivalent force acting on the pump rotor and the associated rotordynamic coefficients of clearance flow will be investigated through theoretical model and numerical analysis.Several factors including inlet pre-swirl,axial flow and axial length are studied.An experimental test rig is made according to the clearance flow in canned motor RCP.The validity of the theoretical and numerical models are tested by experiments.The rotordynamic characteristics of canned motor RCP are evaluated by numerical methods based on the theoretical and numerical models for clearance flow.The detailed contents are as follows:(1)The governing equations of clearance flow in three dimensional cylindrical coordinate system are adopted.The moving reference frame method and turbulence model are discussed.The fluid induced force of the clearance flow is modeled by regression method.Finally,the calculation method of the rotordynamic coefficients is described.(2)Numerical methods are discussed for solving the governing equations and further for obtaining the rotordynamic coefficients of the clearance flow.A coordinate transformation is implemented between the 3 dimensional cylindrical coordinate system and a general curvilinear coordinate system to cope with the whirl motion of the rotor.A finite volume method considering coordinate transformation and a parameter driven auto grid generator are developed.With the coordinate transformation borne in mind,specific methods for dealing with velocity-pressure coupling,gradient reconstruction and implementation of boundary conditions are derived.Certain parallelization method is explored to handle the possible scalability issue which may be faced later in the research.A specific purpose code for clearance flow simulation is developed.In order to validate the software,the numerical results are compared with the data obtained both by general purpose commercial computational fluid dynamics software(CFD)and experimental studies.(3)The rotordynamic coefficients of clearance flow are studied through two major aspects.One is the inlet pre-swirl,the other is the mechanism of the negative direct stiffness.In the study relating the inlet pre-swirl,the numerical treatment of special inlet boundary is discussed.The effects of inlet pre-swirl are investigated by subsequent numerical simulations.In the study considering the negative direct stiffness,efforts are made to explain the mechanism of negative stiffness and its key factors.The negative stiffness phenomenon is studied by its relationship with axial flow and axial length of the clearance flow.(4)To validate the results of the theoretical analysis and numerical simulations,an experiment test rig is constructed.The test rig is designed based on the key geometry parameters of canned motor RCP.It is the first clearance flow test rig around the world that possesses a vertical rotor supported by two bearing.Fluid induced force and associated rotordynamic coefficients of the clearance flow are measured on this test rig with various working conditions.(5)The inlet pre-swirl level inside canned motor RCP is obtained by 3 dimensional simulations on the complex internal flow channels of canned motor RCP.The rotordynamic coefficients of the clearance flow in canned motor RCP are calculated based on the developed models including the boundary treatment of the inlet preswirl.The effects of clearance flow on the rotor system of canned motor RCP are further studied by numerical method.The following work has been done in the current work.A 3 dimensional numerical model for clearance flow and theoretical model for predicting the direct stiffness sign are developed through the study of the clearance flow in canned motor RCP.Numerical analysis shows that the rotordynamic coefficients are effected by inlet pre-swirl.The cross-coupled stiffness coefficient becomes larger as the inlet pre-swirl level goes up,decreasing the stability of the overall system.The clearance flow tends to produce negative direct stiffness if the axial flow is relatively weak or the axial length is large.The direct stiffness of canned motor RCP is proved to be negative both by numerical simulation and theoretical analysis.The calculated first critical speed of the model including clearance flow is lower than that without clearance flow,as the consequence of the negative direct stiffness and the enormous mass coefficient brought by the clearance flow.It is showed by the present study that clearance flow could bring significant influences to the rotordynamic characteristics of canned motor RCP.The results obtained by the current work could be considered as the basis for the future design of canned motor RCPs.