Research On The Mixing And Model Of Coolant In The Rod Bundle With Spacer Grid

In the open fuel assembly of reactor,the coolant of adjacent subchannels has transverse mixing of mass,momentum and energy during the flow process.For the rod bundle with spacer grid,the mixing caused by the geometry of mixing vane of spacer grid is more intense.The transverse mixing between subchannels causes the mass flux of each subchannel to change continuously along the axial direction,which is beneficial to flattening the temperature distribution in the rod bundle and improving the safety margin of the reactor.Therefore,study the flow mixing characteristics in the rod bundle with spacer grid has very important significance for the safety and economy of nuclear reactor thermal-hydraulic analysis.At present,the subchannel analysis method is mainly used in the thermal-hydraulic analysis of rod bundle in reactor,wihle the calculation accuracy is limited by the degree of perfection of the subchannel mixing model,and the high cost and less measurement data for modeling through experimental study.Therefore,in this paper,considering the characteristics of detailed flow field information obtained by numerical simulation,the combination of numerical simulation and subchannel analysis is used to establish the mixing model and coupled to the subchannel analysis model to carry out thermal-hydraulic characteristics analysis of rod bundle with spacer grid.In this paper,the flow field conservation equation of thermal-hydraulic analysis is established based on subchannel analysis method and the two-phase flow theory model is introduced to complete the subchannel analysis model,and the equation is solved by numerical discretization method.Considering the influence of the geometry structure of spacer grid on the interchannel mixing,the geometrical model of spacer grid with different structural was established and detailed three-dimensional flow field data in the rod bundle with spacer grid was obtained by numerical simulation for establishing mixing model.The method of solving the mixing speed between adjacent subchannels is derived by combining the time-average parameters and the turbulent pulsation parameters of the flow field.Meanwhile,considering the different mixing mechanism of upstream and downstream grids and the different degree of mixing between different subchannels,an anisotropic mixing model is established along the axial flow direction based on the matrix theory and coupled to the subchannel analysis model.By comparing the anisotropic mixing model and the isotropic mixing model with the experimental data,it is found that the anisotropic mixing model and the isotropic mixing model are in good agreement with the experimental data in the rod bundle without spacer grid,but the anisotropic mixing model can better match the experimental data in the rod bundle with spacer grid.Thus the reliability and applicability of the anisotropic mixing model in the rod bundle with spacer grid are verified.The subchannel analysis of single-span rod bundle with spacer grid is carried out for isotropic mixing model and anisotropic mixing model,it is found that the subchannel thermal parameters distribution of the two mixing models is the same in the upstream of the grid,while the obvious difference has appeared downstream of the grid.The anisotropic mixing model results in different mass flux distributions of subchannels with the same cross-sectional area,more uniform temperature distribution between subchannels,and an increase of DNBR on the surface of fuel rod.It is indicated that the anisotropic mixing model fully considers the mixing mechanism of different flow regions along the axial direction and the anisotropic mixing between different subchannels,and due to the forced mixing caused by spacer grid,the temperature distribution between the subchannels is flattened and the safety margin of the thermal-hydraulic analysis is improved.The subchannel analysis of the rod bundle with spacer grid of different structures was carried out,including with or without mixing vane and the different arrangement of the mixing vane.Based on the subchannel analysis of rod bundle with or without mixing vane,it is found that the forced mixing effect between subchannels is mainly caused by the mixing vane.The flow field and temperature field between sub-channels are changed continuously,and the DNBR in rod bundle is improved.Based on the subchannel analysis of rod bundles with different mixing vanes,it is found that the influence of different mixing vanes fins on the flow field is small.Type A spacer grid has a stronger mixing on the temperature than type B spacer grid,which makes the temperature distribution between the subchannels more uniform and improved DNBR.The anisotropic mixing model in the rod bundle with spacer grid is proposed based on the numerical simulation method in this paper,and the applicability and reliability of the model are verified by analyzing the thermal-hydraulic characteristics from different angles,and the accuracy of subchannel thermal-hydraulic analysis is improved.Meanwhile,the subchannel analysis of rod bundle with spacer grid of different structures is carried out to provide guidance for the optimization and design of spacer grid.