Investigation On The Effect Of Mixing Vane Spacer Grid On Flow Transition Mechanisms And Heat Transfer Characteristics In Pressurized Water Reactor

The mixing vane is one of the important structures on the spacer grid of the PWR fuel assembly.After the fluid passes through the grid mixing vanes,there will be a secondary flow around the rod bundle formed in the sub-channel.It has a significant effect on strengthening single-phase flow heat transfer and improving critical heat flux.With the rapid development of computational fluid dynamics numerical simulation,it is more and more applied to the analysis of reactor thermal-hydraulic design,especially in the optimization of spacer grid design of fuel assemblies.The application of detailed and large-scale CFD simulation of is one of the current research focuses on reactor thermalhydraulic problems.In this paper,both of experimental and numerical methods were used to investigate the influence of spacer grid mixing vanes on single-phase flow heat transfer and subcooled boiling flow heat transfer in rod bundle channels,fuel assemblies and reactor pressure vessels.The main contents included: PIV experimental study of single-phase flow downstream different mixing vanes;A single-phase CFD simulation study of the effect of mixing vanes on the secondary flow and heat transfer enhancement in the rod bundle channel;Eulerian two-phase numerical simulation study of the effect of mixing vanes on CHF in departure from nucleate boiling.Research on refined CFD models of complete fuel assemblies and large-scale CFD models of reactor pressure vessels.A high Reynolds number water circulation flow system was established.A series of separate and split mixing vanes were printed by 3D printing.The PIV experimental study of turbulence flow in rod bundle sub-channels under different mixing vanes was carried out.The effects of Reynolds number,bending angle and type of mixing vanes on the distribution and evolution characteristics of downstream lateral flow field(secondary flow),longitudinal flow field and flow resistance characteristics were obtained.At the same time,lots of flow field detail data for the calibration and verification of fuel assembly CFD analysis model were obtained.In single-phase CFD simulation of flow and heat transfer in the rod bundle channel,a series of single subchannel(experimental section model)and 5×5 rod bundle channel was established.Firstly,the accuracy of CFD model was verified by PIV experimental results.Secondly,combined with numerical simulation analysis,the formation and evolution mechanism of vortex flow structure downstream separate and split mixing vanes were revealed.The influence of separated mixing vane structural characteristics on the downstream flow field distribution was studied.Finally,based on the comparative analysis of the secondary flow and heat transfer intensity on the fuel rod surface,the influence of the mixing vane bending angle on the heat transfer characteristics on the fuel rod local surface was summarized.The theory of local flow and heat transfer zoning on the fuel rod surface downstream of the grid was proposed,which provided a reference basis for the design and optimization of the mixing vanes bending angle.In two-phase numerical simulation study of the mixing vane effect on CHF in departure from nucleate boiling,a Eulerian two-phase flow boiling CFD model of a 5×5rod bundle channel with spacer grid was established.A method for judging the CHF based on the boiling curve of numerical simulation was proposed.The accuracy of the interphase interaction model,the boiling sub-model,and the feasibility of using the boiling curve to determine the CHF method were verified by typical expermental results.The influence of the type and the bending angle of mixing vanes on the cavitation distribution was explored,which provided a theoretical basis for the structural optimization of the fuel assembly in the direction of increasing the CHF.In the study of the complete fuel assembly,a refined CFD mesh model of a typical17×17 fuel rods PWR fuel assembly was established.Structural details such as springs,dimples and mixing vanes on the spacer grid were preserved.The mixed method of polyhedral mesh and extruded volume mesh was adopted to divide the whole fuel assembly mesh.The number of the mesh was about 190 million.Through the numerical simulation analysis of the flow heat transfer in the whole fuel assembly under typical working conditions,the influence of the mixing vane on the flow and heat transfer in different sub-channels was revealed.The influence of characteristic structures in the fuel assembly,such as mixing vanes,control rods and intermediate mixing grids,on the flow heat transfer distribution in the overall fuel assembly were obtained,which provided a comprehensive data basis for the thermal-hydraulic design of the fuel assembly.In the large-scale and refined CFD model study of the reactor pressure vessel,the structure of the mixing vanes was preserved.By introducing a local resistance coefficient correction model to simplify the springs and dimples on the spacer grid,a simplified mesh of a single fuel assembly that can accurately reflect the flow and heat transfer characteristics of the fuel assembly was realized.A CFD model for the overall flow and heat transfer analysis of the “Hualong No.1” pressure vessel containing 177 sets of fuel assemblies in the entire core was established through the core fuel assembly partition strategy.The number of mesh was about 2 billion and the large-scale parallel computing laws were explored.This is the first simulation of large-scale thermal-hydraulic numerical simulation that achieves fine mesh division of all structural components in the reactor pressure vessel.The research results of this paper have laid a solid foundation for the construction of thermal-hydraulic digital reactors.