Study On Flow And Heat Transfer Characteristics Of Supercritical Carbon Dioxide Inside Helical Cruciform Fuel Assembly Channel

As a coolant with good performance,supercritical carbon dioxide（S-CO₂）has a very broad application prospect.It can be used in nuclear power generation system to realize the miniaturization of energy conversion system.It is one of the important research directions of the fourth generation advanced reactor and mobile small reactor.Compared with traditional fuels,helical cruciform fuel has larger surface-to-volume ratio,improved coolant radial mixing characteristics and shorter radial heat conduction path in fuel pellets,which can operate at higher power density.The research on the flow and heat transfer characteristics of helical cruciform fuel assembly with S-CO₂ as coolant is of great significance for the core design and safety analysis of subminiature nuclear power system for the special applications.However,at the pseudocritical point,the thermophysical properties of S-CO₂ change dramatically with temperature,and the flow and heat transfer characteristics also change.There are differences in the calculation results of S-CO₂ in the existing turbulence models,and there is a lack of a complete turbulence model suitable for supercritical fluids.Therefore,this paper first studies the S-CO₂ turbulent heat transfer model.Based on the experimental data of S-CO₂ flow and heat transfer in a vertical circular tube,this paper verifies the accuracy of the numerical model.It is found that modifying the turbulent Prandtl number based on the existing turbulence model can improve the modeling of turbulence generation and diffusion,and the low Reynolds number SST k-ωturbulence model modified by Kays turbulent Prandtl number relationship can better simulate the flow and heat transfer characteristics of S-CO₂ in the vertical direction,and provide a reliable numerical calculation model for the subsequent flow and heat transfer research of the helical cruciform fuel assembly.Secondly,the mechanism of spiral flow field inhibiting the deterioration of S-CO₂ heat transfer and enhancing turbulent heat transfer is studied.It is a common passive heat transfer enhancement technology to place spiral blades and other swirl elements in the flow field,the helical cruciform fuel rod forms a spiral flow field by its own torsional structure.Because the research of helical cruciform fuel assembly with S-CO₂ as coolant is little and lack of relevant experimental comparison,firstly,with the help of the simplest swirl element-helical twisted tape,the influence of swirl element with periodic helical characteristics on S-CO₂ flow and heat transfer is analyzed.The addition of spiral twisted tape in the circular tube can effectively flatten the local temperature peak caused by the deterioration of S-CO₂ heat transfer.It is evaluated by the relationship of buoyancy effect and flow acceleration effect,and the influence of geometric factors of spiral twisted tape is explored.By comparing the physical fields before and after the addition of the spiral twisted tape,it is found that the spiral flow field affects the flow state in the boundary layer of the wall area,which can reduce the thickness of the viscous sublayer,increase the thickness of the overall turbulent boundary layer,enhance the turbulence intensity near the wall.As a result,strengthening the heat transfer between the wall area and the mainstream area,and realizing the role of enhancing heat transfer.Finally,the flow and heat transfer characteristics of helical cruciform fuel assembly are studied.Based on the new structured meshing method of the coolant channel of the helical cruciform fuel assembly,the influence of the gap between fuel rods on the flow field is analyzed.Taking the 3×3 helical cruciform fuel assembly as an example,the flow and heat transfer characteristics of S-CO₂ flowing through the fuel assembly are studied.In the helical cruciform fuel assembly,the transverse flow is concentrated around the elbow between the two blades of the fuel rod.Most of the transverse flow converges to the windward side of the blade,and the rest flows to the adjacent rods.When the two transverse flows pass through the central gap in the opposite direction at the same time,the transverse mixing efficiency reaches the highest.The transverse flow makes the temperature distribution of the coolant in the fuel assembly more uniform and effectively improves the heat transfer.Compared with the circular fuel rod wrapped with metal wire,the hot spot factor and the fluctuation range are smaller.The maximum ratio of local heat flux to average heat flux is less than 2.0,and there will be no phase change during S-CO₂ heating,which can avoid the early boiling crisis.The center temperature of coolant and fuel rod,resistance coefficient and Nusselt number all show an obvious periodic fluctuation trend with the periodic torsion structure of helical cruciform fuel rod.The relevant research carried out in this paper can provide reference for the core design and safety analysis of subminiature nuclear power system.