Multi-scale Coupling Study Of Natural Circulation Flow Characteristics Of Petal-shaped Rod Bundle

As a high-performance fuel element,petal-shaped fuel rods have a unique spiral structure that increases the heat transfer area compared to traditional cylindrical rods,which can significantly increase the power density of the reactor and has promising applications in small modular reactors.The petal-shaped fuel rods are used in reactor systems that require consideration of the passive safety characteristics during normal or accidental shutdown,and the natural circulation passive safety technology can further enhance the inherent safety of the reactor system.Studies on petal-shaped rod bundle have mainly focused on forced flow conditions.In contrast,the flow heat transfer characteristics of the petal-shaped rod bundle under natural circulation have not been paid much attention.Therefore,based on the multi-scale coupling method,this paper couples a one-dimensional natural circulation system with a three-dimensional petal-shaped rod channel,and investigates the characteristics of the natural circulation system containing a petal-shaped rod channel and the complex flow heat transfer characteristics in the rod bundle.At the same time,the system characteristics and the flow heat transfer characteristics in the channel are compared between the petal-shaped fuel rods and the cylindrical rods after the application of the two different shapes to the natural circulation system.Finally,a structure correction factor and a viscosity correction factor are introduced to correct the correlation equation for the drag coefficient of petal-shaped rod bundle channel.In this paper,the convergence performance of the multi-scale coupling method based on the fixed iterative step data interaction strategy is analyzed.To address the problems of the fixed iterative step data interaction strategy,an adaptive threshold function based on the historical interaction information is established from the perspectives of data transfer strategy and convergence capability,and the natural circulation flow rate update strategy and interaction flow rate dichotomy strategy are proposed.And the method in this paper is compared and analyzed with the fixed iterative step method.The results show that the coupling convergence speed of this paper’s method is improved by more than 78%,and the convergence speed and convergence capacity are not affected by the initial flow velocity.By constructing a natural circulation system containing a petal-shaped rod bundle channel,we investigate the influence of operating parameters on the characteristics of the single-phase natural circulation system and the flow characteristics in the channel.The results show that:the natural circulation mass flow-power satisfies the G～Q^1/2 relationship;the natural circulation mass flow increases with the increase of heating power and inlet temperature;the natural circulation capacity increases by 27.9%when the height difference between the hot and cold sources increases from 2.05 m to 3.05 m;the decrease of fluid viscosity in the assembly leads to the decrease of drag coefficient under the same Re.Petal-shaped fuel rods with different pitches are established,and the petal-shaped rod bundle channel is designed as a cylindrical rod bundle channel by equalizing the fuel rod volume and hydraulic diameter.The flow heat transfer characteristics in the rod channel and the characteristics of the natural circulation system were investigated after the change of the fuel rod pitch and shape.The study shows that the natural circulation mass flow increases by 0.85%when the pitch is increased from 250 mm to 750 mm and by 0.17%when the pitch is increased to 1250mm;the secondary flow rate in the channel decreases and the resistance coefficient and temperature distribution increases;the mass flow of the natural circulation system with petal-shaped rod channel is larger than that with round-tied rod channel;the flow heat transfer characteristics in the channel with the same Re are compared:The secondary flow rate in the petal-shaped rod bundle is higher,the axial Nu number is higher and the degree of inhomogeneity is greater;on the basis of the existing correlation equation for the drag coefficient of petal-shaped rod bundle channel,the flow resistance correlation equation for the petal-shaped rod bundle channel in a wide range of Re is obtained by fitting,and the maximum error is found to be between-6%and 7%after comparing the simulated and experimental values.