Flow-Visualization Experiment And Large Eddy Simulation Of Cross-Flow Mixing In A Wire-Wrapped Rod Bundle

Liquid metal reactors have the advantages of high thermal efficiency,strong safety,and high energy density,and are a new generation of nuclear energy systems that are being developed domestically and internationally.Liquid metal reactors generally use helical wires as positioning devices,which not only fix the fuel elements but also promote the lateral mixing of the coolant,thereby enhancing the heat transfer efficiency between the cladding and the coolant,reducing the cladding temperature,and helping to improve the safety and economy of the reactor.The intensity of lateral mixing of the coolant inside the wire-wrapped assembly is related to factors such as the rod bundle pitch-to-diameter ratio,wire pitch,wire azimuth angle,subchannel type,and Reynolds number,and the phenomenon is quite complex.This paper combines visualization experiments and large eddy simulations to study the internal flow characteristics of wire-wrapped bundles and analyzes the effect of the above factors on the lateral flow in wire-wrapped bundles,and obtains a lateral mixing model between subchannels based on experimental data and large eddy simulation results.First,based on the similarity theory,the dimensionless numbers for the flow of coolant inside the experimental bundle and the prototype fuel assembly are obtained,verifying the feasibility of using water as a substitute for liquid metal in conducting visualization experiments.Then,using visualization technology to measure the lateral flow characteristics of the wire-wrapped bundle’s horizontal cross-section as a whole and locally,the flow characteristics of different Reynolds numbers and wire azimuth angles are analyzed and compared.The experimental results show that a clockwise overall circulation can be observed in the external area of the bundle,and the maximum flow velocity is located in the upper area of the cross-section,gradually decreasing along the direction of circulation.In addition,there are several complex vortex flows and secondary flows on the cross-section,with large-scale vortex flows mainly appearing in the external area of the bundle and small-scale vortex flows generally existing near the wires,and the location of the vortex flows is closely related to the azimuth angle between the wires and the rods.Under different Reynolds number conditions,the lateral flow characteristics are similar,and the magnitude of the lateral flow velocity increases linearly with the Reynolds number.Based on the experimental research,this paper uses the large eddy simulation method to numerically calculate the lateral flow characteristics of the wire-wrapped bundle.First,using the visualization experimental data,Kolmogorov theory analysis,and turbulence kinetic energy solution ratio to verify the accuracy of the large eddy simulation results.Secondly,numerical simulation research is carried out on the lateral flow characteristics of the bundle under different Reynolds numbers and wire pitch conditions.The results of the large eddy simulation calculation show that the average velocity distribution characteristics of the same cross-section under different Reynolds numbers are roughly the same,with the average velocity outside the bundle being higher than that inside the bundle;in terms of the influence of wire pitch,the lateral flow velocity increases with the decrease of the wire pitch.The dimensionless lateral flow velocity between different subchannel gaps has a sinusoidal function distribution along the axial direction,and its distribution law is closely related to the subchannel type and wire pitch,but is less affected by the Reynolds number.Based on the results of the large eddy simulation,a lateral mixing model between subchannels in the wirewrapped bundle is established,and the comparison analysis with experimental measurement results shows that the established lateral mixing model can accurately predict the lateral mixing between subchannels in the wire-wrapped bundle.