Numerical Study On The Flow Field Characteristics Of Gap Vortex Street In The Rod Beam Channel Of A Small Gap-to-diameter Ratio Reactor

The rod beam structure is widely used in nuclear reactors and steam generators and other key equipment of nuclear energy engineering,and its fully developed axial turbulent flow characteristics are of great reference value for the thermal hydraulic design of nuclear reactor cores,safety analysis and numerical research methods.In recent years MIT has proposed a ring fuel for advanced pressurized water reactors that can be cooled both internally and externally.which can lead to large power gains,while the space between the fuel rods is compressed because the outer diameter of the dual-cooled ring fuel rods is larger than the current fuel rod diameter.Existing studies have shown that the gap-to-diameter ratio(the ratio of gap distance to diameter P/D)of a bar beam channel decreases to a certain threshold value to produce a special turbulence phenomenon:large scale,quasi-periodic flow pulsations with vortex streets.known as gap vortex streets.The presence of gap vortex structures in the bar channel can greatly enhance the heat transfer efficiency of the fluid in the channel,but at the same time the presence of these gap vortexes can also induce vortex vibration phenomena.Most of the existing studies are focused on single flow analysis,and there is a lack of detailed research on the problem of vortex vibration caused by the gap vortex street flow field and the effect of heat transfer efficiency.In this paper,a variety of basic bar beam channel models are selected as research objects,and the formation conditions of the gap vortex structure in the bar beam channel,the vortex vibration mechanism and its effect on the heat transfer efficiency in the channel are studied comprehensively,which will provide important references for the future thermal hydraulic research and structural optimization design of the actual reactor core.In this paper,the simplest structure of the rectangular channel model with a single bar is used as the object of study,and the effects of different numerical solution schemes on the calculation results are evaluated by comparing with classical experimental data to determine the optimal numerical method for studying the interstitial vortex street flow field,and the results show that the RSM model combined with the enhanced wall function can well predict the flow characteristics of the interstitial vortex street flow field.The method was then applied to investigate the effect of the wall gap diameter ratio(the ratio between the distance between the upper surface of the fuel rod and the wall and the diameter of the fuel rod,W/D)on the formation of the vortex structure and the turbulent mixing efficiency.The mechanism of the vortex vibration induced by the vortex structure is also investigated in more depth.In the rectangular channel model with a single rod,the interstitial vortex structure gradually disappears with the increase of W/D value.At the same time,in the narrow slit region of the rectangular channel.the appearance of the interstitial vortex structure intensifies the flow mixing between the narrow slit and the two sides of the fuel rod.The alternating action of the interstitial vortex street structures located on both sides of the fuel rod results in alternating high and low pressure regions near the fuel rod,which eventually leads to periodic changes in the load applied to the fuel rod surface.Considering that the rectangular channel model containing a single bar is not a good study of the general laws in the bar beam channel,the four-channel model is chosen as the next object of study.Firstly,by comparing with the classical experimental data,the influence of different turbulence models on the accuracy of convective heat transfer calculation results is analyzed,and a reliable numerical method is also selected for the subsequent calculation,and the reliability of the two-way fluid-solid coupling algorithm is also verified.After that,the effect of the gap-to-diameter ratio(P/D)on the formation of the gap vortex structure and the flow characteristics in the channel is discussed in detail in a four-channel model,based on which the vortex vibration characteristics of the fuel rod and the convective heat transfer characteristics in the channel under different operating conditions are analyzed.When a stable interstitial vortex street exists in the channel,a stable micro-amplitude vibration of the fuel rod will appear,and when the interstitial vortex street structure disappears,the micro-amplitude vibration of the fuel rod will also disappear.Meanwhile,when a stable gap vortex street exists in the channel,it can significantly improve the heat transfer efficiency of the channel,which is due to the presence of the vortex street structure accelerates the fluid mixing and causes the heat at the narrow slit to spread to both sides,resulting in a uniform temperature distribution.Based on the above research results,the 3×3 small-scale rod beam channel in a restricted space is finally investigated.The flow field characteristics,heat transfer characteristics and fuel rod vibration patterns in the sub-channels of the rod bundle at different locations are studied.In terms of flow,the differences of flow characteristics in the sub-channels of the rod bundle at different locations under different operating conditions are studied,and the effects of different parameter combinations on the gap vortex structure are compared.The flow field load distribution of the fuel rods at different locations in the rod bundle channel is also analyzed.In the area of vortex vibration,a numerical solution based on a self-programming solver for the bi-directional fluid-structure coupling is introduced,and the reliability of the solution is verified and vortex vibration analysis is performed for typical operating conditions.Finally,the convective heat transfer characteristics under different operating conditions are analyzed in detail,focusing on the heat transfer efficiency in the sub-channels of the bar beam at different positions and the influence of different parameters on the heat transfer characteristics.From the Nu number distribution,it can be seen that the heat transfer efficiency of the bar beam subchannel located in the middle of the channel is significantly better than that of the sub-channel near the wall,and the temperature field uniformity of the bar beam channel with gap vortex structure is significantly better than that of the channel without gap vortex.