Thermal Design Of Composite Moderator At High Beam Power

The moderator is one of the important large-scale infrastructures of China’s spallation neutron source.With the establishment of the southern light source and the continuous improvement of the beam power of the secondary target station,the accumulated heat load and extreme working conditions will inevitably affect the overall structure of the moderator and the internal flow and heat transfer process.As the most commonly used slow chemical substance,liquid hydrogen has always been in a supercritical state under normal working conditions.Its drastically changing thermophysical properties and unique moderation properties make its heat transfer law particularly complex when flowing in the moderator.Therefore,on the basis of exploring the flow and heat transfer characteristics of supercritical liquid hydrogen in the traditional U-tube moderator,the optimization and thermal design of the later moderator are discussed and studied.Firstly,based on the physical and mathematical modeling of the circular U-tube moderator in the first-stage neutron source target station,the three-dimensional turbulence and heat transfer process of supercritical liquid hydrogen in the U-tube were studied.Finite volume method and mesh independence verified by grid convergence index(GCI).The calculation results of three turbulence models(SST k-w,RNG k-ε,Standard k-ε)are compared with the experimental data to verify the reliability of the model.The results show that in the process of supercritical liquid hydrogen flow,the heat transfer of the elbow part(α=0°～180°)of the U-tube is strengthened,and the heat transfer at the outlet of the elbow is deteriorated.In the elbow section,when the Reynolds number and heat flux are in the range of 27375～54750 and 80k W/m~2～100k W/m~2,respectively,it is found that the influence of Reynolds number on the change of Nusselt number is more significant.When the direction of gravity is consistent with the direction of liquid hydrogen flow(g_x),the Nusselt number of liquid hydrogen in the channel is the highest.It can be concluded that the heat transfer capacity of liquid hydrogen along the x-axis direction is the strongest,followed by the y-axis direction,z-axis direction is the weakest.Secondly,in order to meet the construction requirements of the neutron source two-phase target station,the model is changed on the basis of the one-phase moderator,the tube section is changed from circular to triangular structure,and the heat transfer characteristics of supercritical liquid hydrogen in the moderator with different bend radius ratios are compared and studied.The wall temperature,fluid temperature and Nusselt number along the triangular U-tube under different heat flux and Reynolds number conditions were compared.The results show that the wall temperature,fluid temperature and Nusselt number increase with the increase of heat flux and Reynolds number,and the Nusselt number changes the most.At the same time,in order to take into account the working conditions of the moderator,we discuss the flow and heat transfer law of the moderator under different extreme conditions.It is found that when the thermal load is stable at about 19K-20K,the overall heat transfer capacity is the strongest,which verifies that the neutron source selects 100W as the initial operating condition of the second phase target station.Finally,in order to better assist the optimization design of the above moderator,the decoupling liquid hydrogen moderator was also optimized and upgraded.According to the irradiation distance of the gadolinium plate,four different DMPH models were constructed,and the internal flow and heat transfer were studied under the requirement of the second-stage beam power upgrade to500k W.Then,under different working conditions,the heat transfer capacity of four different DMPH was compared and analyzed.The results show that the greater the irradiation distance of the gadolinium plate,the more uniform the temperature distribution in the cavity,the more conducive to overall heat transfer.When the irradiation distance of the gadolinium plate is 9.5mm,the heat transfer coefficient is the highest and the performance is the best.