Study On Heat Transfer Characteristics Of Pebbles In Three Stacking Modes Of High Temperature Gas-Cooled Reactor

High Temperature Gas-cooled Reactor(HTGR)is the fourth generation nuclear reactor.Its core is a complex pebble bed structure formed by the accumulation of graphite pebbles.The stacking mode is a mixture of simple cubic(SC),body centered cubic(BCC),and face centered cubic(FCC).The different stacking methods directly affect the distribution of core temperature in High-Temperature Gas-cooled Reactors.Therefore,conducting computational research on the heat transfer characteristics of the three pebble stacking methods is of great significance.For pebbles with different stacking methods,the main research is on the local hot spot of the pebble or the change law of the heat transfer coefficient with Reynolds number.The pebble stacking model established is generally small.There is a lack of the study on the change law of the heat transfer coefficient with the pebble layer and the corresponding helium flow characteristics.In real cores,the physical parameters of helium gas change with temperature and pressure,which leads to changes in the flow characteristics of helium gas as it continuously flows through the pebble layers.Therefore,it is necessary to establish pebble models with different number of pebbles and conduct more detailed research on the heat transfer characteristics of the pebbles in the three stacking methods.This article takes the core of the High Temperature Gas-Cooled Reactor Pebblebed Modular as the research object.Firstly,establishing pebble models of three regular stacking with multiple pebble layers,and the reliability of the numerical calculation method was verified through comparison with experiments.Calculating and analyzing the variation of the heat transfer coefficient with the pebble layer under three stacking modes.The results show that the heat transfer coefficient of the three stacking modes has an upward trend in the first three layers,and the heat transfer coefficient of the last layer of SC stacking and FCC stacking is different from that of the previous pebble layers.The heat transfer coefficient of the BCC stacking shows a sudden increase that is not found in the other two types of stacks.Secondly,analyzing the helium flow field.The results show that the change of the heat transfer coefficient of the last layer in SC stacking and FCC stacking is mainly caused by the change of helium reflux.The reason for the sudden increase of the heat transfer coefficient of BCC stack is the phenomenon of helium rotation in the flow process.which starts from the ninth layer,until the thirteenth layer is stable around and continues to the end.The helium rotation phenomenon reduces the speed of the main flow area by about 4.5 m/s.which will extend the contact time and strengthen the heat transfer between helium and the pebble layer.Thus significantly increase the heat transfer coefficient of BCC stacking.Finally,conducting a transient analysis on the pebble model of BCC stacking,revealing the formation process of flow field rotation phenomenon and the influence of helium inlet boundary conditions on the helium flow field rotation phenomenon.The rotation of the helium flow field undergoes a reverse rotation before stabilizing.Reverse rotation forms and stabilizes at around 0.5 seconds until 1.8 seconds.Then the direction of rotation began to change and a final stable rotation phenomenon was formed around 2.0 seconds.At the same time,analyzing the influence of the change in helium inlet velocity on the rotation phenomenon of the helium flow field.Decreasing the inlet speed or increasing the inlet temperature will advance the starting position of the rotation phenomenon towards the inlet direction.