中文摘要：

在球床高温气冷堆中，堆芯燃料球采取随机布置，冷却剂流径复杂，引起局部冷却不均匀，导致燃料球表面形成局部热点，这些热点是导致元件球失效的潜在因素。研究球床中的燃料球表面放热分布和温度分布的不均匀性，对燃料球的合理设计和反应堆的安全性具有重要意义。本文采用CFD方法对单球和球床代表单元进行了数值模拟，研究了单球及不同排列型式球床燃料球表面的放热系数和热点分布。结果表明，单球的放热强度主要受边界层发展的影响，在分离点处，局部Ⅳu最小，温度最高，球表面温度的不均匀程度为42K。球床内部燃料球的放热强度主要受冷却剂交混扰动的影响，在相邻球的接触区域，局部Ⅳu最小，温度最高，球表面温度的不均匀程度为180K。球床的不同排列型式对燃料球热点及放热分布有较大影响。

英文摘要：

In the reactor core of a pebble bed HTGR, fuel pebbles are in random arrangement. The coolant flow passages are complicated, which results in non-uniform local heat removal and local hot spots on the pebble surface. This may be potential factors of fuel pebble failure. Analysis of the non-uniformity of heat transfer and temperature distribution is important for reasonable thermal design of the fuel element and safety of HTGR. In this paper, the flow and heat transfer over a single sphere and typical representative elements of the pebble bed were numerically simulated. Heat transfer coefficients and hot spots on the surface of a single sphere and fuel pebbles in different structure of pebble beds were investigated. The results showed that, heat transfer coefficient distribution over the surface of a single sphere is mainly influenced by the process of boundary layer growth. Minimum local Nusselt number and maximum temperature both appear at the flow separation points. The difference between maximum and minimum temperature of a single sphere surface is 42 K. Heat transfer in pebble beds is mainly governed by flow disturbance and mixing in the gaps among spheres. Minimum local Nusselt number and maximum temperature of a pebble in a pebble bed both appear at contacts between spheres. The difference between maximum and minimum temperatures of the surface of a pebble in a pebble bed is 180 K. Hot spot and heat transfer of pebbles in a pebble bed are influenced by the configuration that how the spheres are packed.