Non-uniform temperature distribution at horizontal pipes cross section leads to an asymmetric thermal stress. This condition would be considered as an ageing mechanism due to fatigue effect, known as thermal stratification. To evaluate this ageing mechanism, the stress analysis should be done in horizontal pipelines like surge-line connected the pressurizer to the hot leg of primary loop of NPPs by consideration of operational or transient conditions. During these conditions, the status of surge-line would be single phase or two-phase stratified flow. To estimate the equivalent stress in horizontal section of surge-line, the developed CFD model for both single and two-phase flow is coupled with mechanical structural analysis. In two phase-condition, the heat transfer coefficient is evaluated by employment of turbulent kinetic energy source function. The results show that the maximum stress is almost same for single and two-phase stratification, but the propagation of cold water layer intensifies the asymmetric stress in two-phase condition.

水平管道横截面的不均匀温度分布导致不对称的热应力。由于疲劳效应，这种情况被认为是一种老化机制，称为热分层。为了评估这种老化机制，应考虑运行或瞬态条件，在水平管道中进行应力分析，例如将稳压器连接到 NPP 一回路热段的喘振管线。在这些条件下，喘振线的状态将是单相或两相分层流。为了估算喘振管线水平部分的等效应力，将开发的单相流和两相流 CFD 模型与机械结构分析相结合。在两相条件下，传热系数采用湍流动能源函数进行评价。结果表明，单相和两相分层的最大应力几乎相同，但冷水层的扩展加剧了两相条件下的不对称应力。