核电管道系统的弯管热分层现象导致的疲劳热应力，容易诱发管道热疲劳。揭示弯管热分层流动与传热机理，预测弯管内壁面热波动而又不破坏弯管整体结构，是核电厂事故预防和安全运行亟待解决的研究课题之一。本课题拟开展热分层三维弯管流动与传热机理、瞬态多宗量导热反问题及相关实验研究。通过大涡模拟数值预测弯管热分层温度和速度波动特性；建立瞬态多宗量导热反问题模型，以红外热像仪测量外壁面温度和热电偶测量浅层温度作为导热反问题的输入条件，同时反演内壁面对流换热系数、流体温度和内壁面温度；对比分析内壁面温度和近内壁面流体温度的大涡模拟结果、导热反问题结果和实验结果，验证和修正理论模型；构建理论计算-实验校验系统，旨在揭示诱发热分层弯管的流动与传热机理，为热分层三维弯管内壁面瞬时第三类边界条件的预测提供无损间接可靠的方法。研究成果有望运用于核电管道系统的优化设计、结构完备性和系统安全性评估等工作。

Thermal stratification in elbows of piping system in nuclear power plant results in thermal fatigue stress which likely induces pipe thermal fatigue. Mechanisms of fluid flow and heat transfer of thermal stratification, prediction of thermal fluctuation on inner wall of the elbow and not to destruct the whole structure of elbow are one of the urgent problems for accident prevention and safe operation in nuclear power plants. In this project, the mechanisms of fluid flow and heat trasnfer of thermal stratification in three-dimensional elbow, the unsteady inverse heat conduction problem with multi-variables, and related experimental study would be carried out. Fluctuation characteristics of termperature and velocity in elbow would be predicted by large-eddy simulation. The mathematics model of unsteady inverse conduction problem with multi-variables would be established. Using measured temperatures of outer wall by thermal infrared imager and superficial wall by thermal couples as input conditions, the inverse heat conduction problem would be solved for simultaneously inversion the unsteady coefficient of heat convection on inner wall, fluid temperature and inner wall temperature. Comparative analysis of large-eddy simulation results, the inverse heat conduction problem results and experimental results of the inner wall temperature and fluid temperature near the inner wall would be completed to validate and revise the theoretical model. Establishment of the theoretical calculation - experimental calibration system is designed to reveal the mechanism of fluid flow and heat transfer which inducing thermal stratification and to provide non-destructive indirect reliable method for the prediction of the unsteady third boundary condition on the inner wall in the three-dimensional elbow with thermal stratification. The research results would be expected to optimize the design of the piping systems and assessment of structure integrity and system security in nuclear power plant.