热分层三维弯管流动与传热机理及瞬态多宗量导热反问题研究

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.

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。

热分层现象普遍存在于石油、化工、能源等过程工业中，特别是在核电站管道系统（弯管及盲管）中，由于冷热流体密度差引起的浮升力与流体惯性力在铅垂方向形成动态的力平衡，伴随核电站长期运行，热分层现象伴随着温度波动，极易诱发管道热疲劳失效，对核电站安全运行构成严重威胁。由于核电站管道结构具有严格的结构完整性要求，不允许在管道上开孔安装热电偶来测量流体和管道内壁面的温度，这就需要研究一种无损间接的管道内流体及管道内壁面的温度测量方法。在基金的资助下，本项目紧密围绕热分层流动与传热机理及瞬态多宗量导热反问题的研究，开展了：（1）核电站典型管道构件T型弯管及波动管内部诱发热分层的流动及传热机理研究；（2）热分层圆管导热反问题研究。通过相关的实验研究和理论研究，揭示了热分层的流动与传热机理，寻找到了削弱热波动的技术途径，通过导热反问题能够准确反演管道内壁面温度或管道内壁面第三类边界条件，研究结果表明：（1）在T型弯管内部安装防涡器对分支管道内温度扩散和热波动有一定的削弱作用；（2）主支管到动量比及冷热流体温差是影响波动管热分层及湍流穿透的重要因素；（3）多宗量导热反问题的具有较高的精度，可以为热分层三维圆管内壁面第三类边界条件的预测提供无损间接的方法。该研究对于揭示诱发管道热疲劳的流动与传热机理、为探究一种无损间接的管道内壁面测温方法、确保管道设备完备性等方面具有重要的科学意义和实际价值。在本项目的资助下，发表论文近20篇，其中SCI收录论文11篇, EI收录论文3篇。

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