近壁边界层对超临界流体拟临界温度附近传热特性影响机理

The special flow and heat transfer phenomenon of supercritical fluid in the vicinity of pseudo critical point has drown more and more attentions. It has been proved that the boundary layer has great effects on the heat transfer enhancement and deterioration of supercritical fluid and revealing the corresponding mechanism has important scientific significance and great practical value. However, it is meager to us due to the lack of correlative fundamental research. In this project, the basic physical features, as well as the key scientific issues involved in this very field will be studied, including the structural evolution of the boundary layer during the flow and heat transfer process, the heat transfer rules among the viscous sub-layer, the buffer sub-layer and the log-law sub-layer, the large temperature increase when the heat transfer deterioration occurs, the formation cause of buoyancy and thermal acceleration and their effects on the flow and heat transfer. .By combining the experimental measurement, visualization tailing and capturing, numerical simulation and theoretical analysis, the influence mechanisms of boundary layer on the heat transfer enhancement and deterioration of supercritical fluid near the pseudo-critical point will be accurately revealed. The relations of several macroscopic parameters, e.g. system parameters and dimensions of flow channel, and several microscopic parameters, e.g. structure of boundary layer, velocity vector distribution near the wall and stress distribution in the boundary layer, will be built. The turbulence model of supercritical fluid, which fully considers the boundary layer effects, will be developed. It is expected to provide part of theory foundation for the development of heat transfer augmentation technique of supercritical fluid.

超临界流体在其拟临界点附近的特殊流动与传热现象日益受到广泛关注，而拟临界点附近传热强化及传热恶化受边界层效应的影响对超临界流体传热机理的揭示有着重要的科学意义和实际价值，但迄今为止相关基础研究不足。本项目拟对管内超临界流体流动与传热过程中近壁边界层的结构演变，粘性底层、缓冲层与对数层间的热量传递规律，传热恶化时的温度飞升等表观现象背后的流动与传热机理，浮升力及热加速度成因及对流动与传热的影响等相关基本物理过程和关键科学问题进行研究。采用实验测量、可视化跟踪捕捉、数值计算与理论分析相结合的手段，准确揭示边界层效应对超临界流体在拟临界点附近传热强化和传热恶化特性的影响机理，构建系统参数及流道尺度等宏观参数时空变化与边界层结构、近壁速度矢量分布，边界层间应力分布等微观特征的内在联系。发展考虑边界层效应的管内超临界流体湍流流动与传热模型，为超临界传热强化技术的发展提供一定的理论基础。

超临界流体在其拟临界点附近的特殊流动与传热现象日益受到广泛关注，而边界层效应对拟临界点附近传热强化及传热恶化的影响对超临界流体传热机理的揭示有着重要的科学意义和实际价值，但迄今为止相关基础研究不足。本项目搭建了可视化超临界CO2流动传热实验平台，通过对超临界CO2传热恶化状态下的可视化实验研究(上升流)，发现当局部壁温出现飞升时，加热面附近流体出现明显的“模糊拖影”，该现象为超临界CO2传热恶化状态下直观的流动状态表现，其本质是加热面附近流体受热达到“类气态”而密度显著降低，在浮升力的作用下，产生了强烈的局部流动加速效应。这一局部流动加速现象定性地反映了超临界传热恶化中典型的“M”型速度分布，并揭示了加热面附近流体能量传递与质量传递对整体换热表现的显著影响。通过圆管内的湍流传热实验，探究在宽浮升力范围下超临界CO2的传热表现，发现超临界CO2发生传热恶化的界限热流密度qonset, min与质量流速G的平方成正比关系，提出了通用传热恶化判据，该判据经过与文献中的实验工况对比验证，具有良好的普适性及外推性。在实验研究基础上进一步开展了数值模拟研究，速、减小局部的湍流切应力进而削弱边界层内的湍流强度来恶化上升流中的传热表现。基于(拟)临界温度附近物性变化与边界层效应的耦合机制，以传统SST 湍流模型为基础，进一步考虑管径与系统压力对传热恶化的影响，提出了可变Prt的模型改进方案。模型通过总计26组超临界CO2实验工况的验证，不仅可以复现传热恶化中出现的壁温突起现象，其预测结果还能反映超临界CO2传热恶化随入口温度Tin及热流密度q升高的发生演变过程。改进模型适用范围广，预测精度明显优于现有的传统湍流模型，对于高效超临界CO2换热器设计及其工业应用具有良好的实用价值。