仿生双孔隙率多孔层的3D构建及内部气液输运特性对临界热流密度的影响机制

The study of critical heat flux (CHF) is of great importance to ensure the safe and efficient operation of nuclear reactors. The micro/nano porous structured heat transfer surface modification is the most effective method of CHF enhancement. In this research, biporous layer with tree-like fractal structure is constructed by 3D printing technology. The different porosity of the biporous structure design could provide flow and diffusion channels for gas and fluid, and on the other hand, this design introduces bionic structure to improve the radial flow, which could reduce the fluid transport resistance during boiling; furthermore, hydrophilicity improvement and the formation of micro/nano porous structure through surface modification increase capillary wicking, which leads to CHF enhancement. The relationship between the parameters of the porous layer, the parameters of fluid transport and the CHF is established. The dynamic characteristics of gas and liquid phases and the wall temperature in time and space dimension in porous layer close to CHF state is recorded by high speed photography visualization and non-contact temperature measurement. On the basis of upper results, the key parameters of porous structure controlling CHF is confirmed via further discussing of physical mechanism of CHF, and the CHF model is calibrated. These results could offer a scientific basis for optimal design about the porous structure for CHF strength. The research can provide a new way of consideration for the design and preparation of the porous structure of CHF enhancement.

临界热流密度（CHF）的研究对于确保核反应堆等涉及沸腾传热装置的安全、高效运行具有极为重要的意义。传热表面微纳多孔化是最为有效的强化CHF的方法，本项目采用3D打印技术构建嵌套树状分形网络的双孔隙率多孔层，拟通过不同孔隙率的多孔结构设计分别为气、液两相提供流动和扩散通道、引入仿生结构降低流体的输运阻力；进一步通过表面修饰形成微纳多孔结构、提高亲水性，增大毛细泵吸力，达到强化CHF的目的。实验研究建立该多孔层结构参数、流体输运特性参数、CHF三者间的关联关系。高速摄影可视化和非接触式温度测试相结合，获得近CHF态时，多孔层内气、液两相及壁面温度在时间和空间维度的动态特性，深入探讨CHF触发的物理机理，提出表征多孔结构输运特性的关键控制参数，修正CHF理论模型，为强化CHF多孔结构的优化设计提供科学依据。本项目的研究可为强化CHF的表面多孔结构的设计和制备提供一个新思路和新技术。

临界热流密度（CHF）研究对于高热流密度电子元器件的散热和安全运行具有极为重要的意义，而传热表面的孔隙结构特征与内部气液两相输运是强化CHF的重要控制因素，双孔隙率多孔层可以组合不同孔隙结构特征优势提升CHF。本项目从双孔隙率多孔层的3D构建方法入手，设计构建了不同孔隙形式与不同孔隙尺度的双孔隙率多孔层。针对金属以及非金属材料，开发了不同的双孔隙率多孔层的3D打印制备方法和工艺，实现了表面微多孔层与结构骨架内部多孔的双孔隙率多孔结构的可控制备。通过传热实验验证了不同结构形式与参数的双孔隙率多孔结构的传热特性，可视化分析了多孔结构内部的气液输运特征，研究发现增大多孔层厚度不利于内部气液输运与传热性能的提升；沸腾传热过程中径向分形孔道强化传热效果有限；存在最佳的多孔结构参数使得CHF获得最大提升，该孔道结构尺寸与气泡离场直径相匹配；微小孔隙基质内嵌套大孔有效改善了气液流动阻力，能够进一步提升传热性能。在实验基础上分析了不同双孔隙率多孔层的CHF的触发机制，基于阻力极限与水动力不稳定性分析构建了新的CHF模型，该模型能够准确预测CHF随孔隙特征变化的规律。本项目的研究成果可为双孔隙率多孔层的构建及其在高热流密度电子元器件高效散热方面的应用提供设计思路与科学参考。