微纳复合结构吸液芯强化低温回路热管传热的研究

Because of the cryogenic working fluid’s low latent heat for evaporation and small surface tension, as well as the adoption of simple wick with monoporous structure for the key component of loop heat pipe (LHP), evaporator, the effective heat transfer coefficient and maximum heat transfer capacity of the present cryogenic loop heat pipe (CLHP) are very limited and cannot satisfy with the ever-increasing heat dissipation demand from the electronic devices, which need to be kept under cryogenic conditions to have a reliable performance. In this study, we propose a novel micro-nano multi-scale wick, which has double-layer of monoporous layer and biporous/bidisperse layer, as well as the chemically etched nano-pores on the sintered particles. The heat transfer and visualizations study will be conducted on a specially designed test section to obtain the variation of heat transfer coefficient and the corresponding vapor-liquid interface position inside the wick. A multi-scale mixed model with the phase change LBM for the parts which will experience phase change during evaporator working and with the macro-scale finite volume method for the residual parts will be developed to do numerical simulations. The conjugated fluid flow, heat transfer and phase change processes, and their affecting factor, such as pore sixe, porosity, thickness of the particle sintered wick and the additional role of nanopore will be investigated through comparative study of experiment and numerical simulation, in order to identify the enhancement mechanisms of the proposed micro-nano multi-scale wick. The optimization of the wick structure and the feature sizes will be conducted and the flat-plate evaporator will be designed and fabricated with the consideration of the high pressure working condition. The start-up and stable working characteristics and performance will be experimentally studied under different working conditions. The proposed study is not only helpful to give insight and provide digital tools on the design of advanced wick for CLHP, but also meaningful for the micro-nano phase change phenomena in other fields.

由于低温工质蒸发潜热低、表面张力小的固有缺陷，目前低温回路热管采用的单孔隙吸液芯蒸发器的传热系数和传热极限都无法满足日益提高的低温器件的散热需求。本项目提出在单孔隙—双孔隙/双分散多层烧结复合芯的颗粒表面通过化学刻蚀法制作纳米结构的新型微纳复合结构吸液芯，建立可视化测试段研究传热系数和气液界面随加热功率的变化规律，开发介观相变网格波尔兹曼法和宏观有限容积法相耦合的多尺度仿真模型，通过实验和仿真对比研究揭示微纳复合结构吸液芯蒸发器内流动、传热和相变过程的耦合传递规律、影响因素（各层烧结芯的孔隙大小、孔隙率和厚度，以及纳米结构的作用）和传热强化机理；在此基础上优化设计和研制高强度平板式蒸发器，研究回路热管启动和运行特性，以及传热系数和传热极限，并考察重力的影响。本项目研究不但可以为低温回路热管蒸发器的研发提供设计参考、理论指导和数字化设计工具，而且对其它领域微纳尺度相变研究也具有重要意义。

回路热管尤其是低温回路热管在航天载荷和地面科学仪器的热管理方面具有越来越重要的应用前景，本项目从低温工质选择方法、吸液芯蒸发传热特性和机理、回路热管内两相分布和相变换热特征、热管启动特性和传热性能各方面开展了理论、数值仿真和实验研究，主要成果包括：１）改进了随机四参数多孔结构生成算法，解决了流动死区、孤立固相小团簇、孔隙分布不均匀性和骨架不连续等问题，建立了三维孔隙尺度多孔吸液芯气蒸发传热的LBM相变仿真模型，获得了热负荷、孔隙率和表面润湿性等对两相界面分布及换热特性的影响规律；2)发展了能还原全部实际热物理性质的LBM相变仿真模型，仿真揭示了超薄纳米多孔薄膜蒸发气液界面自适应变化的动态特性，发现了纳米孔驱液和极限传热能力低于理论值的原因，并获得了孔隙大小和膜厚、表面润湿性等对其的影响机制和规律；3)提出了一种优化的低温换热热管工质品质因素，能够准确判断工质的传热极限能力优略；4)可视化实验获得了吸液芯蒸发器的相变启动模式和充液量和热负荷分布的影响，以及微通道冷凝器的两相流型及影响因素，并筛选了合适的传热关联式；5)探明了深低温平板回路热管的启动特性以及加热功率和工作温度对传热热阻的影响规律，并探索了多蒸发器回路热管的可行性；6)开发了一种基于分布式供液的平板蒸发器，验证了其在启动、稳态传热和大面热源应用方面的优势。本项目的研究方法和研究成果不但可以为低温回路热管蒸发器的研发提供理论指导和设计工具，而且对其它领域微纳尺度蒸发传热的研究也具有重要参考价值。