液氮温区脉动热管运行机理及加热方式与接触角的影响研究

High temperature superconducting magnets have been developed rapidly in recent years. Liquid hydrogen is a promising fuel for space shuttle and vehicles, and the long-term efficient storage is quite significant which affects the extension of space exploration and hydrogen vehicles application. Both of these systems should operate in a cryogenic environment, and thus effective thermal management technology becomes one of the most serious challenges. Cryocoolers are the important equipment for providing the cryogenic environment. However, it only produces the cooling at the tip of the cold-finger. Pulsating heat pipe (PHP) is a new heat transfer device, which has flexible structure and can match cryocooler. It provides a possibility for cooling transfer efficiently in long distance and distributed cooling system. Its application in room temperature has attracted considerable attention. However, the physical properties of cryogenic fluids and the bubble formation conditions are greatly different. As a result, the driving and dragging force of heat transfer and fluid flow inside the same tube are also different. The regularity at room temperature is not necessarily applicable to low temperatures. So, it is essential to investigate cryogenic PHP but the study on it has just started at present. This project aims at the operation mechanism of PHP with liquid nitrogen and the effects of heating mode and contact angle. Based on fluid mechanics, thermodynamics and cryogenic heat transfer theory, the operation mechanism of PHP with liquid nitrogen will be performed. The mechanisms under pulsating condition include the bubble formation and aggregation, the flow pattern transition of vapor plug and liquid slug, and thermal performance. Two PHPs with different tube material but the same geometric dimensions will be built, with one of glass tube and the other of stainless steel. High speed camera will be utilized to visualize the flow characteristics inside the capillary tube of the glass PHP. Flow patterns, temperatures of evaporator section and condenser section, as well as pressure will be measured synchronously. Three different heating modes, step by step heating, one step heating and pulse heating, will be applied on glass PHP to explore the effect of heating modes on internal flow of working fluid. After that, the correlation between flow patterns and thermal characteristics like thermal conductivity, temperature difference, and heat load will be established. Experiment to study the thermal performance of stainless steel PHP with liquid nitrogen will be carried out. And the differences between thermal performances of glass PHP and stainless steel PHP will be studied. Then the effect of contact angle with different materials will be explained. The empirical criteria for start-up, operation and heat transfer limit will be built. The physical model for analyzing the performance of a PHP will be established, and can predict the heat transfer and flow characteristics of the PHP with liquid nitrogen by introducing the empirical criteria. Finally, the solid foundation can be provided for the application of PHP in the field of thermal control such as superconductivity and space exploration.

从被冷却对象的角度，如何把制冷机冷头来之不易的有限冷量高效的远距离地传递到被冷却对象中？低温脉动热管给出了其中的一种解决路径。它是一种新型传热元件，结构灵活、能在远距离和分布式冷却系统中实现冷量的高效传输，其在室温区的研究受到了广泛关注。但由于低温工质物性与气泡生成条件的显著差异，导致其在管内流动和传热时的动力和阻力特性不同，但该领域的研究刚刚起步。本项目拟采用可视化研究手段，以掌握液氮温区脉动热管运行机理及加热方式与接触角的影响为目标，从流体力学、热力学和低温传热学的角度出发，着重研究脉动条件下液氮工质气泡的产生与合并、气液塞流型转换机理，探索不同加热方式对脉动流动的影响；分析液氮工质在不同材质内的运行特质，总结启动、流型转换及极限的经验准则；为最终构建低温温区脉动热管的理论模型奠定基础，研究成果将在超导和空间探测等热控制领域具有广阔的应用前景。

空间探测器、低温推进剂、量子计算以及超导技术等尖端科技的发展对低温传热技术提出了新要求：传热效率高、自重轻以及易维护。脉动热管兼具对流传热与相变传热机制，不仅具有高传热性能，还具备自重轻、无功耗、结构简单、布置灵活以及可在微重力下运行等显著优势。本项目围绕低温脉动热管中尚未解决的科学问题以及其实际应用链条，开展了一系列工作：.理论研究方面，从热力学角度分析低温工质物性以及低温气体非理想性，系统说明低温脉动热管有别于常温脉动热管的特殊性，揭示其充液率的温度相关性，并利用实际气体状态方程（RKS状态方程）描述低温气体的压力方程、能量方程及饱和物性，建立适用于低温脉动热管的一维塞状流模型，并得到液氢温区实验验证，利用模型分析了液氢脉动热管的传热和流动特性，说明其传热机理。.实验研究方面，采用控制变量法开展实验，系统揭示关键参数对液氢脉动热管启动特性和稳态传热性能的影响规律，为不同场景下脉动热管的应用提供重要参考，并拟合针对液氢脉动热管的传热性能经验关联式，为快速估算脉动热管的传热性能提供了一个便利工具。针对可视化研究，利用刻蚀、键合等微纳加工技术，制备了可用于液氮温区的硅基平板脉动热管，并搭建液氮脉动热管可视化实验平台，实现了液氮脉动热管的全流道可视化，为后续的研究打下基础。.实际应用方面，针对脉动热管的冷源性能优化，建立了包括压力函数和体积函数的斯特林制冷机通用模型并分析其运行特性，为优化其热效率提供了理论基础。为拓展脉动热管冷源的选择，对不同压缩机驱动的预冷型液氦JT制冷机进行了测试，获得了稳定的低温制冷性能，证实了其作为脉动热管冷源的可行性。针对低温热控系统的真空和绝热性能优化，测试了多层绝热材料的放气速率，为其调控给出参考。.项目从理论、实验和应用三方面出发，对低温脉动热管进行了全面探究，为其未来的系统化应用奠定了良好的基础。

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