低温脉动热管传热与流动机理研究

The cryocooler has applied in many industrial fields. However, the cooling is produced only at the tip of the cold-finger. Pulsating heat pipe (PHP) is a new heat transfer device, which can match crycooler and achieve cooling transfer efficiently in long distance and distributed cooling system. Its application in medium temperature has attracted considerable attention. The physical properties of cryogenic fluid and the bubble formation conditions are greatly different. As a result, the driving and dragging force of heat transfer and fluid flow are also different. At present, the study on cryogenic PHP has just started. The research on the mechanism inside has not been carried out. In this project, the optimization method for heat transfer performance of the cryogenic PHP is aimed. Based on fluid mechanics, thermodynamics and the modern cryogenic heat transfer theory, the PHP with nitrogen as working fluid will be emphatically studied. The following mechanism under pulsating condition will be analyzed: the bubble formation and aggregation, the flow pattern transition of vapor and liquid, the mass transfer process among liquid film, vapor slug and liquid plug. From two views of experiment and theory, the heat transfer, fluid flow and oscillating characteristic at the steady operation will be revealed, as well as the effects of the main parameters. The mechanism of start-up and heat transfer limit, and the criteria of start-up and heat transfer limit will be explored. The analytical approach to the performance and the range boundaries of steady operation of the PHP with liquid nitrogen will be determined. The solid foundation can be provided for the application of PHP in cryogenic devices, as well as making the application of the cryocooler enlarged.

低温制冷机技术已在众多领域得到了广泛应用，但它仅能在冷头提供冷量，脉动热管是一种能与其匹配的新型传热元件，可在长距离和分布式冷却系统中实现冷量的高效传输。它在中温区的应用已得到了广泛关注，由于低温工质物性和气泡生成条件的差异，导致低温脉动热管传热与流动的动力和阻力呈现不同特征。目前，低温领域的研究才刚刚起步，尚未对其开展机理研究。本项目以掌握低温脉动热管传热性能的优化方法为目标，从流体力学、热力学和现代低温传热理论出发，着重研究以液氮为工质时管内脉动条件下气泡生成与聚合，气液相流型变化，液膜、气塞和液塞之间的质量传递的机制，从实验和理论两方面揭示稳定运行时工质传热、流动与振荡特性及主要参数的影响，明确启动过程和传热极限的机理，寻求启动条件和传热极限的判断准则，探索分析液氮温区脉动热管稳定运行性能及区间边界的方法，为其在低温设备中的应用以及辅助低温制冷机扩宽应用领域奠定坚实基础。

脉动热管是一种新型的传热元件，由于结构灵活、传热效率高，因此能够在长距离、分布式及复杂结构的冷却系统中实现冷量的高效传输。它在中温区的应用已得到了广泛关注，由于低温工质物性和气泡生成条件的差异，导致低温脉动热管传热与流动的动力和阻力呈现不同特征。本项目对低温脉动热管传热和流动的影响因素及其机理进行了研究，主要包括：(1) 从充液率的热力学定义出发，论证了低温脉动热管充液率的温度相关性及其对传热特性的影响，首次提出存在临界充液率FRc，当初始充液率FR0<FRc时，充液率随温度升高而减小或者随温度升高先增大后减小，直至降低到0%，即达到干涸极限；当FR0>FRc时，充液率随温度升高而增大直到100%，即达到液体对流极限。(2) 建立了多气泡-液塞的闭式脉动热管模型，模拟出了低温工质(氢)和常温工质(乙醇)的振荡和传热特性以及内部的流型变化特征，发现相比于常温工质，低温工质流动速度更快，管内新产生的气泡均无法稳定地存在，亦无法迅速膨胀使原有的较长液塞自中间断裂，从而更容易形成稳定的单向流动。(3) 以氢为工质的实验研究发现①在启动之前，PHP两端会出现一个较大的温差，低温PHP在较低热负荷下即可成功启动，这和常温PHP的启动有所区别。②绝热段长度增加到原来的5倍，即传热距离增加近5倍，热阻仅增加约30%，相较于金属导体，PHP在远距离传热上有明显优势。弯头数量和加热方式对有效导热率及壁面温度影响不大，因此PHP可以实现小空间内的高效传热、同时对热负荷有较高的适应性。③验证了传热极限与充液率的关系，并发现最佳充液率与临界充液率极为接近，从理论和实验两方面揭示了不同热负荷条件下存在不同的最佳充液率的机理。上述研究成果将为低温脉动热管在低温设备的冷却系统及辅助低温制冷机扩展应用领域奠定基础。