【作者】 涂正凯；

【导师】 刘伟；

【作者基本信息】 华中科技大学 ， 工程热物理， 2009， 博士

【摘要】 热毛细泵相变回路包括毛细泵抽吸两相环路(CPL)以及环路热管(LHP)，由于采用工质相变传输热量，具有传热能力强，热阻低、等温性好、效率高、无运动部件以及传输距离长等优点，从而使其成为了航天器热控以及电子器件高热流密度散热的有效方式。快速、简单、可靠的启动性能是一个先进的CPL／LHP系统所具备的基本特性，但是国内外的研究表明，CPL／LHP系统在启动过程中存在启动困难的问题，特别是在CPL／LHP小型化以后，侧壁导热对CPL／LHP的启动性能有明显的影响。本文通过搭建小型平板式LHP的实验平台，并在不同的热流密度下进行LHP的启动性能研究。实验结果表明，在常规条件下热流密度为1W／cm~2时系统不能正常启动，而在蒸发器底部加上冰水混合物后，LHP系统能顺利启动。分析指出，侧壁导热和背向导热对平板式CPL／LHP的启动有较大的影响，因此，必须对平板式CPL／LHP的蒸发器进行优化设计。本文建立了蒸发器毛细多孔芯、金属壁面、蒸汽槽道以及液体槽道内传热传质的整场数学模型，并运用SIMPLE程序对小型平板蒸发器进行了整场耦合求解。数值结果表明，由于CPL系统的特殊平板结构所产生的侧壁导热效应使得热流从金属侧壁及下壁反向加热液体补偿腔以及多孔芯内的液体，导致气泡的产生，从而影响系统的正常启动和工作性能，因此，侧壁导热效应在平板型蒸发器的优化设计中必要予以重视。减小液体补偿腔的高度，减小侧壁以及下壁的厚度以及增加毛细芯的高度可以提高CPL的传热能力。在实际的应用过程中，CPL／LHP有较大的温度波动，而目前国内外学者对CPL／LHP温度波动的研究仅仅集中在实验分析上面，理论的研究很少见到报道。本文首先从毛细相变界面的质量、动量和能量守恒方程，提出了相变界面的稳定性判据，然后建立了毛细液柱的Lucas-Washburn方程，第一次指出了毛细芯高度对系统稳定性的影响。研究表明，在高热流密度的情况下，毛细芯高度对系统的提升力的影响不能再忽略，系统工质的选择标准应该综合考虑相变潜热、毛细张力系数以及液体密度的影响。系统温度波动的源头是汽液冷凝界面，而蒸汽压力的波动导致冷凝器内液柱的共振，是系统有较大温度波动的根本原因。由于液体的不可压缩性，越靠近冷凝界面，液体的温度波动幅度越大，蒸发器的温度波动较小是因为其有较大的固有频率，同时由于毛细芯的孔径尺寸远小于压力波动的波长，使得压力波的被反射回去的结果。因此，系统应在一定的工作热流范围内运行，使蒸汽波动的频率远离冷凝器液柱的固有频率，以免导致共振。针对CPL系统在运行中可能存在供液不足的问题，本文在系统中加入微泵引射辅助回路，将微泵和引射器应用于CPL系统，微泵引射回路与储液器共同起作用，在运行条件恶劣时做出响应，辅助主回路循环，以提高系统的控温能力及运行稳定性。数值计算显示，在引射流体时，引射器内会出现气蚀现象，导致气泡进入蒸发器，可能会引起系统的正常运行，甚至烧干，因此，在引进新的引射回路的同时必须在引射器出口和蒸发器入口段之间加入过冷器，使得引射器出口中的气相冷凝成液体进入蒸发器，保证蒸发器的正常工作。在CPL／LHP的一些实际应用中发现，系统压降大于毛细芯提供的毛细压头，，针对这种传统驱动方式不能解释的现象，刘伟等提出了毛细芯热管的驱动模型，并将其推广到毛细泵流体回路(CPL)和环路热管(LHP)系统。文中以甲醇作为工质的算例表明，小型CPL回路蒸汽的流动阻力大于液体的流动阻力，尤其在高热流密度下，蒸汽侧的流动阻力占主导地位，需要较大的相变驱动压头来驱使蒸汽流动。文中还建立了工质流动阻力与界面热力学参数之间的关系式，据此，可计算回路蒸发端和冷凝端的工质温度、压力以及其它热力学参数，对系统的传热和流动性能作出评价，也可通过计算回路的流动阻力，获得冷凝端的流体温度、压力，并评估系统的热阻和均温性，从而进行CPL和LHP的优化设计。更多还原

【Abstract】 The capillary pumped loop (CPL) and the loop heat pipe (LHP) are the two-phase thermal control devices with the latent heat of evaporation of a working fluid to transfer heat and the capillary action for fluid transport, which are capable of transport large heat density and passively transporting heat over large distances with minimal temperature losses, and contain no moving parts. As a result, CPL/LHP becomes more active and interesting in many engineering domains including thermal management of satellites and spacecrafts as well as cooling of electronic devices.A prompt, simple and reliable startup performance is the basic characteristics of advanced CPL/LHP system. Whereas, a number of difficult startup problems of CPL/LHP were found in present literatures due to various reasons. An overall two-dimensional numerical model was developed to address the heat and mass transfer characteristics in the evaporator of mLHP, the numerical results showed that ’side wall thermal conductivity’ had a significant effect on the heat and mass transfer in the evaporator of a mLHP. The start-up characteristics of a flat miniature loop heat pipe (LHP) with different heat loads were experimentally investigated in this paper and the experimental results showed that the mLHP couldn’t start up successfully under normal condition except that a bag of ice-water was located on the bottom of the evaporator with a heat flux of 1W/cm~2. Reasons for the difficulties in its start-up were analyzed and ’side wall effect’ was deduced , hence, a numerical study was addressed to investigate the optimization design of mLHP in the geometrical structure. The numerical results showed that the performance of the flat miniature LHP could be enhanced by improving the geometrical structure in a limited working space. Effects of thickness of the side wall, height of the compensation chamber and the wick on system performance were discussed in detail.Pulsations of the operating temperature, which take place in stable conditions of heat supply and removal, are a characteristic process inherent in some types of closed heat-transfer devices operating on the evaporation-condensation cycle, but the theoretical reports are seldom seen, hence, a mathematical model based on the Lucas -Washburn equation has developed to address the relations of the capillary height, capillary radius and the heat flux in a capillary column in un- gravity, and the formulas deduced as a consequent is used to analyze the influence of the height of the capillary wick in the capillary force and stability in a capillary loop with phase change. According to the theoretical analysis, the height of the wick will lower the capillary force in the loop, and the stability of the loop by introduced a small disturbance into the height of the capillary wick is studied in detail. The result shows that the steady state of the capillary loop with phase change is over-damped in non-gravitational condition. The root of the temperature pulsation is in the condenser, the closer to the condenser, the larger of the amplitude it is, and resonance will occur in the condenser when the frequency of the pressure pulsation is equal to the inherent frequency of the liquid column in the liquid line , therefore, a appropriate heat flux is required to keep the frequency of the liquid column far away from the pressure pulsation frequency in the vapor line.An accessorial loop with a micro-pump and ejector is applied into the Capillary Pumped Loop(CPL) to enhance the heat transfer capability of CPL. A 3-D model is developed to investigate the operation performance of the ejector. The numerical results show that the primary loop will not only can operate normally but the mass flux can be enhanced with a accessorial loop. However, vapor can be found in the outlet of the ejector and it can flow into the evaporator, which is a big disadvantage for system, hence, a sub-cooler is necessary to locate between the outlet of the ejector and the inlet of the evaporator to condense the mixed fluid into liquid.A phase change driving mechanism in Capillary Pumped Loop (CPL) and Loop Heat Pipe (LHP) is pointed out in this paper . A mathematical model has been developed to describe this driving mechanism. The calculating examples presented in the paper with methanol as working fluid shows that flow resistance in vapor side is larger than that in liquid side. Especially in the case of high heat flux, flow resistance in vapor side is dominative part of the whole system pressure drop, so that a remarkable phase change pressure head is needed to drive vapor flow. The formulas reflecting relations between thermodynamics parameters and flow resistance have been developed to quantitatively evaluate characteristics of heat transfer and flow in the system by calculating working fluids’ temperature, pressure and other parameters in the evaporating and condensing interfaces, and to predict thermal resistance and temperature uniformity of the system, thereby guiding the design of CPL and LHP. The model established can be extended to all heat pipes with capillary wick or micro-groovy channels.更多还原