【作者】 李金旺；

【导师】 王立秋； 程林；

【作者基本信息】 山东大学 ， 热能工程， 2011， 博士

【摘要】 环路热管(LHP)是一种新型的热管——利用相变换热的高效传热装置,具有传热能力大、传输距离远、安装和布局方便、反重力运行能力强等诸多优点,在高热流密度电子散热尤其是航空航天热控制领域得到了越来越广泛的应用。本文首先建立了环路热管传热的分析模型,推导得出环路热管稳定工作时的传热热流量可表示为工质的汽化潜热和毛细芯抽吸工质的质量流量的乘积。这从理论上表明工质和毛细芯是影响环路热管传热性能的关键因素,而毛细芯通过毛细力抽吸工质的行为则是其中重要的基础问题。随后建立了多孔结构通过毛细力抽吸工质的模型,推导了毛细抽吸量的数学表达式,并用实验对其进行了验证。研究结果表明,多孔结构通过毛细力抽吸工质时,其抽吸量按照指数增长函数的规律变化,该指数增长函数的偏置值和幅值互为相反数,大小等于多孔结构的孔隙率、横截面积和高度与工质的密度的乘积。该指数增长函数的时间常数的大小则等于多孔结构的相对渗透率和孔隙率的比值、工质的密度以及重力加速度的乘积的倒数,也就是说最终的抽吸量取决于多孔结构总孔隙体积的大小,而抽吸的速率则随着多孔结构的渗透率与孔隙率的比值的增大和工质密度的增大而增大。对推导得出的多孔结构毛细抽吸量的数学表达式分别进行了定性分析实验验证和定量计算实验验证。实验的结果均与用所推导得到的毛细抽吸模型分析得出的结果吻合良好。探讨了所建立的环路热管传热分析模型和多孔结构毛细抽吸模型的相关研究的一些应用。研究结果表明多孔结构毛细抽吸的快慢即是毛细抽吸力和渗透率的综合平衡,本文将这种表征多孔结构通过毛细抽吸力抽吸工质快慢的能力称为毛细抽吸性能。在相同的时间内,多孔结构抽吸的工质越多,也就是抽吸的速度越大,则说明其毛细抽吸性能越好。通常认为用于LHP的毛细芯提供的毛细抽吸力和渗透率均越大越好,但它们是矛盾的、此消彼长的关系,毛细抽吸性能则正好是它们的综合平衡的体现。毛细抽吸性能的实验方法可以用来测量多孔结构的渗透率,该方法与传统的测量渗透率的方法相比具有测试方便、密封要求低、流体驱动力为毛细抽吸力等优点,因而更适合用来衡量热管使用的毛细芯。在多孔结构毛细抽吸性能研究的基础上,进一步研究了室温下工质的表面张力、密度和粘度等物性参数对多孔结构的毛细抽吸性能的影响,结果表明多孔结构通过毛细力抽吸不同的工质时,在多孔结构的高度小于其毛细抽吸的极限高度的情况下,最终的毛细抽吸量的大小由多孔结构内部的总孔隙体积决定,抽吸的速率随着工质的表面张力以及密度的增大而增大、随着工质的粘度的增大而减小。此结果是与用工质传输品质因数在不考虑汽化潜热因式的影响的情况下分析得出的结果相吻合的。工质传输品质因数只衡量了热管工质的性能,本文探明了LHP的传热热流量、工质的传输品质因数和物性参数、毛细芯的毛细抽吸性能之间的关系,提出了用毛细芯的毛细抽吸性能来研究LHP的传热性能的分析方法,这对指导环路热管的设计和优化具有一定的意义。本文所建立的环路热管的传热分析模型和多孔结构的毛细抽吸模型及其实验方法除了具有上述的应用外,它们还可以用于指导毛细芯的优化。例如,本文的优化研究结果表明毛细芯的厚度存在一个最佳值,这与许多文献的研究结果是一致的；在毛细芯的孔径和孔径分布相同、毛细抽吸力也满足要求的情况下,毛细芯的孔隙率越大,对提高环路热管的传热性能也就越有利。基于上述研究结果,使用粉末冶金的方法结合造孔剂技术制备了一批高孔隙率的毛细芯,研究了制备参数对毛细芯孔隙参数的影响。实验结果表明：当使用微晶纤维素(MCC)作为造孔剂时,成型压力每增大10MPa,毛细芯的孔隙率约减小6.32%；造孔剂添加量每增加10wt%,毛细芯的孔隙率约增加6.64%；随着造孔剂添加量的增加,毛细芯的平均孔径变大、孔径分布趋向于更分散。采用瞬态平面热源法,研究了使用不同成型压力和不同造孔剂添加量制备的LHP毛细芯的导热系数、热扩散系数和单位体积热容,探明了这些热常数与孔隙率以及制备时所用的成型压力和造孔剂添加量之间的关系。研究结果表明：随着孔隙率的增大,导热系数呈现出单调下降的趋势,但热扩散系数和单位体积热容却与孔隙率的关系并不明显；含水毛细芯的导热系数和单位体积热容均比相同参数下干态的要大,但热扩散系数的情况却不完全如此；随着成型压力的增大,毛细芯的导热系数显著增大,热扩散系数和单位体积热容变化情况不一；随着造孔剂添加量的增大,毛细芯的导热系数和单位体积热容显著降低,热扩散系数明显增大。采用添加适量造孔剂的方法,可以制备得到较大孔隙率、较大强度、较小导热系数的毛细芯。在研究制备参数对毛细芯孔隙率和热物性参数的影响的基础上,提出了一种制备毛细芯时有效控制其孔隙率等参数的方法并进行了实验验证,结果表明用此控制方法制备孔隙率为75%的毛细芯时其孔隙率误差在6%之内。进一步研究后还发现当调控成型压力和造孔剂含量使得毛细芯的孔隙率相同时,毛细芯可以具有不同的毛细抽吸性能和强度以及相同的导热系数,这为毛细芯的优化制备和环路热管传热性能的强化提供了重要参考。最后探讨了LHP制造中的一些重要问题并实际制造了若干LHP实验件并进行了传热特性实验研究。对完整的LHP进行实验研究,结果表明,热源功率越大,LHP启动越快,最后稳定工作的温度值也越高。通过对环路热管在开环(即没有连接蒸汽管路)的情况下进行实验研究,避开工质循环和冷凝器等方面带来的影响,专注研究毛细芯孔隙率和热源功率对蒸发器启动和运行特性的影响,研究结果表明：在某些条件下,毛细芯蒸发器启动时其温度会出现剧烈的波动；毛细芯孔隙率越大,产生温度波动所对应的热源功率越小；毛细芯相同时,热源功率越大,越容易出现温度波动,并且温度波动的程度越剧烈。温度波动的原因是热源功率和毛细芯孔隙率等参数不匹配。总之,通过本文的研究,可以得到一系列强化环路热管传热性能的解决方案,总的来说就是：根据工作温度等要求,选用较大汽化潜热值的工质；然后根据传输距离和反重力运行高度的需要确定所需的最小毛细抽吸力；最后在确保毛细芯的毛细抽吸力满足要求的前提下,采用较好毛细抽吸性能的毛细芯。本文对多孔结构的毛细抽吸性能展开了深入的研究,结合环路热管的传热分析模型,对毛细芯的孔隙率、厚度、孔径、孔径分布及其与LHP的热源功率等外部参数的匹配等进行了优化研究,并探讨了实现优化结果的具体制备技术。更多还原

【Abstract】 Loop heat pipe (LHP) is a new type of heat pipe which is a high efficient heat transfer device using the phase transition to transfer heat. LHP has the advantages of high capability of transferring heat efficiency in long distance, good ability to work against gravity, and suitable layout and so on. It is now trending widely used in thermal management of satellites and spacecrafts as well as cooling of high heat flux electrical and electronic devices.This study firstly developed a new heat transfer analysis model of LHP. It concluded that the heat transfer capability of LHP could be evaluated by the power of the latent heat of vaporization of working fluid and the mass flow rate of working fluid through the capillary wick. It showed theoretically that both working fluid and capillary wick play an important role in the thermal performance of LHP. The behavior of porous wick pumping working fluid by capillary force is an important fundamental problem. The capillary pumping model of porous structure was developed. The math expression of capillary pumping amount was derived and verified by experiments. The result shows that the changing of capillary pumping amount accords well with an exponential increase equation. The offset and the amplitude of the exponential increase equation are opposite numbers and theirs value is equal to the power of density of the working fluid, porosity, height and cross sectional area of the porous wick. Besides, the reciprocal value of the time constant in the exponential increase equation is equal to the power of density of the working fluid, the ratio of relative permeability to porosity of the porous wick and the acceleration of gravity. That is to say, the final capillary pumping amount is determined by the total volume of the pores in the porous structure; the capillary pumping rate increases with the increasing density of the working fluid and the increasing ratio of relative permeability to porosity of the porous wick. Both experiments of qualitative analysis and quantitative calculation were performed for verifying the capillary pumping model. All the experimental results accorded well with the results analyzed by the capillary pumping model developed before.The applications of the heat transfer analysis model of LHP and the capillary pumping model of porous structure were investigated. The result shows that the rapidness or tardiness of the capillary pumping behavior represents the comprehensive performance of the capillary force and the permeability of the porous wick, and such capability is named capillary pumping performance in this study. The more amount of working fluid can be pumped into the porous wick at the same time, that is to say, the quicker it can pump, the better capillary pumping performance it has. It is used to considering that both capillary force and permeability of the wick are the bigger the better; however, they are antinomy. The capillary pumping performance can be considered as the best balance of them. It was proposed that the capillary pumping performance experiment can be used to measure the permeability of porous structure. Compared with other traditional methods, it needn’t be sealed during the measuring process and the flow was drove by capillary force as it was in a LHP and the measurement was convenient, so it was supposed to be better than the traditional method in evaluating the flow property of porous wicks for loop heat pipe applications.Based on the former study of the capillary pumping performance of porous structure, the effects of thermophysical properties such as surface tension coefficient, viscosity and density of working fluids on the capillary pumping performances of porous structure under room temperature were further studied. The result shows that when porous structure pumps different working fluids, the final capillary pumping amount is determined by the total pore volume of the porous structure when its height is shorter than its capillary rise limit, and the capillary pumping rate increases with the increasing surface tension coefficient and the increasing density of the working fluid, and decreases with the increasing viscosity of the working fluid. This result accords well with the result that analyzing by merit number without considering the effect of latent heat of vaporization of the working fluid. The merit number only evaluates the performance of working fluid. The relationship among heat transfer capability of LHP, merit number and physical properties of working fluid and capillary pumping performance of capillary wick was found in this thesis. The capillary pumping performance method was proposed to evaluate the thermal performance of LHP. It was useful for the design and optimization of LHP.Besides the above applications, the heat transfer analysis model of LHP, the capillary pumping model of porous structure and their experimental methods can also be used in guiding the optimization of capillary wick parameters. For example, the research results show that there is an optimization value of the thickness of capillary wick for LHP. This result accords well with the results reported in many literatures. When the pore diameter and the pore size distribution are the same, and the capillary force also meets the requirements, then, the bigger porosity, the better capillary pumping performance the capillary wick has, which is supposed to be helpful to improve the thermal performance of LHP.Based on the above results, porous wicks with high porosities were prepared by powder metallurgy method and space-holder technology with microcrystalline cellulose (MCC) as the space-holder. The effect of fabricating parameters on properties of porous wicks was investigated. The result shows that porosity increases about 6.32% when forming pressure decreases 10MPa and increases about 6.64% when microcrystalline cellulose addition increases 10wt%. The pore size distribution ranges become wider and the mean pore diameters increase with the increasing space-holder addition.The thermal conductivity, the thermal diffusivity and the volume specific heat of porous wicks was studied by using the transient plane source method. The relationships between thermal properties and porosity, forming pressure and space-holder addition were investigated. The result shows that thermal conductivity decreases with the increasing porosity, but there are no obvious similar relationship between thermal diffusivity, volume specific heat and porosity. Both thermal conductivity and volume specific heat of water saturated state porous wicks are bigger than those of dry state ones, but there is no obvious similar rule for thermal diffusivity. Thermal conductivity increases with the increasing forming pressure, while there are no obvious changing trends for thermal diffusivity and volume specific heat. Thermal conductivity and volume specific heat decrease while thermal diffusivity increases with the increasing space-holder addition. Porous wicks with high porosity, good strength and low thermal conductivity can be prepared by using the space-holder technology.Based on the above studies of the effects of fabricating parameters on the pore properties and the thermal properties of porous wicks, a new method for controlling the parameters of porous wicks during the preparation was proposed and verified by experiment. The result showed that the error of porosity was less than 6% in the case study that used the porosity control method to fabricate porous wicks with the expected porosity of 75%. It was found further that the capillary pumping performances and the strength were different while both thermal conductivities and porosities of the porous wicks prepared by the porosity control method were the same. This is useful for guiding the optimization of porous wicks for LHP.Some important matters in the design and the preparation of LHP were finally studied, and some LHP samples were fabricated and their heat transfer characteristics were investigated. The experimental result of the whole LHP shows that the system starts quicker and the working temperature increases as the increasing heat source power. The experimental study on the open loop heat pipe evaporator (just like a loop heat pipe without linking the vapor line) was carried out wisely without coupling both the effects of the circulation of working fluid and the parameters of condenser. The result shows that the temperature of the evaporator fluctuates in some cases during the startup of the system. The heat source power making the temperature fluctuate decreases with the increasing porosity of the porous wick. When porous wick is the same, temperature is easier to fluctuate and it is much serious in the cases of bigger heat source power. The reason for the generation of the temperature fluctuation is that the parameter (such as porosity) of capillary wick is not matching with the external parameters of the system (such as heat source power).In a word, a series of solutions can be found to improve the thermal performance of LHP in this thesis. In general, firstly choose working fluid with good latent heat of vaporization according to the requirement of working temperature. Secondly, determine the needed capillary pumping force of the capillary wick according to the heat transfer distance and the against gravity height. Finally, select the capillary wicks with better capillary pumping performance and make sure that the capillary force meets the above requirement. The capillary pumping performance of porous structure, together with the heat transfer analysis model of LHP, was studied in detail in this thesis. The optimization of porosity, thickness, pore diameter, pore size distribution of capillary wick and its matching with the external parameters of the LHP such as the heat course power, as well as the preparation technologies for achieving the detail optimization results were performed.更多还原