【作者】 陈强；

【导师】 黄永华；

【作者基本信息】 上海交通大学 ， 制冷及低温工程， 2017， 博士

【摘要】 热管是借助工质的相变潜热来实现高效传热的装置,可以达到比铜等金属高2～3个数量级的等效热导率。其中,吸液芯是热管工作的核心部件,它利用微孔隙产生毛细力驱动液体工质回流并形成循环。近几十年来科技的发展尤其是电子行业的快速发展,发热部件的小型化、紧凑化使得热流密度极速上升,同时允许用于传输热量的通道尺寸也不断被限制,这些发展对传统热管的传热能力提出了很大的挑战。基于这个背景,本文探讨了以碳纳米管阵列作为新一代热管吸液芯结构的可能性,从碳纳米管吸液芯的传导、润湿、流动、以及相变传热等基础层面进行了研究,继而从应用的角度考察了碳纳米管吸液芯在热管中的工作特性,形成了碳纳米管吸液芯的理论研究方法,为解决小尺寸下高密度热量传输的实际问题提供了理论依据。本文的主要研究内容与结论如下:首先,对碳纳米管阵列的热传导特性作了一般性的探讨。碳纳米管中沿其轴向的热传递以声子的弹道传输为主,在室温下其本征热导率比金刚石还高一个数量级。检索整理了文献中大量的基于理论和实验的碳纳米管热导率研究结果,分析了碳纳米管材料热导率的一般规律及影响因素。文献报道的热导率数值往往与具体的样品有很大关联性,缺乏统一的衡量标准或公式。借鉴热力学中对应态原理的思想,提出了一种衡量碳纳米管热导率的无量纲化方法,使得原本相差巨大的测量或理论结果归一为相同的规律,并提出了具有明确物理意义的热导率方程。验证结果表明该方程在从绝对零度附近到室温以上的温区内对单根碳纳米管、碳纳米管定向和非定向阵列甚至石墨材料的热导率都可以较准确地预测,为碳纳米管吸液芯的仿真计算提供了必要的、准确的数据支撑。其次,分别从微观和宏观两个层面研究了碳纳米管阵列的润湿特性,建立了相应的理论模型。液体工质在微纳米孔隙中形成的微弯液面是吸液芯中毛细压的来源。用基于表面能最小化原理的数值计算方法,对比分析了碳纳米管阵列的结构参数、与液体的润湿性等参数等对毛细压特性和弯液面中蒸发特性的影响。针对实际阵列中碳纳米管位置随机分布的特征定量讨论了其对润湿效果的影响,发现位置少量地偏离理想位置有利于蒸发传热,但更大偏离时会使得润湿效果变差。对于大规模碳纳米管阵列或微柱阵列中宏观润湿问题求解的困难,提出“等效表面张力”的概念,基于此建立了具有可操作性的求解方法和相应的数值模型,并借助传统数值工具对阵列中的宏观润湿特性进行了讨论。定量化研究碳纳米管吸液芯中液体工质的弯液面扩展和整体扩散趋势,确认了碳纳米管吸液芯在静态润湿方面的优势。继而,针对碳纳米管吸液芯中亚微米尺度下的传热传质过程,探讨了尺度效应的影响及其修正方法。流动方面,以碳纳米管阵列中一个代表性单元为研究对象,用格子Boltzmann方法模拟了液体在碳纳米管壁面处速度滑移对流场的影响,发现尽管主要的流动阻力来自于壁面的阻挡作用,但滑移仍然对流动产生了显著的促进作用。进一步的计算表明毛细压和毛细长度都不同程度地受到特征尺度和速度滑移的影响。传热方面,分析了代表性单元中的热阻网络以及其中的主要热阻要素,计算了碳纳米管阵列传导热阻与阵列密度的关系,并研究了纳米尺度下蒸发效应的作用,发现不蒸发区域和薄膜区域的相对影响大大增加,在两者同时作用下,整体的蒸发速率得到了显著增强,最高提升达到了两个数量级。碳纳米管吸液芯内液体流动和传热过程都由于尺度效应的影响得到了不同程度的增强,使得其吸液和传热能力进一步提升。进而,从动力学角度研究了液体工质在碳纳米管阵列中经毛细力驱动的动态扩散规律。以可视化研究的方法用高速摄像机拍摄了乙醇和丙酮两种工质在碳纳米管阵列中的毛细扩散过程,发现了在不同高度的阵列中存在稳定均匀扩散和不稳定随机扩散两种不同的模式。对稳定均匀扩散过程,基于Washburn的经典扩散模型提出了修正的三阶段扩散假设,结合所提出的毛细力模型和滑移修正的渗透率模型,准确描述了Washburn扩散阶段的时间变化规律,验证了理论模型的正确性。引入蒸发因素的考虑,定量地解释了在第三阶段与Washburn扩散过程偏离的原因。对于阵列润湿蒸干后发现的自组装行为,探讨了对吸液芯工作性能的影响。最后,从热管整体工作性能的角度研究了碳纳米管吸液芯特性的影响。为此,首先基于格子Boltzmann方法建立了可以对热管动态运行过程进行高效数值模拟的理论模型,在典型工况下以文献结果对模型的准确性和正确性进行了验证。用该模型对碳纳米管吸液芯最典型的低渗透率特性进行了参数化研究,结果表明在毛细压足以克服流动阻力时,低渗透率使得吸液芯中流动发生重新分布,但对传热过程的影响不显著。然而,由于减小尺度时毛细压的增加不足以平衡阻力的增加,容易出现干烧是主要的性能瓶颈。进一步地,提出了改进的两级多孔碳纳米管吸液芯结构,使得其在保留高毛细压优势的同时大幅降低了流动阻力,从而提升了热管在小通道下的热流传输能力。通过数值模拟采用两级碳纳米管吸液芯的超薄热管工作特性,证明了它在毫米尺度以下传热通道具有传输高热流密度的能力。更多还原

【Abstract】 Heat pipes are efficient heat transfer devices that utilize the transportation of latent heat of the working substance.They can reach effective thermal conductivities that are over 100 folds higher than that of metals such as copper.Wick is the essential component of heat pipes,which offers the capillary pressure generated from the micro pores to drive the circulation of the liquid.In the last decade,due to the fast progress of technology,especially in the electronics industry,the heating components are being diminished in size and enhanced in power,while the allowed heat transfer channels are continuously being restricted.All those developments pose great challenges on the heat transfer capability of conventional heat pipes.Base on such a background,this paper studied the possibility of adopting carbon nanotube(CNT)array as wicking structure of the next-generation heat pipes,and conducted investigations on the fundamental aspects including the conduction,wetting,liquid transfer and phase transition of carbon nanotube wicks.The theoretical approaches presented in this paper may offer references for solving the practical problem of high-density heat transfer within small sizes.The main research content and conclusions are summarized as follows:Firstly,the heat conduction characteristics of carbon nanotube array has been discussed in general.The axial heat conduction in CNT is dominated by ballistic phonon transportation,thus the intrinsic thermal conductivity is one magnitude higher than diamond.Theoretical and experimental results on the thermal conductivity of CNT has been collected and organized from literature,with which the general characteristics and influence factors of CNT conductivity have been discussed.The previously reported conductivity values tend to have great dependence on samples,with no consistent evaluation criteria or equations.Similar to the corresponding states principle,a non-dimensionalization method has been proposed to evaluate the conductivity of CNT,which makes the vastly diverged measurement and theoretical results collapse into a single curve.A thermal conductivity equation with clear physical interpretation is proposed.Verification over the equation shows that it can accurately predict the thermal conductivity of single CNT,aligned and unaligned CNT array,and even graphite material over the temperature range of absolute zero to over room temperature.The thermal conductivity results provide necessary and accurate data support for the theoretical study of carbon nanotube wicks.Secondly,the wetting characteristics of CNT array has been studied theoretically from both the microscale and macroscale.The micro-meniscii are the sources of the capillary pressure which drives the liquid flow inside the wick.With the numerical method based on the principle of minimizing surface energy,the influences of the configuration parameters of CNT array and wetting parameters upon the capillary pressure and evaporation characteristics have been comparatively analyzed.The effect of random distribution of CNT positions on the wetting characteristics has been quantitatively calculated,which showed that minor offset from ideal positions slightly enhances the evaporation,while larger offsets lead to worsened wetting effects.For the problem in the simulation of macroscale wetting in large number of CNT or micro-pillar array,the idea of “equivalent surface tension” has been proposed,based on which an operational approach and corresponding numerical model have been developed.The macroscale wetting characteristics are then discussed with the calculation via the conventional numerical tool.Quantitative results on the extension of the micro-meniscus and the expanding tendency of the bulk liquid confirm the advantage of carbon nanotube wick in static wetting perspective.Thirdly,for the heat and mass transfer processes of CNT wick under sub-micrometer scale,the scale effects and the correction methods have been investigated.From the perspective of flow,a representative CNT unit from the array has been identified as the physical model,and lattice Boltzmann method has been adopted to simulate the liquid flow to evaluate the effect of velocity slip at the CNT wall.Results show that even though the main flow resistance comes from the blocking of the array,velocity slip still leads to significant enhancement over the flow.Further calculations show that capillary pressure and the capillary length are both affected by the characteristic length scale and the velocity slip.From the perspective of heat transfer,the thermal resistance network in a CNT unit and the dominating resistance components have been analyzed.The dependence of the conductive resistance on the density of the array has been calculated.Analysis over the effects of the nanoscale evaporation shows that the overall evaporation rate is enhanced due to the combined effects of the non-evaporating and thin-film regions,in which the highest enhancement is over two magnitudes.The liquid flow and heat transfer in carbon nanotube wick are enhanced by different levels due to the scale effects,which help further elevate the capability of wicking liquid and transfering heat.Further,the dynamic capillary spreading characteristics of liquid in the CNT array has been studied.Visualization approach has been adopted using a high-speed camera to record the capillary spreading process with ethanol and acetone as the working substances,showing that there are two difference spreading modes in arrays with different heights.For the steady and uniform spreading mode,a corrected three-phase spreading assumption is proposed based upon the classic Washburn spreading model.Combined with the previously obtained capillary model and permeability model corrected with slip effect,the temporal variation law in the Washburn regime has been accurately described,which also validates the correctness of the theoretical model.The inclusion of the evaporation effects help quantitatively explain the deviation from the Washburn curve during the third regime of the spreading.The effects of the self-assembly behavior after the evaporative dry-out has also been discussed.Finally,the characteristics of CNT wick has been studied from the perspective of heat pipes.A highly optimized theoretical model based on lattice Boltzmann method is developed,which can efficiently simulate the transient performance of heat pipes.The accuracy and correctness of the model have been validated through a typical working condition.Using the heat pipe model,a parametric study on the low-permeability wick is conducted,which is an essential characteristic of the CNT wick.Results show that when the capillary pressure is sufficient to overcome the flow resistance,low permeability causes redistribution of the flow profiles,but has relatively insignificant effect on the heat transfer.However,since the increase of capillary pressure is unable to balance the increase of the flow resistance,ease of dry out become the main performance bottleneck for CNT wick.Therefore,improved structure of bi-porous carbon nanotube wick has been proposed,so that the advantage of high capillary pressure can be retained while the liquid flow resistance can be largely reduced,which helps enhance the capability of heat pipes to transfer heat.Numerical investigation on a heat pipe with bi-porous CNT wick proved its ability to transfer high-density heat flux at sub-millimeter channels.更多还原