【作者】 李启明；

【导师】 王补宣；

【作者基本信息】 清华大学 ， 动力工程及工程热物理， 2011， 博士

【摘要】 电子元器件的集成化和小型化，迫切需要发展更高效的散热技术。振荡热管因其良好的传热特性，成为热门散热技术之一。在基液中添加纳米颗粒形成稀颗粒体积浓度的纳米流体可望改善振荡热管的实际运行性能。这种纳米流体振荡热管受到学术界和工业界的广泛关注，然而相关基础物理现象和过程特性尚未得到充分认识。本文针对纳米流体振荡热管内部传递过程的基础性问题开展深入的研究，分析纳米颗粒改善传递性能的机理，为纳米流体振荡热管的实际应用提供必要基础。实验研究中搭建了可视化振荡热管测试系统，分别对纯流体和纳米流体振荡热管的运行性能进行研究。采用竖直振荡热管，对工质流动的振荡特性进行了系统的研究，结果表明，振荡热管在正常高效运行时，内部流动呈现小振幅慢速振荡和大振幅快速振荡交替进行的形式，交替转变受加热段液柱内的剧烈核化和快速相变所控制。在相同的加热功率下，纳米流体振荡热管能在较低的加热段温度下发生这种转变，从而有利于热量的快速传递。采用水平振荡热管，对工质的流型特性进行了细致的研究，结果表明，纯流体工质传热效果较差，流型以简单液柱状流动为主，容易出现加热段蒸干现象。纳米流体在热流密度相对较低时，内部也以简单液柱状流动为主，但传热能力相对略强；热流较高时，发生流型转变，出现了泡状流、弹状流甚至环状流，流型的转变，提升了振荡热管的热流极限，改善了传热性能。以经典核化理论为基础，引入溶液热力学的相关理论，建立了纳米流体的核化模型。通过热力学分析得出，体相中的颗粒在汽液界面处的聚集减小了核胚的半径，同时降低了核化势垒，有利于核化的发生；纳米颗粒在三相接触线薄液膜区的有序排列以及在壁面的沉积，改善了流体在壁面的湿润性，有利于液体的蒸发。并通过毛细管内液汽相变实验证实：纳米流体更易核化形成汽泡，汽泡/汽柱的生长速率更快。在纯流体振荡热管模型的基础上，建立了纳米流体振荡热管运行的简化模型，进行了数值模拟，发现纳米颗粒对流体核化和液汽相变特性的改善是提高振荡传热性能的一个重要因素，为进一步开展纳米流体振荡热管性能的研究及优化提供了基础。更多还原

【Abstract】 As electronic devices continuously decrease in size, traditional cooling methodsand technologies are facing great challenges and an urgent demand appears for higherefficiency cooling methods. Oscillating heat pipes, for its high heat-removal efficiency,attract many researchers’ attention. It is found that capabilities of oscillating heat pipecould be improved by adopting nanoparticle suspension and/or nanofluids of diluteparticle volume concentration as the working fluid. The nanofluid oscillating heat pipeoffers a new future to electronics cooling. To provide a knowlegement base fordeveloping higher heat-efficiency cooling technologies, basic phenomena and transportcharacteristics of the nanofluid oscillating heat pipe need to be explored to understandthe mechanism of the improvement of heat efficiency by nanoparticles addition.A series of experimental observations were conducted on oscillating heat pipeswith nanfluids and DI water for comparing their operating performance and internalflow phenomena. In a vertical oscillating heat pipe system, internal flow oscillationswere studied. it was found that when the oscillating heat pipe operated normally, theinternal flow was alternately small-amplitude slow oscillation and large-amplitude fastoscillation. The transition depended on drastic nucleation and quick phase change insidethe liquid slug in the evaporator. At the same heat load, nanoparticles addition made thedrastic nucleation and quick phase change to happen at relatively low temperatures. Inanother horizontal oscillating heat pipe system, the internal flow patterns were the focus.In the DI water heat pipe, simple column flow was mainly observed and the evaporatordried out at a relatively high heat load. In the nanofluid heat pipe, simple column flowwas also mainly observed at relatively low heat load but the heat transfer performancewas improved compared with the DI water; however at relatively high heat loads,several interesting flow patterns were observed: bubbly flow, slug flow and annularflow. The flow-pattern changes ensured oscillatory flow inside the oscillating heat pipeand thereby increased operation the limit of operation heat flux and improved the heattransfer performance of the oscillating heat pipe.Based on the classical nucleation theory and the thermodynamic theory of solution,a new model of nanofluid nucleation was built up. It was found that nanoparticles accumulation at the vapor-liquid interface decreased the embryo bubble radius and thepotential barrier of nucleation, and facilitated liquid nucleation. Additionally,nanoparticles accumulation in the three-phase contact region and deposition duringnucleate boiling on the wall both improved wettability of nanofluid on the wall, whichimproved liquid evaporation. An experiment about liquid-vapor phase change inside acapillary tube was conducted to further validate the nucleation and wettingcharacteristics of nanofluid: nucleation was easier and growth rates ofbubbles/vapor-plugs were faster in nanofluids than in pure liquids.Based on pure fluid oscillating heat pipe computation models, a simplified modelfor nanofluid oscillating heat pipe was built up to model heat and mass transfer innanofluid heat pipes. Results showed that the easy liquid-vapor nucleation andphasechange was the key factor for the improvement of heat transfer of the nanofluidheat pipe. This model can provide essential information for further investigations onnanofluid oscillating heat pipes.更多还原