【作者】 陈俊；

【导师】 史琳；

【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】 纳米流体是指将纳米颗粒悬浮于基液中形成的一类换热流体；纳米流体在强化传热方面表现出许多优良特性，而输运性质是研究其强化传热的基础，成为了国内外研究热点。目前研究者大多采用实验的方法，实验结果间差异性较大。本文主要利用分子动力学模拟方法，研究了以氩、水和制冷剂为基液的纳米流体的导热系数、粘度等输运参数的作用机理，以加深对纳米流体特性的认识。本文首先推导了适用于纳米流体输运参数的自相关函数公式，建立了纳米流体导热系数、粘度等输运参数的模拟体系，编制了计算程序；并对模拟稳定性问题进行了分析，提出了分子平均自由力程的新概念，进而推导出用于模拟过程中最佳步长选择的计算公式和方法，为实现纳米流体分子模拟的稳定性奠定了基础。对纳米流体的制备进行了研究。实验观察了纳米颗粒在去离子水、制冷剂中的分散情况，总结了比较合适的制备方法，讨论了最佳振荡时间。模拟观察了纳米颗粒在基液中的布朗运动、颗粒吸附液体层、颗粒团聚的现象，并讨论了颗粒尺度、体系温度对分散稳定性的影响。实验测量了多种纳米流体的导热系数和粘度。实验结果为分子动力学模拟选取模拟条件提供了依据。利用本文建立的纳米流体分子动力学模拟体系，对以氩、水和制冷剂为基液的纳米流体的导热系数、粘度进行了模拟研究。对颗粒体积百分比、温度、颗粒尺寸、颗粒形状、比表面积及球形度等可能影响纳米流体有效导热系数、有效粘度的因素进行了分析，得到了纳米流体有效导热系数、有效粘度与不同因素之间的变化关系。在机理分析过程中，提出了单位体积输运参数增强比值TPeV (TransportProperties Enhancement Ratio by Nanoparticles Volume Fraction)，便于分析纳米颗粒对纳米流体的作用。从TPeV角度分析，纳米流体存在一个最佳体积百分比和最大体积百分比。对纳米流体导热系数、粘度内部作用进行了分解分析。发现基液在颗粒作用下的导热系数的增强，是纳米流体导热系数增大的主因，而纳米流体分子间作用势能是改变基液导热系数的本质。发现基液在颗粒的作用下属性的改变是影响纳米流体粘度的主要原因，当颗粒体积百分比超过某一值后基液流动性急剧改变。更多还原

【Abstract】 Nanofluids, a sort of researching working fluid with nanoparticles inside, havebeen proposed as a route for surpassing the performance of currently available heattransfer liquids in the near future. This dissertation presents reference data, experimentaldata, molecular dynamics simulation results and theroretical analysis of the mechanismof nanofluids transport properties, such as thermal conductivity, viscosity and diffusioncoefficient.The auto correlation function formulas of nanofluids’ transport parameters werededuced in this dissertation and a simulation system was established to investigatenanofluids’ transport parameters, including thermal conductivity, viscosity anddiffusion coefficient, according to the deduced formulas. According to analysis thestability of the transport parameters integral functions, a new conception was proposed—mean-free-force-path. Combined the mean-free-force-path with related formulas andtheories, a new formula was deduced which used to calculate the time step in moleculardynamics simulations.The nanofluids’ dispersivity and stablility were also investigated throughexperiments and simulations. The dispersion stability of nanoparticles indeionized water and refrigerants were investigated by experiments. The suitablepreparation method was summarized and the optimum oscillation time wasdiscussed. Molecular dynamics simulations were used to investigate Brownianmotion of the nanoparticles in base fluid. The absorbed liquid molecular layeraround the nanoparticle and aggregation of nanoparticles were also investigated,and other factors, such as the particle size and temperature, were also discussed.The nanofluids’ transport properties, with the base fluid of water or refrigerantR123or R141b, were investigated by combined with reference data, experimental data,molecular dynamics simulations results and theoretical analysis of the mechanism. Mostof the important factors, such as temperature, volume fraction, nanoparticle size,nanoparticle shape, surface area and sphericity, were discussed in the dissertation whicheffect the nanofluids’ thermal conductivity and viscosity.A new parameter, TPeV, was introduced to analysis the mechanism ofnanoparticles in the nanofluid and TPeV is the abbreviation of Transport Properties Enhancement Ratio by Nanoparticles Volume Fraction. CombinedTPeV with the reference data, we believe that there exist the best volumefraction and maxium volume fraction of nanoparticles in nanofluid.The collective heat flux vector of nanofluids’ thermal conductivity was separatedinto the liquid contribution, the nanoparticles contribution, and the interaction betweenliquid and nanoparticles; the results show that the contribution of the nanoparticles tothe nanofluids’ thermal conductivity meet the Maxwell’s effective medium theory, butthe liquid parts’ thermal conductivity was higher than the fluid with no nanoparticlesitself. The heat flux vector was also decomposed into the kinetic energy, theintermolecular potential, the pair virial function and the interaction between allfunctions; the results show that the most remarkable aspect is the increase ofpotential contribution which plays an insignificant single base fluid. The stresstensor of viscosity can also separated into the liquid contribution, the nanoparticlescontribution, and the interaction between liquid and nanoparticles by work fluids; theresults show that there is little difference of viscosity between Einstein’ theory andnanofluids with few nanoparticles volume fraction; when the volume fraction is higherthan a certain value, the effective viscosity become to increase sharply which mainlydue to the changing of fluid’s rheology by enough nanoparticles.更多还原