纳米粒子构建表面的超疏水性能实验研究

采用疏水纳米粉体压片法和岩心吸附法构建了具有微纳米结构的表面,测试了这些表面的接触角,拍摄了水滴在吸附纳米粒子的岩石表面的滚动过程照片,采用扫描电子显微镜(scanning electron microscope,SEM)检测了表面的微结构.实验结果表明：无机纳米粒子经弱疏水性材料修饰后,其表面润湿性由强亲水变为强疏水;疏水纳米粒子吸附表面的接触角均大于120°,滚动角约7°,显示出超疏水特性;SEM照片显示,这些超疏水表面是具有不规则微纳米结构的气固复合面,符合Cassie-Baxter的复合表面模型. 关键词： 超疏水 纳米粒子 微纳米结构表面 接触角     

纳米结构影响沸腾换热特性的分子动力学模拟

采用分子动力学方法模拟液体在纳米结构表面的快速沸腾过程.主要研究了纳米结构表面粗糙度以及栏栅形和棋盘形两种排列方式对液体快速沸腾过程以及换热特性的影响.研究结果表明,随着纳米结构表面粗糙度的增加,栏栅形和棋盘形纳米结构表面液体沸腾起始时间均提前.当栏栅形和棋盘形纳米结构表面粗糙度相同时,棋盘形纳米结构表面会进一步缩短液体沸腾起始时间.形成这种现象的原因是纳米结构表面粗糙度的增加,增加了固液接触面积,提高了初始时刻热通量,减小了固液界面热阻,导致表面附近液体动能增大,增大了液体高度方向的温度梯度,有利于液体发生沸腾.当纳米结构表面粗糙度相同时,棋盘形纳米结构表面具有较小的界面热阻,从而缩短了沸腾所需要的时间.     

混合润湿性柱状纳米结构对铜板上纳米氩膜沸腾传热的影响

混合润湿性对固/液相互作用有显著影响,因此对提高相变过程中的传热速率有积极作用.采用分子动力学模拟方法研究了柱状纳米结构表面混合润湿性对池沸腾传热的影响.分析了混合润湿性和纳米结构柱高对液体起始沸腾时间和温度的影响及其机理.结果表明,疏水比例和柱高会影响爆沸的起始温度和时间.与纯亲水壁相比,增加疏水比改变了固液界面性质,可以降低沸腾温度,更容易突破势能壁垒,使液体起始沸腾时间提前,并且随着疏水比的增加,不同柱高下的沸腾温度降低;当疏水比相同时,增加柱高扩大了混合润湿性的影响,也能降低沸腾起始温度并使液体起始沸腾时间提前.这为设计微纳粗糙结构和混合润湿表面以强化沸腾传热提供了思路.     

两步法制备超疏水性ZnO纳米棒薄膜

采用两步法制备了超疏水性ZnO纳米棒薄膜,在用磁控溅射在普通玻璃衬底上生长一层ZnO籽晶层基础上,利用液相法制备了空间取向高度一致的ZnO纳米棒阵列,经修饰后由亲水性转变为超疏水性.用扫描电子显微镜观察了纳米棒的表面结构,用接触角测量仪测出水滴在ZnO纳米棒薄膜表面的接触角为151°±05°,滚动角为7°.用Cassie模型对ZnO纳米棒薄膜的超疏水性进行了验证. 关键词： ZnO纳米棒 超疏水 两步法     

二氧化钛纳米管界面沸腾特性

TiO2纳米管阵列表面具有超亲水,高比表面积的特点.本文实验研究其作为沸腾换热表面的池沸腾特性.通过与Ti金属表面池沸腾实验的比较,表明TiO2纳米管阵列表面具有优秀的沸腾换热能力.     

纳米颗粒吸附岩心表面的强疏水特征

通过将疏水的纳米颗粒吸附在岩心微通道壁面,可以形成具有类荷叶表面的双重微结构表面,从而在注水开发的过程中在岩心微通道壁面产生水流滑移,达到降低注水压力、增加注水量的目的.研究纳米颗粒吸附岩心切片表面的强疏水特征对纳米颗粒吸附法减阻技术具有重要的意义.本文简要叙述了荷叶、蚊子腿以及水黾腿的超疏水特征;介绍了制备具有亚微米、纳米双重微结构的强疏水表面的纳米颗粒吸附法;给出了规则排列时纳米颗粒吸附岩心切片表面的强疏水特征的物理机制,根据真实的纳米颗粒吸附岩心切片,给出了接触角的范围,计算结果与实验数据一致.岩心流动实验结果表明,经纳米颗粒分散液处理后,岩心的平均水相渗透率提高94%.     

碳纳米管阵列超双疏性质的发现

用高温裂解酞菁金属络合物方法制备了几种具有不同形貌的阵列碳纳米管膜 ,并对其超疏水和超双疏性质进行了研究 .对于具有均匀长度和外径的阵列碳纳米管膜 ,文章作者发现 ,在未经任何处理时 ,其表现出超疏水和超亲油性质 ,与水的接触角为 15 8 5± 1 5° ,与油的接触角为 0± 1 0°.经氟化处理后 ,则表现出超双疏性质 ,与水和油的接触角分别为 171± 0 5°和 16 1± 1 0° .对具有类荷叶结构的阵列碳纳米管膜 ,其表面形貌与荷叶的十分接近 ,且在未经任何处理时所表现出的超疏水性也与荷叶的非常接近 ,与水的接触角为 16 6° ,滚动角为 8° .这种超疏水和超双疏性质是由表面的纳米结构以及微米结构和纳米结构的结合产生的 .这一发现为无氟超疏水表面 界面材料的研究提供了新的思路     

超疏水表面膜态池沸腾传热实验研究

通过纳米颗粒沉积法在不锈钢球上制备了静态接触角大于160°的超疏水表面,并采用淬火法对该不锈钢球在过冷度为0℃~25℃的去离子水中的瞬态池沸腾传热过程进行了实验研究。结果表明,由于表面的超疏水特性,整个瞬态沸腾过程直至极低表面过热度时始终处于膜态沸腾状态。不锈钢球的冷却速率随着过冷度的增大而提高,其膜态沸腾的热流密度和平均努塞尔数也随着过冷度的增加而近似呈线性增长的趋势。对汽膜演化过程的可视化观察发现,在过冷度较大时蒸汽产生量减少,导致汽膜层扰动减弱、汽液相界面趋于平稳。     

纳米结构超疏水表面冷凝现象的三维格子玻尔兹曼方法模拟

采用三维多相流格子玻尔兹曼方法 (lattice Boltzmann method, LBM),对纳米结构超疏水表面液滴的冷凝行为进行模拟研究.通过Laplace定律和光滑表面的本征接触角理论对三维LBM模型进行定量验证.模拟分析了超疏水表面纳米阵列的几何尺寸和润湿性的局部不均匀性对冷凝液滴形核位置和最终润湿状态的影响规律.结果表明,较高的纳米阵列使液滴在纳米结构间隙的上部侧面和底部优先形核长大,通过采用上下不均匀的间隙可避免液滴在底部形核长大,而在上部侧面形核的冷凝液滴在生长过程中向上运动,其润湿状态由Wenzel态转变为Cassie态;较低的纳米阵列使液滴在纳米结构底部优先形核长大,液滴的最终润湿状态为Wenzel态;润湿性不均匀的纳米结构表面使液滴在阵列顶端亲水位置处优先形核长大,成为Cassie态.冷凝液滴在不同几何尺寸的纳米结构表面上的最终润湿状态的模拟结果与文献报道的实验结果符合良好.通过模拟还发现,冷凝液滴在生长过程中的运动行为与液滴统计平均作用力的变化有关.本文的LBM模拟再现了三维空间中液滴的形核、长大和润湿状态转变等物理现象.     

纳米特性表面圆柱型高速喷流沸腾临界热流密度的实验研究

制备了分别具有亲水和疏水特性的2种纳米特性表面,对具有不同固液接触角的传热面上圆柱型高速水喷流沸腾的沸腾临界热流密度(CHF)进行了系统的稳态实验研究,重点考察了喷流速度,过冷度和传热面固液接触角对CHF的影响。通过研究整理了传热面固液接触角和CHF之间的实验关系。对作者过去提出的饱和液与过冷液圆柱喷流沸腾CHF的半理论关系式进行了扩展和改进,使公式扩展到高流速范围和广泛的固液接触角范围。改进的关系式与实验数据符合得很好.     

Eulerian–Eulerian simulation of non-uniform magnetic field effects on the ferrofluid nucleate pool boiling

The nucleate pool boiling heat transfer of ferrofluid on a horizontal plate in the presence of a non-uniform magnetic field has been studied numerically using Eulerian–Eulerian approach. Also, the wall partitioning model was extended to consider the boiling surface modification by the nanoparticles deposition on the heated surface. Adding nanoparticles causes deterioration in the boiling heat transfer coefficient and void fraction. Moreover, applying the magnetic field intensifies these reductions.     

Experimental investigation into the pool boiling heat transfer of aqueous based γ-alumina nanofluids

This paper is concerned about pool boiling heat transfer using nanofluids, a subject of several investigations over the past few years. The work is motivated by the controversial results reported in the literature and the potential impact of nanofluids on heat transfer intensification. Systematic experiments are carried out to formulate stable aqueous based nanofluids containing γ-alumina nanoparticles (primary particle size 10–50 nm), and to investigate their heat transfer behaviour under nucleate pool boiling conditions. The results show that alumina nanofluids can significantly enhance boiling heat transfer. The enhancement increases with increasing particle concentration and reaches ∼ ∼40% at a particle loading of 1.25% by weight. Discussion of the results suggests that the reported controversies in the thermal performance of nanofluids under the nucleate pool boiling conditions be associated with the properties and behaviour of the nanofluids and boiling surface, as well as their interactions.This revised version was published online in August 2005 with a corrected issue number.     

Experimental investigation into the pool boiling heat transfer of aqueous based γ-alumina nanofluids

This paper is concerned about pool boiling heat transfer using nanofluids, a subject of several investigations over the past few years. The work is motivated by the controversial results reported in the literature and the potential impact of nanofluids on heat transfer intensification. Systematic experiments are carried out to formulate stable aqueous based nanofluids containing γ-alumina nanoparticles (primary particle size 10–50 nm), and to investigate their heat transfer behaviour under nucleate pool boiling conditions. The results show that alumina nanofluids can significantly enhance boiling heat transfer. The enhancement increases with increasing particle concentration and reaches ∼ ∼40% at a particle loading of 1.25% by weight. Discussion of the results suggests that the reported controversies in the thermal performance of nanofluids under the nucleate pool boiling conditions be associated with the properties and behaviour of the nanofluids and boiling surface, as well as their interactions.     

The influence of an electric field on the heat transfer rate during film boiling of stagnant fluids

This paper describes the influence of an electric field on the heat transfer rate during film boiling of stagnant fluids (pool boiling) on a horizontal surface. Modelling the influence of an electric field on the heat transfer during film boiling requires the knowledge of the equilibrium shape of the liquid-vapour interface. This equilibrium shape is calculated using an iterative solution technique. In each iteration step a fourth order Runge-Kutta technique is used to calculate the shape of the interface and a boundary element method is used to calculate the electric field. It is shown that the vapour bubbles formed on this interface become elongated in the presence of an electric field. This effect is accounted for in the existing heat transfer models in order to describe the influence of the electric field on the heat transfer coefficient during film boiling.     

A fractal study for nucleate pool boiling heat transfer of nanofluids

In this paper, a fractal model for nucleate pool boiling heat transfer of nanofluids is developed based on the fractal distribution of nanoparticles and nucleation sites on boiling surfaces. The model shows the dependences of the heat flux on nanoparticle size and the nanoparticle volume fraction of the suspension, the fractal dimension of the nanoparticle and nucleation site, temperature of nanofluids and properties of fluids. The fractal model predictions show that the natural convection stage continues r...     

Flow boiling heat transfer of alumina nanofluids in single microchannels and the roles of nanoparticles

This study investigates flow boiling heat transfer of aqueous alumina nanofluids in single microchannels with particular focuses on the critical heat flux (CHF) and the potential dual roles played by nanoparticles, i.e., (i) modification of the heating surface through particle deposition and (ii) modification of bubble dynamics through particles suspended in the liquid phase. Low concentrations of nanofluids (0.001–0.1 vol.%) are formulated by the two-step method and the average alumina particle size is ~25 nm. Two sets of experiments are performed: (a) flow boiling of formed nanofluids in single microchannels where the effect of heating surface modification by nanoparticle deposition is apparent and (b) bubble formation in a quiescent pool of alumina nanofluids under adiabatic conditions where the role of suspended nanoparticles in the liquid phase is revealed. The flow boiling experiments reveal a modest increase in CHF by nanofluids, being higher at higher nanoparticle concentrations and higher inlet subcoolings. The bubble formation experiments show that suspended nanoparticles in the liquid phase alone can significantly affect bubble dynamics. Further discussion reveals that both roles are likely co-existent in a typical boiling system. Properly surface-promoted nanoparticles could minimize particle deposition hence little modification of the heating surface, but could still contribute to the modification in heat transfer through the second mechanism, which is potentially promising for microchannel applications.     

NUCLEATE POOL BOILING UNDER DC ELECTRIC FIELD

This article deals with an experimental study of the influence of a DC uniform electric field on the nucleate boiling heat transfer. Electrohydrodynamic (EHD) effects on heat transfer coefficients for dielectric liquids are quantitatively investigated by performing experiments on various liquids with different properties. In these experiments, n-pentane, R-113, and R-123 are used as working fluids and the boiling phenomenon takes place on a horizontal plane copper surface. The experimental results have shown: (1) a threefold increase of nucleate pool boiling heat transfer coefficients, (2) a threefold increase of the critical heat flux (CHF), and (3) the disappearance of the hysteresis phenomenon. For nucleate pool boiling and CHF regimes, heat transfer laws based on dimensionless numbers are proposed. The results obtained by the proposed EHD model are in good agreement with the experimental results.     

Prediction of heat transfer of nanofluid on critical heat flux based on fractal geometry

Analytical expressions for nucleate pool boiling heat transfer of nanofluid in the critical heat flux (CHF) region are derived taking into account the effect of nanoparticles moving in liquid based on the fractal geometry theory. The proposed fractal model for the CHF of nanofluid is explicitly related to the average diameter of the nanoparticles, the volumetric nanoparticle concentration, the thermal conductivity of nanoparticles, the fractal dimension of nanoparticles, the fractal dimension of active cavities on the heated surfaces, the temperature, and the properties of the fluid. It is found that the CHF of nanofluid decreases with the increase of the average diameter of nanoparticles. Each parameter of the proposed formulas on CHF has a clear physical meaning. The model predictions are compared with the existing experimental data, and a good agreement between the model predictions and experimental data is found. The validity of the present model is thus verified. The proposed fractal model can reveal the mechanism of heat transfer in nanofluid.     

光管及窄环隙流道池沸腾换热实验研究

本文在常压下以水为工质,对管外及窄环隙流道池沸腾换热进行了可视化实验,结果表明：汽泡扰动并不是沸腾换热系数高的主要原因,液体过冷使核沸腾的换热能力降低,窄隙流道内的沸腾换热机理与普通的大容积沸腾没有明显区别。文中还根据观察和测量到的结果对一些换热现象重新进行了解释。