【作者】 王宏；

【导师】 廖强；

【作者基本信息】 重庆大学 ， 工程热物理， 2008， 博士

【摘要】 大量研究表明滴状凝结是一种高效传热方式,其传热系数是相应膜状凝结传热系数的几倍至几十倍。凝结表面上液滴的运动和聚合过程对凝结液滴的生长和脱离有重要的影响。相对于传统凝结表面依靠重力排除凝结液滴而言,在梯度表面能材料表面上,凝结液滴可以以自迁移方式运动,从而提供了重力以外的排除凝结液滴方式。因此,对于水平放置或者其它失重状态下的梯度能表面,都能够及时排除其表面上的凝结液滴,有效促进滴状凝结换热系数的提高。此外,梯度表面能材料上的液滴自驱动运动在燃料电池、热管换热器、微机电等领域亦具有潜在的重要应用价值。目前,液滴在梯度表面能材料表面上运动机理及凝结换热特性等方面开展的研究工作还非常有限。为了研究液滴聚合影响梯度表面能材料上液滴运动的机理,本文首先采用了可视化实验手段系统的研究了大气环境中等温条件下均质表面上的液滴聚合特性,获得了液滴聚合过程中聚合液滴接触线、液桥半径及接触角随时间变化的规律;分析了界面性质、液滴物性、表面倾角以及液滴大小等因素对液滴聚合的影响,并将等径与非等径液滴的聚合进行了比较。结果表明两个液滴聚合后呈衰减性振荡,这是由两液滴凹凸液面压差造成的振荡,导致液滴内部的粘性耗散,其能量来自于液滴聚合后,气液界面面积的减少而释放出的表面能。固体的表面属性对液滴聚合过程中液桥半径和接触角的变化,以及聚合振荡时间均有显著的影响;对于相同固体表面上液滴聚合,液滴自身属性对聚合过程影响表现为:液滴的粘性的越大,液桥半径和接触角振荡的频率和振幅越小,振荡时间越短,两侧接触线收缩的幅度越小。最后,本文对液滴聚合中液桥半径随时间的变化进行拟合( 0 <τ<τ0),建立实验关联式,结果表明:液滴聚合初始阶段,液桥半径随时间的的变化满足R y= atb形式,符合R y∝tb定律。a和b的值与液滴直径的大小、液滴的粘性、表面性质和表面的倾角等因素有关,一般来说0<a<1,0<b<0.5。采用VOF方法对水平均质表面上等径液滴聚合进行了模拟,其计算得到液滴形态变化和液桥变化与实验结果基本一致。在均质表面液滴聚合特性研究的基础上,本文采用化学气相沉积方法制备梯度表面能材料,实现了液滴在该表面上的自迁移,通过对梯度表面能材料表面上液滴运动行为的研究,获得了液滴运动速度、接触角的变化规律,并从能量转换的角度分析了引起液滴自迁移的主要原因。结果表明蒸馏水液滴在水平梯度表面能材料表面上的峰值运动速度能达到42 mm/s,运动距离约为3 mm。同时,液滴的运动可以分为加速运动区和减速运动区两个阶段;当液滴峰值速度较小而减速运动区较大时,液滴运动会呈现蠕动的现象。驱使液滴运动的最主要动力是来自于作用在三相接触线上的非平衡表面张力的合力,由润湿梯度引起的液滴轮廓非对称性分布所导致的液滴内部流体环流的作用也是主要的驱动力之一。从液滴运动过程中能量转换的关系分析,液滴的固-气界面能和重力势能减少,释放出来的能量转化为液滴的动能、液-气界面能、固-液界面能以及耗散功,而固-气界面表面能的减少是促使液滴自发定向运动的主要原因。采用VOF方法对水平梯度表面能材料上液滴运动进行模拟,得到液滴形态变化规律与实验结果基本吻合。通过对大气环境中水平梯度表面能材料上液滴聚合过程的实验研究,发现了梯度表面材料上液滴聚合与均质表面上液滴聚合具有显著的区别,结果表明液滴聚合过程是加速液滴在梯度表面上运动的主要原因,同时发现薄液膜的存在亦对液滴运动具有有效的促进作用。通过对饱和水蒸汽在梯度表面能材料表面上滴状凝结换热实验,发现水蒸汽凝结状态下的凝结液滴峰值运动速度达到200 mm/s,表面换热系数先随过冷度增大而增大,到达一个最大值后,反而随之减小;凝结表面倾角变大,表面换热系数也随之增大,且过冷度对表面换热系数的影响越大;表面能梯度越大,表面换热系数也越大。通过改进均质表面上滴状凝结换热理论,并结合梯度表面能材料表面的液滴分布及凝结换热特性,得到了一维水平梯度表面能材料表面上的滴状凝结换热计算模型;通过实验观察确定模型的重要参数-最大液滴半径随过冷度的变化关系,并分别计算得到十二烷基三氯硅烷和辛基三氯硅烷制备的水平梯度表面能材料表面上的滴状凝结平均表面换热系数。模型计算结果与实验值基本吻合。并采用该模型对甲醇和乙醇蒸汽分别在一维矩形梯度表面能材料表面上凝结换热性能进行了预测,对水、甲醇和乙醇蒸汽分别在圆形径向梯度表面能材料表面上凝结换热性能进行了预测。更多还原

【Abstract】 The boiling and condensation heat transfer, as one of the most efficient cooling method, have been paid more attention. Dropwise condensation is a most efficient heat transfer because of its higher surface heat transfer coefficient. Wettability is one of the most important properties of a solid surface and can be applied to propel drops to spread or move on the solid surface without external force. The characteristics of the droplet movement on the gradient surface energy supply a method of eliminating the condensation expect for the gravity. Therefore, the gradient surface energy could be capable of eliminating the condensation in time and improve the condensation heat transfer as for horizontal surface or other gravity free state surface. It can be expected that the interesting and inspiring research on liquid drops motion on the gradient energy surface will significantly contribute to condensation heat transfer enhancement of heat exchanger, biological applications involving cell cytometry studies as well as design and operation of microfluidic devices. Therefore, this research possesses high learning value and engineering applicable value. Presently, research on this aspect is just in the first step in the international academic, and the experiment and mechanism research on this field is limited.In the present study, we adopted the visualized experiments to investigate the coalescence of droplets on the homogenous surface under ambient conditions. The evolution of the three-phase contact line, liquid bridge and contact angle of the coalescing droplets with time were analyzed. The results showed that coalescing drops behaved as a typical damped oscillation after coalescence as a result of differential pressure in concave convex liquid level of the two drops. The energy consumption resulted from viscous dissipation in process of internal flow within liquid drop was compensated by the released surface energy due to the decrease of drop interface area in coalescing. The property of the solid surface had obvious influence on the characters of the coalescence. When the coalescence on the same solid surface, the higher viscosity fluid exhibited the lower oscillation amplitude and frequency of liquid bridge as will as the contact angle, and oscillated for a short time, and decreased the amplitude of the shrinkage of contact lines. Finally, we used a function to fit the evolution of the liquid bridge at the initial rapid growth of a meniscus between the droplets( 0 <τ<τ0), and established experimental correlative equation. The evolution of the liquid bridge with time satisfied with the function, R y= atb. It conformed to the law R y∝tb which could be conveniently governed by adjusting the parameters a, b, and the value of a and b were related with diameter of drops, viscosity, character and inclination of surface, and 0<a<1,0<b<0.5 generally. The volume-of-fluid (VOF) method employed to investigate the coalescence of equal-size water droplets on the homogenous surface, and the results of the numerical investigation agreed with the results of experiments on the variation of the shape and the liquid bridge of the coalescence.Following the works, the self-propelled liquid droplet on the gradient surface was recorded by high-speed camera with 500 fps (Redlake MotionXtra HG-100K) and analyzed by professional graphic software. The variation of the velocity and contact angle of the droplet was investigated on the horizontal gradient surface, and system energy transition was used to understand the mechanics of the liquid droplet motion on the gradient energy surface. The main findings and conclusions indicated that the liquid droplets were self-propelled to move horizontally or uphill from hydrophobic zone to hydrophilic zone on both horizontal and inclined surface with gradient surface energy, accompanying with deformation from hemisphere to flat. The peak velocity of water droplet of 2μl in volume reached up to 42 mm/s on the horizontal surface and the distance was 3 mm. The motion of the droplet on the gradient energy surface was divided into accelerating stage and decelerating stage. The velocity of the droplet increased to reach a peak value in accelerating stage and decreased accompanying with creep motion in decelerating stage for a long time on horizontal surface. In addition, the creep motion was visualized significantly in both accelerating and decelerating stages on the inclined gradient energy surface. The driving force was induced to integrate the unbalance surface tension acting on the whole three-phase contact line. The droplet motion was resulted from the energy transition among interfacial energy, kinetic energy, gravitational potential energy, and viscous dissipation energy. The interfacial energy released from the deformation of the droplet is the main source for the droplet motion. But the effect of gravitational potential energy could be neglected in the present study. The VOF method employed to investigate the movement of the droplet on the horizontal gradient surface, the results of the numerical investigation for the variation of the shape of the droplet basically agreed with the results of experiments.In the further works, the coalescence of droplets on the gradient surface was studied by the visualizing experiments under ambient conditions. The main conclusion showed that the coalescence process was the primary factor to accelerate droplets moving on the gradient surface. The condensation on the gradient surface was investigated by the visualization experiments, also. Dropwise condensation was conducted on the surfaces with gradient surface energy at various inclination angles of 0o, 30o, 60o, and 90o, respectively. The growth, coalescence, motion, and detachment of the condensate droplets were recorded by the high-speed video imaging system. The results showed that the condensate droplets larger than about 1 mm in diameter could move at a peak speed of 200 mm/s from hydrophobic area to hydrophilic area on the horizontal condensing surface with gradient surface energy. The velocity of the condensate droplet was much higher than that of droplet on the surface without gradient surface energy in ambient conditions. A series of parametric was studied, including the effects of heat transfer temperature difference, inclination angle of condensing surface, and gradient of surface energy on the condensation heat transfer were performed in virtue of the photographic results. The experimental results showed that the condensation heat transfer coefficient increased to reach a maximum value and decreased afterwards with increasing heat transfer temperature difference. Larger inclination angle of the condensing surface induced higher condensation heat transfer coefficient due to the action of gravity on the departure, and the motion of the droplet while larger gradient of surface energy led to earlier departure and faster motion of droplet, hence a higher condensation heat transfer coefficient. Finally, a theoretical model was developed for the dropwise condensation heat transfer on the horizontal surface with gradient surface energy. The performance of the methanol and ethanol vapour condensation on the rectangular surface with gradient surface energy were predicted respectively, and the performance of the water, methanol and ethanol vapour condensation on the rectangular surface with gradient surface energy were predicted respectively also. There are some new ideals about the heat transfer model of individual drop and the size distribution model of condensate drop on homogeneous condensation surface.更多还原