【作者】 周慧；

【导师】 黄护林；

【作者基本信息】 南京航空航天大学 ， 热能工程， 2010， 硕士

【摘要】 珠状凝结相比其他的传热方式具有非常高的换热系数,实现水蒸汽珠状凝结的换热器将能极大地提高换热效率,减小换热面积,降低成本,因此在工业上有广阔的应用前景。本文首先总结了实现水蒸汽珠状凝结的表面改性方法,并通过分析对比认为结合分子自组装技术和离子束动态混合注入技术,将改性后的高分子聚合物有序地固定在金属表面,形成梯度化的固体表面能将是以后表面改性技术的发展方向。其次采用数值模拟的方法,分别从不同基底材料对凝结换热系数的影响以及表面改性材料的热应力分析、汽液界面热毛细效应对液滴内部流动换热的影响、液滴在基底表面接触角对液滴脱落直径和速率的影响三个方面对影响水蒸汽珠状凝结的换热因素进行了研究,研究结果表明:⑴凝结壁面冷凝侧换热系数基本不受基底热导率的影响。基底表面大液体覆盖处以及表面改性材料与金属基底交界面存在由于热膨胀不一致而引起的热应力集中,最大应力值为17.13 MPa。⑵热毛细效应会引起液滴内部流体对称的“涡流”,这种涡流能有效的促进液滴内部的换热,尤其是在液滴边缘区域。液滴内部温度等值线在热毛细作用下由“水平”分布变为“圆弧”分布,且上下表面平均温差由16℃缩小为3.5℃。热毛细作用下直径为2mm的液滴,其底面换热系数相对不考虑热毛细效应影响时提高了47.1%。⑶接触角滞后随表观平衡接触角和表面粗糙度的增大而增大。当表面粗糙度达到0.01以上时,接触角滞后变化不明显。液滴的脱离直径随接触角滞后的增大而增大。液滴与基底之间平衡接触角越大,液滴脱落速度越快。更多还原

【Abstract】 Compared with other heat transfer means, dropwise condensation has a very high heat transfer coefficient,which can greatly improve the efficiency of heat transfer, reduce the heat transfer area and costs for heat exchanger. So the dropwise condensation of water vapor will be promising in the industrial applications.First, this paper summarized the surface modification methods achieving dropwise condensation of water vapor, and found that the formation of gradient surface energy would be the future development direction of surface modification technology which could be obtained by molecular self-assembly and ion beam dynamic mixing technology to make the appropriate follow-up treatment. Second, the factors that impact on heat transfer of dropwise condensation were studied by numerical simulation method, such as: the condensation heat transfer coefficient and the thermal stress distribution on different substrate materials, the heat transfer inside the droplet considering the thermocapillary convection on gas-liquid interface, the effect of droplet shedding diameter and rate due to droplet contact angle of the substrate surface. Some conclusions were drawn:(1) Condensing heat transfer coefficients didn’t change with the thermal conductivity of the substrate. Thermal stress caused by thermal expansion inconsistencies was localed at the substrate covered by large liquid and interface of surface modification materials and metal, the maximal value was 17.13 MPa .(2) The symmetry vortex was found in the droplet caused by thermocapillary convection, which could effectively promote the heat transfer inside the droplet, especially at the edge of the droplet. The temperature contour in the droplet turned from the“level-line”to“arc-line”due to the thermocapillary convection. The temperature difference of the up-down surface varied from 16℃to 3.5℃. The substrate heat transfer coefficient of a 2mm diameter droplet increased to 47.1% by thermocapillary convection.(3) The hysteresis contact angle increased with the apparent equilibrium contact angle and surface roughness. The hysteresis contact angle almost kept a constant value at the surface roughness reached 0.01. The falling diameter of the droplet increased with the hysteresis contact angle. The larger the balance contact angle between droplet and substrate was, the faster droplets fell.更多还原