Simultaneous dropwise and filmwise condensation on hydrophilic microstructured surfaces
Simultaneous dropwise and filmwise condensation on hydrophilic microstructured surfaces
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DOI：
10.1016/j.ijheatmasstransfer.2017.06.023
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发表时间：
2017-11-01
影响因子：
5.2
通讯作者：
Mitra, Sushanta K.
中科院分区：
文献类型：
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作者：
Orejon, Daniel;Shardt, Orest;Mitra, Sushanta K.
虽然已经发现液体通过微结构的芯吸或扩散对于诸如纺织品、微电子或散热器的应用是有希望的，但是这种结构化表面对冷凝相变的影响受到较少的关注。在亲水性表面上，并为一个固定的微柱纵横比（高度/直径），支柱之间的间距被发现有强烈的影响冷凝的动力学和冷凝物的最终形态。在柱之间具有大间距的微柱的情况下，冷凝物最初逐滴生长，此后，随着冷凝的发展，冷凝物克服柱的高度淹没基底，并且冷凝以膜状冷凝（FWC）方式连续。相比之下，当柱之间的间距减小时，液滴以滴状冷凝（DWC）的方式在柱的顶部处发生膜状冷凝和连续成核、生长和离开。在这种构造中，微结构的几何形状约束柱之间的冷凝物，并且冷凝物界面在微柱高度之上的上升在热力学上是不利的，而柱的顶部充当成核位点。我们将后者的冷凝行为称为同时滴状/膜状冷凝。这些观察是由环境扫描电子显微镜的出色的空间和时间分辨率实现的。提出了一个传热模型，以证明更大的传热性能的同时滴/膜状冷凝行为在这些表面上相比，单独膜状冷凝。增强的传热是可实现的，这是由于在微结构内保持薄膜的能力以及在微柱顶部处的主动滴状冷凝。我们报告的第一次滴状冷凝的发生在一个完全亲水性的润湿性配置没有一个疏水涂层的援助。我们的研究结果铺平了道路的微观结构的发展，以增强冷凝传热。(C)2017爱思唯尔有限公司版权所有
While wicking or spreading of a liquid through microstructures has been found to be promising for applications such as textiles, microelectronics or heat sinks, the effects of such structured surfaces on condensation phase change has received less attention. On a hydrophilic surface and for a fixed micropillar aspect ratio (height/diameter), the spacing between pillars is found to have a strong impact on the dynamics of condensation and on the final morphology of the condensate. In the case of micropillars with a large spacing between pillars, the condensate grows initially dropwise, and thereafter, as condensation develops, the condensate overcomes the pillars' height flooding the substrate, and condensation continuous in a filmwise condensation (FWC) fashion. In contrast, filmwise condensation and the continuous nucleation, growth, and departure of drops at the pillars' tops in a dropwise condensation (DWC) fashion occurs when the spacing between pillars is decreased. In this configuration, the geometry of the microstructures constrains the condensate between the pillars and rise of the condensate interface above the micropillars' height is not thermodynamically favorable, while the top of the pillars act as nucleation sites. We refer to this latter condensation behavior as simultaneous dropwise/filmwise condensation. These observations were enabled by the excellent spatial and temporal resolution of Environmental Scanning Electron Microscopy. A heat transfer model is proposed to demonstrate the greater heat transfer performance of the simultaneous dropwise/filmwise condensation behavior on these surfaces when compared to solely filmwise condensation. The enhanced heat transfer is realizable due to the ability to maintain a thin film within the microstructures and to the active dropwise condensation at the micropillars' tops. We report for the first time the occurrence of dropwise condensation on a completely hydrophilic wettability configuration without the assistance of a hydrophobic coating. Our findings pave the way to the development of microstructures for enhanced condensation heat transfer. (C) 2017 Elsevier Ltd. All rights reserved.