Department of Mechanical Engineering , Massachusetts Institute of Technology , 77 Massachusetts Avenue , Cambridge , Massachusetts 02139 , United States.
Shiley-Marcos School of Engineering , University of San Diego , San Diego , California 92110 , United States.
Langmuir. 2018 Apr 17;34(15):4658-4664. doi: 10.1021/acs.langmuir.7b04203. Epub 2018 Apr 5.
Vapor condensation is routinely used as an effective means of transferring heat or separating fluids. Filmwise condensation is prevalent in typical industrial-scale systems, where the condensed fluid forms a thin liquid film due to the high surface energy associated with many industrial materials. Conversely, dropwise condensation, where the condensate forms discrete liquid droplets which grow, coalesce, and shed, results in an improvement in heat transfer performance of an order of magnitude compared to filmwise condensation. However, current state-of-the-art dropwise technology relies on functional hydrophobic coatings, for example, long chain fatty acids or polymers, which are often not robust and therefore undesirable in industrial conditions. In addition, low surface tension fluid condensates, such as hydrocarbons, pose a unique challenge because common hydrophobic condenser coatings used to shed water (with a surface tension of 73 mN/m) often do not repel fluids with lower surface tensions (<25 mN/m). We demonstrate a method to enhance condensation heat transfer using gravitationally driven flow through a porous metal wick, which takes advantage of the condensate's affinity to wet the surface and also eliminates the need for condensate-phobic coatings. The condensate-filled wick has a lower thermal resistance than the fluid film observed during filmwise condensation, resulting in an improved heat transfer coefficient of up to an order of magnitude and comparable to that observed during dropwise condensation. The improved heat transfer realized by this design presents the opportunity for significant energy savings in natural gas processing, thermal management, heating and cooling, and power generation.
蒸汽冷凝通常被用作一种有效的传热或分离流体的手段。膜状冷凝在典型的工业规模系统中很常见，在这些系统中，由于许多工业材料具有高表面能，凝结的流体形成了一层很薄的液膜。相比之下，在滴状冷凝中，凝结物形成离散的液滴，这些液滴会生长、合并和脱落，从而使传热性能提高一个数量级。然而，目前的滴状冷凝技术依赖于功能疏水性涂层，例如长链脂肪酸或聚合物，这些涂层往往不坚固，因此在工业条件下并不理想。此外，低表面张力的流体凝结物，如碳氢化合物，带来了独特的挑战，因为用于除去水的常见疏水性冷凝器涂层（表面张力为 73 mN/m）通常不排斥表面张力较低的流体（<25 mN/m）。我们展示了一种使用多孔金属芯吸作用驱动的重力驱动流动来增强冷凝传热的方法，该方法利用了冷凝物对表面的亲和力，并且还消除了对冷凝物疏水性涂层的需求。充满冷凝物的芯吸具有比在膜状冷凝中观察到的流体膜更低的热阻，从而导致传热系数提高了一个数量级，与在滴状冷凝中观察到的传热系数相当。这种设计实现的改进传热为天然气处理、热管理、加热和冷却以及发电中的节能提供了机会。
