Modeling and Optimization of Superhydrophobic Condensation
Modeling and Optimization of Superhydrophobic Condensation
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DOI：
10.1115/1.4024597
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发表时间：
2013-11-01
影响因子：
--
通讯作者：
Wang, Evelyn N.
中科院分区：
文献类型：
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作者：
Miljkovic, Nenad;Enright, Ryan;Wang, Evelyn N.
用于滴状冷凝的超疏水微/纳米结构表面最近受到了极大的关注，这是由于其通过在低于毛细管长度的长度尺度处经由聚结诱导的液滴跳跃而脱落水滴来增强传热性能的潜力。然而，实现这种行为的最佳表面设计需要捕获目前缺乏的运输过程的细节。虽然已经针对平坦疏水性表面开发了全面的模型，但由于动态液滴与结构的相互作用，它们不能直接应用于微/纳米结构表面上的冷凝。在这项工作中，我们开发了一个统一的模型，通过将单个液滴的传热，尺寸分布和润湿形态的超疏水结构表面上的滴状冷凝。两个液滴尺寸分布，这是有效的液滴进行聚结诱导液滴跳跃，并表现出恒定或可变的接触角液滴生长。不同的紧急液滴润湿形态，Cassie跳跃，Cassie非跳跃，或Wenzel，通过耦合的结构几何形状与成核密度，并考虑当地的能量障碍润湿。模型结果表明，一个特定的几何形状范围（0.5-2 μ m），允许形成的聚结诱导跳跃液滴与190%的整体表面热通量增强比传统的平坦滴状冷凝表面。随后，研究了四种典型的自组装单层促进剂涂层对总热流密度的影响。表现出聚结诱导的液滴跳跃的表面对涂层润湿特性（接触角滞后）不敏感（< 5%），这与依赖于重力液滴去除的表面相反。此外，具有低促进剂涂层接触角滞后（< 2度）的平坦表面在结构的长度尺度高于一定尺寸（> 2 μ m）时优于结构化超疏水表面。这项工作提供了一个统一的微/纳米结构的超疏水表面上的滴状冷凝模型，并提供了指导方针的结构化表面的设计，以最大限度地提高传热。
Superhydrophobic micro/nanostructured surfaces for dropwise condensation have recently received significant attention due to their potential to enhance heat transfer performance by shedding water droplets via coalescence-induced droplet jumping at length scales below the capillary length. However, achieving optimal surface designs for such behavior requires capturing the details of transport processes that is currently lacking. While comprehensive models have been developed for flat hydrophobic surfaces, they cannot be directly applied for condensation on micro/nanostructured surfaces due to the dynamic droplet-structure interactions. In this work, we developed a unified model for dropwise condensation on superhydrophobic structured surfaces by incorporating individual droplet heat transfer, size distribution, and wetting morphology. Two droplet size distributions were developed, which are valid for droplets undergoing coalescence-induced droplet jumping, and exhibiting either a constant or variable contact angle droplet growth. Distinct emergent droplet wetting morphologies, Cassie jumping, Cassie nonjumping, or Wenzel, were determined by coupling of the structure geometry with the nucleation density and considering local energy barriers to wetting. The model results suggest a specific range of geometries (0.5-2 mu m) allowing for the formation of coalescence-induced jumping droplets with a 190% overall surface heat flux enhancement over conventional flat dropwise condensing surfaces. Subsequently, the effects of four typical self-assembled monolayer promoter coatings on overall heat flux were investigated. Surfaces exhibiting coalescence-induced droplet jumping were not sensitive (< 5%) to the coating wetting characteristics (contact angle hysteresis), which was in contrast to surfaces relying on gravitational droplet removal. Furthermore, flat surfaces with low promoter coating contact angle hysteresis (< 2 deg) outperformed structured superhydrophobic surfaces when the length scale of the structures was above a certain size (> 2 mu m). This work provides a unified model for dropwise condensation on micro/nanostructured superhydrophobic surfaces and offers guidelines for the design of structured surfaces to maximize heat transfer.