Transient heat transfer and droplet dynamics during dropwise condensation on lubricant-infused surfaces

蒸汽冷凝在自然界和许多工业应用中无处不在，包括发电、空调和集水。通常，水凝结成厚（~mm）的膜，作为热传递的热障。相比之下，水滴式冷凝，其中冷凝物形成离散的液滴，可以导致一个数量级的改善传热性能与膜式模式相比。所谓的润滑注入表面（LIS），由注入薄油层的微孔表面组成，促进滴状冷凝，从而可以实现更小、更高效的能源和水收集系统。然而，由于对液滴-润滑剂相互作用、液滴成核和生长以及润滑剂排水的不完全理解，LIS在商业应用中的广泛实施受到限制。本项目阐明了这些动力学，并研究了它们对冷凝传热速率的影响。此外，该项目还培养了人们对科学的兴趣，并通过在当地一所女子中学的推广项目和为有才华的高中生提供暑期研究实习机会，帮助女性和少数族裔克服进入工程领域的障碍。这个综合实验和数学研究项目提供了一个全面的理解传热，相变和流体动力学的关系，在润滑注入表面的冷凝。将高速成像与高分辨率显微技术相结合，克服了以往阻碍水滴凝结过程成核和传热直接观察和定量的局限性。数学建模补充实验，以开发预测工具和设计标准的冷凝传热增强。具体而言，主要有三个研究目标：(1)确定注入润滑油表面的成核率密度；(2)利用高速红外成像测量相变和扫油液滴强制对流对总体传热率的贡献；(3)通过直接吸积量化隐形对液滴生长的影响。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Vapor condensation is ubiquitous in nature and in many industrial applications, including power generation, air conditioning, and water harvesting. Commonly water condenses as a thick (~mm) film, acting as a thermal barrier to heat transfer. In contrast, dropwise condensation, in which the condensate forms discrete droplets, can lead to an order-of-magnitude improvement in heat transfer performance compared to the filmwise mode. So-called lubricant-infused surfaces (LIS), which consist of a microporous surface infused with a thin oil layer, promote dropwise condensation, and can consequently lead to smaller and more efficient energy and water harvesting systems. However, the widespread implementation of LIS in commercial applications is limited due an incomplete understanding of droplet-lubricant interactions, droplet nucleation and growth, and lubricant drainage. This project elucidates these dynamics, and studies their influence on condensation heat transfer rates. Furthermore, this project fosters interest in science and help overcome the barriers for women and underrepresented minorities entering the field of engineering via outreach programs at a local girls-only middle school and summer research internships for talented high school students.This integrated experimental and mathematical research project provides a comprehensive understanding of the relationship of heat transfer, phase change, and fluid dynamics during condensation on lubricant-infused surfaces. It overcomes previous limitations that hindered the direct observation and quantification of nucleation and heat transfer during dropwise condensation by combining high-speed imaging with high-resolution microscopy. Mathematical modeling complements experiments to develop predictive tools and design criteria for condensation heat transfer enhancement. Specifically, the three main research aims are: (1) Determine the nucleation rate density on lubricant-infused surfaces, (2) Measure the contributions of phase change and forced convection from sweeping droplets on overall heat transfer rates using high-speed infrared imaging, and (3) Quantify the effect of cloaking on droplet growth via direct accretion.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.