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都是普遍存在的，包括发电、空调和水收集。通常情况下，水凝结成一层厚(~毫米)的薄膜，起到热传递的热障作用。相反，滴状冷凝，其中冷凝物形成离散的液滴，与膜状模式相比，可以导致换热性能的数量级改善。所谓的润滑剂注入表面(LIS)，由注入薄油层的微孔表面组成，促进滴状冷凝，从而可以产生更小、更高效的能量和水收集系统。然而，由于对液滴-润滑剂的相互作用、液滴的成核和生长以及润滑剂的排放缺乏了解，液滴信息系统在商业应用中的广泛应用受到了限制。本项目阐明了这些动力学，并研究了它们对冷凝换热速率的影响。此外，这个项目培养了人们对科学的兴趣，并通过当地一所女子中学的外展计划和有才华的高中生的暑期研究实习，帮助克服妇女和少数族裔进入工程领域的障碍。这个综合的实验和数学研究项目全面了解了在注入润滑剂的表面上冷凝过程中的热传递、相变和流体动力学的关系。它通过将高速成像与高分辨率显微镜相结合，克服了以往阻碍直接观察和量化滴状冷凝过程中的成核和传热的限制。数学建模是对实验的补充，以开发预测工具和强化冷凝换热的设计标准。具体地说，三个主要研究目标是：(1)确定注入润滑剂的表面上的成核率密度；(2)利用高速红外成像测量来自扫掠液滴的相变和强迫对流对总体热传率的贡献；以及(3)通过直接吸积来量化遮盖对液滴生长的影响。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.