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普遍存在，包括发电，空调和水收集。通常水会凝结成一层厚（~mm）的薄膜，作为热传递的热屏障。相比之下，滴状冷凝，其中冷凝物形成离散的液滴，可以导致一个数量级的传热性能的改善相比，膜状模式。所谓的润滑剂注入表面（LIS）由注入薄油层的微孔表面组成，可促进滴状冷凝，因此可实现更小、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。本计画阐明这些动力学，并研究其对凝结热传率的影响。此外，该项目还通过在当地女子中学开展的外展计划和为有才华的高中生提供的暑期研究实习机会，培养对科学的兴趣，并帮助克服妇女和代表性不足的少数民族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对传热、相变、以及在注入润滑剂的表面上冷凝期间的流体动力学。它克服了以前的限制，阻碍了直接观察和定量的成核和传热过程中滴状冷凝相结合的高速成像与高分辨率显微镜。数学建模补充实验，开发预测工具和设计标准，冷凝传热增强。具体而言，三个主要研究目标是：（1）确定注入润滑剂的表面上的成核速率密度，（2）使用高速红外成像测量来自清扫液滴的相变和强制对流对总传热速率的贡献，以及（3）通过直接吸积量化隐形对液滴增长的影响。该奖项反映了NSF的法定使命，并被认为是值得的。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来提供支持。

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.