Marangoni condensation of binary refrigerant mixtures

每年，数万亿千瓦时的能量作为废热损失。在气候变化严重的时期，要在今后几十年内实现碳中和，就必须更有效地利用资源，包括利用低品位热源发电或更有效的空调和制冷系统。在所有这些应用中，在工作流体（主要是制冷剂）的冷凝期间的低传热速率限制了整个系统的效率并且增加了冷凝器单元的尺寸和成本。差的热性能源于这些非水工作流体的低表面张力，这导致它们冷凝为膜，在蒸汽和冷凝器盘管之间产生热阻。克服这一挑战的一个可能的解决方案在于使用稀释的制冷剂混合物，由于混合物性质的独特温度依赖性，该混合物能够通过诱导导致形成单个液滴的不稳定性来显著增加传热速率。该项目旨在探索这种所谓的“马兰戈尼冷凝”的基本原理和限制，并证明冷凝传热速率的显着增强。此外，该项目还将提高人们对热流体科学在应对气候变化方面的作用的认识，并通过当地中学的外展计划和有才华的高中生的暑期研究实习，帮助克服妇女和代表性不足的少数民族进入工程领域的障碍。通过一个综合的实验和模拟的方法，这个项目试图回答两个主要问题：（1）什么是可靠的Marangoni冷凝的关键参数，以及如何不稳定性和液滴生长取决于这些参数？以及（2）我们能否人为地引发低表面张力流体（如非水制冷剂混合物）的不稳定性和液滴形成？为了回答这些问题，将开发一种新的实验装置，专门用于表征制冷剂混合物的冷凝，该装置将最先进的高速干涉显微镜与高速红外热成像和传统的基于探针的传热测量相结合。质量、动量、能量和物质守恒方程的标度分析和微扰方法，沿着努塞尔理论的膜状冷凝将补充实验，创造一个物理-该奖项反映了NSF的法定使命，并已被认为是值得支持的，通过评估使用基金会的学术价值和更广泛的影响审查标准。

Every year, trillions of kilowatt-hours of energy are lost as waste heat. In times of severe climate change, becoming carbon-neutral within the next few decades will necessitate the more efficient use of resources, including power generation from low-grade heat sources or more efficient air conditioning and refrigeration systems. In all of these applications the low rates of heat transfer during condensation of the working fluids, mainly refrigerants, limit the overall system efficiency and increase the size and cost of the condenser units. The poor thermal performance stems from the low surface tension of these non-aqueous working fluids, which causes them to condense as a film, creating a thermal resistance between the vapor and the condenser coils. One possible solution to overcome this challenge lies in the use of dilute refrigerant mixtures, which – due to a unique temperature-dependence of mixture properties – are able to increase heat transfer rates dramatically by inducing instabilities that lead to the formation of individual droplets. This project aims to explore the underlying fundamentals and limits of this so-called “Marangoni condensation” and demonstrate significant enhancement in condensation heat transfer rates. Furthermore, this project will foster awareness of the role of thermal-fluid sciences in combating climate change and help overcome barriers for women and underrepresented minorities entering the field of engineering via outreach programs at a local middle school and summer research internships for talented high school students. Through an integrated experimental and modeling approach, this project seeks to answer two main questions: (1) What are the critical parameters for reliable Marangoni condensation, and how do instabilities and droplet growth depend on these parameters? and (2) Can we artificially trigger instabilities and droplet formation for low-surface tension fluids, such as non-aqueous refrigerant mixtures? To answer these questions, a new experimental setup specifically aimed at characterizing condensation of refrigerant mixtures, which combines state-of-the-art high-speed interference microscopy with high-speed infrared thermography and traditional probe-based heat transfer measurements, will be developed. Scaling analysis and a perturbation approach to the mass, momentum, energy, and species conservation equations, along with the Nusselt theory for film-wise condensation will complement the experiments to create a physics-based model to predict the occurrence of Marangoni condensation for a given pair of working fluids and thermal operating conditions.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.