Marangoni condensation of binary refrigerant mixtures

每年，数万亿千瓦时的能源被浪费为余热。在气候变化严重的时期，要在未来几十年内实现碳中性，就必须更有效地利用资源，包括利用低档热源发电或更高效的空调和制冷系统。在所有这些应用中，工作流体(主要是制冷剂)在冷凝过程中的低传热率限制了整个系统的效率，并增加了冷凝器单元的尺寸和成本。较差的热性能源于这些非水工质的低表面张力，这会导致它们冷凝成膜，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是使用稀释制冷剂混合物，由于混合物性质的独特的温度依赖性，这种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提高热传率。该项目旨在探索这种所谓的“Marangoni冷凝”的基本原理和限制，并展示冷凝换热速率的显著增强。此外，该项目将培养人们对热-流体科学在应对气候变化中的作用的认识，并通过当地一所中学的外联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族裔进入工程领域的障碍。通过综合的实验和建模方法，该项目试图回答两个主要问题：(1)可靠的Marangoni冷凝的关键参数是什么，以及不稳定性和液滴生长如何依赖这些参数？以及(2)我们是否可以人为地引发低表面张力流体的不稳定性和液滴的形成，例如非水的制冷剂混合物？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，它结合了最先进的高速干涉显微镜、高速红外热像和传统的基于探头的热传递测量。标度分析和质量、动量、能量和物种守恒方程的微扰方法，以及膜状冷凝的Nusselt理论将补充实验，以创建一个基于物理的模型来预测给定的一对工作流体和热操作条件下Marangoni冷凝的发生。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。

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.