Marangoni condensation of binary refrigerant mixtures

Washington University, Saint Louis MO

Investigators

Abstract

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.