9/12/2019 Julia Stackler 5 min read
Written by Julia Stackler
With a recent publication led by postdoctoral scholar Dr. Soumyadip Sett, MechSE Assistant Professor Nenad Miljkovic has demonstrated a method of achieving stable dropwise condensation of ethanol and hexane, and has developed the fundamental design principles for creating durable lubricant-infused surfaces for enhanced condensation heat transfer of low-surface-tension fluids.
The paper, “Stable Dropwise Condensation of Ethanol and Hexane on Rationally-Designed Ultra-Scalable Nanostructured Lubricant-Infused Surfaces,” was published in Nano Letters, a publication of the American Chemical Society (ACS). The work was authored by Sett and co-authored by seven colleagues in Miljkovic’s group, along with two researchers from BP. Two of the co-authors, including the second author, are MechSE undergraduates. The work was primarily funded by British Petroleum (BP) and the Office of Naval Research.
In recent studies pertaining to advances in micro- and nanoscale surface fabrication technologies, much of the focus has been on water-repellent interfaces, which enhance steam condensation heat transfer by promoting dropwise condensation. Dropwise condensation is an effective mechanism of heat transfer in which the condensate liquid collects in the form of many tiny (< 1mm) droplets of varying diameters and does not wet the cooling surface.
While significant work in this field has focused on steam, few studies have succeeded in achieving dropwise condensation of non-aqueous liquids, especially those with low surface tension such as alcohols and hydrocarbons. Water has a relatively high surface tension at room temperature (73 mN/m) and typical state-of-the-art surfaces for steam condensation have a combination of non-polar chemical coatings in conjunction with structures which lower the overall surface energy. This significant difference in energy between water and the surface prevents film formation and promotes dropwise condensation. However, for fluids with low surface tensions (< 30 mN/m), the surface energies of the fluid and the surface become comparable, presenting a significant challenge in developing surfaces that will promote continuous droplet shedding, a prerequisite for dropwise condensation.
To enable stable dropwise condensation of such low surface tension fluids, Sett, Miljkovic, and colleagues at BP developed ultra-scalable copper oxide (CuO) surfaces with nanoscale structures infused with rationally selected fluorinated lubricants. Because of the additional lubricant layer, liquid droplets – even those having low surface tension – easily roll off, enabling rapid droplet shedding. This uniqueness leads to stable dropwise condensation.
The team chose ethanol and hexane as the working fluids because they are a good representation of general alcohol and alkane behavior, and their heat transfer performance serves as a benchmark for other low-surface-tension fluids. In contrast to previous studies, based on lubricant viscosity, surface tension, and vapor pressure, the team selected different grades of fluorinated lubricants for designing the surfaces. In doing so, heat transfer measurements in pure ethanol and hexane vapor revealed a 100% and 200% enhancement in condensation heat transfer and heat transfer coefficient, respectively, when compared to state-of-the-art condensation which occurs on condenser surfaces (termed filmwise condensation).
The team tested the surfaces durability and heat transfer performance for more than seven hours, proving heat transfer performance enhancement can be sustained over long periods of time.
Condensation of fluids with low surface tension is crucial to several industrial operations, such as in the distillation of crude oil in the oil and gas industry, in air conditioning and refrigeration systems and stand-alone small power plants, both of which use refrigerants as the working fluid. For BP, the process of separating hydrocarbons from the extracted crude oil – which involves boiling and condensation at different temperatures – limits plant efficiency. The results of this research – which has also been filed for a provisional patent – suggest promising guidelines for enhancing the efficiency of such operations and enable the design of smaller condensers.
“Some recent works have already shown that slippery coatings can promote the dropwise mode of condensation for low surface tension fluids, which enhances the heat transfer compared to filmwise condensation. However, these previous works observed surface failure after only one hour of condensation, due to the lubricant being sheared from the surface over time. In this new work by Professor Miljkovic’s group, they realized that by using higher viscosity oils, the slippery dropwise mode of condensation can now be sustained for at least seven hours of time with no apparent degradation in the heat transfer performance. This is important because it demonstrates the practical viability of using slippery condensers for longer time scales to enhance heat transfer for various industrial applications. Their current setup was only able to produce vapor from ethanol or hexane for seven hours; in the future it would, of course, be important to test whether the dropwise mode is stable for hundreds or even thousands of hours,” said Miljkovic’s colleague in the field, Jonathan Boreyko, an assistant professor of mechanical engineering at Virginia Tech, and who was not involved in the study.
“As the world is witnessing the long overdue transition towards alternative energy sources and biofuels, the need for efficient condensation and separation of fluids with low surface tension is of utmost necessity. Development of such novel surfaces to promote higher efficiency of vapor-to-liquid phase change, leading to increased overall process efficiency and smaller plant size, will have significant impact on energy reduction and costs,” Sett said.
Sett earned his PhD in mechanical engineering from the University of Illinois at Chicago (UIC) in 2016, and a BS in power engineering from Jadavpur University, India. He joined Miljkovic’s Energy Transport Research Lab in 2016.
Photo at top: Dr. Sett in the lab. Photo by Julia Stackler for MechSE.