ABSTRACT: Micro and nanostructures have long been recognized to promise heat transfer enhancement in phase-change processes for a wide range of applications including thermal management, building environment control, water harvesting, desalination, and industrial power generation. In this talk, I will focus on fundamental investigations of water vapor condensation on superhydrophobic surfaces, as well as the demonstration of such surfaces for enhanced condensation heat transfer performance. Through the use of fabrication, characterization, modeling, and experiments, we examined the role of surface structure on emerging droplet morphology, nucleation density, droplet growth rate, and departure characteristics including a coalescence-induced jumping mechanism. The insights gained from the fundamental studies led to the development of a scalable functionalized oxide nanostructure technique on copper surfaces capable of sustaining jumping droplet condensation. Accordingly, we demonstrated a 25% higher overall heat flux and 30% higher condensation heat transfer coefficient compared to state-of-the-art hydrophobic condensing surfaces. In addition, we discovered that these jumping droplets during condensation are also electrostatically charged due to electric double layer separation. With the aid of electric fields, the charge on the droplets was quantified, and the mechanism for the charge accumulation was studied. Finally, we demonstrated electric-field-enhanced condensation, whereby an external electric field was used to force charged departing droplets away from the surface. The results experimentally demonstrated a 50% higher overall heat transfer coefficient compared to the no-field jumping surface and a 90% higher heat transfer coefficient when compared to state-of-the-art dropwise condensing surfaces. This work not only shows significant condensation heat transfer enhancement, but it offers improved fundamental understanding of wetting and condensation on micro/nanostructures as well as practical implementation of these structures. The insights gained promise new surface engineering approaches to enhance the performance of various thermal management and energy production applications.
BIO: Dr. Nenad Miljkovic is currently a Postdoctoral Associate in the Device Research Lab at the Department of Mechanical Engineering at the Massachusetts Institute of Technology. He specializes in phase-change heat transfer, interfacial phenomena, and solar energy conversion. He earned his PhD and M.S. degrees from the Massachusetts Institute of Technology in 2013 and 2011, respectively. His PhD work focused on the design and characterization of micro/nanostructured surfaces for enhanced condensation heat transfer, while his M.S. work involved the development and analysis of a novel solar energy conversion system called the Hybrid Solar Thermoelectric (HSTE). In 2012, he was the recipient of the ASME Micro/Nano Heat Transfer Heat and Mass Transfer International Conference Best Paper Award (First Prize), and in 2013 he received the Wunsch Foundation Silent Hoist and Crane Award for outstanding graduate research during his PhD. His work has been extensively covered by journal and media outlets, including MRS Bulletin, MIT Front Page News, MIT Energy Futures Magazine NSF News, Bloomberg, CBS Smart Planet, New England Post, International Business Times, and Reuters. He has 6 patents granted or pending, and has given more than 30 talks at international conferences.