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International Heat Transfer Conference 15

ISSN: 2377-424X (online)
ISSN: 2377-4371 (flashdrive)

Electric-Field-Enhanced Jumping-Droplet Condensation

Nenad Miljkovic
Department of Mechanical Science and Engineering, University of Illinois at Urbana–Champaign, Urbana, IL, 61801, USA; Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana–Champaign, Urbana, Illinois 61801, USA; International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan

Daniel J. Preston
Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

Ryan Enright
Bell Labs Ireland, Alcatel-Lucent Ireland Ltd.

Evelyn N. Wang
Device Research Laboratory, Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02149, USA

DOI: 10.1615/IHTC15.cds.008896
pages 882-896


KEY WORDS: Condensation, Heat transfer enhancement, condensation, wetting, superhydrophobic, nanostructured design, heat transfer enhancement, droplet charging, electric field, vapor entrainment

Abstract

When condensed droplets coalesce on a nanostructured superhydrophobic surface, the resulting droplet can jump due to the conversion of surface energy into kinetic energy, and enhance condensation heat transfer up to 30% compared to state-of-the-art dropwise condensing surfaces. However, after the droplets jump away from the surface, the vapor flow towards the condensing surface that satisfies conservation of mass increases the drag on the jumping droplets, which can lead to complete droplet reversal and return to the surface. This effect limits the possible heat transfer enhancement because larger droplets form upon droplet return to the surface and impedes heat transfer until they can be either removed by jumping again or finally shedding via gravity. By characterizing individual droplet trajectories during condensation on superhydrophobic nanostructured copper oxide surfaces, we show that this vapor flow entrainment dominates droplet motion for droplets smaller than R ~ 30 µm at moderate heat fluxes (q” > 2 W/cm^2). Subsequently, we demonstrate electric-field-enhanced (EFE) condensation, whereby an external electric field is applied to prevent jumping droplet return. This concept is possible based on our recent discovery that these jumping droplets gain a net positive charge due to charge separation of the electric double layer at the hydrophobic coating. As a result, with scalable superhydrophobic copper oxide surfaces, we experimentally demonstrated a 50% higher overall condensation heat transfer coefficient compared to that on a jumping surface with no applied field for low supersaturations (<1.12). This work not only shows significant condensation heat transfer enhancement, but it offers insights into new avenues for improving the performance of self-cleaning and anti-icing surfaces as well as thermal diodes.

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