TY - CONF
T1 - Electric-field-enhanced jumping-droplet condensation
AU - Miljkovic, Nenad
AU - Preston, Daniel J.
AU - Enright, Ryan
AU - Wang, Evelyn N.
N1 - Funding Information:
We gratefully acknowledge funding support from the MIT S3TEC Center, an Energy Frontier Research Center funded by the Department of Energy, Office of Science, Basic Energy Sciences under Award # DEFG02- 09ER46577, and the Office of Naval Research (ONR) with Dr. Mark Spector as program manager. D.J.P. acknowledges funding received by the National Science Foundation Graduate Research Fellowship under Grant No. 1122374. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors(s) and do not necessarily reflect the views of the National Science Foundation. R.E. acknowledges funding received from the Irish Research Council for Science, Engineering, and Technology, cofunded by Marie Curie Actions under FP7. We also acknowledge the support from the National Science Foundation through the Major Research Instrumentation Grant for Rapid Response Research (MRI-RAPID) for the microgoniometer.
PY - 2014
Y1 - 2014
N2 - When condensed droplets coalesce on a superhydrophobic nanostructured surface, the resulting droplet can jump due to the conversion of excess surface energy into kinetic energy. This phenomenon has been shown to enhance condensation heat transfer by up to 30% compared to state-of-the-art dropwise condensing surfaces. However, after the droplets jump away from the surface, the existence of vapor flow towards the condensing surface increases the drag on the jumping droplets, which can lead to droplet reversal of direction and return to the surface. This effect limits the possible heat transfer enhancement because larger droplets form upon droplet return to the surface that impede 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/cm2). Subsequently, we demonstrate electric-field-enhanced (EFE) condensation, whereby an externally applied electric field prevents jumping droplet return. This concept leverages our recent insight that these 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 CuO surfaces, we experimentally demonstrate a 50% higher overall condensation heat transfer coefficient compared to that on a jumpingdroplet surface with no applied field for low supersaturations (<1.12). This work not only shows significant condensation heat transfer enhancement, but also offers avenues for improving the performance of selfcleaning and anti-icing surfaces as well as thermal diodes.
AB - When condensed droplets coalesce on a superhydrophobic nanostructured surface, the resulting droplet can jump due to the conversion of excess surface energy into kinetic energy. This phenomenon has been shown to enhance condensation heat transfer by up to 30% compared to state-of-the-art dropwise condensing surfaces. However, after the droplets jump away from the surface, the existence of vapor flow towards the condensing surface increases the drag on the jumping droplets, which can lead to droplet reversal of direction and return to the surface. This effect limits the possible heat transfer enhancement because larger droplets form upon droplet return to the surface that impede 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/cm2). Subsequently, we demonstrate electric-field-enhanced (EFE) condensation, whereby an externally applied electric field prevents jumping droplet return. This concept leverages our recent insight that these 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 CuO surfaces, we experimentally demonstrate a 50% higher overall condensation heat transfer coefficient compared to that on a jumpingdroplet surface with no applied field for low supersaturations (<1.12). This work not only shows significant condensation heat transfer enhancement, but also offers avenues for improving the performance of selfcleaning and anti-icing surfaces as well as thermal diodes.
KW - Condensation
KW - Droplet charging
KW - Electric field
KW - Heat transfer
KW - Nanostructure
KW - Superhydrophobic
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U2 - 10.1615/ihtc15.cds.008896
DO - 10.1615/ihtc15.cds.008896
M3 - Paper
AN - SCOPUS:85086949723
T2 - 15th International Heat Transfer Conference, IHTC 2014
Y2 - 10 August 2014 through 15 August 2014
ER -