Superhydrophobic micro/nanostructured surfaces for dropwise condensation have recently received significant attention due to their potential to enhance heat transfer performance by shedding water droplets via coalescence-induced droplet jumping at length scales below the capillary length. However, achieving optimal surface designs for such behavior requires capturing the details of transport processes that is currently lacking. While comprehensive models have been developed for flat hydrophobic surfaces, they cannot be directly applied for condensation on micro/nanostructured surfaces due to the dynamic droplet-structure interactions. In this work, we developed a unified model for dropwise condensation on superhydrophobic structured surfaces by incorporating individual droplet heat transfer, size distribution, and wetting morphology. Two droplet size distributions were developed, which are valid for droplets undergoing coalescence-induced droplet jumping, and exhibiting either a constant or variable contact angle droplet growth. Distinct emergent droplet wetting morphologies, Cassie jumping, Cassie nonjumping, or Wenzel, were determined by coupling of the structure geometry with the nucleation density and considering local energy barriers to wetting. The model results suggest a specific range of geometries (0.5–2 μm) allowing for the formation of coalescence-induced jumping droplets with a 190% overall surface heat flux enhancement over conventional flat dropwise condensing surfaces. Subsequently, the effects of four typical self-assembled monolayer promoter coatings on overall heat flux were investigated. Surfaces exhibiting coalescence-induced droplet jumping were not sensitive (<5%) to the coating wetting characteristics (contact angle hysteresis), which was in contrast to surfaces relying on gravitational droplet removal. Furthermore, flat surfaces with low promoter coating contact angle hysteresis (<2 deg) outperformed structured superhydrophobic surfaces when the length scale of the structures was above a certain size (>2 μm). This work provides a unified model for dropwise condensation on micro/nanostructured superhydrophobic surfaces and offers guidelines for the design of structured surfaces to maximize heat transfer. Keywords: superhydrophobic condensation, jumping droplets, droplet coalescence, condensation optimization, environmental scanning electron microscopy; micro/nanoscale water condensation, condensation heat transfer.
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November 2013
This article was originally published in
Journal of Heat Transfer
Research-Article
Modeling and Optimization of Superhydrophobic Condensation
Nenad Miljkovic,
Nenad Miljkovic
Department of Mechanical Engineering,
Massachusetts Institute of Technology
,77 Massachusetts Avenue
,Cambridge, MA 02139
Search for other works by this author on:
Ryan Enright,
Ryan Enright
Department of Mechanical Engineering,
Massachusetts Institute of Technology
,77 Massachusetts Avenue
,Cambridge, MA 02139
;Stokes Institute,
Limerick,
University of Limerick
,Limerick,
Ireland
Search for other works by this author on:
Evelyn N. Wang
Evelyn N. Wang
1
Department of Mechanical Engineering,
e-mail: enwang@mit.edu
Massachusetts Institute of Technology
,77 Massachusetts Avenue
, Cambridge, MA 02139
e-mail: enwang@mit.edu
1Corresponding author.
Search for other works by this author on:
Nenad Miljkovic
Department of Mechanical Engineering,
Massachusetts Institute of Technology
,77 Massachusetts Avenue
,Cambridge, MA 02139
Ryan Enright
Department of Mechanical Engineering,
Massachusetts Institute of Technology
,77 Massachusetts Avenue
,Cambridge, MA 02139
;Stokes Institute,
Limerick,
University of Limerick
,Limerick,
Ireland
Evelyn N. Wang
Department of Mechanical Engineering,
e-mail: enwang@mit.edu
Massachusetts Institute of Technology
,77 Massachusetts Avenue
, Cambridge, MA 02139
e-mail: enwang@mit.edu
1Corresponding author.
Contributed by the Heat Transfer Division of ASME for publication in the Journal of Heat Transfer. Manuscript received March 31, 2012; final manuscript received September 2, 2012; published online September 23, 2013. Assoc. Editor: Sujoy Kumar Saha.
J. Heat Transfer. Nov 2013, 135(11): 111004 (14 pages)
Published Online: September 23, 2013
Article history
Received:
March 31, 2012
Revision Received:
September 2, 2012
Citation
Miljkovic, N., Enright, R., and Wang, E. N. (September 23, 2013). "Modeling and Optimization of Superhydrophobic Condensation." ASME. J. Heat Transfer. November 2013; 135(11): 111004. https://doi.org/10.1115/1.4024597
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