Modeling and Optimization of Superhydrophobic Condensation
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, a...
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| Format: | Article |
| Language: | en_US |
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ASME International
2014
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| Online Access: | http://hdl.handle.net/1721.1/84986 https://orcid.org/0000-0001-7045-1200 |
| _version_ | 1811078728833302528 |
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| author | Miljkovic, Nenad Enright, Ryan Wang, Evelyn N. |
| author2 | Massachusetts Institute of Technology. Department of Mechanical Engineering |
| author_facet | Massachusetts Institute of Technology. Department of Mechanical Engineering Miljkovic, Nenad Enright, Ryan Wang, Evelyn N. |
| author_sort | Miljkovic, Nenad |
| collection | MIT |
| description | 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. |
| first_indexed | 2024-09-23T11:04:32Z |
| format | Article |
| id | mit-1721.1/84986 |
| institution | Massachusetts Institute of Technology |
| language | en_US |
| last_indexed | 2024-09-23T11:04:32Z |
| publishDate | 2014 |
| publisher | ASME International |
| record_format | dspace |
| spelling | mit-1721.1/849862022-09-27T16:58:53Z Modeling and Optimization of Superhydrophobic Condensation Miljkovic, Nenad Enright, Ryan Wang, Evelyn N. Massachusetts Institute of Technology. Department of Mechanical Engineering Miljkovic, Nenad Miljkovic, Nenad Enright, Ryan Wang, Evelyn N. 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. United States. Dept. of Energy. Office of Basic Energy Sciences (Award DE-FG02-09ER46577) National Science Foundation (U.S.) (Award ECS-0335765) 2014-02-18T19:36:57Z 2014-02-18T19:36:57Z 2013-09 2012-09 Article http://purl.org/eprint/type/JournalArticle 0022-1481 http://hdl.handle.net/1721.1/84986 Miljkovic, Nenad, Ryan Enright, and Evelyn N. Wang. “Modeling and Optimization of Superhydrophobic Condensation.” Journal of Heat Transfer 135, no. 11 (November 1, 2013): 111004. https://orcid.org/0000-0001-7045-1200 en_US http://dx.doi.org/10.1115/1.4024597 Journal of Heat Transfer Creative Commons Attribution-Noncommercial-Share Alike http://creativecommons.org/licenses/by-nc-sa/4.0/ application/pdf ASME International Nenad Miljkovic |
| spellingShingle | Miljkovic, Nenad Enright, Ryan Wang, Evelyn N. Modeling and Optimization of Superhydrophobic Condensation |
| title | Modeling and Optimization of Superhydrophobic Condensation |
| title_full | Modeling and Optimization of Superhydrophobic Condensation |
| title_fullStr | Modeling and Optimization of Superhydrophobic Condensation |
| title_full_unstemmed | Modeling and Optimization of Superhydrophobic Condensation |
| title_short | Modeling and Optimization of Superhydrophobic Condensation |
| title_sort | modeling and optimization of superhydrophobic condensation |
| url | http://hdl.handle.net/1721.1/84986 https://orcid.org/0000-0001-7045-1200 |
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