Wetting Mode Evolution of Steam Dropwise Condensation on Superhydrophobic Surface in the Presence of Noncondensable Gas

2011 ◽  
Vol 134 (2) ◽  
Author(s):  
Xuehu Ma ◽  
Sifang Wang ◽  
Zhong Lan ◽  
Benli Peng ◽  
H. B. Ma ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Contact Angle ◽  
Superhydrophobic Surface ◽  
Contact Angle Hysteresis ◽  
High Surface ◽  
Dropwise Condensation ◽  
Noncondensable Gas ◽  
Order Of Magnitude ◽  
Rapid Removal ◽  
Condensation Surface

It is well known that heat transfer in dropwise condensation (DWC) is superior to that in filmwise condensation (FWC) by at least one order of magnitude. Surfaces with larger contact angle (CA) can promote DWC heat transfer due to the formation of “bare” condensation surface caused by the rapid removal of large condensate droplets and high surface replenishment frequency. Superhydrophobic surfaces with high contact angle (> 150°) of water and low contact angle hysteresis (< 5°) seem to be an ideal condensing surface to promote DWC and enhance heat transfer, in particular, for the steam-air mixture vapor. In the present paper, steam DWC heat transfer characteristics in the presence of noncondensable gas (NCG) were investigated experimentally on superhydrophobic and hydrophobic surfaces including the wetting mode evolution on the roughness-induced superhydrophobic surface. It was found that with increasing NCG concentration, the droplet conducts a transition from the Wenzel to Cassie-Baxter mode. And a new condensate wetting mode—a condensate sinkage mode—was observed, which can help to explain the effect of NCG on the condensation heat transfer performance of steam-air mixture on a roughness-induced superhydrophobic SAM-1 surface.

scholarly journals Dropwise condensation on solid hydrophilic surfaces

2020 ◽  
Vol 6 (2) ◽  
pp. eaax0746 ◽  
Author(s):  
Hyeongyun Cha ◽  
Hamed Vahabi ◽  
Alex Wu ◽  
Shreyas Chavan ◽  
Moon-Kyung Kim ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Contact Angle ◽  
Contact Angle Hysteresis ◽  
Bond Number ◽  
Condensation Heat Transfer ◽  
Past Century ◽  
Dropwise Condensation ◽  
The Past ◽  
Order Of Magnitude ◽  
Droplet Sizes

Droplet nucleation and condensation are ubiquitous phenomena in nature and industry. Over the past century, research has shown dropwise condensation heat transfer on nonwetting surfaces to be an order of magnitude higher than filmwise condensation heat transfer on wetting substrates. However, the necessity for nonwetting to achieve dropwise condensation is unclear. This article reports stable dropwise condensation on a smooth, solid, hydrophilic surface (θa = 38°) having low contact angle hysteresis (<3°). We show that the distribution of nano- to micro- to macroscale droplet sizes (about 100 nm to 1 mm) for coalescing droplets agrees well with the classical distribution on hydrophobic surfaces and elucidate that the wettability-governed dropwise-to-filmwise transition is mediated by the departing droplet Bond number. Our findings demonstrate that achieving stable dropwise condensation is not governed by surface intrinsic wettability, as assumed for the past eight decades, but rather, it is dictated by contact angle hysteresis.

scholarly journals Modeling and Optimization of Superhydrophobic Condensation

2013 ◽  
Vol 135 (11) ◽  
Author(s):  
Nenad Miljkovic ◽  
Ryan Enright ◽  
Evelyn N. Wang
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Contact Angle ◽  
Contact Angle Hysteresis ◽  
Unified Model ◽  
Transport Processes ◽  
Superhydrophobic Surfaces ◽  
Dropwise Condensation ◽  
Structured Surfaces ◽  
Nanostructured Surfaces

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.

A Heat Transfer Model of Dropwise Condensation Underneath a Horizontal Substrate

2011 ◽  
Vol 199-200 ◽  
pp. 1604-1608
Author(s):  
Yun Fu Chen
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Contact Angle ◽  
Fractal Model ◽  
Condensation Heat Transfer ◽  
Droplet Growth ◽  
Transfer Model ◽  
Dropwise Condensation ◽  
Small Contact Angle ◽  
Horizontal Substrate

For finding influence of the condensing surface to dropwise condensation heat transfer, a fractal model for dropwise condensation heat transfer has been established based on the self-similarity characteristics of droplet growth at various magnifications on condensing surfaces with considering influence of contact angle to heat transfer. It has been shown based on the proposed fractal model that the area fraction of drops decreases with contact angle increase under the same sub-cooled temperature; Varying the contact angle changes the drop distribution; higher the contact angle, lower the departing droplet size and large number density of small droplets; dropwise condensation translates easily to the filmwise condensation at the small contact angle ;the heat flux increases with the sub-cooled temperature increases, and the greater of contact angle, the more heat flux increases slowly.

scholarly journals Preparation of Superhydrophobic Steel Surfaces with Chemical Stability and Corrosion

Coatings ◽  
2019 ◽  
Vol 9 (6) ◽  
pp. 398 ◽  
Author(s):  
Chongwei Du ◽  
Xiaoyan He ◽  
Feng Tian ◽  
Xiuqin Bai ◽  
Chengqing Yuan
Keyword(s):  
Contact Angle ◽  
Corrosion Resistance ◽  
Carbon Steel ◽  
Chemical Stability ◽  
Superhydrophobic Surface ◽  
Contact Angle Hysteresis ◽  
Water Contact ◽  
Simple Method ◽  
Q235 Carbon Steel ◽  
Steel Surfaces

Corrosion seriously limits the long-term application of Q235 carbon steel. Herein, a simple fabrication method was used to fabricate superhydrophobic surfaces on Q235 carbon steel for anticorrosion application. The combination of structure and the grafted low-surface-energy material contributed to the formation of superhydrophobic steel surfaces, which exhibited a water contact angle of 161.6° and a contact angle hysteresis of 0.8°. Meanwhile, the as-prepared superhydrophobic surface showed repellent toward different solutions with pH ranging from 1 to 14, presenting excellent chemical stability. Moreover, the acid corrosive liquid (HCl solution with pH of 1) maintained sphere-like shape on the as-prepared superhydrophobic surface at room temperature, indicating superior corrosion resistance. This work provides a simple method to fabricate superhydrophobic steel surfaces with chemical stability and corrosion resistance.

scholarly journals Review of Micro–Nanoscale Surface Coatings Application for Sustaining Dropwise Condensation

Coatings ◽  
2019 ◽  
Vol 9 (2) ◽  
pp. 117 ◽  
Author(s):  
Shoukat Alim Khan ◽  
Furqan Tahir ◽  
Ahmer Ali Bozdar Baloch ◽  
Muammer Koc
Keyword(s):  
Heat Transfer ◽  
Noble Metals ◽  
Power Plants ◽  
Contact Angle Hysteresis ◽  
Surface Hydrophobicity ◽  
Future Research ◽  
Dropwise Condensation ◽  
Nanostructured Surfaces ◽  
Research Guidelines ◽  
Coating Methods

Condensation occurs in most of the heat transfer processes, ranging from cooling of electronics to heat rejection in power plants. Therefore, any improvement in condensation processes will be reflected in the minimization of global energy consumption, reduction in environmental burdens, and development of sustainable systems. The overall heat transfer coefficient of dropwise condensation (DWC) is higher by several times compared to filmwise condensation (FWC), which is the normal mode in industrial condensers. Thus, it is of utmost importance to obtain sustained DWC for better performance. Stability of DWC depends on surface hydrophobicity, surface free energy, condensate liquid surface tension, contact angle hysteresis, and droplet removal. The required properties for DWC may be achieved by micro–nanoscale surface modification. In this survey, micro–nanoscale coatings such as noble metals, ion implantation, rare earth oxides, lubricant-infused surfaces, polymers, nanostructured surfaces, carbon nanotubes, graphene, and porous coatings have been reviewed and discussed. The surface coating methods, applications, and enhancement potential have been compared with respect to the heat transfer ability, durability, and efficiency. Furthermore, limitations and prevailing challenges for condensation enhancement applications have been consolidated to provide future research guidelines.

Pulsating and Bouncing off of Dropwise Condensate on Superhydrophobic Surface

2011 ◽  
Author(s):  
Xuehu Ma ◽  
Sifang Wang ◽  
Zhong Lan ◽  
Benli Peng ◽  
Tao Bai ◽  
...  
Keyword(s):  
Superhydrophobic Surface ◽  
High Speed ◽  
High Speed Camera ◽  
Dropwise Condensation ◽  
High Concentration ◽  
Long Distance ◽  
Noncondensable Gas ◽  
Pinning Effect

The steam dropwise condensation (DWC) characteristics on superhydrophobic plates were investigated experimentally in the presence of a high concentration noncondensable gas (NCG, >80mol%). The behaviors of condensate droplets on the roughness-induced superhydrophobic surface were observed with a photron high speed camera attached to a microscope. Pulsating features are found during droplets coalescence movement. Bouncing off of coalesced droplet was also observed induced by the strong effect of pulsating motion to overcome the pinning effect of the surface micro-nanostructures. Induced by the pulsating effect of droplets coalescence, the droplet can move at a long distance to join a coalesced droplet.

scholarly journals Glaze Icing on Superhydrophobic Coating Prepared by Nanoparticles Filling Combined with Etching Method for Insulators

2015 ◽  
Vol 2015 ◽  
pp. 1-7 ◽  
Author(s):  
Chao Guo ◽  
Ruijin Liao ◽  
Yuan Yuan ◽  
Zhiping Zuo ◽  
Aoyun Zhuang
Keyword(s):  
Contact Angle ◽  
Photoelectron Spectroscopy ◽  
Superhydrophobic Surface ◽  
Power Transmission ◽  
Contact Angle Hysteresis ◽  
Transmission System ◽  
Sliding Angle ◽  
Power Transmission System ◽  
Etching Method ◽  
Xps Characterization

Icing on insulators may cause flashover or even blackout accidents in the power transmission system. However, there are few anti-icing techniques for insulators which consume energy or manpower. Considering the water repelling property, the superhydrophobic surface is introduced for anti-icing of insulators. Among the icing forms, the glaze icing owns the highest density, strongest adhesion, and greatest risk to the power transmission system but lacks researches on superhydrophobic surface. In this paper, superhydrophobic surfaces with contact angle of 166.4°, contact angle hysteresis of 0.9°, and sliding angle of less than 1° are prepared by nanoparticle filling combined with etching method. The coated glass slide and glass insulator showed excellent anti-icing performance in the glaze icing test at −5°C. The superhydrophobicity and anti-icing property of the coatings benefit from the low surface energy and hierarchical rough structure containing micron scale pits and nanoscale coralloid bulges supported by scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) characterization.

Dropwise Condensation Heat Transfer on Superhydrophobic Surface in the Presence of Non-Condensable Gas

2010 ◽  
Author(s):  
Xuehu Ma ◽  
Sifang Wang ◽  
Zhong Lan ◽  
Aili Wang ◽  
Benli Peng
Keyword(s):  
Heat Transfer ◽  
Superhydrophobic Surface ◽  
Vertical Plate ◽  
Experimental Results ◽  
Dropwise Condensation ◽  
Heat Transfer Characteristics ◽  
Steam Condensation ◽  
High Concentration ◽  
Transfer Characteristics ◽  
Wetting Behaviors

Roughness-induced superhydrophobic surface was applied to promote dropwise condensation (DWC) on a vertical plate in the presence of non-condensable gas (NCG). The DWC heat transfer characteristics were investigated and the wetting behaviors of the condensate droplets were observed visually. The experimental results have shown that the roughness-induced superhydrophobic surface would enhance the heat transfer characteristics of steam condensation in the presence of NCG with high concentration. The underlined mechanism is analyzed in terms of the droplet wetting modes.

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