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.

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.

Dropwise Condensation on Carbon Steel Surface

2016 ◽  
Author(s):  
Fangyu Cao ◽  
Sean Hoenig ◽  
Chien-hua Chen
Keyword(s):  
Heat Transfer ◽  
Carbon Steel ◽  
Power Plants ◽  
Heat Dissipation ◽  
Low Cost ◽  
Condensation Heat Transfer ◽  
Working Fluid ◽  
High Heat Flux ◽  
Dropwise Condensation ◽  
Two Phase

The increasing demand of heat dissipation in power plants has pushed the limits of current two-phase thermal technologies such as heat pipes and vapor chambers. One of the most obvious areas for thermal improvement is centered on the high heat flux condensers including improved evaporators, thermal interfaces, etc, with low cost materials and surface treatment. Dropwise condensation has shown the ability to increase condensation heat transfer coefficient by an order of magnitude over conventional filmwise condensation. Current dropwise condensation research is focused on Cu and other special metals, the cost of which limits its application in the scale of commercial power plants. Presented here is a general use of self-assembled monolayer coatings to promote dropwise condensation on low-cost steel-based surfaces. Together with inhibitors in the working fluid, the surface of condenser is protected by hydrophobic coating, and the condensation heat transfer is promoted on carbon steel surfaces.

A DSMC Model on Droplet Clusters to Explore the Limiting Factors of Modified Surfaces Promoting Enhanced Dropwise Condensation

2015 ◽  
Author(s):  
Hector Mendoza ◽  
Van P. Carey
Keyword(s):  
Heat Transfer ◽  
Cross Section ◽  
Limiting Factors ◽  
Future Research ◽  
Computational Domain ◽  
Dropwise Condensation ◽  
Cold Wall ◽  
Ambient Conditions ◽  
Droplet Distribution ◽  
Droplet Sizes

Condensation is a physical process that occurs when a vapor is cooled and/or compressed to its saturation limit. Condensation becomes important in a variety of engineering applications such as in heat exchangers used for distillation purposes. In such instances, higher condensation efficiencies are desirable. Research to improve condensation has focused on dropwise condensation as it has been shown that it can be significantly more efficient than filmwise condensation. Recent investigations of dropwise condensation on nanostructured surfaces suggest that enhanced dropwise condensation can be attained as the average droplet sizes are reduced for clusters growing through dropwise condensation. This, in turn, significantly enhances the heat transfer coefficients of dropwise condensation. This paper summarizes a computational model developed to explore the mechanisms leading to this enhanced dropwise condensation. A Direct Simulation Monte Carlo (DSMC) approach is used here to investigate the mechanisms and limitations of enhanced dropwise condensation for these surfaces aiming to reduce the average droplet sizes of condensation. For computational purposes, several idealizations are assumed by the model, which include: (1) The condensation droplet clusters are assumed to have uniform size, corresponding to an average droplet size observed in actual dropwise condensation scenarios; (2) Due to the assumed uniform droplet distribution, symmetry can be observed from the droplet cluster, so a small but symmetrical cross section of the droplet distribution is used for the computational domain; and (3) Supersaturated steam condensing on a cold wall is assumed for most of the simulations. The mechanisms at play that are deliberately explored are: (1) The effects of surface wettability by using a model that considers droplet conduction variations with varying contact angle; (2) The changes of interfacial resistance with droplet curvature by introducing a surface tension model based on the Tolman length; and (3) The dynamic interactions between neighboring droplets by choosing our computational domain to be a symmetrical cross section that encompasses surrounding droplets in an appropriate fashion. The ambient conditions that were investigated were: (1) Varying atmospheric pressure; (2) Varying amounts of wall subcooling for the droplets; (3) Varying accommodation for water molecules condensing on the droplet; and (4) The introduction of air into the assumed supersaturated steam condensing on the cold wall. To investigate the overall and combined effects of the aforementioned mechanisms on enhanced dropwise condensation through reduced droplet sizes, the simulations were run for droplets with radii between 1 micrometer down to 5 nanometers. The model predictions indicate that the larger droplet transport trend of increasing heat transfer with decreasing droplet sizes breaks down as droplet sizes become smaller due to more prominence of the mechanisms hindering condensation for the reduced droplet sizes. As the model breaks down, a peak heat transfer is reached, and heat transfer is further reduced as the average droplet sizes continue to decrease. The predictions of this particular DSMC model are compared to previous work investigating similar effects. The implications of our observations and potential impact to current and future research in the area is discussed in detail.

An Exploration of Transport Within Microdroplet and Nanodroplet Clusters During Dropwise Condensation of Water on Nanostructured Surfaces

2014 ◽  
Vol 136 (12) ◽  
Author(s):  
Hector Mendoza ◽  
Sara Beaini ◽  
Van P. Carey
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Droplet Size ◽  
Experimental Studies ◽  
Dropwise Condensation ◽  
Maximum Heat ◽  
Nanostructured Surfaces ◽  
Maximum Heat Transfer ◽  
The Impact

Experimental studies of dropwise condensation have generally indicated that higher heat transfer coefficients correspond to smaller mean sizes for droplets growing through condensation on the surface. Recent investigations of dropwise condensation on nanostructured surfaces suggest that optimizing the design of such surfaces can push mean droplet sizes down to smaller values and significantly enhance heat transfer. This paper summarizes a theoretical exploration of the limits of heat transfer enhancement that can be achieved by pushing mean droplet size to progressively smaller sizes. A model analysis is developed that predicts transport near clusters of water droplets undergoing dropwise condensation. The model accounts for interfacial tension effects on thermodynamic equilibrium and noncontinuum transport effects, which become increasingly important as droplet size becomes progressively smaller. In this investigation, the variation of condensing heat transfer coefficient for droplet clusters of different sizes was explored for droplet diameters ranging from hundreds of microns to tens of nanometers. The model predictions indicate that the larger droplet transport trend of increasing heat transfer coefficient with decreasing mean droplet size breaks down as droplet size becomes smaller. The model further predicts that as drop size becomes smaller, a peak heat transfer coefficient is reached, beyond which the coefficient drops as the size continues to diminish. This maximum heat transfer coefficient results from the increasing importance of surface tension effects and noncontinuum effects as droplet size becomes smaller. The impact of these predictions on the interpretation of dropwise condensation heat transfer data, and the implications for design of nanostructured surfaces to enhance dropwise condensation are discussed in detail.

DSMC Modeling of Dropwise Condensation With Variable Wettability

2012 ◽  
Author(s):  
Hector Mendoza ◽  
Van P. Carey
Keyword(s):  
Heat Transfer ◽  
Computational Time ◽  
Future Research ◽  
Direct Monte Carlo Simulation ◽  
Effective Diameter ◽  
Dropwise Condensation ◽  
Atmospheric Conditions ◽  
Interfacial Resistance ◽  
Transfer Coefficients ◽  
Droplet Sizes

Recent heat transfer studies on nanostructured surfaces have shown that enhanced dropwise condensation heat transfer can be attained through clusters with reduced droplet sizes. A Direct Monte Carlo Simulation (DSMC) approach is used here to investigate the limitations of dropwise condensation as droplet sizes are reduced. The model is idealized by assuming uniform droplet distribution with an effective diameter. To minimize computational time, the model further uses symmetry to focus the analysis on a quarter segment of a single droplet condensing on a cold wall. The effects of surface wettability are explored by accounting for variations in droplet conduction with contact angle, while changes of interfacial resistance with droplet curvature are also incorporated into the model. To investigate the effect of reduced droplet sizes, the simulations were run for droplets with radii between 1μm down to 5nm. The simulations were run for standard atmospheric conditions at different levels of subcooling and accommodation. In all instances, the same behavior is observed as non-continuum effects become significant at reduced diameters, causing heat transfer coefficients to reach a maximum. The predictions of the DSMC model are compared to previous work attempting to capture similar effects. The significance of the observations for current and future research in dropwise condensation is discussed in detail.

An Exploration of Transport Within Micro and Nano Droplet Clusters During Dropwise Condensation of Water on Nanostructured Surfaces

2011 ◽  
Author(s):  
Hector Mendoza ◽  
Sara Beaini ◽  
Van P. Carey
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Droplet Size ◽  
Experimental Studies ◽  
Dropwise Condensation ◽  
Maximum Heat ◽  
Nanostructured Surfaces ◽  
Maximum Heat Transfer ◽  
The Impact

Experimental studies of dropwise condensation have generally indicated that higher heat transfer coefficients correspond to smaller mean sizes for droplets growing through condensation on the surface. Recent investigations of dropwise condensation on nanostructured surfaces suggest that optimizing the design of such surfaces can push mean droplet sizes down to smaller values and significantly enhance heat transfer. This paper summarizes a theoretical exploration of the limits of heat transfer enhancement that can be achieved by pushing mean droplet size to progressively smaller sizes. A model analysis is developed that predicts transport near clusters of water droplets undergoing dropwise condensation. The model accounts for interfacial tension effects on thermodynamic equilibrium and noncontinuum transport effects, which become increasingly important as droplet size becomes progressively smaller. In this investigation, the variation of condensing heat transfer coefficient for droplet clusters of different size was explored for droplet diameters ranging from hundreds of microns to tens of nanometers. The model predictions indicate that the larger droplet transport trend of increasing heat transfer coefficient with decreasing mean droplet size breaks down as droplet size becomes smaller. The model further predicts that as drop size becomes smaller, a peak heat transfer coefficient is reached, beyond which the coefficient drops as the size continues to diminish. This maximum heat transfer coefficient results from the increasing importance of surface tension effects and non-continuum effects as droplet size becomes smaller. The impact of these predictions on the interpretation of dropwise condensation heat transfer data, and the implications for design of nanostructured surfaces to enhance dropwise condensation are discussed in detail.

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 Growth Dynamics During Dropwise Condensation on Nanostructured Superhydrophobic Surfaces

2012 ◽  
Author(s):  
Nenad Miljkovic ◽  
Ryan Enright ◽  
Evelyn N. Wang
Keyword(s):  
Heat Transfer ◽  
Growth Rate ◽  
Heat Transfer Enhancement ◽  
Droplet Growth ◽  
Initial Growth ◽  
Dropwise Condensation ◽  
Transfer Enhancement ◽  
Capillary Length ◽  
Nanostructured Surfaces ◽  
Stable Surfaces

Condensation on superhydrophobic nanostructured surfaces offers new opportunities for enhanced energy conversion, efficient water harvesting, and high performance thermal management. Such surfaces are designed to be Cassie stable, which minimize contact line pinning and allow for passive shedding of condensed water droplets at sizes smaller than the capillary length. In this work, we investigated in situ water condensation on superhydrophobic nanostructured surfaces using environmental scanning electron microscopy (ESEM). The “Cassie stable” surfaces consisted of silane coated silicon nanopillars with diameters of 300 nm, heights of 6.1 μm, and spacings of 2 μm, but allowed droplets of distinct suspended (S) and partially wetting (PW) morphologies to coexist. With these experiments combined with thermal modeling of droplet behavior, the importance of initial growth rates and droplet morphology on heat transfer is elucidated. The effect of wetting morphology on heat transfer enhancement is highlighted with observed 6× higher initial growth rate of PW droplets compared to S droplets. Consequently, the heat transfer of the PW droplet is 4–6× higher than that of the S droplet. To compare the heat transfer enhancement, PW and S droplet heat transfer rates are compared to that of a flat superhydrophobic silane coated surface, showing a 56% enhancement for the PW morphology, and 71% degradation for the S morphology. This study provides insight into importance of local wetting morphology on droplet growth rate during superhydrophobic condensation, as well as the importance of designing CB stable surfaces with PW droplet morphologies to achieve enhanced heat transfer during dropwise condensation.

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