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 Near Field Condensation

2020 ◽  
Author(s):  
Xiao Yan ◽  
Feipeng Chen ◽  
Chongyan Zhao ◽  
Yimeng Qin ◽  
Xiong Wang ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Population Density ◽  
Near Field ◽  
Condensation Heat Transfer ◽  
Droplet Growth ◽  
Dropwise Condensation ◽  
Droplet Transfer ◽  
Droplet Sizes ◽  
Departure Size ◽  
Contact Line Pinning

Abstract Dropwise condensation represents the upper limit of condensation heat transfer. Promoting dropwise condensation relies on surface chemical functionalization, and is fundamentally limited by the maximum droplet departure size. A century of research has focused on active and passive methods to enable the removal of ever smaller droplets. However, fundamental contact line pinning limitations prevent gravitational and shear-based removal of droplets smaller than 250 µm. Here, we break this limitation through near field condensation. By de-coupling nucleation, droplet growth, and shedding via droplet transfer between parallel surfaces, we enable the control of droplet population density and removal of droplets as small as 20 µm without the need for chemical modification or surface structuring. We identify droplet bridging to develop a regime map, showing that rational wettability contrast propels spontaneous droplet transfer from condensing surfaces ranging from hydrophilic to hydrophobic. To demonstrate efficacy, we perform condensation experiments on surfaces ranging from hydrophilic to superhydrophobic. The results show that near field condensation with optimal gap spacing can limit the maximum droplet sizes and significantly increase the population density of sub-20 µm droplets. Theoretical analysis and direct numerical simulation confirm the breaking of classical condensation heat transfer paradigms through enhanced heat transfer. Our study not only pushes beyond century-old phase change limitations, it demonstrates a promising method to enhance the efficiency of applications where high, tunable, gravity-independent, and durable condensation heat transfer is required.

Electrowetting-Based Coalescence of Droplets During Dropwise Condensation of Humid Air

2019 ◽  
Author(s):  
Enakshi Wikramanayake ◽  
Vaibhav Bahadur
Keyword(s):  
Heat Transfer ◽  
Electric Field ◽  
Electric Fields ◽  
Applied Electric Field ◽  
Growth Dynamics ◽  
Condensation Heat Transfer ◽  
Droplet Growth ◽  
Dropwise Condensation ◽  
Condensation Rate ◽  
Humid Air

Abstract Dropwise condensation yields higher heat transfer coefficients by avoiding the thermal resistance of the condensate film, seen during filmwise condensation. This work explores further enhancement of dropwise condensation heat transfer through the use of electrowetting to achieve faster droplet growth via coalescence of the condensed droplets. Electrowetting is a well understood microfluidic technique to actuate and control droplets. This work shows that AC electric fields can significantly enhance droplet growth dynamics. This enhancement is a result of coalescence triggered by various types of droplet motion (translation of droplets, oscillations of three phase line), which in turn depends on the frequency of the applied AC waveform. The applied electric field modifies droplet condensation patterns as well as the roll-off dynamics on the surface. Experiments are conducted to study early-stage droplet growth dynamics, as well as steady state condensation rates under the influence of electric fields. It is noted that this study deals with condensation of humid air, and not pure steam. Results show that increasing the voltage magnitude and frequency increases droplet growth rate and overall condensation rate. Overall, this study reports more than a 30 % enhancement in condensation rate resulting from the applied electric field, which highlights the potential of this concept for condensation heat transfer enhancement.

scholarly journals Dropwise condensation on bioinspired hydrophilic-slippery surface

RSC Advances ◽  
2018 ◽  
Vol 8 (69) ◽  
pp. 39341-39351 ◽  
Author(s):  
L. Guo ◽  
G. H. Tang
Keyword(s):  
Heat Transfer ◽  
Contact Angle ◽  
Water Transport ◽  
Nucleation Rate ◽  
Copper Surface ◽  
Dropwise Condensation ◽  
Sliding Angle ◽  
Small Contact Angle ◽  
Slippery Surface

A hydrophilic-slippery copper surface is fabricated, reconciling two required factors, enhanced condensation and efficient water transport. Nucleation rate, droplet mobility and heat transfer are enhanced by the small contact angle and sliding angle.

scholarly journals A Review of Research on Dropwise Condensation Heat Transfer

2021 ◽  
Vol 11 (4) ◽  
pp. 1553
Author(s):  
Xuechao Hu ◽  
Qiujie Yi ◽  
Xiangqiang Kong ◽  
Jianwei Wang
Keyword(s):  
Heat Transfer ◽  
Theoretical Models ◽  
High Heat ◽  
Condensation Heat Transfer ◽  
Formation Mechanisms ◽  
Transfer Model ◽  
Dropwise Condensation ◽  
High Heat Transfer ◽  
Nucleation Sites ◽  
Enhancing Heat Transfer

Dropwise condensation is considered to be an effective method of enhancing heat transfer due to its high heat transfer performance. However, because the effect of dropwise condensation is affected by many complex factors, there is no systematic review summarized on the law of dropwise condensation heat transfer by scholars. In this paper, the main methods and problems of promoting dropwise condensation were reviewed based on the dropwise condensation mechanism and theoretical model. The three different hypotheses about the mechanism of dropwise condensation and the heat transfer model of dropwise condensation based on the hypothesis of nucleation sites were summarized. The methods for promoting dropwise condensation and the problems that influence dropwise condensation heat transfer are introduced in this paper. The research showed that many researchers focused on how the surface fabricated forms dropwise condensation rather than whether it enhances heat transfer. In this paper, we point out that the droplet shedding rate is the key to enhancing dropwise condensation heat transfer. Much more research on droplet formation mechanisms and theoretical models of different surfaces is supposed to be carried out in the future.

The Effect of Thermal Resistance for Dropwise Condensation on Hydrophobic Micro-Pillared Structures

2012 ◽  
Author(s):  
Sara S. Beaini ◽  
Hector Mendoza ◽  
Van P. Carey
Keyword(s):  
Heat Transfer ◽  
Contact Angle ◽  
Surface Energy ◽  
Thermal Resistance ◽  
Droplet Growth ◽  
Dropwise Condensation ◽  
Textured Surfaces ◽  
Critical Question ◽  
Low Surface Energy ◽  
The Continuum

Superhydrophobic/hydrophobic surfaces, developed to promote dropwise condensation, can be produced by modifying the surface chemically with low surface energy films, and/or structurally by fabricating micro-textured surfaces. Some research has reported the increased thermal resistance from the added chemical layer and its effect on condensation heat transfer. A critical question of interest is the thermal resistance due to micro-pillared structures and their influence on droplet growth during condensation as compared to smooth or non-textured surfaces. Though idealized, this paper presents a theoretical and computational model for evaluating and quantifying the effects of the pillared structures thermal resistance, as well as the continuum versus non-continuum mechanisms affecting droplet growth during dropwise condensation. The model is used to compare different micro-pillared surfaces, cited in the literature, and to predict which micro-pillar dimensions contribute to slower condensate growth despite the higher contact angle advantage during dropwise condensation.

Submicron Superhydrophobic Surface Treatment Suitable for High Performance Condensers: A Modeling

2009 ◽  
Author(s):  
Sunwoo Kim ◽  
Kwang J. Kim ◽  
John M. Kennedy ◽  
Jiong Liu ◽  
Ganesh Skandan
Keyword(s):  
Heat Transfer ◽  
Contact Angle ◽  
Theoretical Model ◽  
Size Distribution ◽  
Drop Size ◽  
Drop Size Distribution ◽  
Condensation Heat Transfer ◽  
Dropwise Condensation ◽  
Superhydrophobic Coating ◽  
Single Droplet

The effect of the drop-contact angle on dropwise condensation heat transfer of saturated steam on a single horizontal copper tube with the superhydrophobic coating was investigated theoretically. The theoretical model is established by combining heat transfer through a single droplet with a well-known drop size distribution theory. The analysis of single droplet heat transfer incorporates resistances due to vapor-liquid interface, drop curvature, conduction through the drop, and conduction through the superhydrophobic coating layer. Each resistance is expressed as a function of the contact angle. The total resistance for a drop with a fixed radius increases as the contact angle increases. A population balance model is used to develop a drop distribution function for the small drops that grow by direct condensation. Drop size distribution for large drops that grow mainly by coalescence is obtained from the empirical equation proposed by Le Fevre and Rose (1966). The results indicate that the contact angle has a strong correlation with the maximum drop radius, which plays a pivotal role in determining drop size distribution. A high contact angle leads to a significant reduction in the radius of the largest drop that is about to fall down due to gravity and sweep away drops in its path. Thus, there are more areas on the condensing surface for small drops, allowing for greater heat transfer. Also, it is shown that surface wettability affects the performance of dropwise condensation heat transfer and our theoretical model successfully predicts this phenomenon.

Dropwise Condensation Heat Transfer Model: Effect of the Liquid-Solid Surface Free Energy Difference

2006 ◽  
Author(s):  
Xue-Hu Ma ◽  
Zhong Lan ◽  
Yu Zhang ◽  
Xing-Dong Zhou ◽  
Tian-Yi Sun
Keyword(s):  
Heat Transfer ◽  
Contact Angle ◽  
Free Energy ◽  
Heat Transfer Coefficient ◽  
Energy Difference ◽  
Interfacial Interaction ◽  
Condensation Heat Transfer ◽  
Dropwise Condensation ◽  
Free Energy Difference ◽  
Simulation Results

Dropwise condensation heat transfer performance depends not only on the condensing conditions, but also on the interfacial interaction between condensate and condensing surface material. Based on the well-established Rose’s model, a modified model of dropwise condensation heat transfer is proposed by considering the interfacial interaction between liquid and solid, and established by rebuilding the space conformation of drop distributing into time conformation. The simulation results indicate that the heat transfer coefficient increases with the surface free energy difference increasing and the contact angle hysteresis decreasing. The larger contact angle and the smaller departure drop size result in the higher heat transfer coefficient. Different interfacial effect gives rise to the different heat transfer curves. For the identical solid-liquid-vapor system, the simulation results agree very well with the present experimental data and those reported in literature. The controversy among experimental results in literature might be well understood with the concept of the present paper.

Influence of Surface Wettability on Dropwise Condensation Heat Transfer

2011 ◽  
Vol 228-229 ◽  
pp. 869-873
Author(s):  
Yun Fu Chen
Keyword(s):  
Heat Transfer ◽  
Contact Angle ◽  
Surface Wettability ◽  
Condensation Heat Transfer ◽  
Size Distributions ◽  
Dropwise Condensation ◽  
Number Density ◽  
Drop Radius ◽  
Proposed Model ◽  
Drop Size Distributions

For finding influence of the surface wettability on dropwise condensation heat transfer, a model for dropwise condensation heat transfer has been established based on the drop size distributions and the heat transfer rate through a single drop with considering influence of contact angle to heat transfer. It has been shown based on the proposed model that up to a drop radius of 5μm, the rate of decrease in the drop population density is not as steep as the rate for a drop radius greater than 10μm, because coalescence between drops starts taking place. Varying the contact angle changes the drop distribution; higher the contact angle, lower the departing droplet size and large number density of small droplets. Heat flux first increases and then decreases with increasing contact angle under the temperature difference condition.

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