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.

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.

Wettability Effect on Droplet Growth Behaviors During Dropwise Condensation

2015 ◽  
Author(s):  
Jae Bin Lee ◽  
Seong Hyuk Lee ◽  
Chang Kyoung Choi ◽  
Jungho Lee
Keyword(s):  
Heat Transfer ◽  
Contact Angles ◽  
Area Fraction ◽  
Droplet Growth ◽  
Condensation Process ◽  
Dropwise Condensation ◽  
Self Assembled Monolayer ◽  
Low Surface Energy ◽  
The Heat Transfer Coefficient ◽  
Off Time

The present study examines the transient characteristics of droplet growth and heat transfer during dropwise condensation process on different hydrophobic surfaces. The self-assembled monolayer (SAM) of n-octadecyl mercaptan was coated on the surface to change the surface wettability with the contact angles of 148° and 124°. A Canon EOS 7D camera and an Infinity K2 lens were used to capture the spontaneous images during condensation. From the experiment, three regimes were observed: in the first regime where the droplets were formed with nearly homogenous pattern, the heat transfer coefficient increased rapidly. In the second regime, both the droplet size and the liquid area fraction increased. In the third regime, coalescence among larger droplets was dominant, causing the decrease in the liquid area fraction with time. Moreover, the measured fall-off time was faster in the case with higher contact angle because of relatively low surface energy.

Molecular Dynamics Simulations of Interfacial Heat and Mass Transfer at Nanostructured Surface

2007 ◽  
Author(s):  
Gyoko Nagayama ◽  
Masako Kawagoe ◽  
Takaharu Tsuruta
Keyword(s):  
Heat Transfer ◽  
Molecular Dynamics ◽  
Boundary Condition ◽  
Thermal Resistance ◽  
Hydrodynamic Resistance ◽  
Hydrophilic Surface ◽  
Dynamics Simulations ◽  
Solid Liquid Interface ◽  
Solid Liquid ◽  
The Continuum

The nanoscale heat and mass transport phenomena play important roles on the applications of nanotechnologies with great attention to its differences from the continuum mechanics. In this paper, the breakdown of the continuum assumption for nanoscale flows has been verified based on the molecular dynamics simulations and the heat transfer mechanism at the nanostructured solid-liquid interface in the nanochannels is studied from the microscopic point of view. Simple Lennard-Jones (LJ) fluids are simulated for thermal energy transfer in a nanochannel using nonequilibrium molecular dynamics techniques. Multi-layers of platinum atoms are utilized to simulate the solid walls with arranged nanostructures and argon atoms are employed as the LJ fluid. The results show that the interface structure (i.e. the solid-like structure formed by the adsorption layers of liquid molecules) between solid and liquid are affected by the nanostructures. It is found that the hydrodynamic resistance and thermal resistance dependents on the surface wettability and for the nanoscale heat and fluid flows, the interface resistance cannot be neglected but can be reduced by the nanostructures. For the hydrodynamic boundary condition at the solid-liquid interface, the no-slip boundary condition holds good at the super-hydrophilic surface with large hydrodynamic resistance. However, apparent slip is observed at the low hydrodynamic resistance surface when the driving force overcomes the interfacial resistance. For the thermal boundary condition, it is found that the thermal resistance at the interface depends on the interface wettability and the hydrophilic surface has lower thermal resistance than that of the hydrophobic surfaces. The interface thermal resistance decreases at the nanostructed surface and significant heat transfer enhancement has been achieved at the hydrophilic nanostructured surfaces. Although the surface with nanostrutures has larger surface area than the flat surface, the rate of heat flux increase caused by the nanostructures is remarkable.

Optimization of Mixed Hydrophilic and Hydrophobic Surfaces Under Laminar Flow Conditions

2020 ◽  
Author(s):  
Brian Frymyer ◽  
Alparslan Oztekin
Keyword(s):  
Heat Transfer ◽  
Laminar Flow ◽  
Thermal Resistance ◽  
Energy Barrier ◽  
High Energy ◽  
Film Condensation ◽  
Dropwise Condensation ◽  
Flow Conditions ◽  
Mass Collection ◽  
Laminar Flow Conditions

Abstract When condensation first forms on a surface, it starts as tiny droplets. As the surface continues to collect condensation, the droplets grow together and form a film. The film increases the thermal resistance of the system. It is possible to remove the fluid from the condensing surface before it develops into a film. Dropwise condensation has the capability of providing up to an order of magnitude higher heat transfer than film condensation. A hydrophobic surface is capable of sustaining dropwise condensation but creates a high energy barrier that restricts nucleation. A hydrophilic surface has a low energy barrier for nucleation but retains the water quickly transitioning to film condensation. A hydrophilic and hydrophobic patterned surface creates a surface with a low nucleation energy barrier and is capable of sustaining dropwise condensation. Surface patterns are evaluated under laminar flow conditions to maximize mass collection. The surfaces are evaluated using a thermal model, which includes an equivalent thermal resistance for diffusion. Laminar flow rates are evaluated using Reynolds numbers from 1,218 to 4 × 105. Hydrophilic nodules sizes are evaluated from 0.1 mm to 3.7 mm. Under natural convection flow, mass collection can be increased by 20% with respect to film heat transfer.

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.

scholarly journals Towards AquaSun practical utilization: strong adhesion and lack of ecotoxicity of solar-driven antifouling sol-gel coating

2021 ◽  
Author(s):  
Antonino Scurria ◽  
Silvia Sfameni ◽  
Gabriella Di Carlo ◽  
Mario Pagliaro ◽  
Anna Maria Visco ◽  
...  
Keyword(s):  
Contact Angle ◽  
Surface Energy ◽  
Sol Gel ◽  
Scratch Resistance ◽  
Practical Utilization ◽  
Marine Coatings ◽  
Low Surface Energy ◽  
H2o2 Formation ◽  
Strong Adhesion ◽  
Ecotoxicity Tests

The outcomes of adhesion and ecotoxicity tests carried out on metal specimens faithfully representing the surface of real ships, including the primer and tie coat layers typically applied on ship hull prior to deposition of the antifouling paint, show the practical applicability of "AquaSun" antifouling sol-gel coatings. Newly developed AquaSun coatings share superhydrophicity (contact angle >115) and exceptionally high scratch resistance (ASTM 5B). Coupled to the ecofriendly antifouling mechanism based on continuous H2O2 formation upon exposure to solar light and foul release due to low surface energy, these results open the route to the practical utilization of these novel marine coatings.

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 Robust Superhydrophobic Surface on Polypropylene with Thick Hydrophobic Silica Nanoparticle-Coated Films Prepared by Facile Compression Molding

Energies ◽  
2021 ◽  
Vol 14 (11) ◽  
pp. 3155
Author(s):  
Oyunchimeg Erdene-Ochir ◽  
Doo-Man Chun
Keyword(s):  
Contact Angle ◽  
Surface Energy ◽  
Alkaline Medium ◽  
Superhydrophobic Surface ◽  
Thick Layer ◽  
Scratch Test ◽  
Superhydrophobic Surfaces ◽  
Sliding Angle ◽  
Hydrophobic Silica ◽  
Low Surface Energy

Superhydrophobic surfaces have been extensively studied for their unique interfacial interaction between water and the surface, and they can be used for self-cleaning, drag reduction, anti-icing, and other applications. To make the superhydrophobic surfaces, nano/microscale structures and a low surface energy should be realized. The development of a durable superhydrophobic surface was hindered by the vulnerability of the surface to mechanical contact. To improve the robustness of the superhydrophobic surface toward mechanical damage, the hydrophobic polypropylene (PP) surface was coated with a thick layer of hydrophobic silica nanoparticles (SNPs) using a simple compression molding process. The thick layer consists of SNPs and PP, and the roles of SNPs and PP are nano/microscale structures with a low surface energy and binder for nanoparticles, respectively. This revealed improvement in the superhydrophobic tendency, with an apparent contact angle of about 170° and a sliding angle of less than 5°. The morphology and the corresponding elemental analysis of the PP/SNPs coated films were investigated using field emission scanning electron microscopy and energy-dispersive spectrometry. The mechanical durability of the superhydrophobic surface was evaluated by the scotch tape test and scratch test with sandpaper. The coated films with SNPs showed the superhydrophobic behavior after 25 tape tests. In addition, the coated films with SNPs showed a contact angle greater than 150° and a sliding angle less than 10° after a 100-cm scratch test with 1000 grit sandpaper, under a weight of 500 g, on an area of 40 × 40 mm2. The chemical stability of PP/SNPs coated films was also investigated in acidic, neutral, and alkaline medium solutions. The films showed good stability under the acidic and neutral medium solutions even after 24 h, but an alkaline medium could damage the surface. The obtained results demonstrated the robustness of the superhydrophobic coating with SNPs.

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