SINGLE DROPLET GROWTH MODEL FOR DROPWISE CONDENSATION CONSIDERING NON-CONDENSABLE GAS

2018 ◽  
Author(s):  
Shaofei Zheng ◽  
Ferdinand Eimann ◽  
Christian Philipp ◽  
Ulrich Gross
Keyword(s):  
Growth Model ◽  
Droplet Growth ◽  
Dropwise Condensation ◽  
Single Droplet

scholarly journals Droplet Growth Model for Dropwise Condensation on Concave Hydrophobic Surfaces

ACS Omega ◽  
2020 ◽  
Vol 5 (35) ◽  
pp. 22560-22567
Author(s):  
Tongqian Zhang ◽  
Zengzhi Zhang
Keyword(s):  
Growth Model ◽  
Droplet Growth ◽  
Dropwise Condensation ◽  
Hydrophobic Surfaces

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.

Filmwise-to-Dropwise Condensation Transition Enabled by Patterned High Wetting Contrast

2015 ◽  
Vol 137 (8) ◽  
Author(s):  
Youmin Hou ◽  
Miao Yu ◽  
Xuemei Chen ◽  
Zuankai Wang
Keyword(s):  
Liquid Film ◽  
Droplet Growth ◽  
Pinning Force ◽  
Dropwise Condensation ◽  
Ambient Conditions ◽  
Spherical Droplet ◽  
Design And Optimization ◽  
Morphology Transition ◽  
Hydrophobic Coating ◽  
Condensed Water

Recent advances in condensing surfaces with hybrid architectures of superhydrophobic/hydrophilic patterns allow us to decrease the nucleation energy barrier and spatially control the water condensation. However, the condensed water is susceptible to the large pinning force of the hydrophilic area, leading to an ultimate flooding. Here, we demonstrate a hierarchical nanostructured surface with patterned high wetting contrast to achieve a natural transition from filmwise-to-dropwise condensation, which reconciles the existing problems. The energy-dispersive X-ray spectroscopy (EDX) indicates that the fluorinated hydrophobic coating conformably covers the nanostructures except for the tops of micropillars, which are covered by hydrophilic silicon dioxide (FIG 1), resulting in an extreme wetting contrast. Condensation on the hybrid surface was observed in the environmental scanning electron microscope (ESEM) and ambient conditions with controlled humidity. Water preferentially nucleates on the top of micropillars and exhibits a rapid droplet growth (FIG 2). The enhancement is attributed to the filmwise-to-dropwise transition induced by the unique architectures and wetting features of the hybrid surface (FIG 3). The water embryos initially nucleate on the hydrophilic tops and quickly grow to a liquid film covering the whole top area. Since the superhydrophobic surrounding confines the spreading of condensed water, the localized liquid film gradually transits to an isolated spherical droplet as it grows. Remarkably, the condensate morphology transition activates an unusual droplet self-propelling despite the presence of abundant hydrophilic patches. It is important to note that such coalescence-induced jumping is dependent on the size of hydrophilic patches, that is, for larger hydrophilic patches, the energy released by coalescence may not overcome the increased droplet pinning, resulting in an immobile coalescence (FIG 4). The droplet departure ensures the recurrence of filmwise-to-dropwise transition, thus prevents the water accumulation in continuous condensation. These visualizations reveal the undiscovered impact of heterogeneous wettability and architectures on the morphology transition of the condensed water, and provide important insights into the surface design and optimization for enhanced condensation.

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 Hierarchical Superhydrophobic Copper for Sustained Dropwise Condensation

2015 ◽  
Vol 137 (8) ◽  
Author(s):  
Xuemei Chen ◽  
Justin A. Weibel
Keyword(s):  
Growth Dynamics ◽  
Time Lapse ◽  
Droplet Surface ◽  
Droplet Growth ◽  
Environmental Scanning ◽  
Dropwise Condensation ◽  
Droplet Motion ◽  
Surface Design ◽  
Industrial Systems ◽  
The Hill

Engineering surfaces that sustain continuous dropwise condensation, and are composed of materials commonly employed in heat transfer applications, are of great interest for scaled-up industrial systems. We fabricate hierarchical micro/nano-structured superhydrophobic surfaces on copper substrates. Condensate droplet growth dynamics on the as-fabricated samples were investigated using an environmental scanning electron microscope (ESEM; FEI Quanta 3D, ~6 torr, ~3 °C stage). Time-lapse ESEM images show that the condensate droplets preferentially nucleate at the bases of the hill-shaped microstructures (40 s). The droplets at the microstructure bases coalesce; merged droplets rise and appear to be suspended atop adjacent microstructures (180-220 s). These droplets, when triggered by coalescence, can gain sufficient kinetic energy by a reduction in droplet surface area/energy to spontaneously depart from the substrate. This droplet motion sweeps additional droplets in the trajectory and exposes fresh space for formation of new droplets (220-250 s). These droplet growth and departure dynamics are facilitated by the combination of microscale and nanoscale roughness features on the surface, and the behavior provides important insight into surface design requirements for sustaining dropwise condensation in thermal management applications.

Theoretical and Experimental Studies on the Pseudo-Dropwise Condensation of a Binary Vapor Mixture

1996 ◽  
Vol 118 (1) ◽  
pp. 140-147 ◽  
Author(s):  
K. Hijikata ◽  
Y. Fukasaku ◽  
O. Nakabeppu
Keyword(s):  
Surface Tension ◽  
Experimental Studies ◽  
Marangoni Effect ◽  
Copper Plate ◽  
Droplet Growth ◽  
Dropwise Condensation ◽  
Vapor Mixture ◽  
Ethanol Mixture ◽  
Absolute Value ◽  
Critical Marangoni Number

When a water–ethanol binary mixture condenses on a flat plate, one observes that the liquid film condensate rises locally and eventually forms many droplets on the film. Usually, filmwise condensation is expected because both substances are completely soluble in each other and they wet a copper plate well. This paper presents the droplet growth mechanism during so-called pseudo-dropwise condensation. Instability analysis is used to determine the transition from filmwise condensation to pseudo-dropwise condensation theoretically. In a stress balance at the vapor–liquid interface, the analysis considers not only the surface tension itself, but also the surface tension variation due to changes in temperature and concentration, assuming saturation conditions at the interface. Numerical results indicate that the Marangoni effect plays a more important role than the absolute value of the surface tension in pseudo-dropwise condensation. The change in surface tension with temperature is not always negative; it becomes positive for certain mixtures due to the dependence on concentration. Pseudo-dropwise condensation is only realized when surface tension increases with temperature. This analysis qualitatively predicts the critical Marangoni number experimentally observed during water–ethanol mixture condensation.

scholarly journals Behavior of condensed droplets growth and jumping on superhydrophobic surface

2019 ◽  
Vol 128 ◽  
pp. 07003
Author(s):  
Sihang Gao ◽  
Fuqiang Chu ◽  
Xuan Zhang ◽  
Xiaomin Wu
Keyword(s):  
Growth Model ◽  
Superhydrophobic Surface ◽  
High Speed ◽  
Fluid Model ◽  
Chemical Deposition ◽  
Droplet Growth ◽  
Condensation Process ◽  
Camera System ◽  
Simulation And Experiment ◽  
Volume Of Fluid Model

Droplets on the superhydrophobic surface can fall off the surface spontaneously, which greatly promote dropwise condensation. This study considers a continuous droplet condensation process including droplet growth and droplet jumping. A droplet growth model considered NCG is developed and droplet jumping is simulated using VOF (Volume Of Fluid) model. Al–based superhydrophobic surfaces are prepared using chemical deposition and etching method. The Al-based superhydrophobic surface has a contact angle of 157°±1° and a rolling angle of 2°±1°. An observation experiment is designed to observe droplet jumping on superhydrophobic surface using a high– speed camera system. The result of droplet growth model shows a good match with experimental data in mid-term of droplet growth. Fordroplet jumping, simulation and experiment results show that droplet jumping of different diameter hasa universality in a non–dimensional form. The jumping process can be divided into 3 stages and droplet vibration is observed.

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