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.

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.

Effect of Open Micro-Channels on External Condensation Heat Transfer

2016 ◽  
Author(s):  
Brandon Hulet ◽  
Andres Martinez ◽  
Melanie Derby ◽  
Amy Rachel Betz
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Film Thickness ◽  
Transfer Coefficient ◽  
Heat Fluxes ◽  
Condensation Heat Transfer ◽  
Dropwise Condensation ◽  
Micro Channels ◽  
Lower Heat ◽  
The Heat Transfer Coefficient

This research experimentally investigates the heat transfer performance of open-micro channels under filmwise condensation conditions. Filmwise condensation is an important factor in the design of steam condensers used in thermoelectric power generation, desalination, and other industrial applications. Filmwise condensation averages five times lower heat transfer coefficients than those present in dropwise condensation, and filmwise condensation is the dominant condensation regime in the steam condensers due to a lack of a durable dropwise condensation surface. Film thickness is also of concern because it is directly proportional to the condenser’s overall thermal resistance. This research focuses on optimizing the channel size to inhibit the creation of a water film and/or to reduce its overall thickness in order to maximize the heat transfer coefficient of the surface. Condensation heat transfer was measured in three square channels and a plane surface as a control. The sizes of the square fins were 0.25 mm; 0.5 mm; and 1 mm, and tests were done at a constant pressure of 6.2 kPa. At lower heat fluxes, the 0.25mm fins perform better, whereas at larger heat fluxes a smooth surface offers better performance. At lower heat fluxes, droplets are swept away by gravity before the channels are flooded. Whereas, at higher heat fluxes, the channels are flooded increasing the total film thickness, thereby reducing the heat transfer coefficient.

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.

Steam Condensation Enhancement by Applying Self-Assembled Monolayer (Chlorosilane) on Titanium Corrugated Tubes

2019 ◽  
Vol 27 (01) ◽  
pp. 1950009
Author(s):  
Nae-Hyun Kim ◽  
Ho-Seung Han
Keyword(s):  
Heat Transfer ◽  
Condensation Heat Transfer ◽  
Film Condensation ◽  
Dropwise Condensation ◽  
Transfer Enhancement ◽  
Heat Transfer Characteristics ◽  
Steam Condensation ◽  
Self Assembled Monolayer ◽  
Transfer Method ◽  
Condensation Heat Transfer Coefficient

It is well known that dropwise condensation enhances the condensation heat transfer coefficient significantly compared with film condensation. In the present study, dropwise condensation heat transfer characteristics on titanium corrugated tubes were investigated. Two corrugated tubes with different corrugation pitch and depth were tested at the steam pressures of 5 and 10[Formula: see text]kPa. To promote dropwise condensation, silane-based SAM was coated. For bare corrugated tubes, significant enhancement of condensation heat transfer was noted, especially for the 2.1/0.2 (corrugation pitch/corrugation depth in mm) tube. For SAM-coated tubes, the heat transfer enhancement was significant (2.61 at 5[Formula: see text]kPa and 2.45 at 10[Formula: see text]kPa) for the smooth tube. For the corrugated tubes, however, the enhancement decreased to 1.78 and 2.22 for 8.7/0.4 tube and to 1.26 and 1.52 for 2.1/0.2 tube. The present results suggest that corrugations may not be as an effective heat transfer method for dropwise condensation as it is for film condensation. This result was supported by the photos taken by mist spray, which suggested that surface tension drained condensation by corrugations is not a major heat transfer mechanism for dropwise condensation on corrugated tubes.

Experimental Study and Visualization of Dropwise Condensation on SUS316 Surface Under Non-Condensable Gas Condition

2016 ◽  
Author(s):  
Jae Young Choi ◽  
Yong Hoon Jeong ◽  
Noriyuki Watanabe
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Heat Exchanger ◽  
Transfer Coefficient ◽  
Surface Coating ◽  
Drop Size ◽  
Dropwise Condensation ◽  
Light Water ◽  
Cooling Surface ◽  
The Heat Transfer Coefficient

The passive safety features of nuclear power plant against station blackout (SBO) and intact containment integrity are the main key issues after Fukushima accident. As a corresponding safety system, passive containment cooling system (PCCS) received attention as one of the candidate systems applying to advanced light water reactors. Next generation of light water reactor, AP-1000 and ESBWR have suggested their own PCCS design. However, PCCS have difficulty in its heat exchanger volume due to low heat transfer coefficient of condensation under the presence of non-condensable gas condition. Several attempts had been studied worldwide to enhance the heat transfer coefficient of PCCS and this paper focused on dropwise condensation, which has much higher heat transfer coefficient than those found with filmwise condensation. Historically, surface coating or applying organic promoter on the cooling surface were typically used to induce dropwise condensation, but those method had disadvantage of their duration time. In general, surface coating and organic promoter were considered to have few years and few days of their duration, respectively. Therefore, an aim of our experiment was to determine whether SUS316 without any surface treatment is able to utilize dropwise condensation for PCCS heat exchanger. Following studies were compared to the filmwise condensation on SUS316 with same condition and also the experiment results reported by other researchers. The overall results determined how much the heat transfer coefficient was enhanced. To analyze the behavior of dropwise condensation, visualized images of cooling surface with droplets were graphically processed. All experiments were conducted on 13mm diameter of vertical-oriented flat surface with 6mm thickness. Air concentration (non-condensable concentration) was estimated by the partial pressure ratio of steam and air. Subcooled temperature, the difference between steam saturated temperature and surface temperature, was tested from 1 to 30 degree Celsius. High-speed camera visualized the condensate on the cooling surface with several magnification and frame speed. All measurement was measured after the whole system reached to equilibrium state and sustained it more than 30 minutes. Each data was recorded for 60 seconds and time-averaged its measurement. Experiment results indicated that SUS316 surface without any surface treatment could sustain dropwise condensation phase over 12 hours of experiment under low subcooled temperature below approximately 20 degree Celsius. Also, dropwise condensation had around 3 to 4 times enhanced heat transfer coefficients than those of filmwise condensation. The result of dropwise condensation with pure steam condition was well proportional to the power of subcooled temperature. Otherwise, the result with non-condensable gas showed heat transfer coefficient, shaded in high uncertainty of systematic error, seems to be decreased in low subcooled temperature under 5 degree Celsius. Higher sensitivity of non-condensable concentration upon heat transfer coefficient was observed on dropwise condensation more than filmwise condensation. The variation of drop-size distribution with the periodic time passage after surface sweeping was observed and patterns of drop-size distribution were repeated after the surface sweeping regardless of experimental conditions. The study provides feasibility and benefit of the utilization of dropwise condensation to PCCS heat exchanger if the system has designed to be operated within low subcooled temperature.

scholarly journals Condensation on Superhydrophobic Copper Oxide Nanostructures

2012 ◽  
Author(s):  
Ryan Enright ◽  
Nicholas Dou ◽  
Nenad Miljkovic ◽  
Youngsuk Nam ◽  
Evelyn N. Wang
Keyword(s):  
Heat Transfer ◽  
Copper Oxide ◽  
Environmental Scanning Electron Microscopy ◽  
Hydrophobic Surface ◽  
Characteristic Size ◽  
Chemical Oxidation ◽  
Contact Angles ◽  
Water Desalination ◽  
Self Assembled Monolayer ◽  
Oxide Nanostructures

Condensation is an important process in both emerging and traditional power generation and water desalination technologies. Superhydrophobic nanostructures promise enhanced condensation heat transfer by reducing the characteristic size of departing droplets via a surface-tension-driven mechanism [1]. In this work, we investigated a scalable synthesis technique to produce oxide nanostructures on copper surfaces capable of sustaining superhydrophobic condensation and characterized the growth and departure behavior of condensed droplets. Nanostructured copper oxide (CuO) films were formed via chemical oxidation in an alkaline solution. A dense array of sharp CuO nanostructures with characteristic heights and widths of ∼1 μm and ∼300 nm, respectively, were formed. A gold film was deposited on the surface and functionalized with a self-assembled monolayer to make the surfaces hydrophobic. Condensation on these surfaces was then characterized using optical microscopy (OM) and environmental scanning electron microscopy (ESEM) to quantify the distribution of nucleation sites and elucidate the growth behavior of individual droplets with a characteristic size of ∼1 to 10 μm at low supersaturations. Comparison of the observed behavior to a recently developed model for condensation on superhydrophobic surfaces [2, 3] suggests a restricted regime of heat transfer enhancement compared to a corresponding smooth hydrophobic surface due to the large apparent contact angles demonstrated by the CuO surface.

Characteristics of Droplet Growth Behavior on Hydrophobic Micro-textured Surfaces

2015 ◽  
Vol 137 (8) ◽  
Author(s):  
Jae Bin Lee ◽  
Joo Hyun Moon ◽  
Minhaeng Cho ◽  
Seong Hyuk Lee
Keyword(s):  
Surface Tension ◽  
Surface Texturing ◽  
Copper Plate ◽  
Growth Behavior ◽  
Numerical Control ◽  
Droplet Growth ◽  
Dropwise Condensation ◽  
Textured Surfaces ◽  
Cnc Machine ◽  
Self Assembled Monolayer

For occurring dropwise condensation, the droplet growth behavior such as single droplet growth, coalescence, and fall-off of large droplets play a major role in regard to condensation heat transfer and water harvesting. The present study visualized the droplet growth behavior of dropwise condensation which might be controlled by the surface tension. We used three copper plate with different hole area fraction (i.e., ϕ=0, 0.148 and 0.439). Surface texturing was conducted by using a μ-computer numerical control (μ-CNC) machine and composed of micro-holes (diameter: 300 μm, depth: 200 μm) with 691 μm, 401 μm pitch. To make the hydrophobic surfaces, the copper surfaces were coated self-assembled monolayer (SAM). From the results, it was found that for only SAM coated surface, random coalescence occurred and affected the droplet growth significantly, whereas for textured SAM coated surfaces, the droplet started to be grown up at the textured holes and coalesced near the holes, indicating that capillary effects might affect the droplet growth mechanism. In particular, the fall-off time at which a coalesced droplet is removed away from the surface increased when the textured surfaces was used, because of surface tension effect increased by the textured holes.

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