• Nuclear Engineering and Technology
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• v.51 no.1
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• pp.84-94
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• 2019

Aluminum substrates are irradiated with chromium ions and the steam condensation heat transfer performance on these surfaces is examined. Filmwise condensation is induced on the surface of aluminum specimens irradiated with chromium ion dose of less than $10^{16}ions/cm^2$ while dropwise condensation occurs on the specimens irradiated with chromium ion dose of $5{\times}10^{16}ions/cm^2$ in the range of ion energy from 70 to 100 keV. The heat transfer coefficient of the surfaces on which dropwise condensation occurs appeared to be approximately twice as much as the prediction by Nusselt's film theory. In a durability test, dropwise condensation lasts over six months and the heat transfer coefficient is also maintained.

• The Magazine of the Society of Air-Conditioning and Refrigerating Engineers of Korea
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• v.15 no.2
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• pp.188-195
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• 1986

Condensation heat transfer can be classified in dropwise condensation and filmwise condensation, and for the industrial purpose, the former is more useful than the latter because of the higher heat transfer rate. But it is difficult to maintain the dropwise condensation continuously since most of the metal surfaces become wetted after exposure to a condensing vapor over an extended period of time. To maintain dropwise condensation continuously , various surface coatings and promoters have been used recently, but these methods must be reconsidered about the durability of condensing surface. Therefore, in this study, evaporating method of various pure metals on the condensing surface has been performed to maintain dropwise condensation. The results have showed that the heat transfer rate of silver evaporating surface is higher than any other metal evaporating in dropwise area. Transition temperature and filmwise condensation curves are uniform regardless of kinds of evaporating metals.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.13 no.1
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• pp.25-29
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• 2001

A study has been conducted to investigate the effect of the heat transfer surface position in the range of dropwise condensation, filmwise condensation, and glacial condensation. For dropwise condensation promoter, the heat transfer surface was evaporated by gold. As a result, heat transfer rate is almost same where the position of heat transfer surface is between 45 and 135 degree. It is found that heat transfer rate was reduced as subcooled degree was increased. And it is also found that if the subcooled degree becomes lower, the position of heat transfer surface is more effective. Adversely, if the subcooled degree becomes higher, the effectiveness of surface position is getting relatively lower. Regardless of the position, the transition temperatures from dropwise condensation to filmwise condensation is in the vicinity of 80K.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.29 no.12
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• pp.621-627
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• 2017

Nitrogen ion with various levels of dose and irradiation energy was irradiated on aluminum surfaces. Contact angle of surface was increased and surface color was changed by nitrogen ion implantation. During steam condensation experiment using nitrogen ion implanted specimen, dropwise condensation initially occurred on specimens. However, condensation mode eventually changed into filmwise condensation. The color of the surface was also changed from yellow-brown to silver-white. This change of surface color and condensation mode were results of hydrolysis reaction between condensate and nitrogen ion implanted on aluminum surfaces.

• Journal of ILASS-Korea
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• v.23 no.3
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• pp.149-153
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• 2018

The present study investigates the heat transfer characteristics of droplet growth during dropwise condensation on the hydrophobic copper surface. We use the copper specimen coated by the self-assembled layer and conduct the real-time measurement of droplet size and spatial distribution of condensates during condensation with the use of the K2 lens (long distance microscope lens) and CMOS camera. The temperatures are measured by three RTDs (resistance temperature detectors) that are located through the holes made in the specimen. The surface temperature is estimated by the measured temperatures with the use of the one-dimensional conduction equation. It is observed that the droplets on the surface are growing up and merging, causing larger droplets. The experimental results show that there are three distinct regimes; in the first regime, individual small droplets are created on the surface in the early stage of condensation, and they are getting larger owing to direct condensation and coalescence with other droplets. In the second and third regimes, the coalescence occurs mainly, and the droplets are detached from the surface. Also, the fall-off time becomes faster as the surface wettability decreases. In particular, the heat transfer coefficient increases substantially with the decrease in wettability because of faster removal of droplets on the surfaces for lower wettability.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.19 no.4
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• pp.517-524
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• 2018

This purpose of this research is to observe the influence of non-condensable gas (NCG) on a horizontal super-hydrophobic aluminum tube and compare it with a bare aluminum tube. To achieve super-hydrophobic characteristics, an aluminum tube was coated with a Self-Assembled Monolayer (SAM). The overall heat transfer coefficient U was used to represent the condensation performance. The NCG mass fraction was the main variable, and its range was 0.08 to 0.45. The condensation performance of the SAM tube and bare tube increased with decreasing mass fraction of NCG. The SAM tube showed 1.9 to 2.5 times larger dropwise condensation performance than the bare tube. When the mass fraction of NCG decreased in the SAM tube, the rate of increase of the SAM tube was lower because flooded condensation occurred. In addition, filmwise condensation occurred in the SAM tube when more active condensation was generated, and its performance was lower than that of the bare aluminum tube. The flooded and filmwise condensation in the SAM tube is explained by the pinning effect. In conclusion, controlling the condition of the condenser is necessary to improve the condensation performance by surface modification a SAM.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.157.2-157.2
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• 2016

Water repellent surfaces may enhance the condensation by efficiently removing the condensed droplets. However, such surfaces may lose their original performance as they are exposed to external mechanical stresses. In this work, we fabricated spray-coated mechanically robust superhydrophobic surfaces using treated titanium dioxide (Type 1) or silica particles (Type 2). Then we compared the mechanical robustness of such surfaces with the silane-coated superhydrophobic surface and PEEK coated surface using a controlled-sand blasting method. The results show that the spray-coated samples can maintain the same level of the contact angle hysteresis than silane-coated superhydorphobic surface after sand blasting at 2 bar. The spray-coating method was applied to the tube type condenser and the condensation behaviors were observed within the environmental chamber with controlled pressure, humidity and non-condensable gas. Previously-reported droplet jumping was observed in the early stage of the condensation event, but soon the droplet jumping stopped and only dropwise condensation was observed since the condensed droplets were pinned on the cracks at spray-coated surfaces. The static contact angle decreases from $158.0^{\circ}$ to $133.2^{\circ}$, and hysteresis increases from $3.0^{\circ}$ to $23.5^{\circ}$ when active condensation occurs on such surfaces. This work suggests the benefits and limitation of spray-coated superhydrophobic condensers and help develop advanced condensers for practical use.

• Journal of the Korean institute of surface engineering
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• v.54 no.2
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• pp.90-95
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• 2021

Surface modification technique enabling the control of condensation provides various benefit in various engineering systems, such as heat transfer, desalination, power plants, and so on. In this study, lubricant oil-impregnation into Teflon-coated nanoporous anodic oxide layer of aluminum to enhance a de-wetting and mobility of water droplet on surface. Due to the surface treatment improving water-repellency, the condensation mode is changed to dropwise, thus the frequency of sliding condensed water droplet on surface is increased. For these reasons, the surface of oil-impregnated Teflon-coated nanoporous anodic aluminum oxide shows significantly enhanced condensation heat transfer compared to bare aluminum surface. In addition, the porosity of anodic aluminum oxide affected the mobility of water droplet even with oil-impregnation and Teflon-coating, indicating that the optimization of porous structure of anodic oxide is required for maximizing the condensation heat transfer.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.33 no.4
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• pp.288-297
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• 2009

A serpentine channel geometry often used in a fuel cell has a strong pressure gradient between adjacent channels in specific regions. The pressure gradient helps some amount of reactant gas penetrate through a gas diffusion layer(GDL). As a result, the overall serpentine flow structure is slightly different from the intention of a designer. The purpose of this paper is to examine the effect of serpentine flow structure on current density distribution. By using a commercial code, STAR-CD, a numerical simulation is performed to analyze the fuel cell with high aspect ratio of active area. To increase the accuracy of the numerical simulation, GDL permeabilities are measured with various compressive forces. Three-dimensional flow field and current density distribution are calculated. For the verification of the numerical simulation results, water condensation process in the cathode channel is observed through a transparent bipolar plate. The result of this study shows that the region of relatively low current density corresponds that of dropwise condensation in cathode channels.

• 한국신재생에너지학회:학술대회논문집
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• 2007.11a
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• pp.147-151
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• 2007

A serpentine channel geometry often used in a polymer electrolyte membrane fuel cell has a strong pressure gradient between adjacent channels in specific regions. The pressure gradient helps some amount of reactant gas penetrate through a gas diffusion layer(GDL). As a result, the overall serpentine flow structure is slightly different from intention of a designer. The purpose of this paper is to examine the effect of serpentine flow structure on current density distribution. By using a commercial code, STAR-CD, a numerical simulation is performed to analyze the fuel cell with relatively high aspect ratio active area. To increase the accuracy of the numerical simulation, GDL permeabilities are measured with various compression conditions. Three-dimensional flow field and current density distribution are calculated. For the verification of the numerical simulation results, water condensation process in the cathode channel is observed through a transparent bipolar plate. The result of this study shows that the region of relatively low current density corresponds to that of dropwise condensation in cathode channels.