Frosting Characteristics on Hydrophilic and Superhydrophobic Copper Surfaces

2016 ◽  
Vol 138 (2) ◽  
Author(s):  
Chan Ho Jeong ◽  
Jae Bin Lee ◽  
Seong Hyuk Lee ◽  
Jungho Lee ◽  
Seung Mun You ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Copper Substrate ◽  
Copper Surface ◽  
Contact Angles ◽  
Film Condensation ◽  
Time Interval ◽  
Copper Surfaces ◽  
Static Contact ◽  
Temperature Detector ◽  
Bare Copper

The main objective of this study is to examine the frosting characteristics affected by the surface wettability. Two different copper surfaces – bare and nano structured - were prepared for the experiments. Their static contact angles are 74° (bare: without surface treatment) and 154° (nano-structured), respectively. The temperature of the copper substrate was measured by using resistance temperature detector (RTD) sensors embedded inside small holes drilled at 1 mm underneath the surface. During the phase change, the temperature of the copper substrates remained -7.8±0.6°C and the ambient temperature was set as 24±0.5°C with the relative humidity of 45%. Images were captured by using the CMOS camera with the 5 second time interval. Film condensation occurred because of higher wettability of the bare copper surface. Film condensates were frozen at the early stage and frost crystal grew in the vertical direction. On the other hand, dropwise condensates formed on the nano-structured copper surface remained as the supercooled liquid phase for 44 minutes owing to its low wettability. After 4 minutes, frosting on the bare copper substrate was triggered and propagated until it covered the whole surface. The frosting was significantly delayed on the superhydrophobic copper surface due to the lower surface free energy. The different porous media composed of frost which directly influence the heat transfer characteristics was formed on each surfaces. Therefore, additional investigation for heat transfer phenomenon on superhydrophobic surface should be conducted.

Pool Boiling Heat Transfer of Water on Hydrophilic Surfaces With Different Wettability

2016 ◽  
Author(s):  
Adam R. Girard ◽  
Jinsub Kim ◽  
Seung M. You
Keyword(s):  
Heat Transfer ◽  
Boiling Heat Transfer ◽  
Pool Boiling ◽  
Copper Surface ◽  
Contact Angles ◽  
Copper Surfaces ◽  
Nanoscale Structures ◽  
Flat Surfaces ◽  
Hydrophilic Surfaces ◽  
Alkali Solutions

The effect of wettability on boiling heat transfer (BHT) coefficient and critical heat flux (CHF) in pool boiling of water on hydrophilic surfaces having different contact angles was investigated. Hot alkali solutions were utilized to promote cupric and cuprous oxide growth which exhibited micro and nanoscale structures on copper surfaces, with thicknesses on the order of a couple of micrometers. These structure and surface energy variations result in different levels of wettability and roughness while maintaining the effusivity of the bare copper surface. The study showed that the BHT coefficient has an inverse relationship to wettability; the BHT coefficient decreases as wettability increases. Furthermore, it was shown that this dependency between BHT coefficient and wettability is more significant than the relationship between BHT coefficient and surface roughness. The CHF was also found to increase with increases in wettability and roughness. For the most hydrophilic surface tested in this study, CHF values were recorded near the 2,000 kW/m2 mark. This value is compared with maximum values reported in literature for water on non-structured flat surfaces without area enhancements. Based on these results it is postulated that there exists a true hydrodynamic CHF limit for pool boiling with water on flat surfaces, very near 2,000 kW/m2, independent of heater material, representing an 80% increase in the limit suggested by Zuber [1].

Droplets Jumping from a Hybrid Superhydrophilic and Superhydrophobic Surface

2017 ◽  
Vol 139 (2) ◽  
Author(s):  
Hai Wang ◽  
Quang Nguyen ◽  
Jae W. Kwon ◽  
Jing Wang ◽  
Hongbin Ma
Keyword(s):  
Contact Angle ◽  
Copper Substrate ◽  
Copper Surface ◽  
Film Condensation ◽  
Condensation Process ◽  
Nanostructured Surface ◽  
Additional Energy ◽  
Ammonium Persulphate ◽  
Condition Effect ◽  
Superhydrophilic Surface

The wetting condition effect of the condensation process on a hybrid superhydrophobic and superhydrophilic copper surface as shown in Fig. 1a was experimentally investigated. The superhydrophilic surface (Fig. 1b) consists of micro-flowers (CuO) and nanorods (Cu(OH)2) obtained by immersing the copper substrate into alkaline solution of 2.5 M sodium hydroxide and 0.1 M ammonium persulphate, and the superhydrophobic nanostructured surface (Fig. 1c) was formed by spin coating the Cytop on the hierarchically structured CuO / Cu(OH)2 surface. Experimental results show that the film condensation started on the superhydrophilic region while the dropwise condensation of tiny droplets with an average contact angle of 160° were formed on the superhydrophobic region. Because the film condensation was confined within the superhydrophilic region of 1 mm x 1 mm, the contact angle of this droplet became larger and larger. When a tiny droplet developed on the superhydrophobic area joins with the big droplet formed on the superhydrophilic surface (square region), the coalesced droplet obtains additional energy and jumps off from the condensing surface.

Experimental Study on Heat Transfer Characteristics of Circular Jet Impingement Boiling on the Variety of Structured Copper Surfaces in Stagnation Zone

2016 ◽  
Author(s):  
Mayank Modak ◽  
Vishal Nirgude ◽  
Avadhesh K. Sharma ◽  
Santosh K. Sahu
Keyword(s):  
Heat Transfer ◽  
Jet Impingement ◽  
Copper Surface ◽  
Heat Transfer Characteristics ◽  
Two Phase ◽  
Copper Surfaces ◽  
Transfer Rates ◽  
Copper Target ◽  
Pin Fin ◽  
Transfer Characteristics

In the present work an attempt has been made to study the heat transfer characteristics of single circular jet on a variety of enhanced surfaces. In the present investigation three different copper target surfaces of various surface modifications: bare copper surface, pin fin enhancement surface and a flat surface coated with alumina porous layer. The heat transfer performance of each surface is studied in two phase boiling operation at different flow rates (3959 < Re < 7900). The comparison indicates that both the surface modification have enhanced the boiling heat transfer rates.

Bubble Formation on Superhydrophobic Micropatterns

2011 ◽  
Author(s):  
Xinwei Wang ◽  
Siwei Zhao ◽  
Hao Wang ◽  
Tingrui Pan
Keyword(s):  
Superhydrophobic Surface ◽  
Bubble Formation ◽  
Copper Surface ◽  
Contact Angles ◽  
Controlled Experiments ◽  
Bare Copper

In this paper, boiling phenomena on a copper surface coated with superhydrophobic micropatterns have been investigated. The micropatterns consisted of chemically inert nanoparticles (PTFE) were in square patterns 180 μm on each side with contact angles above 150° for the superhydrophobic behavior. Boiling experiments were conducted on the patterned copper surface with or without degassing treatment prior to heating, from which bubbles were found to form only on the superhydrophobic sites. As controlled experiments, a uniformly-coated superhydrophobic surface as well as a bare copper surface was also tested in comparison.

Experimental Study of the Gas Entrapment Process in Closed-End Microchannels

2004 ◽  
Author(s):  
Ana V. Pesse ◽  
Gopinath R. Warrier ◽  
Vijay K. Dhir
Keyword(s):  
Contact Angle ◽  
Ccd Camera ◽  
Contact Angles ◽  
Surface Forces ◽  
Time Interval ◽  
Physical Mechanisms ◽  
Test Pieces ◽  
Nucleation Sites ◽  
Static Contact ◽  
Gas Entrapment

Earlier studies have shown that for cavities present on any heater surface to become active nucleation sites during boiling, they should entrap gas. The liquid penetrates the cavity due to the capillary and surface forces, but the exact physical mechanisms have not been fully quantified. The physical mechanisms of the gas entrapment process in closed-end microchannels, representing nucleation sites, are investigated in this study. Aside from the fluid properties, the width, length and depth of the cavities, as well as the static contact angle of the test liquid with the solid are considered as main parameters that influence the gas entrapment process. Test pieces consisted of micromachined silicon dices with glass bonded on top. Widths of 50, 30, 15 and 5μm were chosen based on size distribution probability. The mouth angle was 90° in all cases. Test pieces were held horizontally under a microscope equipped with a CCD camera. A drop of liquid was placed at the entrance of the microchannel and capillary and surface forces drive the liquid into the microchannel. Experiments show two main filling behaviors: (1) A uniform meniscus forms at the entrance and moves inwards, (2) Two menisci: one at the entrance and the other at the closed end of the microchannel. In some cases droplet formation at the walls was observed. A single meniscus typically forms for higher contact angles, while two menisci form for lower contact angles. In all cases, after a sufficient time interval (hours to days) the microchannel was completely flooded. In general, for a given depth, wider microchannels take longer to fill. Surface cleanliness and fabrication process also play a role in modifying the contact angle and hence the time taken to fill the microchannel. A comparison of the interface advancement in the microchannel with a simple mass diffusion model shows reasonable agreement.

Study on Condensation Heat Transfer of Micro Structured Surfaces

2009 ◽  
Author(s):  
Hiroyasu Ohtake ◽  
Yasuo Koizumi ◽  
Soichiro Miyake
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Surface Film ◽  
Copper Surface ◽  
Heat Fluxes ◽  
Condensation Heat Transfer ◽  
Silicon Surfaces ◽  
Copper Surfaces ◽  
Structured Surfaces ◽  
Mems Technology

Condensation heat transfer experiments for steam were performed by using mirror-finished copper surfaces, mirror-finished silicon surfaces and silicon surfaces with micro grooves or micro pins on it. The micro-grooves and the micro-pins were created by the MEMS technology. The film- and also the drop-wise condensation were observed on the copper surface. The film-wise condensation heat flux was in good agreement with the values of the Nusselt equation. It was approximately one-tenth of the drop-wise condensation heat flux. The condensation on the mirror-finished silicon surface was the drop-wise condensation. The heat flux was approximately one-tenth of the drop-wise condensation heat flux on the copper surface. The condensation on the micro-grooved and the micro-pin silicon surfaces was film-wise. The condensation heat fluxes were approximately one-tenth of the copper surface film-wise condensation heat flux. When the contact angle was smaller than 70 degree, the condensation was film-wise and when larger than the value, drop-wise. It seemed that the hollow parts of the micro-grooved or the micro-pin surface were filled with condensate first after the condensation was initiated. It made the surface hydrophilic and the condensation film-wise.

Nucleate Boiling of Dielectric Liquids on Hydrophobic and Hydrophilic Surfaces

2015 ◽  
Author(s):  
Huseyin Bostanci ◽  
Nihal E. Joshua
Keyword(s):  
Heat Transfer ◽  
Performance Enhancement ◽  
Copper Substrate ◽  
Nucleate Boiling ◽  
High Heat ◽  
Copper Surface ◽  
Working Fluid ◽  
High Heat Flux ◽  
Layer By Layer ◽  
Hydrophilic Surfaces

An experimental study was conducted to investigate the effect of hydrophobic, hydrophilic and mixed hydrophobic/hydrophilic surfaces in nucleate boiling heat transfer. A dielectric liquid, HFE-7100, was used as the working fluid in the saturated boiling tests. A total of 12 test samples were used in this study, featuring four types of boiling surfaces with a common copper substrate; (1) plain, smooth copper surface (as reference), (2) hydrophobic patterned or fully-covered surface, (3) hydrophilic patterned or fully-covered surface, and (4) mixed hydrophobic/ hydrophilic patterned surface. All test samples were prepared on 10 mm × 10 mm × 2 mm copper substrates with matching size thick film resistors attached onto the opposite side, to generate heat and simulate high heat flux electronic devices. The fabrication of hydrophobic surfaces involved common photolithography techniques to apply 100 μm thick Teflon layer. Hydrophilic surfaces were prepared by depositing a TiO2 layer through a two-step process involving layer by layer self-assembly (L-B-L) and liquid phase deposition (L-P-D) techniques. Test samples with the mixed hydrophobic/hydrophilic surfaces were obtained by first applying Teflon hydrophobic patterns, and then by covering the remaining substrate area with hydrophilic coating. The effect of pattern and pitch size was investigated by varying the circular pattern dimensions between 40, 100 and 250 μm and corresponding pitch dimensions between 80, 200 and 500 μm. The results indicated that hydrophobic and hydrophilic surfaces have distinct benefits, and mixed hydrophobic/hydrophilic surfaces offer an optimum performance enhancement, providing: (a) early transition to boiling regime with no temperature overshoot at boiling incipience, (b) up to 10.6 kW/m2°C HTC (representing 82% increase), and (c) up to 28 W/cm2 CHF level (representing 47% increase). The studied enhanced surfaces therefore demonstrated a practical surface modification method for heat transfer enhancement in immersion cooling applications.

Effect of Polymer Coating on Vapor Condensation Heat Transfer

2020 ◽  
Vol 142 (4) ◽  
Author(s):  
Mete Budakli ◽  
Thamer Khalif Salem ◽  
Mehmet Arik ◽  
Barca Donmez ◽  
Yusuf Menceloglu
Keyword(s):  
Heat Transfer ◽  
Saturation Pressure ◽  
Copper Substrate ◽  
Copper Surface ◽  
Vapor Condensation ◽  
Condensation Heat Transfer ◽  
Condensation Process ◽  
Limiting Factor ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients

Abstract Condensation heat transfer coefficients (HTCs) are rather low compared to thin film evaporation. Therefore, it can be a limiting factor for designing heat transfer equipment. In this work, heat transfer characteristics of water vapor condensation phenomena were experimentally studied on a vertically aligned smooth copper substrate for a range of pressures and temperatures for two different liquid wettability conditions. The heat transfer performance is dominated by the phase change process at the solid–vapor interface along with the liquid formation mechanism. Compared to heat transfer results measured at an untreated copper surface, heat transport is augmented with a thin layer of perfluoro-silane coating over the same substrate. In this work, the effect of saturation pressure on the condensation process at both surfaces has been investigated by analyzing heat transfer coefficients. The results obtained experimentally show an increase in contact angle (CA) with the surface coating. A heat transfer augmentation of about 26% over uncoated surfaces was obtained and surfaces did not show any degradation after 40 h of operation. Finally, current results are compared with heat transfer values reported in open literature.

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