Condensation Heat Transfer Enhancement on Surfaces with Interlaced Wettability (original) (raw)
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EFFECTS OF ROUGHNESS AND SURFACE WETTABILITY ON CONDENSATION IN PRESENCE OF NONCONDENSABLE GASES
The field of condensation heat transfer has drawn enormous research interest spanning over many years, since it plays a pivotal role in a large variety of engineering applicationsranging from power plant condensers to passive containment cooling systems in nuclear power plants to HVAC systems [1].Most of the reported studies that have assessed the role of surface wettability in condensation heat transfer have dealt with pure steam[2]while the study of condensation performance in presence of noncondensable gases (NCG) has been far fewer in number [3]. The literature clearly lacks an explicit relationship between the surface wettability, surface roughnessand NCG mass fraction in the context of dropwise condensation (DWC). Herein, we have investigated the condensation phenomenon in presence of NCG (here air) and has linked the DWC and its transition to filmwise condensation (FWC) and variation of overall heat transfer coefficient (HTC) with the condenser surface wettability and roughness. Aluminum (grade 6061) plates of 40×40 mm 2 dimension were chosen as sample condenser plates, which were imparted different surface finish and wettability, viz.,mirror-finish (Sample-1) superhydrophilic (SHPL, Sample-2) and superhydrophobic (SHPB, Sample-3). While the Sample 1 is used as-received, the roughness and surface energy of Samples 2 and 3have been tuned through wet chemical etching and surface treatment. The aluminum plates were first immersed in a 3M HCl solution (Merck) for 10-15 min. The etchant created roughness on the surface through the reaction: 2 Al(s) + 6 HCl(aq) = 2 AlCl 3 (aq) + 3 H 2 (g) The etched surface is then passivated through immersion in boiling water (1 h) resulting in a formation of a thin layer of aluminum oxide hydroxide (böhmite) on the surface of the substrate, rendering a SHPL surface (Sample-2). For preparing Sample-3, freshly-prepared SHPLplates are further immersed in an ethanol solution of 1H,1H,2H,2H-Perfluorodecyltriethoxysilane (1-2%, Sigma) for 2 hrs. A self-assembled monolayer of the fluoro-alkyl silane (FAS) deposits on the surface to render the surface superhydrophobic. The surface roughness created by chemical etching is observed (see Fig 1) under a microscope (Magnus) and the roughness parameters (Ra value in µm) are measured using a surface profilometer (Taylor Hobson precision, surtronic 3+). The, surface wettability are characterized by measuring the static and dynamic contact angles of sessile water droplets using a standard goniometer (Holmarc), which are shown as insets in Fig. 1 Fig.1:Microscopic view of surface roughness created by wet chemical etching. Inset images show contact angle (CA) values and the average surface roughness (Ra values, in m). Condensation experiments on the three types of samples were conducted in a temperature-and humidity-controlled environment chamber (see Fig. 2). Condenser surfaces were sub-cooled to pre-set Sample-2 (sandblasted, HCl-treated, passivated),Ra =4.54 Sample-3 (sandblasted, HCl-treated, passivated, FAS-coated), Ra = 3.08 Sample-1 (Mirror-finish), Ra = 0.197
A B S T R A C T Most materials of practical interest are neither completely wetting nor completely non-wetting. " Surface wett-ability " then refers to the degree that a surface is hydrophilic (i.e. water-loving) or hydrophobic (i.e. water-fearing). Through careful design, it is possible to alter the natural wettability of a surface to be more water-loving or water-fearing. This is principally achieved by modifying the surface chemistry and/or surface roughness. In some cases, modifying the surface may bring operational benefit or advantage. For example, aluminum and copper (which are used in the construction of heat exchangers) tend to retain water in application , which can degrade performance. Modifying the surface however to be superhydrophilic can help to spread out the condensate, reduce the air-side pressure drop, and facilitate drainage. Moreover, by creating a wettability pattern or gradient, it is possible to predetermine the initiating sites for condensation on a surface as well as facilitate droplet motion and/or control the water droplet movement path. In the first part of this review, the current state of the art of surface wettability modification and control techniques are presented, which includes topographical manipulation, chemical modification, as well as methods for creating gradient surfaces and patterned wettability. In the second part of this review, possible applications and the potential impact of these methodologies in energy systems are discussed with a special focus on heating, ventilation, air conditioning , and refrigeration (HVAC&R) systems and components.
Processes
The jumping-droplet phenomenon occurring on superhydrophobic (SHPhob) surfaces under special conditions may be beneficial for numerous systems using condensation, due to the reported increased heat transfer coefficients. One technique to create a SHPhob surface is coating, which can be applied to larger areas of existing elements. However, challenges are associated with coating stability and the realization of continuous dropwise condensation. This research examined the condensation of steam at different flow rates (2, 4 and 6 g/min) and its influence on heat flux and water contact angles on the SHPhob spray-coated aluminum samples. Special emphasis on the impact of time was addressed through a series of one and five-hour condensation experiments on the samples with different storage periods (coated either one year ago or shortly before testing). Over the experimental series at a higher steam flow rate (6 g/min), heat flux decreased by 20% through the old-coated samples and water co...
Condensation heat transfer on patterned surfaces
International Journal of Heat and Mass Transfer, 2013
An experimental study of condensation heat transfer was carried out on a 25.4 mm diameter surface using steam as the condensing fluid. Three surface conditions were studied: hydrophilic, hydrophobic, and a surface with patterns of distinct hydrophilic and hydrophobic regions. The effects of inlet vapor velocity, mass flux, and hydraulic diameter on the heat transfer coefficients were investigated. The inlet vapor velocity was varied from about 0.05 m/s to about 5 m/s and the hydraulic diameter was varied from 4.5 mm to 32.5 mm. Depending on the surface condition, the heat transfer coefficients showed different responses to the varying parameters of the experiments. For the hydrophilic surface, the heat transfer coefficient was observed to be up to 2.5 times lower than that for the hydrophobic surface with all other parameters unaltered. On the other hand, the surface with a pattern of distinct hydrophobic and hydrophilic regions showed heat transfer coefficients that were higher than that of the hydrophilic surface and lower than that of the hydrophobic surface. In both the patterned and the hydrophobic surfaces, the heat transfer coefficient was observed to increase significantly with mass flux, while for the Condensation Heat Transfer on Patterned Surfaces 2 hydrophilic surface, the heat transfer coefficient was observed to be affected much less by the mass flux. In all cases, the heat transfer coefficients increased with increasing heat flux and decreased with increasing wall sub-cooling. The effect of average quality of the steam showed little effect on the heat transfer coefficients.
Condensation heat transfer of a hybrid hydrophilic–hydrophobic surface with different arrangements
Chemical Engineering Communications, 2021
Condensation, a process in which the phase changes from vapor to liquid, plays a crucial role in various industrial systems. So far, several techniques have been proposed to improve the characteristics of condensation. The manipulation of the wettability of a surface is a novel scheme which reduces the heat resistance of the droplets on the surface. The present study experimentally explored the effect of hybrid hydrophilic–hydrophobic surfaces on the formation of primary nucleation, the movement of droplets, and heat transfer. The hydrophobic and hybrid surfaces fabricated by sol-gel and deep coating method. Three distinct surfaces were considered including (i) a hydrophobic surface, (ii) a hybrid hydrophilic–hydrophobic surface with a vertical arrangement (HHSVA), and (iii) a hybrid hydrophilic–hydrophobic surface with a horizontal arrangement (HHSHA). It was found that the HHSVA markedly enhanced the heat and mass transfer compared with the hydrophobic and the HHSHA configurations. Compared with that of the hydrophobic surface, the heat transfer of the HHSVA and the HHSHA increased by 31.9% and decreased by 25.9%, respectively. Then, the heat transfer on these hybrid surfaces depends on the orientation of surfaces. The optimized orientation can increase the heat transfer and improve the droplet’s dynamic parameter on hybrid surfaces.
Enhancement of condensation heat transfer with patterned surfaces
International Journal of Heat and Mass Transfer, 2014
An experimental study of condensation heat transfer on surfaces with patterns of distinct hydrophilic and hydrophobic regions has been carried out. The patterned surfaces of 25.4 mm diameter comprised of 25% hydrophilic region and 75% hydrophobic region by area. Experiments were performed on surfaces with several feature sizes and shapes of the patterns. The feature sizes varied from 0.25 mm to 1.50 mm. Two types of pattern shapes were studied: circular hydrophilic regions on an otherwise hydrophobic surface (island-patterns), and hydrophilic region resembling a tree on a background of hydrophobic region (tree-pattern). Depending on the type of pattern on the condensation surface, the heat transfer coefficients were either higher or lower than that of the completely hydrophobic surface. For the range of inlet vapor velocities (about 0.05 m/s to 5 m/s), among all the surfaces, the highest heat transfer coefficient was observed for the patterned surface with the feature size of 0.25 mm, which was higher than that of a completely hydrophobic surface.
International Journal of Thermal Sciences, 2019
Dropwise condensation (DWC) is a promising heat transfer mechanism in new trends of thermal management and power generation systems to enhance the heat transfer during condensation. Creation of surfaces which can promote dropwise condensation is one of the main issues. For this purpose, a heat transfer surface that can maintain stable dropwise condensation, using hybrid organic-inorganic sol-gel silica coatings functionalized with methyl groups over an aluminum substrate, is developed and tested. This coating displays mildly hydrophilic behavior. Condensation of steam flowing on this surface occurs in dropwise mode with heat transfer coefficient values equal to 150-180 kW m −2 K −1 in the heat flux range between 150 and 510 kW m −2. The importance of the coating thermal resistance is discussed in the paper. The measured heat transfer coefficient is high compared to previous studies of DWC on metallic-and in particular aluminum-substrates. This type of surfaces paves the way to a cheap and green route to promote stable DWC on aluminum substrates without using fluorocarbons or controlled roughness patterns. 100 kW m −2) and produces very high HTC on superhydrophobic surfaces [21,26-28]. Normally a surface with still low surface tension but only nanometric roughness may be preferable to achieve high HTC and high heat flux. The present paper reports a study of DWC on a smooth metallic
2010 14th International Heat Transfer Conference, Volume 2, 2010
Roughness-induced superhydrophobic surface was applied to promote dropwise condensation (DWC) on a vertical plate in the presence of non-condensable gas (NCG). The DWC heat transfer characteristics were investigated and the wetting behaviors of the condensate droplets were observed visually. The experimental results have shown that the roughness-induced superhydrophobic surface would enhance the heat transfer characteristics of steam condensation in the presence of NCG with high concentration. The underlined mechanism is analyzed in terms of the droplet wetting modes.
Effect of Porous Coating on Condensation Heat Transfer
2015
The effects of porous coating on both dropwise and filmwise condensation were studied. The hydrophobic surface was achieved by Self-Assembled Monolayer (SAM), a family of coatings that spontaneously form one molecular layer thick coatings on the surface. The porous surface was formed via sintered copper powder. Using a condensation heat transfer test apparatus developed at Advanced Cooling Technologies, Inc. (ACT), experiments were performed to evaluate the condensation heat transfer on samples having different surface treatments at temperatures range from 35 to 60 o C, a typical condensation temperature for power plant air-cooled condensers. The coating types evaluated included sintered powder porous surfaces with and without hydrophobic SAM coatings. A bare uncoated surface was served as a baseline. The experimental results showed: (1) porous coating can further improve the condensation heat transfer coefficient for dropwise condensation (surface with SAM coating), (2) porous coated surface exhibited a surprisingly high heat transfer coefficient for filmwise condensation mode (surface without a SAM coating). Droplet motion/ removal (desirable for enhanced condensation) by tailoring the surface properties via porous coating was observed and it can potentially be used to further improve the condensation heat transfer.