Investigation of Dropwise Condensation Heat Transfer on Laser-Ablated Superhydrophobic/Hydrophilic Hybrid Copper Surfaces

Song, Zitao; Lu, Mingxiang; Chen, Xuemei

doi:10.1021/acsomega.0c01995

Cited by 27 publications

(8 citation statements)

References 46 publications

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“…It was shown that superhydrophilic regions on biphilic surfaces promoted the heat transfer performance by reducing the delay in neighboring droplet coalescence. Condensation experiments on hybrid superhydrophobic and hydrophilic surfaces fabricated with a combination of wet etching and laser ablation methods were performed . Accordingly, the distance between hydrophilic islands affected both droplet growth rate and coalescence-induced droplet departure and was an important parameter for enhancing condensation heat transfer.…”

Section: Introductionmentioning

confidence: 99%

Copper-Based Superhydrophobic Nanostructures for Heat Transfer in Flow Condensation

Chehrghani

Abbasiasl

Sadaghiani

et al. 2021

ACS Appl. Nano Mater.

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Superhydrophobic surfaces enhance the condensation heat transfer performance by facilitating removal of condensed droplets and coalescence-induced droplet jumping. During the last decade, studies on superhydrophobic surfaces have been mostly limited to working conditions, which are ideal for electron microscopy techniques. Therefore, the behavior of condensed droplets in the presence of a vapor flow with different vapor qualities and their implementation in flow condensation heat transfer enhancement have received rather little attention, which is the motivation behind this study. Thus, beside heat transfer analysis, we performed a visualization study on flow condensation in a minichannel and investigated droplet dynamics, including a histogram of droplet diameter distribution at different time intervals and stages of a condensation cycle consisting of nucleation growth and departure, droplet departure diameters, cycle time, and droplet number density. The droplet departure diameter decreases with steam mass flux, leading to a shift to smaller radii in droplet size distribution, which enhances condensation heat transfer. Enhancements up to 33% in the heat transfer coefficient were obtained at lower steam qualities for the tested superhydrophobic surface compared to the reference plain hydrophobic surface. We demonstrated that in addition to the high vapor shear stress exerted by the flow on the interface of the vapor and the condensed droplet, the advantage of an inherently unique water repellency feature of the nanostructured superhydrophobic surface can be utilized in flow condensation for enhancing condensation heat transfer.

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Section: Introductionmentioning

confidence: 99%

Copper-Based Superhydrophobic Nanostructures for Heat Transfer in Flow Condensation

Chehrghani

Abbasiasl

Sadaghiani

et al. 2021

ACS Appl. Nano Mater.

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“…Nonuniform wettability control promotes the formation of condensed nuclei in hydrophilic spots, allowing control of nucleation density and liquid–solid contact area. As adhesion becomes dominant with increasing nucleation density, the spacing and arrangement of hydrophilic spots are important to improve the frequency of desquamation, induced by coalescence. − In addition, the heat transfer coefficient can be increased by providing a large liquid–solid contact area at the hydrophilic spots. − …”

Section: Introductionmentioning

confidence: 99%

Condensation Behavior of Hierarchical Nano/Microstructured Surfaces Inspired by Euphorbia myrsinites

Baba

Sawada

Tanaka

et al. 2021

ACS Appl. Mater. Interfaces

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In nature, many extant species exhibit functionalized surface structures during evolution. In particular, wettability affects the functionalization of the surface, and nano/microstructures have been found to enable functions, such as droplet jumping, thereby making self-cleaning, antifog, antibacterial, and antireflection surfaces. Important efforts are underway to understand the surface structure of plant leaves and establish rational design tools for the development of new engineering materials. In this study, we focused on the hierarchical nano/microstructure of the leaves of Euphorbia myrsinites (hereinafter, E. myrsinites), which has a hierarchical shape with microsized papillae, covered with nanosized protruding wax, and observed the condensation behavior on the leaf surface. Si is vertically etched via reactive ion etching (RIE) to artificially mimic the hierarchical nano/microstructures on the leaves of E. myrsinites. We made four types of artificial hierarchical structures, with micropillars having pillar diameters of 5.6 and 16 μm (pillar spacing of 20 and 40 μm, respectively) and heights of 6.5 and 19.5 μm, and nanopillars formed on the surface. The optical observation with a microscope revealed a very high density of condensed droplets on the artificial surface and a stable jumping behavior of droplets of 10 μm or more. Furthermore, in the samples with a micropillar diameter of 5.6 μm and a micropillar height of 19.5 μm, the droplets that had jumped and fallen thereupon bounced off, thereby preventing reattachment. As a result, no droplets of 35 μm or more could exist even after 10 min. In addition, it was clear that a small underlying droplet of less than 10 μm was generated at the bottom of the relatively large secondary droplet existing on the large micropillar of 16 μm, and a frequent coalescence of the droplets occurred. This study revealed the phenomenon of condensation on the surface of plants as well as made it possible to improve the heat exchange process by significantly promoting the heat transfer of condensation using artificial surfaces.

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“…48 Recently, another study demonstrated that the hydrophilic island spacing was an important parameter for enhancing condensation heat transfer and influenced both the droplet growth rate and coalescence-induced droplet departure. 49 It is noteworthy to mention that the application of biphilic surfaces is not limited to condensation heat transfer and has been the focus of research on boiling heat transfer 50,51 and freezing 52 as well. In addition, noteworthy efforts have been recently made in developing numerical models to investigate hybrid surfaces for their thermal performance and droplet dynamics.…”

Section: Introductionmentioning

confidence: 99%

“…In addition, the superhydrophilic region on biphilic surfaces could promote the heat transfer performance by reducing the delay in neighboring droplet coalescence . Recently, another study demonstrated that the hydrophilic island spacing was an important parameter for enhancing condensation heat transfer and influenced both the droplet growth rate and coalescence-induced droplet departure . It is noteworthy to mention that the application of biphilic surfaces is not limited to condensation heat transfer and has been the focus of research on boiling heat transfer , and freezing as well.…”

Section: Introductionmentioning

confidence: 99%

Biphilic Surfaces with Optimum Hydrophobic Islands on a Superhydrophobic Background for Dropwise Flow Condensation

et al. 2021

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Sustaining dropwise condensation is of great importance in many applications, especially in confined spaces. In this regard, superhydrophobic surfaces enhance condensation heat transfer performance due to the discrete droplet formation and rapid removal. On the other hand, droplets tend to nucleate easier and faster on hydrophobic surfaces compared to superhydrophobic ones. To take advantage of the mixed wettability, we fabricated biphilic surfaces and integrated them to small channels to assess their effect on thermal performance in flow condensation in small channels. Hydrophobic islands in the range of 100–900 μm diameter were fabricated using a combination of wet etching, surface functionalization, and physical vapor deposition (PVD) techniques. Condensation experiments were performed in a minichannel with a length, width, and height of 37, 10, and 1 mm, respectively. Here, we report optimum island diameters for the hydrophobic islands in terms of the maximum thermal performance. We show that considering the optimum point for each steam mass flux corresponding to the best heat transfer performance, the condensation heat transfer coefficient is increased by 51, 48, 42, 40, and 36% compared to the plain reference hydrophobic surface for steam mass fluxes of 10, 20, 30, 40, and 50 kg/m2 s, respectively. The optimum island diameters are obtained as 200, 300, 400, 400, and 500 μm, with the ratios of hydrophobic to superhydrophobic surface areas (A* = A hydrophobic/A superhydrophobic) of 3.2, 7.6, 14.4, 14.4, and 24.4%, for steam mass fluxes of 10, 20, 30, 40, and 50 kg/m2 s, respectively. The liquid film forming on the liquid–vapor interface acts as an insulation layer and generates thermal resistance, and bridges appear on the patterned areas and deteriorate the thermal performance. Therefore, it is crucial to characterize the role of droplet mobility on biphilic surfaces to avoid the occurrence of bridging. Through visualization, we demonstrate that the optimum conditions correspond to enhanced droplet nucleation and rapid sweeping regions, where droplet pinning and bridging do not occur. The trends in condensation heat transfer with surface mixed wettability can be divided into three regions: enhanced droplet nucleation and rapid sweeping, highly pinned droplet, and bridging droplet segments. We reveal that the interfacial heat transfer augmentation in the enhanced droplet nucleation and rapid sweeping region is due to both spatial control of droplet nucleation and an increase in the sweeping period. Furthermore, by fitting the experimental data, a correlation for predicting the optimum island diameter for biphilic surfaces is proposed for condensation heat transfer in confined channels, which will be a valuable guideline for engineers and researchers working on the design and development of thermal systems.

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Investigation of Dropwise Condensation Heat Transfer on Laser-Ablated Superhydrophobic/Hydrophilic Hybrid Copper Surfaces

Cited by 27 publications

References 46 publications

Copper-Based Superhydrophobic Nanostructures for Heat Transfer in Flow Condensation

Copper-Based Superhydrophobic Nanostructures for Heat Transfer in Flow Condensation

Condensation Behavior of Hierarchical Nano/Microstructured Surfaces Inspired by Euphorbia myrsinites

Biphilic Surfaces with Optimum Hydrophobic Islands on a Superhydrophobic Background for Dropwise Flow Condensation

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