Experimental condensation study of vertical superhydrophobic surfaces assisted by hydrophilic constructal-like patterns

Oestreich, Jens; Geld, C.W.M. van der; Oliveira, J.L.G.; Silva, A.K. da

doi:10.1016/j.ijthermalsci.2018.09.024

Cited by 24 publications

(13 citation statements)

References 34 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…In humid air, an accurate measurement of HTC during condensation is challenging compared to that in the pure steam environment. The HTC during condensation in humid air is on the order of ∼ O [10 2 W m –2 K –1 ] . Therefore, the heat flux ( q ″) transferred to the cooling system from the condensing surface considered in this work is comparatively low.…”

Section: Resultsmentioning

confidence: 99%

“…The order of the maximum possible temperature drop across the coating can be calculated from the heat flux value through the energy conservation principle. The reported heat flux value for a hydrophilic surface was q ″ ≈ 1200 W m –2 under experimental conditions of T s = 10 °C, T env = 50 °C, and RH = 80% . If we consider a maximum coating thickness of ∼0.5 μm and thermal conductivity of k coat = 1 W m –1 K –1 , then the maximum temperature drop across the coating is on the order of 0.6 mK.…”

Section: Resultsmentioning

confidence: 99%

See 1 more Smart Citation

Condensation of Humid Air on Superhydrophobic Surfaces: Effect of Nanocoatings on a Hierarchical Interface

Thomas

Mahapatra

2021

Langmuir

View full text Add to dashboard Cite

Vapor condensation is a well-known phase-change phenomenon observed in nature as well as in different industrial applications. Superhydrophobic surfaces (SHSs) with low hysteresis can efficiently drain off the condensate and rejuvenate the nucleation sites further. In this work, three distinct SHSs were fabricated by nanocoating three hydrophobic agents, viz., perfluoro-octyl-triethoxy-silane (PFOTS), perfluoro-octanoic-acid (PFOA), and commercial Glaco solution on a hierarchical aluminum surface. The surface morphology of all surfaces was investigated, and its effects on the wetting, droplet departure, and overall heat-transfer coefficient (HTC) during condensation phenomena in the humid air (>95% noncondensable gases) were analyzed. The contact angle hysteresis of all three surfaces was very low (∼5°); however, different wetting behaviors were observed during the condensation, depending on the adhesion of the condensate drop with nanoscale textures in the microcavities. Dropwise condensation (DWC) was observed in silane and Glaco-coated surfaces. A gravity-assisted sweeping mechanism removed the condensate from the silane-coated surface. In contrast, the condensate was ejected out of the plane of the Glaco-coated surface by droplet jumping. The PFOA-coated surface has shown DWC initially and floods in the later stages due to highly pinned condensed droplets. This study reports an enhancement of ∼35 to ∼110% in the HTC for the SHS-exhibiting gravity-assisted sweeping mechanism compared to the droplet-jumping mechanism. The present work will provide substantial insights into the fabrication of efficient hierarchical interfaces for water-energy nexus applications.

show abstract

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Condensation of Humid Air on Superhydrophobic Surfaces: Effect of Nanocoatings on a Hierarchical Interface

Thomas

Mahapatra

2021

Langmuir

View full text Add to dashboard Cite

show abstract

“…Tree-like hydrophilic patterns on superhydrophobic bases with different ratios of the superhydrophobic/hydrophilic area fraction could also be another alternative pattern. 43 The study showed that approximately 7.4% enhancement in the condensation heat transfer could be achieved for the area fraction of 70%.…”

Section: Introductionmentioning

confidence: 95%

“…The ratio of the hydrophobic to hydrophilic strips could also play a vital role in enhancing heat transfer. , Optimizing this ratio could improve the condensation thermal performance by controlling the droplet departure frequency and droplet area coverage. Tree-like hydrophilic patterns on superhydrophobic bases with different ratios of the superhydrophobic/hydrophilic area fraction could also be another alternative pattern . The study showed that approximately 7.4% enhancement in the condensation heat transfer could be achieved for the area fraction of 70%.…”

Section: Introductionmentioning

confidence: 98%

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

et al. 2021

View full text Add to dashboard Cite

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.

show abstract

“…With the rapid development of surface engineering, various superhydrophobic surfaces with higher contact angle and lower contact angle hysteresis have been designed and prepared to accelerate surface renewal and enhance condensation heat transfer. − However, long-term effective dropwise condensation could not be maintained due to the collapse and breakage of nanograss . On account of being capable to remove condensate droplets spontaneously by capillary force, condensing surfaces with heterogeneous wettability have attracted a great deal of attention and have been designed to enhance condensation heat transfer. − Ghosh et al designed a bioinspired surface with hydrophilic–superhydrophilic patterns, driving condensate discharge with capillary force to control the maximum condensate size. Oestreich et al prepared hydrophilic constructal-like patterns on superhydrophobic test sections to help remove condensate droplets.…”

Section: Introductionmentioning

confidence: 99%

Lattice Boltzmann Modeling of Condensation Heat Transfer on Downward-Facing Surfaces with Different Wettabilities

Wang

Chen

et al. 2020

Langmuir

View full text Add to dashboard Cite

The model of vapor condensation heat transfer on downward-facing surfaces with different wettabilities is built by a two-dimensional (2D) lattice Boltzmann method. Dynamic evolution of condensate microdroplets on different wettability surfaces is simulated and the influence on heat transfer performance is analyzed. Moreover, the mechanism of a heterogeneous wettability surface enhancing condensation heat transfer is explored by investigating the condensate behaviors in the process of condensation. The numerical results indicate that as the contact angle of the homogeneous wettability surface increases, the initial nucleation time of the condensate is prolonged, while the departure time of the condensate is reduced significantly. The temperature adjacent to the gas−liquid interface, especially in the three-phase contact line region, is much higher than elsewhere due to the release of latent heat during condensation. Coalescence and detachment behaviors of condensate droplets cause the average heat flux to fluctuate locally with time. For the hybrid wettability surface, if the proportion of hydrophobic regions is small, the condensation heat transfer performance will be deteriorated. However, increasing the hydrophobic−hydrophilic ratio has a positive effect on enhancing heat transfer. It is found that a critical hydrophobic−hydrophilic ratio exists to optimize the heat transfer performance. For the gradient wettability surface, directional migration induced by capillary force facilitates the removal of condensate droplets, thereby enhancing the condensation heat transfer. Furthermore, a larger wetting gradient benefits to further improve the heat transfer performance. The results are valuable for optimally designing the heat transfer enhancement of vapor condensation on functionalized surfaces with heterogeneous wettability.

show abstract

Experimental condensation study of vertical superhydrophobic surfaces assisted by hydrophilic constructal-like patterns

Cited by 24 publications

References 34 publications

Condensation of Humid Air on Superhydrophobic Surfaces: Effect of Nanocoatings on a Hierarchical Interface

Condensation of Humid Air on Superhydrophobic Surfaces: Effect of Nanocoatings on a Hierarchical Interface

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

Lattice Boltzmann Modeling of Condensation Heat Transfer on Downward-Facing Surfaces with Different Wettabilities

Contact Info

Product

Resources

About