Superhydrophobic Si Nanowires for Enhanced Condensation Heat Transfer
Authors: Ming-Chang Lu, Chien-Chang Lin, Ching-Wen Lo, Cheng-Wei Huang, and Chi-Chuan Wang
Journals: International Journal of Heat and Mass Transfer
Year of Publication: 2017 2016 Impact Factor: 2.458
Abstract Condensation is an essential process in various industrial systems. Enhancing condensation by employing superhydrophobic (SHB) surfaces had drawn significant attention in recent years because of the emerging technology for surface engineering. However, the efficacy of SHB surfaces in condensation is controversy in the literature. The observed deteriorated heat transfer on SHB in condensation is presumably a result of highly pinned Wenzel droplets or flooding formed on the SHB surfaces. Si nanowire (SiNW) array-coated surface which can simultaneously provide a large number of nucleation sites and prevent condensate from penetrating into the nano-structure is a promising candidate for enhancing condensation. In this work, heat transfer on the SHB SiNW surface was investigated. At low subcooling, jumping of liquid droplets accompanied with a high droplet departure frequency resulted in a large heat transfer coefficient (HTC) of 88 ± 16 kW/m2 K on the SHB surface. This value is one of the highest reported condensation HTCs in the literature. It was 155% and 87% higher than those on the plain hydrophilic and hydrophobic surfaces, respectively. Heat transfer decreased with the rise of subcooling due to an increased condensate surface coverage ratio. However, condensate can still be rapidly shed away from the SHB SiNW surface at high subcooling, which render the comparatively larger HTC of 18.6 ± 2 kW/m2 K on the SHB SiNW surface as opposed to plain hydrophobic and hydrophilic surfaces. It was evidenced that SHB surface could have a superior heat and mass transfer performance than hydrophobic surface provided that the liquid droplets on the SHB could be shed away efficiently.
Scale Effect on Dropwise Condensation on Superhydrophobic Surfaces
Authors: Ching-Wen Lo, Chi-Chuan Wang and Ming-Chang Lu
Journals: ACS Applied Materials & Interface
Year of Publication: 2014 2016 Impact Factor: 7.504
Abstract Micro/nano (two-tier) structures are often employed to achieve superhydrophobicity. In condensation, utilizing such a surface is not necessarily advantageous because the macroscopically observed Cassie droplets are usually in fact partial Wenzel in condensation. The increase in contact angle through introducing microstructures on such two-tier roughened surfaces may result in an increase of droplet departure diameter and consequently deteriorate the performance. In the meantime, nanostructures roughened surfaces could potentially yield efficient shedding of liquid droplets, whereas microstructures roughened surfaces often lead to highly pinned Wenzel droplets. To attain efficient shedding of liquid droplets in condensation on a superhydrophobic surface, a Bond number (a dimensionless number for appraising dropwise condensation) and a solid-liquid fraction smaller than 0.1 and 0.3, respectively, are suggested.
Spatial Control of Heterogeneous Nucleation on the Superhydrophobic Nanowire Array
Authors: Ching-Wen Lo, Chi-Chuan Wang and Ming-Chang Lu
Journals: Advanced Functional Materials
Year of Publication: 2014 2016 Impact Factor: 12.124
Abstract We report the ability to spatial control heterogeneous nucleation on a superhydrophobic surface by manipulating the free energy barrier to nucleation through parameterizing regional roughness scale on the Si nanowire array-coated surface. Water vapor preferentially condenses on the designed microgrooves on the Si nanowire surface and continuous shedding of the drop-wise condensate is observed on the surface. The nucleation site density can also be manipulated by tailoring the density of the microgroove on the surface. Moreover, the cycle time on the Si nanowire array with microgrooves is approximately ten times smaller than that on a plain Si surface. This suggests that potentially high heat and mass transfer rates can be achieved on the surface. The insight from this study has implications in enhancing energy efficiency in a wide range of thermal energy conversion systems.