Dropwise condensation heat transfer process optimisation on superhydrophobic surfaces using a multi-disciplinary approach

Abstract

Dropwise condensation has superior heat transfer efficiency than filmwise condensation; however condensate evacuation from the surface still remains a significant technological challenge. The process of droplets jumping, against adhesive forces, from a solid surface upon coalescence has been studied using both experimental and Computational Fluid Dynamics (CFD) analysis. Both Lattice Boltzmann (LBM) and Volume of Fluid (VOF) methods have been used to evaluate different kinematic conditions of coalescence inducing a jump velocity. In this paper, an optimisation framework for superhydrophobic surface designs is presented which uses experimentally verified high fidelity CFD analyses to identify optimal combinations of design features which maximise desirable characteristics such as the vertical velocity of the merged jumping droplet from the surface and energy efficiency. A Radial Basis Function (RBF)-based surrogate modelling approach using design of experiment (DOE) technique was used to establish near-optimal initial process parameters around which to focus the study. This multidisciplinary approach allows us to evaluate the jumping phenomenon for superhydrophobic surfaces for which several input parameters may be varied, so as to improve the heat transfer exchange rate on the surface during condensation. Reliable conditions were found to occur for droplets within initial radius range of r=20-40 m and static contact angle s~160º. Moreover, the jumping phenomenon was observed for droplets with initial radius of up to 500 m. Lastly, our study also reveals that a critical contact angle for droplets to jump upon coalescence is c~1 40º. KeywordsCondensation heat transfer, Super-hydrophobic surface, Jumping droplets velocity, Multi-disciplinary optimisation. IntroductionDrop-wise condensation processes, where condensation occurs through small droplets on a solid surface, has been demonstrated to significantly improve heat transfer rates in comparison to filmwise condensation (where a whole surface is covered by a thin film of liquid) [1,2]. Drop-wise condensation usually takes place on hydrophobic or super-hydrophobic surfaces as demonstrated by Boreyko and Chen [3]. One of the main technological challenges is to create such a surface, so as to allow condensation and evacuation of the droplets to take place in a continuous manner. Droplet coalescence is a complex physical phenomenon and optimisation of kinematic conditions leading to surface dewetting and jumping of droplets is of paramount importance for processes like heat transfer, de-icing, atmospheric water harvesting or dehumidification [4][5][6]. Dropwise condensation heat transfer performance can be enhanced by allowing condensed droplets to be removed rapidly from the surface to minimize the thermal barrier [7]. Recently, researchers showed that superhydrophobic surfaces provide a higher mobility of condensates, which may enhance the heat transfer performance [1][2][3][4][5][6][7]. Coalescence-induced jumping phenomena occur on superhydrophobic surfa...

Computational Modelling of Droplet Dynamics Behaviour in Polymer Electrolyte Membrane Fuel Cells: A Review

“…In the condensates formation study, it uses a hydrophilic surface for observing the dynamics coalescences with the neighbouring condensates [53][54][55]. Such study helps to develop a more efficient way of removing condensates from fin surfaces [27][28][29][30]. For hydrophobic surface, the applications are, e.g., anti-icing surface [56][57][58][59][60], droplet impact on hydrophobic surface [61][62][63], microliter droplet evaporation [64][65][66], nanoliter droplet dispensers [67][68][69], droplet sliding behaviour on different surface roughness [70][71][72] and different surface structures [73][74][75].…”

“…Therefore, an optimum condition exists for the droplet to jump at the highest velocity. Khatir et al [29] investigated different droplet radius; ranging from 100 to 515 microns. As shown in Fig.…”

“…As shown in Fig. 7, Khatir et al [29] compared the numerical results of VOF and LBM and the experiments. Khatir et al [29] estimated that the droplet with a radius of 35-40 µm would jump at the highest velocity on a surface with θ s = 160 o .…”

“…7, Khatir et al [29] compared the numerical results of VOF and LBM and the experiments. Khatir et al [29] estimated that the droplet with a radius of 35-40 µm would jump at the highest velocity on a surface with θ s = 160 o . The peak condition of VOF results were closer to the experimental results of Boreyko and Chen [111] s' rather than Peng et al [109]s'.…”

“…The superhydrophobic surface was used mainly for the category of droplet jumping upon coalescence (type-C). In some studies, the θ s was as low as 130 o [28][29][30] and as high as 180 o [25,27,29].…”