Enhanced dropwise condensation on downward-facing cross-shaped pillar-structured surfaces with mixed wettability

Abstract

In this paper, a novel downward-facing cross-shaped pillar-structured surface with mixed wettability is conceived for enhancing dropwise condensation. A three-dimensional thermal lattice Boltzmann model is employed to investigate the condensation performance on the downward-facing cross-shaped pillar-structured surface with mixed wettability and the associated enhancement mechanism of dropwise condensation. The numerical investigation shows that the cross-shaped pillar-structured surface with mixed wettability exhibits much better condensation performance than the square pillar-structured surface with mixed wettability and the flat surface with mixed wettability due to the synergistic effects of structural effects and mixed wettability, which can promote the droplet nucleation and accelerate the condensate removal. Moreover, for different contact angles of the pillar top (θtop), there exists a competition between the droplet nucleation and the condensate removal on the downward-facing cross-shaped pillar-structured surface. It is found that, when θtop=60°, an optimal droplet dripping rate can be achieved due to a suitable balance between a relatively large mass of detached droplets and a short condensation cycle time. Furthermore, the aspect ratio (γ) has an important influence on the droplet dripping rate, i.e., as γ increases, the droplet dripping rate first exhibits small fluctuations, then increases rapidly before γ=1.0, and after that experiences a slight variation. The large droplet dripping rate achieved at γ=1.0 is mainly attributed to the fact that an optimum structure of the concave corner can promote the droplet nucleation, increase the length of the triple-phase contact line, advance the appearance of droplet coalescence, and finally accelerate the condensate removal.