Enhancing dropwise condensation on downward-facing surfaces through the synergistic effects of surface structure and mixed wettability

Abstract

In this paper, the condensation performance and the dynamic behavior of condensed droplets on a downward-facing structured surface with mixed wettability are numerically investigated using a thermal multiphase lattice Boltzmann model, with a focus being placed on exploring the enhancement mechanism of dropwise condensation on downward-facing structured surfaces. The numerical investigation shows that the downward-facing structured surface with mixed wettability exhibits much better condensation performance than those with homogeneous wettability owing to the synergistic effects of surface structure and mixed wettability, which increase the droplet departure frequency and prevent the flooding phenomenon. Furthermore, it is found that the dynamic behavior of condensed droplets on the downward-facing structured surface with mixed wettability can be divided into three stages, i.e., the nucleation-growth stage, the coalescence-slip stage, and the stick-departure stage. Particularly, there exists a competition between the time of the first stage and that of the third stage in terms of the contact angle of the pillar top (θtop). The former reduces but the latter increases with decreasing θtop, because the contact lines are always pinned at the edges of the pillar top during the third stage when θtop is small. An optimal θtop is therefore found, which provides the best droplet dripping rate by achieving a suitable balance between a large droplet departure volume and a relatively short condensation cycle time.