Droplet Boiling on Micro-Pillar Array Surface ─ Transition Boiling Regime.

Droplet boiling on the heating surface is a representative phenomenon in two-phase spray cooling under low volumetric fluxes. In particular, droplet boiling in the transition boiling regime holds the advantages of avoiding heat transfer deterioration in a film boiling regime and achieving comparable high heat transfer capacity in a nucleate boiling regime. While it is known to consist of intermittent liquid contact with the surface and surface dryout, quantifying the ensuing transient heat transfer performance and droplet behavior is very illusive. In this study, droplet boiling in the transition boiling regime on a micropillar array surface is investigated systematically, using the lattice Boltzmann model built up in the lab. The major contents discussed include the transient behaviors of the droplet, motion of the liquid bridge, and pinning/depinning of the three-phase contact line (TPCL), as well as the corresponding heat transfer performance. The evolution of a vapor film pierced by micropillars is analyzed from the views of morphological change and pressure distribution. The thickness of the vapor film is determined by the vapor generation rate dominated by the contact area and effective thermal conductivity, and the vapor escape rate by the permeability. The low permeability under a large pillar side length is responsible for the pressure buildup below the droplet, thus facilitating droplet rebound. The competition between capillary pressure and vapor film pressure dominates the trigger mode of the droplet rebound, i.e., fracture of the liquid bridge or filament and depinning of TPCL. The micropillar array surface is optimized to pursue the best cooling performance by assessing the impact from micropillar geometric dimensions on droplet contact time and area.

Droplet Wetting Propagation on a Hybrid-Wettability Surface.

Droplet impinging on the boundary between hydrophilic and hydrophobic regions of a hybrid-wettability surface is studied both experimentally and numerically in the present paper. The interfacial evolution and dynamic feature and the corresponding underlying mechanisms behind are mainly analyzed. Because of the unbalanced surface energy in the vicinity of a boundary, the droplet undergoes spreading-receding in the hydrophobic region before migration toward the hydrophilic region. This results in an increase first but then a decrease in the spreading factor in the hydrophobic region, while it increases continuously in the hydrophilic region. In addition, increasing Weber number leads to the increase in both the spreading factor and migration displacement of the droplet in the hydrophobic region, but the latter decreases in the hydrophilic region, resulting from different momentums of secondary spreading. The experimental determinations are verified in detail by a series of numerical simulations performed based on the single variable method by fixing contact angles in different regions separately and excluding the impact momentum. It is shown that the highly unsymmetrical pressure field is exactly one important reason for droplet migration on the hybrid-wettability surface. Despite the weak dependence of the spreading factor on the hydrophilic contact angle in the hydrophobic region, it has an appreciably positive effect on droplet migration, which is confirmed by the increased pressure gradient with its action area in the hydrophobic region when decreasing the hydrophilic contact angle. This paper advances the fundamental understanding for droplet migration on the hybrid-/gradient-wettability surface.