Behavior of a water drop impinging on heated porous surfaces

•Behavior of an impinging water droplet against heated porous substrates was examined.•A tentative post-impingement regime map was constructed.•Cooling performance was mainly influenced by the wetted-area size and the permeation time.•Optimum permeability to maximize the cooling performance was available. In this work, thermal and hydrodynamic behavior of a water drop impinging on heated porous surfaces was investigated experimentally. Four porous substrates having different permeability and surface roughness were prepared by sintering small glass beads with different sizes and the surface temperature was varied from 60°C to 300°C. The impinging velocity was varied from 0.8m/s to 2.3m/s while the drop diameter was fixed at 2.6mm. Two primary impingement regimes were identified: contact and non-contact regimes, each in the low and the high temperature ranges, respectively. The contact regime, in which the drop evaporated or boiled while maintaining contact with the surface, was further divided into three sub-regimes: internal evaporation, internal boiling, and surface nucleate boiling. In the non-contact (surface film boiling) regime, the drop was levitated but at the lower wall temperature with the larger-bead substrates due to active nucleation on the rougher surfaces. Larger impinging velocity resulted in higher transition temperature from the contact to the non-contact regimes, which is due to the increase of the impact pressure at the liquid–solid interface. Time variation of the surface temperature consisted of three stages: right after the drop impact, the surface temperature sharply decreased and then increased with time to reach a temporal thermal-equilibrium between the permeated liquid and the porous solid (stage I). Then the surface temperature gradually decreased until the evaporation was completed (stage II), and finally increased up to the initial wall temperature (stage III). The total evaporation time decreased with the higher impact velocity because of the larger spreading and wet-diameter ratios. Also, there was an optimum glass-bead size (of the substrate) to minimize the evaporation time. In summary, the spreading and wet-diameter ratios, and the time for complete permeation turned out to be the major indicators of the cooling performance, which were strongly influenced by both the impact condition and the structural characteristics of the porous substrates.