Behavior of a water drop impinging on heated porous surfaces.

Abstract

In the present study, thermal and hydrodynamic behaviors of a water drop impinging on heated porous surfaces were investigated experimentally. Four porous substrates having different permeability and surface roughness were prepared by sintering glass beads with different sizes (67.2 μm ‒ 284 μm). The substrate surface temperature was varied from 60ºC to 300ºC and the diameter of the water drop was fixed at 2.6 mm. The impinging Weber number was varied from 25 to 200 by changing the impact velocity from 0.8 m/s to 2.3 m/s. A post-impingement regime map was constructed based on the observation. Basically two impingement regimes were identified: contact regime in the low temperature range and non-contact regime in the high temperature range. 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, levitation of the drop was observed, but occurred at relatively lower wall temperature with the larger-bead substrates due to active nucleation on the rougher surfaces. The increase in the Weber number resulted in the higher transition temperature from the contact to the non-contact regimes. This is considered to be due to the increase of pressure at the liquid-solid interface at the moment of impact. The surface temperature varied with time experiencing three phases. The surface temperature sharply decreased and then increased with time to reach a thermal equilibrium state between the penetrated liquid drop and the porous solid structure (phase I). Then the surface temperature decreased again during the evaporation of the liquid (phase II), and finally increased upto the initial wall temperature (phase III) after the completion of evaporation. The maximum temperature drop in the phase II occurred in the internal boiling sub-regime. The total evaporation time of the drop decreased with the higher impact velocity and with the substrate made of smaller glass-beads, since the larger spreading ratio and wet-area diameter ratio could be achieved. The spreading ratio, the wet-area diameter ratio, and the complete penetration time turned out to be the major indicators of the cooling performance, which were strongly influenced by both the impact condition and the characteristics of the porous substrates.