EVAPORATION BEHAVIOR OF A WATER DROPLET IMPINGING ONTO HEATED POROUS SUBSTRATES

Abstract

In the present study, the evaporation behavior of a water droplet impinging onto heated porous substrates was investigated both experimentally and analytically. Four porous substrates having different permeability (3.5 x 10(-12) m(2) 56.5 x 10(-12) m(2)) were prepared by sintering uniform-sized glass beads and the surface temperature was varied from 50 degrees C to 90 degrees C. The temperature of the droplet prior to impact was 30 degrees C and the impinging velocity was changed from 0.8 to 2.3 m/s while the drop diameter was fixed at 2.6 mm. As the temperature difference between water droplet and substrate surface was increased by a factor of 3 (from 20 degrees C to 60 degrees C), the time of complete evaporation decreased by 75% on average. As the impact velocity was increased by about threefold, the evaporation time became shorter by 30% in average. Furthermore, as the permeability of the substrate was increased to 16 times the lowest value, the evaporation took longer by about 30%. In addition, numerical simulation was conducted to estimate the evaporation time with appropriate assumptions. The general trend of time variations of the temperatures at the surface and inside the porous substrates was predicted reasonably well. The advection of the humid-air flow through the porous structure on the temperature variation of the substrate increases with the higher substrate temperature but the effect is relatively minor. Rather, the major role of the porous structure is to maintain the maximum wetted diameter throughout the evaporation process by pinning of the edge. The numerical simulation with various simplifying assumptions overestimated the evaporation time by 25% compared to the measured values.