Leidenfrost temperature on porous wick surfaces: Decoupling the effects of the capillary wicking and thermal properties

The Leidenfrost temperature T_LFP of falling water droplet was studied on sintered porous wick surfaces. Various surface factors were analyzed to identify those that significantly contribute to increasing the T_LFP. To decouple the effects of capillary wicking on porous wick surfaces, the results obtained using ethanol as a working fluid were compared to the results obtained using water. When ethanol was used, the capillary wicking did not differ significantly between the porous wick surfaces. The evaporation time of the droplets was measured at high temperatures (100–600 °C) to evaluate the T_LFP. The effect of surface permeability on the absorption of the vapor layer through a porous wick surface had a negligible influence on the T_LFP. Within the range of low thermal effusivity of the heating surface, an analysis of the interface temperature shows that in liquid ethanol, the thermal properties dominate the T_LFP as well as the overall boiling regime. Similarly, in water (for which the capillary wicking effect cannot be ignored), the T_LFP and film boiling regime were determined by the thermal effusivity. In both liquids, the thermal effusivity was the dominant determinant of the T_LFP, regardless of the capillary wicking rate. However, the capillary wicking significantly affected the boiling heat transfer in water, until it reached the transition boiling regime.