THE EFFECTS OF EXTENDED THIN-FILM EVAPORATION AND EXTERNAL DIFFUSION RESISTANCE DURING THE CONSTANT DRYING RATE PERIOD

Extended thin film evaporation with external diffusion resistance is analyzed for the constant rate period of the drying process, in which a polar liquid evaporates from porous bodies made of glass. The extended thin film is defined as the liquid film in which the disjoining pressure dominates the fluid flow field and works as the driving force replenishing the evaporating liquid. The results of the analysis shows that due to the existence of the evaporating thin liquid film, the evaporation from the extended thin film can compensate the reduction of evaporation rate caused by the increase of the dry spots and keep the drying rate the same as or even greater than that of the completely wetted surface. The external diffusion resistance makes the vapor concentration near the porous solid surface remain constant and therefore keeps the drying rate nearly constant. Experimental data in the literature are in good agreement with the results of analysis of this work. This implies that both the extended thin film evaporation and the external diffusion resistance are important mechanisms for a drying process.