Sustained high evaporation rates from porous media consisting of packed circular rods

Abstract

Studies of drying from a conventional porous medium, consisting of spheres, have shown the existence of three periods. In the first period evaporation rate is high and essentially depends on the atmospheric demand. Relatively simpler geometry, such as polygonal capillaries, pins liquid along the corner and retains high evaporation rate till a certain extent. We report an experimental study of evaporation from a new, but yet, simpler rod-based porous medium (RBPM) consisting of closely packed vertical circular rods. This configuration can be thought of an ‘extreme case’ of a polygonal capillary where the internal angle is zero (00). Infrared heating at about 1000 W/m2 causes evaporation from an initially saturated RBPM kept in an acrylic box. We find sustained high evaporation rates until almost all the water is depleted, a feature very different from either a conventional porous medium or a polygonal capillary. Near-zero radii contacts between the rods are able to source the liquid, against gravity, to the open end throughout the rod length (75 mm) and thus capillary depinning in all the experiments were forced due to limited liquid content. Using a novel fluorescein dye visualization technique and a simple mathematical model, we show that the corner films present in the near-zero radii contacts between rods results in the high sustained evaporation rate.