Experimental investigation of effect of orientation and surface roughness on drying of porous media consisting of rods

Abstract

Studies of drying from a conventional porous medium (CPM), consisting of spheres, have shown the existence of three periods. In the first period drying rate is high and essentially depends on the atmospheric demand while in the last stage mass transfer happens from within the porous medium and is solely governed by the internal properties. In a previous study (Kumar and Arakeri) it was shown, using a cluster of closely packed smooth rods stacked vertically, that stage 1 was sustained till nearly the whole liquid gets evaporated. Near-zero radii contacts, between the rods, offering infinite height rise was shown to be the reason behind such high sustained evaporation rates and elongated duration of stage 1. Here we show that the orientation of the rods dramatically changes the evaporation process from this vertically oriented rod-based porous medium (VRBPM). Unlike the vertical case, in the horizontal case (HRBPM) smooth rods do not support stage 1. In this case transition to stage 2 was due to the liquid-vapour meniscus receding continuously in the network from the start of the experiment. Rough rods pinned the liquid along its rough surfaces and stage 1 is sustained although its duration is much smaller compared to the vertical case. Maintained hydraulic connections against gravity along the roughness of the rods thus provides a better insight towards understanding stage 1 in a conventional porous medium consisting of complicated pore geometry and their connectivity. In all the experiments, infra-red heating at about 1000 W/m2 causes evaporation from an initially saturated RBPM kept in an acrylic box.