Retardation of a Heat Front in a Porous Medium Containing an Evaporating Liquid

Abstract

We have investigated heat transfer in a layer of silica gel impregnated with a liquid (water, aqueous solutions of calcium and magnesium chlorides, formic acid, and carbon tetrachloride). The layer was arranged on a substrate impenetrable for vapor and it was heated from above by a concentrated light flux. It has been found that the evaporation of the liquid contained in the pores of silica gel substantially slows down the propagation of the heat front into the layer so that the effective thermal conductivity of the layer can be reduced to 0.01 W/(m·K); this value is approximately 4–20 times smaller than the values typical of the majority of standard heat-insulating materials. The time of the front lag depends on the layer thickness, density of the incident heat flux, amount of liquid in the pores, and evaporation heat of the liquid. The observed trends in the motion of the front have been described by a simple one-dimensional model that takes into account phase transition (liquid evaporation) in the interior of the porous matrix.