Modelling and simulations of wet porous medium convective drying

Abstract

In the convective drying of a wet porous medium by hot air, the medium surface exposed to the air fluid may experience three stages namely the fully wet, partly wet, and fully dry stages. At the fully wet stage, the medium surface is fully covered by a water film, with the convective moisture transfer taking place all over the medium surface; at the partly wet stage, the medium surface is partly covered by water film, with the convective moisture transfer occurring only at the wet surface where free water is present. In this research, a theoretical model is presented to simulate the convective drying of a wet porous medium, and experiments on hot air convective drying of a wet porous sand layer are carried out to investigate the sand layer temperature and water content variations during the drying, in which the sand layer thickness is 4 mm, the initial water film thickness is 0.4 mm, and the hot air temperatures are 45, 60 and 75℃, respectively. Inverse calculations are conducted to get some insight into the water film coverage function based on the experimental data, which is necessary for quantitatively evaluating the effective moisture transfer surface area when calculating the convective moisture transfer at the partly wet stage. By combining the coverage function with the wet porous medium drying model, a comprehensive model is obtained. Calculations are implemented to simulate the convective drying of the wet porous sand layer, and the effects of the two constants included in that function on the wet sand layer drying characteristics are discussed. The calculation results are compared with the experimental data, they agree very well, supporting the effectiveness o