Water transport in wet porous materials is expressed as the simultaneous transport of adsorbed water, capillary water and water vapor. The sum of the surface flow coefficient and the liquid water transfer coefficient is defined as the water transfer coefficient in this paper.The water transfer coefficient was measured by a modified method which is a kind of constant-volume method for adsorption equilibrium. Anactivated alumina whosepores are fine wasused as the experimental material. Based on the water transfer model previously proposed by the authors, the observed dependence of the water transfer coefficient on water content and temperature were successfully interpreted.
Intr oductionThe drying mechanismof an adsorptive porous material is so complicated in comparison with that of a nonadsorptive one that it has not been quantitatively understood. In particular, the transport properties of water in adsorptive materials has been reported in only a few previous papers.lj3'4) The water in the adsorptive porous material exists as adsorbed water on the solid surface, condensed water in fine pores, and water vapor in the void. The adsorbed water is transferred by surface flow caused by the gradient of adsorbed amount, the condensed water by capillary flow caused by the gradient of capillary pressure, and the water vapor by gaseous diffusion caused by the pressure gradient of water vapor. But the total pressure inside the material becomes larger than the atmospheric pressure even under ordinary drying conditions. Therefore, we have to take the enhancement of this total pressure gradient on the transfer rate of the condensed and vapor water into account.In the present paper, the sum of surface flow coefficient and liquid water transfer coefficient defined by the water content gradient is called the "water transfer coefficient." This coefficient has been measured so far with coexisting air.3' 4) As it is difficult to estimate a vapor flux under such a condition, however, the reliability of those measured coefficients is deficient in the region of low water content. Therefore, the water transfer coefficient is measured in the present study under the condition that air does not exist, and is interpreted by the model which the authors previously proposed.6j8)
The present work stresses the significance of the effective medium theory in the computation of the macroscopic transport coefficients from the microgeometry of porous media. The porous ‘‘material’’ is simulated as a two-dimensional network of interconnected slits of irregular shape and a random distribution using the Voronoi–Delaunay tesselation technique. The calculation procedure for the macroscopic transport coefficients is based on two concepts, the first one being the approximation of the microscopic field by a smooth field (SFA), and the second one being the average of the network random parameters to a mean/effective value by the effective medium theory (EMT). For the latter we apply an improved version of the EM equation derived for regular lattices by Kirkpatrick [Rev. Mod. Phys. 45, 4 (1973)]. This equation takes into account the irregularity in the slit-length distribution and is applicable on both regular and irregular lattices. The EMT/SFA results of the improved version for ordinary diffusion (apparent diffusivity) are in very good agreement with the numerical ones.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.