Moist Porous Media Research Articles

Transient Simultaneous Heat and Mass Transfer in Moist, Unsaturated Soils

The transient heat and mass transfer in a moist porous medium adjacent to a cylindrical heat source is analyzed in order to characterize the thermal stability of the medium. In the context of this paper, thermal instability occurs in a moist porous medium as a result of significant drying due to excessive thermally induced moisture movement. A dry zone is created which propagates into the medium and thereby inhibits dissipation of heat from the source. The drying of the porous medium adjacent to the heat source is predicted to occur in two distinct stages. During the first stage the rate of moisture movement initially decreases until a “critical moisture content” is reached. Thereafter the drying rate generally increases until complete drying of the material in the vicinity of the heat source has occurred. The value of the critical moisture content is found to be essentially independent of the strength (heat transfer rate per unit length) of the heat source. The parameters which most significantly influence the transport processes are identified and correlated on the basis of numerical solutions of the governing equations. The critical moisture content and critical heat flux are defined and used to quantify thermal stability limits. The correlations are validated through comparison with experimental measurements made on a native soil. The thermal stability model developed in this work establishes the relationship between the thermal stability limits for large-diameter and small-diameter heat sources. From this relationship the thermal stability associated with large-diameter sources can be accurately predicted from experimental measurements using small-diameter sources and small samples.

The Freezing of Plant Tissue

A theory of the freezing process of the tissue of pear fruit is presented, based on the cell permeability theory and on the thermodynamics of moisture in porous media. According to this theory the nature of the temperature v. time curve during freezing is determined by the speed of advance of the ice front into the tissue. Curves for freezing and thawing calculated from the theory are compared wit,h experimental curves.

Application of the Network Model to Gas Diffusion in Moist Porous Media

Millington's description of pore structure in porous media is compared with conclusions based on the network theory. Experimental gas-diffusion data for moist sand indicate that this porous material has a network structure in which each pore is connected to about 15 other pores.

Simultaneous transfer of heat and moisture in porous media

Simultaneous differential equations for the transfer of heat and moisture in porous media under the combined influence of gravity and gradients of temperature and moisture content are developed. These equations are a generalization of those derived by Philip and de Vries [1957]. In particular a consistent distinction is made between changes of moisture content in the liquid and the vapor phase.The interaction between heat and moisture transfer in steady state heat conduction is discussed in detail using numerical values for two soils, a clay loam and a medium sand. The behavior is found to be dependent on the boundary conditions for moisture transfer, on the direction of the temperature gradient and on the ratio of two moisture diffusivities enterbag in the analysis.