Investigation on Vapor Diffusion Resistance Factor in Porous Materials

In the study of heat insulation properties of porous materials, simulation methods that account for both the structure of the porous material and the processes of heat and mass transfer occurring therein are widely employed. In this paper, the effective thermal conductivity of such a heterogeneous system is determined based on the methods of generalized conductivity and reduction to a unit cell. The analysis of heat and mass transfer processes in a multicomponent system is conducted by reducing it to a binary system comprising a solid skeleton and pore space. The thermal conductivity component within the pore space is dependent upon the temperature and structural characteristics of the material, the tortuosity of the pores, and the narrowing of the pore channels. This is typically characterized by a parameter known as the diffusion resistance factor, μ. This factor is an important characteristic of various heat-insulating and construction materials, and thus the development of engineering methods for calculating this parameter is a matter of urgency. In this study, a methodology for calculating μ is proposed. This methodology involves the use of a unit cell comprising a porous material with an interpenetrating structure. The unit cell is separated into two sections: one impermeable to diffusing vapor current streams (termed "adiabatic splitting") and the other perpendicular to vapor flow and exhibiting the same concentration value on its surface (termed "isothermal splitting"). The "combined" method of splitting, which employs sections of both types, yielded the most robust correlation with experimental data. Experimental studies of the thermal conductivity (λ) of wet porous materials have demonstrated that at a specific temperature, the value of λ is independent of moisture content. This indicates that in this instance, the thermal conductivity of the vapor-air mixture within the pore space is equivalent to the thermal conductivity of the liquid phase, particularly that of water.This fact enables the determination of μ for a porous material through the measurement of its thermal conductivity at varying humidity and temperature levels. The λ of the test samples was determined by means of a comparative λ-calorimeter, which has been specifically designed to study the heat-conducting properties of dispersed materials within the range from 0.04 to 80 W/(m·K). The practical consequence of the proposed studies is to guarantee energy savings and a reduction in heat losses when utilising or processing composite materials, which are multicomponent and inhomogeneous systems. Examples of such materials include composite building materials and oil-bearing rocks.