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Transient Conductive and Radiative Heat Transfer through a Porous Layer with One Boundary Subject to a Time-Varying Temperature Condition

This paper reports the results of an analysis on transient heat transfer through a layer of porous material subject to a time-varying temperature boundary condition. The purpose of the analysis was to study the radiation effect on transient heat transfer through porous building materials such as thermal insulations. The physical situation considered was that of a planar porous material with one boundary held at a fixed temperature and the other subjected to a cyclic temperature with a 24-hour period. Two heat transfer models were used in the analysis. They were (1) a purely conductive model which accounted for radiation through the use of an apparent thermal con ductivity, (2) a combined conductive and radiative model which included the two- flux equations to account for radiation in a more rigorous manner. The governing equations were solved numerically using a finite-difference approximation. The ac cumulated heat flows at both boundaries of the porous material were obtained. Ef fects due to parameters such as the boundary emissivities and the extinction coeffi cient of the porous material were illustrated. The results predicted by the two heat transfer models were compared.