Effective Thermal Diffusivity of Porous Media in the Wall Vicinity

Transient heat transfer from an impulsively heated vertical constant heat flux plate embedded in a stationary saturated porous medium is studied experimentally and analytically to determine near-wall thermal diffusivity. The effective diffusivity is shown to depend on the properties of the constituent materials and the near-wall particle morphology. For porous media comprising randomly stacked spheres, the near-wall region is characterized by fewer particle contacts with the wall than in the bulk medium, and this difference is the source of larger thermal diffusivity in the context of volume-averaged values, which apply to the bulk property far from the wall. For combinations of different spherical solids and interstitial fluids, which give a range of fluid:solid conductivity ratio from 0.5 to 2400, early-time transient temperature profiles can be predicted using the thermal conductivity of the interstitial fluid. A conjugate heat transfer analysis accurately predicts the time the conductive front takes to travel through the impermeable wall and quantifies the effect of conduction along the wall on the local and overall Nusselt numbers. The present results raise the possibility of reinterpretation of much of the porous media heat transfer experiments in the literature.