Diffusion and viscous flux in porous solids

In this study, diffusion and viscous flow of gases through porous solids were investigated by single-pellet moment technique. A dynamic version of the Wicke-Kallenbach type of a diffusion cell was used. A pulse of an inert tracer (He) was injected to the carrier gas(N2) following through the upper face of the pellet and the response peak leaving the other end face was detected. The moment expressions were derived for this system for non-isobaric conditions. The experiments were carried out with a-alumina pellets of two different lengths and of porosity 0.68. Under isobaric con ditions, the effective diffusion coefficient was found to be constant and independent of the amount of the tracer injected. Both Knudsen and molecular diffusion regime was found to be important for the system studied and the tortuosity factor of 4 was evaluated. Under non-isobaric conditions, the pressure difference between two faces of the pellet was changed from 2 cmHg to 60 cmHg. In this study, a new dimensionless group (Ö) introduced which gave the relative significance of viscous flow compared to the diffusion flow. The value of 6 changed from 0.05 to 0.8 while the effective diffusion coefficient changed from 0.0233 cm /s to 0.0201 cmr/s. Darcy cons tant was determined as 0.069xl014 m^. It was shown that the relative importance of viscous flux was about the same as compared to the diffusion flux when the pressure difference between the two end faces was about 60 cmHg.