Image modeling of mixed granular porous media

Two-dimensional image modeling of binary and ternary mixed porous media was used to study the effect of porosity and tortuosity on effective diffusion coefficient and permeability were separated. Different volume fractions of large and small particles in the mixtures as well as different particle size ratio were investigated. The following results were obtained: 1). The porosity changes with variation of the volume fraction of large particles in the mixture. The function of the porosity vs. volume fraction of large particles passes through a minimum. 2). The dependence of tortuosity on the volume fraction of large particles has a maximum, which does not coincide with the minimum value of the porosity. 3). The introduction in the binary mixture of a third fraction of particles with a much smaller diameter led to a dramatic decrease of porosity. For a volume fraction of smaller particles as low as 0.05-0.1 in the total volume of particles the porosity was 20-30% of the corresponding mono-sized porous media. This deviation increased with the increase in the particle size ratio. 4). This analysis shows that the characterization of a mixed bed must take in account other properties besides particle size. Different beds with the same equivalent particle diameter can have different porosity or tortuosity. 5). The ratio Ð/T, a term included both in the permeability and effective diffusivity expressions, also passes through a minimum. Therefore, a change in the volume fraction of large particles in the binary mixture may give rise to a 70% decrease in the effective diffusivity of the porous medium an up to 85% decrease for ternary mixtures. This effect is more significant in the case of permeability.Junta Nacional de Investigação Científica e Tecnológica (JNICT). Fundação para a Ciência e a Tecnologia (FCT) - PRAXIS XXI - BBC/6440/95