Effective Diffusion Coefficient in Porous Media

Recent calculations by Prager of upper bounds for the effective diffusion coefficient (or conductivity) in porous media, in terms of certain statistical parameters of the random geometry, are reformulated so as to apply specifically to a bed of spherical particles. The calculations are simplified by considering an idealized bed in which centers are randomly situated without restricting the spheres to nonoverlapping locations. The result, applicable to randomly overlapping spheres of either uniform or nonuniform sizes, gives the upper bound for the effective diffusion coefficient as φD0/[1−½lnφ], where D0 is the actual diffusion coefficient in a fluid which fills the void regions of the bed and φ is the void fraction. This result is compared with experimental results of various investigators for nonoverlapping spheres and also with Hashin and Shtrikman's expression for the best upper bound that can be calculated without taking the statistics of a particular random geometry into account.