DISPERSION IN PERIODIC POROUS-MEDIA - EXPERIENCE VERSUS THEORY FOR 2-DIMENSIONAL SYSTEMS

A method is presented for the experimental study of dispersion in satu rated porous media for two-dimensional spatially periodic systems. The use of stereophotolithography laser as the basis of the porous media fabrication process made this investigation feasible. The porous media consist of a set of circular cylinders accurately positioned in an or dered (in line) or in a disordered (random) unit cell periodically rep roduced in the plane of the study. For two directions of the average f luid velocity in the in-line array and one direction of the velocity v ector in the disordered array, dye is injected in the form of a pulse at the entrance to the medium. The tracer concentration variations wit h time are measured by a video camera and are averaged over a unit cel l. The measured time distributions are compared with computations usin g the macroscopic convection-diffusion equation in order to estimate t he longitudinal dispersion coefficient. The variations of this coeffic ient with particulate Peclet number in the three geometries investigat ed are compared with the available numerical results. All the results agree, showing a significant influence of the direction of the average fluid velocity for the ordered medium. Concerning the random medium, the results appear to indicate that, in spite of its periodic characte r, this type of medium is capable of describing the behaviour of disor dered porous media. This original technique is highly promising for th e explanation of dispersion mechanisms in porous media. (C) 1997 Elsev ier Science Ltd.