Transport in Porous Media 27: 243–264, 1997. 243 c 1997 Kluwer Academic Publishers. Printed in the Netherlands. A Simple Model for the Variation of Permeability due to Partial Saturation in Dual Scale Porous Media Y. DE PARSEVAL,K. M. PILLAI and S. G. ADVANI Department of Mechanical Engineering, University of Delaware, Newark, DE 19716, U.S.A. (Received: 27 September 1995; in final form: 24 September 1996) Abstract. The main focus of this work is to model macroscopically the effects of partial saturation upon the permeability of dual scale fibrous media made of fiber bundles when a Newtonian viscous fluid impregnates it. A new phenomenological model is proposed to explain the discrepancies between experimental pressure results and analytical predictions based on Darcy’s law. This model incorporates the essential features of relative permeability but without the necessity of measuring saturation of the liquid for its prediction. The model is very relevant for the small scale industrial systems where a liquid is forced to flow through a fibrous porous medium. It requires four parameters. Two of them are the two permeability values based on the two length scales. One length scale is of the order of magnitude of the individual fiber radius and corresponds to the permeability of the completely staurated medium, the other is of the order of magnitude of the distance between the fiber bundles and corresponds to the permeability of the partially saturated medium. The other two parameters are the lengths of the two partially saturated regions of the flow domain. The two lengths of the partially saturated region and the permeability of the fully saturated flow domain can be directly measured from the experiments. The excellent agreement between the model and the experimental results of inlet pressure profile with respect to time suggests that this model may be used to describe the variation of the permeability behind a moving front in such porous media for correct pressure prediction. It may also be used to characterize the fibrous medium by determining the two different permeabilities and the relative importance of the unsaturated portion of the flow domain for a given architecture. Key words: permeability, Darcy’s law, unsaturated flow, dual scale, resin transfer molding, liquid injection molding. 1. Introduction Nowadays, mold filling is the governing step in a number of important liquid injection molding processes to manufacture composite materials. One such widely used process in resin transfer molding (RTM). The crucial step of processing in RTM is the filling of a mold packed with reinforcing glass or carbon fibers with resin. The resin is a viscous Newtonian liquid at the time of mold filling. The mold filling process is sufficiently slow such that local Reynolds number at the fiber level is much smaller than 1, thereby allowing the use of Darcy’s law for modeling the flow of the liquid in RTM [1]. In order to improve the design of the mold, numerical simulation of the mold filling process is undertaken [2]. The typical approach followed in these simulations is the use of single phase Darcy’s law for