MODELING VOID FORMATION DYNAMICS IN FIBROUS POROUS MEDIA WITH THE LATTICE BOLTZMANN METHOD. Michael A. A. Spaid and Frederick R. Phelan, Jr. Polymers Division, National Institute of Standards and Technology, Gaithersburg, MD., USA.,

The preform materials used in liquid composite molding (LCM) applications such as resin transfer molding (RTM) are heterogeneous in the sense that the network of fiber bundles (tows) which make up the porous media are themselves porous. As resin penetrates the heterogeneous fiber microstructure, void formation--the entrapment of air pockets within the fiber microstructure--may occur due to the heterogeneous nature of the medium and the competition between capillary and viscous forces. The phenomenon of void formation is well-known, however it has been difficult to clearly characterize by either theoretical or experimental means, primarily due to the two-phase nature (resin/air) of the transient flow processes. In this work, a novel numerical technique based on the lattice Boltzmann (LB) method is developed to model unsaturated flow dynamics in heterogeneous porous media. It is shown that a traditional lattice Boltzmann method may be modified to solve the Stokes and Brinkman equations for flow in a heterogeneous porous medium. As a test of the modified lattice Boltzmann model, steady transverse flow (saturated) through a square array of porous circular cylinders is investigated, with cell permeabilities comparing favorably with known results. The multiphase model is developed by combining the Stokes/Brinkman LB method with the multiphase LB algorithm described by Shan and Chen [X. Shan and H. Chen, Phys. Rev. E 47, 1815 (1993)]. Simulations of multiphase infiltration in a model geometry consisting of a square array of circular porous tows are presented. Unsaturated permeabilities obtained from the numerical simulations are reported, which are found to be significantly lower than the corresponding saturated values.