Determination of transport parameters for multiphase flow in porous gas diffusion electrodes using a capillary network model

The changes of relative permeability and capillary pressure as a function of liquid water phase saturation, two key parameters in two-phase PEMFC models, are investigated using a capillary network model incorporating an invasion percolation algorithm with trapping. The two-dimensional capillary network accounts for capillary dominated drainage and cluster formation. It is shown that relative permeability is constant for low saturation, but follows a power law of saturation for high saturations, with an exponent of about 2.4 that is independent of network size or heterogeneity. An increase of the network size and reduction in heterogeneity tend to reduce the relative permeability, and relative permeabilities of much less then unity are obtained even for saturations as large as 0.8. Capillary pressure on the other hand does not vary with saturation and network size, but is influenced by heterogeneity only. This suggests that regardless of the interface shape and size, the capillaries at the interface maintain a constant average radius causing the capillary pressure to remain constant. It is finally shown that with appropriate scaling and for a given network heterogeneity, the normalized capillary pressure, single-phase permeability and relative permeability can be deduced for other choices of porous medium physical scales without requiring a new set of simulations. (C) 2007 Elsevier B.V. All rights reserved.