Modeling Capillary Transport in Thin Porous Media Using a Composite Continuum-Pore Network Formulation: Effect of Water Blockage on Gas Diffusion and Convection

Abstract

In this work, a composite continuum-pore network formulation is presented to model two-phase transport in thin fibrous porous media [1,2]. The composite model incorporates a control volume (CV) mesh at the layer scale, which embeds a structured cubic pore network. Capillary transport is simulated using the discrete pore network, considering the Purcell toroid model to determine the local entry capillary pressures in the fibrous material [3]. In addition, the pore-network model is used to determine analytically local anisotropic effective transport properties (local effective diffusivity and permeability), which are mapped onto the CV mesh to simulate transport within the porous layer using a continuum formulation. The relative effect of local water blockage on gas diffusion and convection is taken into account using a power-law function of the form (1-s)n, where s is the local liquid saturation. Good agreement is found between the predicted global effective transport properties (global effective diffusivity and permeability) under dry and wet conditions and previous experimental data reported in the literature for Toray TGP-H series carbon paper. Moreover, water saturation distributions are compared with results obtained using X-ray computed tomography [4]. As shown by García-Salaberri et al. [4,5], local water accumulation is found to play an important role on gas diffusion and convection. As a result, the exponent n increases from nearly n=2 at the local, pore scale to nearly n=3 at the global, layer scale. Currently, work is underway to implement the composite model in multiphysics models of a polymer electrolyte fuel cell and a redox flow battery to examine the impact of material heterogeinities on performance and durability.[1] P.A. García-Salaberri, I.V. Zenyuk, J.T. Gostick, A.Z. Weber, Modeling Gas Diffusion Layers in Polymer Electrolyte Fuel Cells Using a Continuum-based Pore-network Formulation, ECS Trans. 97 (7) 615.[2] P.A. García-Salaberri, Modeling diffusion and convection in thin porous transport layers using a composite continuum-network model: Application to gas diffusion layers in polymer electrolyte fuel cells, Int. J. Heat Mass Trans. (2020), accepted.[3] J.T. Gostick, Random Pore Network Modeling of Fibrous PEMFC Gas Diffusion Media Using Voronoi and Delaunay Tessellations, J. Electrochem. Soc 160 (2013) F731–F743.[4] P.A. García-Salaberri, G. Hwang, M. Vera, A.Z. Weber, J.T. Gostick, Effective diffusivity in partially-saturated carbon-fiber gas diffusion layers: Effect of through-plane saturation distribution, Int. J. Heat Mass Trans. 86 (2015) 319–333.[5] P.A. García-Salaberri, J.T. Gostick, G. Hwang, A.Z. Weber, M. Vera, Effective diffusivity in partially-saturated carbon-fiber gas diffusion layers: Effect of local saturation and application to macroscopic continuum models, J. Power Sources 296 (2015) 440–453.. Figure 1. Global relative effective diffusivity in the through-plane direction, gTP=Deff TP,wet/Deff TP,dry, as a function of average saturation, savg, computed with the composite continuum-pore network model. The gas species concentration distributions corresponding to different average saturations are shown on the top.. Figure 1