Two-phase flow in porous media ? Simplified tubes network modeling for GDL in PEM-FC

I. SCHMIDHALTER; P. A. AGUIRRE

Santa Fe

Congreso; AAIQ - X Congreso Argentino de Ingeniería Química CAIQ2019; 2019

Asociación Argentina de Ingenieros Químicos (AAIQ)

A simplified phenomenological model to link the pressure field of gas phase with liquid phase flow profile in GDL (Micro Porous Layer (MPL) + Substrate) is developed. A commercial non-woven or paper GDL from Sigracet, whit or without PTFE treatment and with or without MPL is modelled as a simplified tube network. Porous media in GDL as a conical tubes bundle with a pore diameter distribution was considered. Gas phase has been considered as a continuous phase, and liquid phase as a disperse phase. The pore radius distribution is obtained from the post-processing data of Mercury Intrusion Porosimetry (MIP) experiment of SGL GDL samples. We work with GDL 25BC from Sigracet. This GDL has a hydrophobized substrate with 5 wt% of PTFE loading, along with a standard MPL on one side. MPL and Substrate layer in GLD are differentiated with different geometrical and physical properties in each layer. The influence of PTFE treatment in contact angle is considered. Poiseuille Flow in gas and liquid phase is considered. The GDL in through-plane direction is split into two regions: MPL + Substrate, with 3 characteristic pore radio on each region (Low, middle and high porous range radios). Total possible combinations of linking radios are considered for the conical tubes configuration. Difference between dynamic and static contact angle is neglected and isotropic solid is considered. Solid materials hydrophilic or hydrophobic are both feasible to be considered with the model. Pressure drops in gas phase could be in the same direction or opposite direction than liquid water flow. Liquid water flow could exist in: Conical divergent tube, conical convergent tube, or cylindrical tube depending on contact angle and pressure drops in gas phase. Liquid water flow through cylindrical tubes involves a friction pressure drop and, regarding the pore sizes, capillary pressure drops should also be considered. Depending on the contact angle between the materials and liquid water, both pressure drops could be at the same directions or with opposite directions. We assume GDL material as straight oriented conical tubes bundle along through-plane direction, and we consider the total pressure drop for the tubes as unique and constant in steady state. Then, both pressure drops for each tube in networks is balanced taking overall pressure drop between both sides of the GDL as a constant. The Young-Laplace equation for menisci in cylindrical geometry is applied to computing capillary pressure drop (ΔPci). Each pore radio (i) has a particular capillary pressure. For friction pressure drops in each pore radio (ΔPfi), Darcy?Weisbach equation under laminar flow regime is considered. For simplification, we assume for this work a unique value of length for pore size, Lp. Furthermore, we defined an equivalent pore length proportional to the thickness of the evaluated material in each sub-domain, Lm. Then, the number of pores with size ri (Npi) is calculated using MIP experimental data. Results show that, the lowest the equivalent pore length, the largest the Npi, and the lowest the Reynolds number of liquid flow for this pore size. Furthermore, through minimization of frictional losses from the system, liquid and gas flow distribution are computed for a given porous media characterizations. Thereby, the liquid water hold-up for the porous media becomes determined