Stochastic Microstructure Reconstruction Model

Detailed description of a porous microstructure is an essential prerequisite for unveiling the influence of pore morphology on the underlying two-phase behavior. This can be achieved either by 3-D volume imaging or by constructing a digital microstructure based on stochastic reconstruction models. Non-invasive techniques, such as X-ray micro-tomography, are the popular methods for 3-D 

The stochastic reconstruction method is based on the idea that an arbitrarily complex porous structure can be described by a binary phase function which assumes a value 0 in the pore space and 1 in the solid matrix29 The intrinsic randomness of the phase function can be adequately qualified by the low order statistical moments, namely porosity and two-point autocorrelation function.29 The porosity is the probability that a voxel is in the pore space. The two-point autocorrelation function is the probability that two 

The pore/solid phase is further distinguished as transport and dead phase. The basic idea is that a pore phase unit cell surrounded by solid phase-only cells does not take part in species transport and hence in the electrochemical reaction and can, therefore, be treated as a dead pore and similarly for the electrolyte phase.25 The interface between the transport pore and the transport electrolyte phases is referred to as the electrochemically active area (ECA) and the ratio of ECA and the nominal CL cross-sectional area provides the ECA-ratio . It is be noted that in this chapter, ECA is normalized with the apparent electrode area and therefore differs from the definition in terms of the electrochemically active area per Pt loading reported elsewhere in the literature. 

The multi-faceted functionality of a GDL includes reactant distribution, liquid water transport, electron transport, heat conduction and mechanical support to the membrane-electrode-assembly. 

Cross-section averaged pore/solid volume fraction distributions along the CL thickness 

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The mesoscopic modeling approach consists of a stochastic reconstruction method for the generation of the CL and GDL microstructures, and a two-phase lattice Boltzmann method for studying liquid water transport and flooding phenomena in the reconstructed microstructures. 

The steady-state flow numerical experiment was primarily designed to evaluate the phasic relative permeability relations. The numerical experiment is devised within the two-phase lattice Boltzmann modeling framework for the reconstructed CL microstructure, generated using the stochastic reconstruction technique described earlier. Briefly, in the steady-state flow experiment two immiscible fluids are allowed to flow simultaneously until equilibrium is attained and the corresponding saturations, fluid flow rates and pressure gradients can be directly measured and correlated using Darcy s law, defined below. 

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