Transport simulation effective property estimation

Akey performance limitation in the polymer electrolyte fuel cell (PEFC) originates from the multiple, coupled and competing, transport interactions in the constituent porous components. The suboptimal transport behavior resulting from the underlying complex and multifunctional microstmctures in the catalyst layer (CL), gas diffusion layer (GDL) and microporous layer (MPL) leads to water and thermal management issues and undesirable performance loss. Therefore, it is imperative to understand the profoimd influence of the disparate porous microstmctures on the transport characteristics. In this chapter, we highhght the stochastic microstmcture reconstmction technique and direct transport simulation in the CL, GDL and MPL porous stmctmes in order to estimate the effective transport properties and imderstand the microstmctural impact on the imderlying transport behavior in the PEFC. 

PM) model, based on the mathematical morphology analysis of the digital porous structure, was presented. The PM model is a fast direct simulation method that provides two-phase distribution in the reconstmcted microstmctures. The DNS model was used to predict the effective diflfusivity in the unsaturated and partially saturated non-woven uncompressed and compressed GDL microstmctures. Furthermore, the estimation of the effective thermal conductivity of the carbon paper GDL microstmctures was presented. Finally, it is important to emphasize that understanding the influence of the underlying microstmctures on the effective properties and transport behavior is of paramount importance toward the development of high-fidelity, predictive performance models for the PEFC. 

Several attempts have been made to simulate transport in realistic fully atomistic MD simulations of water/Nafion mixtures. Vishnyakov and Neimark [72-74] investigated alkali transport in aqueous and methanolic solution (and in mixed solvents) in the presence of Nafion. They found indications for the existence of the fluctuative bridging mechanism. The group by Kokhlov and Khalatur has also performed extensive yet unpublished studies of simple ion transport in Nafion. Goddard and coworkers [75] compared structural and dynamical properties of two different copolymerisation patterns, in order to estimate the effect of statistical vs. regular copolymerisation of TFE with the sulfonated vinyl ether. 

Alternatives to Soave s approach and group contribution adaptations would be better focused on capabilities not offered by such approaches. For example, pure component properties like vapor pressure are assumed to be available when applying Soave s methodology. In the coming world of chemical product design, this assumption may not be satisfactory. Molecular simulation offers the prospect of being able to make these predictions for transport properties as well as equilibrium properties. Proximity effects would also be naturally included within molecular modeling. While the National Research Council has estimated that such predictive capability may not be available for a decade or two, viable preliminary versions may come much sooner than that. 

It is obvious that it is practically not feasible to exactly quantify the physico-chemical aquifer properties but estimates and simplifications, e.g. average or effective values must be applied. In view of an application of the SMART modelling strategy and concept for transport predictions, it is important to know how wrong estimates of the physicochemical properties of the aquifer material will take effect on the simulation results. Therefore, the objective of a first suite of simulations is to assess the impact of the composition of the aquifer material and its physicochemical properties on the transport of PHE and to examine the sensitivity of lithological composition and grain... 

Quantitative estimation of the effective material properties and constitutive closure relations is of paramount importance for high-fidelity macroscopic, voliune-averaged computational models deployed in the PEFC performance simulations. The microstractural heterogeneity (e g. morphology, pore connectivity, pore size distribution, anisotropy) inherent in the PEFC components (CL, GDL, MPL) poses a profound impact on the effective transport properties, such as effective diffusivity in the unsaturated and partially saturated (e g. pore blockage by liquid water)... 

Improvements to the model have been made by Lawson et al. [44-46]. The improvements use estimates of thermal conductivity, specific heat and thermo-optical properties (transmittance, reflectance and absorptance) obtained from the thermal data collected from the testing of a variety of fabrics typically used in fire fighters protective clothing. A detailed mathematic model is developed to study transient heat and moisture transfer through multilayered fabric assemblies with or without air gaps. First principles are used to derive the governing equations for transient heat and moisture transfer. The equations also account for the effect of moisture on thermodynamic and transport properties. Numerical simulations are used to study heat and mass transfer. A software tool (Protective Clothing Performanee... 

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