Effect of fiber orientation on the effective mass diffusivity of fibrous porous media

Fibrous porous media, such as carbon felt and carbon paper, are frequently used as the electrodes of aqueous redox flow batteries. The mass transport properties of these fibrous porous media, including effective mass diffusivity, dispersion coefficient, and mass transfer coefficient, are key design parameters to guide the design of the porous electrodes. Specifically, a proper designed non-isotropic porous geometry can enhance the mass transport properties and maximize the coupled transport of mass, electrons, and ions simultaneously. To investigate the effect of fiber orientation on the effective mass diffusivity of fibrous porous media, we adopt lattice Boltzmann method and perform three-dimensional pore-scale simulations. The microstructures of the fibrous porous media are represented by cylindrical fibers with different orientations, including one isotropic porous medium, one anisotropic porous medium preferentially streamwise-oriented, and one anisotropic porous medium preferentially transverse-oriented. The fiber orientations are characterized by the angle between fibers and streamwise direction. Results are presented in the form of effective mass diffusivity as a function of fiber orientation. A comparison between the simulated effective mass diffusivity and the empirical relation is also performed. These findings can deepen our understanding towards the mass transport phenomena in the pores of fibrous porous media and provide guidance for further optimization of pore geometry of the electrodes of aqueous redox flow batteries.