All aspects of fluid flow, heat transfer, and mass transfer in the flow channels and porous electrodes are handled by Ansys Fluent.

The default multicomponent diffusion model in Ansys Fluent is used to calculate the mass diffusion coefficient of species in the mixture.

As noted in [17], the average pore size of gas diffusion layer in fuel cell devices is on the same order as the length of the mean free path, making the Knudsen diffusion play a big role in computing the effective gas diffusivity.

The multi-component Knudsen diffusion coefficient is computed as:

(20–105)

where

= effective radius of the pore

= universal gas constant

= temperature

and = molecular weights of species and , respectively

Finally, the effective gas diffusivity coefficient is computed as:

(20–106)

where

= tortuosity (the average path length over the actual length)

= porosity

= binary diffusivity