How is the Mass Diffusivity evaluated based on Kinetic Theory within Ansys Fluent?
When the Species Transport Model is invoked, one of the additional fluid property inputs required is Mass Diffusivity (m2/s).
Of the various options to input this value, one of them is using kinetic theory to evaluate the mass diffusivity. When using kinetic theory, the solver does not prompt any other user inputs. The mass diffusivity gets evaluated internally using the Chapman-Enskog formula.
For more details with an example, refer to the following document.