The Flux node can be used to specify the total mass flux across a boundary. The total inward flux is defined in the manner of:

In Equation 8-6, N0,i (SI unit: Pa·s/m) is an arbitrary flux expression for species i and can be a function of ωi, temperature, pressure or even electric potential. Set the mass flux of all species except the one computed from the mass constraint. This ensures that the sum of the mass fractions is equal to one (see Species). This node is available for exterior boundaries. The Flux node can, for example, be used to describe a heterogeneous reaction or a separation process occurring at the boundary.

Specify the Inward flux for each species individually. Select the check box for the species to prescribe a flux and enter a value or expression for the flux in the corresponding field. To use another boundary condition for a specific species, click to clear the check box for the flux of that species. Use a positive value for an inward flux.