The Flux node is available on exterior boundaries and can be used to specify the mass flux across a boundary. The boundary mass flux for each species is defined in the manner of:

The prescribed flux j0,i can contain an arbitrary flux expression. It can for example be used to represent a heterogeneous reaction or a separation process occurring at the boundary, and be a function of ωi, temperature, pressure or even electric potential.

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.

Set Flux type to External convection to prescribe a mass flux to or from an exterior domain (not modeled) assumed to include convection. The exterior can for example include a forced convection to control the temperature or to increase the mass transport. In this case the prescribed mass flux corresponds to

where is a mass transfer coefficient and ωb,i is the bulk mass fraction, the typical mass fraction far into the surrounding exterior domain.

When using the Reacting Flow coupling feature to model coupled momentum and mass transport, boundary conditions that results in a non-negligible mass flux can be consistently handled by accounting for the The Stefan Velocity.

Select Account for Stefan velocity to update the Stefan velocity in accordance with the prescribed flux. Examples of cases that may benefit from this are for example when modeling surface reactions or phase change on an exterior boundary.