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

where n denotes the outward pointing normal of the boundary. 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. The flux can for example be a function of ωi, the temperature, the pressure or even the electric potential.

When defining conditions on boundaries between different phases, see the section Prescribing Conditions on Fluid-Solid Interfaces.

By default, the flux due to diffusion and migration in an electric field is prescribed. This is the appropriate flux condition when the relative velocity at the boundary is zero. When the fluid velocity is not equal to that of the boundary, for example due to a fluid injection, it is often convenient to prescribe the total flux including convection. To do so select Include in the Convection section. In this case the prescribed flux is defined as:

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 Stefan Velocity.

Select Account for Stefan velocity to update the Stefan velocity in accordance with the prescribed flux. Examples of cases that benefit from this are for example when modeling surface reactions or phase change on an exterior boundary. The Stefan velocity represents the mixture velocity resulting from mass fluxes at the wall. This section is not available when Include is selected in the Convection section.

Specify the Inward flux for each species individually. Select the check box for the species to prescribe a flux for and enter a value or expression 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 kω,i is a mass transfer coefficient and ωb,i is the bulk mass fraction, the typical mass fraction far into the surrounding exterior domain.