Flux
This node can be used to specify the species flux across a boundary. The prescribed flux of a species c is defined as:
When the mass transport includes migration of ionic species, the flux is defined as:
The flux prescribed, J0, is an arbitrary user-specified flux expression. It can for example represent a flux due to chemical reactions or phase change, or the transport to or from a surrounding environment currently not included model. J0 can be constant or a function of the any dependent variable, for example the concentration, temperature, pressure or the electric potential φ.
Inward Flux
Select the Species check box for the species to specify and enter a value or expression for the inward flux in the corresponding field. Use a minus sign when specifying a flux directed out of the system. To use another boundary condition for a specific species, click to clear the check box for that species.
External convection
Set Flux type to External convection to prescribe a 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 kc is a mass transfer coefficient and cb is the bulk concentration, the typical concentration far into the surrounding exterior domain.