The prescribed flux of a species c is by default defined as

where n denotes the outward pointing normal of the boundary. When the mass transport includes migration of ionic species, the flux is defined as

The flux prescribed, J0, can include any arbitrary user-specified expression. It can be constant or a function of a dependent variable or independent variable. Common examples are a flux dependent of the concentration, temperature, pressure, or the electric potential ϕ.

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, it is often convenient to prescribe the total flux, including the convection. To do this select Include in the Convection section.

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.