The out-of-plane flux node is used to prescribe a flux in an out-of-plane domain, and it can be used to reduce a model geometry to 2D, or even 1D, when the concentration variation is small in one or more directions. This could be the case for example when the object to model is thin or slender. Figure 8-1 shows examples of possible situations in which this type of geometry reduction can be applied.

For a 1D component this node adds a single out-of-plane molar flux J0,z,i for species i. For a 2D component two fluxes can be prescribed for each species; J0,u,i for the upside of the domain, and J0,d,i for the downside of the domain.

The fluxes are added as a source contribution to the right-hand side of the transport equation. For example, in 2D components with diffusion, convection, and out-of-plane flux present, the mass transfer of species i is defined by the mass conservation equation

where Sopf,i is the out-of-plane source for species i

where kc is a mass transfer coefficient and cb is the bulk concentration, the typical concentration far into the surrounding exterior domain. The prescribed flux, J0, can include any arbitrary user-specified expressions. It can be a constant or a function of a dependent variable or independent variables.

For 1D components, enter the cross-sectional perimeter Pc to get the out-of-plane flux

The default value of Pc is the circumference. Either keep the default value, for a circular cross-section shape, or edit the value to get a user-defined shape of the out-of-plane cross-section.

The available flux type options are General inward flux and External convection. Select the Species check box for the species for which to specify the flux, 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.

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.

The available options are General inward flux and External convection. The settings are the same as for the Upside Inward Flux section.