The boundary conditions associated with the Multicomponent Mixing module are as follows:

This condition means that there is no flux or gradient of components normal to the boundary. This boundary condition typically corresponds to a solid boundary in the model.

• When the mixture boundary condition is set to Zero Flux under Multicomponent Mixing module, the boundary conditions are fixed to Zero Flux for the components (as indicated by the blue font).
• This is the default boundary condition for Multicomponent Mixing module, when the boundary condition under Flow module is Wall and Rotating Wall.

This condition allows to set the mass fractions of the components at an opening.

• When the mixture boundary condition is set to Specified Value under Multicomponent Mixing module, the boundary conditions are fixed to Specified Value for the components (as indicated by the blue font). The mass fraction is specified for Value under each component.
• This is the default boundary condition for Multicomponent Mixing module, when the boundary condition under Flow module is Specified Velocity, Specified Volumetric Flux, Specified Total Pressure or Specified Pressure Inlet. It would not typically make physical sense to prescribe a Specified Value condition for a boundary that was not an opening.

Volumetric flux is the volumetric amount of mixture crossing a selected boundary or interface per second. This boundary condition for the Multicomponent Mixing module is the not a default condition and can be set for any Flow module boundary condition, but only makes sense for an opening.

• For this boundary condition, the value of the Volumetric flux for a given component needs to be set under the Components menu.
• The Specified Volumetric Flux is the fixed boundary condition for the individual components when the mixture boundary condition is set to Specified Volumetric Flux.

This condition means that there is no shear, no normal component of velocity and no normal gradient of pressure at the boundary. This boundary condition usually corresponds to a physical symmetry in the model.

• When the mixture boundary condition is set to Symmetry under Multicomponent Mixing module, the boundary conditions are fixed to Symmetry for the components (as indicated by the blue font).
• This is the default condition for Multicomponent Mixing when the boundary condition under Flow module is Symmetry.

This condition allows to set the mass fractions of the components at an opening.

• When the mixture boundary condition is set to Outlet under Multicomponent Mixing module, the boundary conditions are fixed to Outlet for the components (as indicated by the blue font). The mass fraction is specified for Value under each component.

• This is the default boundary condition for Multicomponent Mixing module, when the boundary condition under Flow module is Specified Pressure Outlet or Resistor Capacitor. It would not typically make physical sense to specify an Outlet condition for a boundary that was not an opening.

The Outlet boundary condition under Mixture allows the user to set the mass fraction of any component that would re-enter the domain in the event of back flow at a flow outlet.

The mass fraction set at an outlet does not dictate the mass fraction that exits. The mass fraction of the components for any fluid exiting the domain is based on the internal (upwind) mass fraction. The mass fraction specified at the outlet can influence the internal mass fraction via diffusion. Typically, this effect is insignificant unless the outflow is either very small or the conductivity very high.