The boundary condition parameters for the Multicomponent Mixing module apply to boundaries in the Flow Analysis Tree.

The boundary conditions appear in the Properties panel when you select a boundary in the Flow Analysis Tree under General Boundaries.

The Zero Flux boundary condition corresponds to a solid boundary in the model. Zero Flux means that there is no flux or gradient of components normal to the boundary. When you set the mixture boundary condition to Zero Flux, then the boundary conditions for the components are also fixed to Zero Flux. This is the default boundary condition for Multicomponent Mixing module when the boundary condition for Flow module is Wall or Rotating Wall.

Use Specified Value boundary condition to set mass fractions of the components at an opening. When you set the mixture boundary condition to Specified Value, then boundary conditions for the components are also fixed to Specified Value. The mass fraction is specified for Value under each component. This is the default boundary condition for Multicomponent Mixing module when you set the boundary condition for Flow module to Specified Velocity, Specified Volumetric Flux, Specified Total Pressure, or Specified Pressure Inlet. It is not recommended to prescribe a Specified Value condition for a boundary that is not an opening.

Volumetric flux is the rate at which the volumetric amount of mixture flows over a selected boundary or interface per second. Use Specified Volumetric Flux boundary condition to set the volumetric flux for any Flow module boundary condition at an opening. This corresponds to the volumetric flow boundary condition for the Flow module. For the Specified Volumetric Flux boundary condition, the Value of the volume fraction for each component is set under Component > Specified Value.

Symmetry means that there is no shear, no normal component of velocity, and no normal gradient of pressure at the boundary. A Symmetry boundary condition usually corresponds to a physical symmetry in the model. When you set the mixture boundary condition to Symmetry, the boundary conditions are fixed to Symmetry for the components. This is the default condition for Multicomponent Mixing when the boundary condition under Flow module is Symmetry.

Use Outlet boundary condition to set the mass fractions of the components at an opening. When you set the mixture boundary condition to Outlet, then the boundary conditions are fixed to Outlet for the components. The mass fraction is specified for Value under each component. Outlet is the default boundary condition for Multicomponent Mixing module when you set the boundary condition for the Flow module to Specified Pressure Outlet or Resistor Capacitor. Setting an Outlet boundary condition for a boundary that is not an opening is not recommended.

Outlet boundary condition allows you to set the mass fraction of any component that reenters the domain during backflow at a flow outlet.

The mass fraction value that you set at an outlet does not determine the value of mass fraction that exits. The mass fraction of the components for any fluid that exits the domain is based on the internal or upwind mass fraction. The mass fraction that you specify 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.

The interface conditions for the Multicomponent Mixing module are similar to those for the boundaries, only if the state of the volume on one side of the interface is Blanked for multicomponent mixing. If you set Multicomponent Mixing to Active on both sides of an interface, then options for the Multicomponent Mixing module are not available for that interface.