COMSOL 6.4 - Inflow

The boundary condition sets the mass fraction or mass flow rate of specified heavy species at a boundary. Use this feature to set the composition of gas mixture going into a system. This feature does not set the fluid velocity at a boundary. For that, the plasma model needs to coupled with a fluid flow interface.

For the interface, when solving for plasmas with chemistries that contain more than one element (for example, Ar and O2) the mass fraction of each element is not conserved if no constraint is used. This problem is similar in nature to the one found when solving for Navier–Stokes equations in steady state without fixing the pressure somewhere. The boundary condition fixes the mass fraction or the other equivalent quantities of specified species and it can be used as a strategy to constrain the mass fraction of some species at a given boundary even if no fluid flow exists in the system.

An important aspect to remember is that the feature does not apply a constraint to the species being computed and if some other species mass fractions attain important values the mass fraction of the mass constraint species can differ from the value specified. This can be avoided by setting to a small value the mass fraction of the species that attain large values at the inflow boundary.

In the : select what quantity to constrain:

On the table specify the names of the species to constrain and the value of the selected quantity. As an example, if the inflow mixture of Ar/Cl2 has a mole fraction of 0.1/0.9 the table should be like:

Table 5-3: mole fraction at the inflow boundary.
Species name	Mole fraction (1)
Ar	0.1
Cl2	0.9

Note that the names must correspond to the exact name in the Species name field in the Species feature.

Assume that there is significant dissociation of Cl2 so that Cl is an important species. Also assume that Ar uses the option . In this case the mass fraction for Ar is not solved for and there is no constraint applied to set the mass fraction at 0.1. This means that only the mass fraction of Cl2 is constrained at the Inflow boundary. Since the Cl mass fraction is high everywhere the mass fraction of Ar will not be 0.1 at Inflow boundary. This could be avoided by extending the inflow channel so that there is no plasma and no dissociation in the region where the Inflow boundary is applied. Another solution for this issue is to constrain the mole fraction of Cl to a small value as shown below.