Optionally, for at most one species, instead of defining a transport equation, you can use the closure equation. By definition, a multicomponent system is said to be closed when all species, including the carrier, inert or not, are considered. Such is the case for nondilute mixtures. When systems are closed, the sum of the mass fractions of all species is one. Using this fact, the mass fraction of a single species can be computed if the mass fractions of all other species are known.
The steps for defining a closure equation for a species are as follows:
Create a sub-task
Select the Closure sub-task type.
Closure
Select the species for which you want to solve the closure equation. This will give the sub-task the name of "Closure of species", where species is the species you have selected.
The closure equation does not require any material data or boundary conditions, so no further inputs are required.