Suppose a reacting flow consists of a mixture with i = 1, …,
Q species and
j =
1, …,
N reactions.
Equation 3-131 then describes the mass transport for an individual species:
where, ρ (SI unit: kg/m
3) denotes the mixture density and
u (SI unit: m/s) the mass averaged velocity of the mixture. The remaining variables are specific for each of the species,
i, being described by the mass transfer equation:
The relative mass flux vector ji can include contributions due to molecular diffusion, mass flux due to migration in an electric field, and thermal diffusion.
Q − 1 of the species equations are independent and possible to solve for using
Equation 3-44. To compute the mass fraction of the remaining species, COMSOL Multiphysics uses the fact that the sum of the mass fractions is equal to
1: