Multicomponent Mass Transport

Suppose a reacting flow consists of a mixture with i = 1, …, Q species and j = 1, …, N reactions. Equation 8-1 then describes the mass transport for an individual species:

(8-20)

where, ρ (SI unit: kg/m3) 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:

•

ωi is the mass fraction (1)

•

ji (SI unit: kg/(m2·s)) is the mass flux relative to the mass averaged velocity, and

•

Ri (SI unit: kg/ (m3·s)) is the rate expression describing its production or consumption.

The relative mass flux vector ji can include contributions due to molecular diffusion and thermal diffusion.

Summation of the transport equations over all present species gives Equation 8-21 for the conservation of mass

(8-21)

assuming that

Using the mass conservation equation, the species transport for an individual species, i, is given by:

(8-22)

Q − 1 of the species equations are independent and possible to solve for using Equation 8-22. 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:

(8-23)