Multicomponent Diffusion: Mixture-Average Approximation
The mixture-averaged diffusion model assumes that the relative mass flux due to molecular diffusion is governed by a Fick’s law type expression
(3-46)
where ρi is the density and xi the mole fraction of species i. The diffusion hence depends on a single concentration gradient and is proportional to a diffusion coefficient . The diffusion coefficient describes the diffusion of species i relative to the remaining mixture and is referred to as the mixture-averaged diffusion coefficient. Equation 3-46 can be expressed in terms of mass fractions as
using the definition of the species density and mole fraction
,
Assuming isobaric and isothermal conditions, the following expression for the mixture-averaged diffusion coefficient can be derived from the Maxwell-Stefan equations (Ref. 3):
The mixture-averaged diffusivities are explicitly given by be the multicomponent Maxwell-Stefan diffusivities Dik. As a consequence, no matrix inversion operation is required as for the Maxwell-Stefan diffusion model (when using four or more species). For low-density gas mixtures, the Dik components can be replaced by the binary diffusivities for the species pairs present.
When using the mixture-averaged diffusion model, the species mass transport equations are
Apart from molecular diffusion, transport due to thermal diffusion and migration of charged species in an electric field are accounted for through the third and fourth terms on the right-hand side, respectively. Here
zi (dimensionless) is the charge number
um, i the mobility of the ith species, and
(SI unit: V) is the electric potential.