In the present analysis it is assumed that the effect of pressure and temperature on mass diffusion flux velocities (ui, vi) is small. Hence these fluxes are neglected. Only the effect of concentration gradient is considered on the mass diffusion fluxes. Diffusion fluxes are calculated by Fick’s law, which gives the mechanism of mass diffusion in microscopic point of view. This law correlates mass diffusion flux with concentration gradient as
where D12 is binary diffusion coefficient of species one and two and ui and vi are the diffusion velocities in x and y directions, respectively. The binary diffusion coefficient is function of molecular diameters and temperature.
The above set of governing equations involves two diffusion parameters, which are viscosity and binary diffusion coefficient. In case of single specie, viscosity as the function of temperature can be calculated by Sutherland’s formula as,
where S is the Sutherland’s constant, which depends upon the fluid. In case of multiple species, viscosity can be calculated by Wilke’s mixture rule, as
where Φij is collision integral, Xi are mole fractions and mi are species viscosities. The calculation of mixture viscosity needs the viscosity of individual specie. We can find various ways to calculate viscosity of specie using Sutherland’s formula.