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Simplification of the kinetic theory (Chapman and Enskog theory) approach neglecting the second-order effects.

This equation has been extensively tested (Amdur and Mason, 1958; Bromley and Wilke, 1951; Cheung, 1958; Dahler, 1959; Gandhi and Saxena, 1964; Ranz and Brodowsky, 1962; Saxena and Gambhir, 1963a; Strunk, et al., 1964; Vanderslice, et al. 1962; Wright and Gray, 1962). In most cases, only nonpolar mixtures were compared, and very good results obtained. For some systems containing hydrogen as one component, less satisfactory agreement was noted. Wilke's method predicted mixture viscosities that were larger than experimental for the H2-N2 system, but for H2-NH3, it underestimated the viscosities.
Gururaja, et al. (1967) found that this method also overpredicted in the H2-O2 case but was quite accurate for the H2-CO2 system.
Wilke's approximation has proved reliable even for polar-polar gas mixtures of aliphatic alcohols (Reid and Belenyessy, 1960). The principal reservation appears to lie in those cases where Mi>>Mj and etai>>etaj.

etam = gasMixtureViscosity(yi, M, eta)

yi	Type: MoleFraction[:] (mol/mol) Description: Mole fractions
M	Type: MolarMass[size(yi, 1)] (kg/mol) Description: Mole masses
eta	Type: DynamicViscosity[size(yi, 1)] (Pa·s) Description: Pure component viscosities

etam	Type: DynamicViscosity (Pa·s) Description: Viscosity of the mixture