## 8.4.4 Composition-Dependent Viscosity for Multicomponent Mixtures

If you are modeling a flow that includes more than one chemical species (multicomponent flow), you have the option to define a composition-dependent viscosity. (Note that you can also define the viscosity of the mixture as a constant value or a function of temperature.)

To define a composition-dependent viscosity for a mixture, follow these steps:

1.   For the mixture material, choose mass-weighted-mixing-law or, if you are using the ideal gas law for density, ideal-gas-mixing-law in the drop-down list to the right of Viscosity. If you have a user-defined function that you want to use to model the viscosity, you can choose either the user-defined method or the user-defined-mixing-law method for the mixture material in the drop-down list.

2.   Click Change/Create.

3.   Define the viscosity for each of the fluid materials that comprise the mixture. You may define constant or (if applicable) temperature-dependent viscosities for the individual species. You may also use kinetic theory for the individual viscosities, or specify a non-Newtonian viscosity, if applicable.

4.   If you selected user-defined-mixing-law, define the viscosity for each of the fluid materials that comprise the mixture. You may define constant, or (if applicable) temperature-dependent viscosities, or user-defined viscosities for the individual species. For more information on defining properties with user-defined functions, see the separate UDF Manual .

The only difference between the user-defined-mixing-law and the user-defined option for specifying density, viscosity and thermal conductivity of mixture materials, is that with the user-defined-mixing-law option, the individual properties of the species materials can also be specified. (Note that only the constant, the polynomial methods and the user-defined methods are available.)

If you are using the ideal gas law, the solver will compute the mixture viscosity based on kinetic theory as

 (8.4-11)

where

 (8.4-12)

and is the mole fraction of species .

For non-ideal gas mixtures, the mixture viscosity is computed based on a simple mass fraction average of the pure species viscosities:

 (8.4-13)

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