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. More information on defining properties with user-defined functions can be found in 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:
![[ANSYS, Inc. Logo]](fluentlogo.gif)
is the mole fraction of species
.
![\phi_{ij} = \frac{\left [ 1+ \left (\frac{\mu_i}{\mu_j} \rig... ... [8 \left (1 + \frac{M_{w,i}}{M_{w,j}} \right )\right]^{1/2}}](img2092.gif)
