8.3.7 Composition-Dependent Density for Multicomponent Mixtures

If you are solving species transport equations, set properties for the mixture material and for the constituent fluids (species), as described in detail in Section  15.1.3. To define a composition-dependent density for a mixture, do the following:

1.   Select the density method:

• For non-ideal-gas mixtures, select the volume-weighted-mixing-law method for the mixture material in the drop-down list to the right of Density in the Create/Edit Materials dialog box.

• If you are modeling compressible flow, select ideal-gas for the mixture material in the drop-down list to the right of Density in the Create/Edit Materials dialog box.

• If you are modeling incompressible flow using the ideal gas law, select incompressible-ideal-gas for the mixture material in the Density drop-down list in the Create/Edit Materials dialog box.

• If you have a user-defined function that you want to use to model the density, you can choose either the user-defined method or the user-defined-mixing-law method for the mixture material in the drop-down list.

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.)

2.   Click Change/Create.

3.   If you have selected volume-weighted-mixing-law, define the density for each of the fluid materials that comprise the mixture. You may define constant or (if applicable) temperature-dependent densities for the individual species.

4.   If you selected user-defined-mixing-law, define the density for each of the fluid materials that comprise the mixture. You may define constant, or (if applicable) temperature-dependent densities, or user-defined densities for the individual species. More information on defining properties with user-defined functions can be found in the separate UDF Manual.

If you are modeling a non-ideal-gas mixture, ANSYS FLUENT will compute the mixture density as

where $Y_i$ is the mass fraction and $\rho_i$ is the density of species $i$.

For compressible flows, the gas law has the following form:

where,

$p$	=	the local relative (or gauge) pressure predicted by ANSYS FLUENT
$R$	=	the universal gas constant
$Y_i$	=	the mass fraction of species $i$
$M_{w,i}$	=	the molecular weight of species $i$
$p_{\rm op}$	=	the Operating Pressure

In ANSYS FLUENT, if you choose to define the density using the ideal gas law for an incompressible flow, the solver will compute the density as

where,

$R$	=	the universal gas constant
$Y_i$	=	the mass fraction of species $i$
$M_{w,i}$	=	the molecular weight of species $i$
$p_{\rm op}$	=	the Operating Pressure