8.13 Kinetic Theory Parameters

You may choose to define the following properties using kinetic theory when the ideal gas law is enabled:

• viscosity (for fluids)

• thermal conductivity (for fluids)

• specific heat capacity (for fluids)

• mass diffusion coefficients (for multi-species mixtures)

If you are using kinetic theory for a fluid's viscosity (Equation  8.4-9), you will need to input the kinetic theory parameters $\sigma$ and $\epsilon/k_B$ for that fluid. These parameters are the Lennard-Jones parameters and are referred to by ANSYS FLUENT as the "characteristic length'' and the "energy parameter'' respectively.

When kinetic theory is applied to calculation of a fluid's thermal conductivity only, no inputs are required.

If you are going to calculate a fluid's specific heat using kinetic theory (Equation  8.7-5), you will need to input the degrees of freedom for the fluid material.

If you use kinetic theory to define a mixture material's mass diffusivity (Equation  8.9-16), you will need to input $\sigma_i$ and $(\epsilon/k_B)_i$ for each chemical species $i$.

Inputs for Kinetic Theory

The procedure for using kinetic theory is as follows:

1.   Select kinetic-theory as the property specification method for the Viscosity, Thermal Conductivity, or heat capacity Cp of a fluid material, or for the Mass Diffusivity of a mixture material.

2.   If the material for which you have selected the kinetic theory method for one or more properties is a fluid material, you must set the kinetic theory parameters for that material. If you are using kinetic theory for the mass diffusivity of a mixture material, you will define the kinetic theory parameters for each of the constituent species (fluid materials).

The parameters to be set are as follows:

• L-J Characteristic Length

• L-J Energy Parameter

• Degrees of Freedom (only required if kinetic theory is used for specific heat)

See the beginning of this section to find out which parameters are required to calculate each property using kinetic theory.

Characteristic length is defined in units of Angstroms. The energy parameter is defined in units of absolute temperature. Degrees of freedom is a dimensionless input. All kinetic theory parameters are set to zero by default. Appropriate values for different materials can be found in the literature (e.g., [ 33]).