The thermal diffusion coefficients are evaluated in the following section on multicomponent properties. This section describes a relatively inexpensive way to estimate the thermal diffusion of light species into a mixture. This method is included here for the sake of backward compatibility. However, this approximate method is considerably less accurate than the thermal diffusion coefficients that are computed from the multicomponent formulation. It is therefore recommended that users employ the multicomponent formulation when thermal diffusion is likely to be important.

A thermal diffusion ratio can be defined such that the thermal diffusion velocity is given by

(5–52)

where is a spatial coordinate. The mole fractions are given by , and the are mixture diffusion coefficients Equation 5–43 . In this form we only consider thermal diffusion in the trace, light component limit (specifically, species having molecular mass less than 5). The thermal diffusion ratio [47] is given by

(5–53)

where

(5–54)

Three ratios of collision integrals are defined by

(5–55)

(5–56)

(5–57)

We have fit polynomials to tables of , , and .[36]

In the Transport Pre-processor (where the pure species properties are fit) we also fit the temperature dependent parts of the pairs of the thermal diffusion ratios for each light species diffusing into all the other species. That is, we fit for all species pairs in which . Since the depend weakly on temperature, we fit to polynomials in temperature, rather than the logarithm of temperature. The coefficients of these fits are written onto the Transport Linking File.