To expedite the evaluation of transport properties in Ansys Chemkin, we fit the temperature dependent parts of the pure species property expressions. Then, rather than evaluating the complex expressions for the properties, only comparatively simple fits need to be evaluated.

We use a polynomial fit of the logarithm of the property versus the logarithm of the temperature. For the viscosity

(5–35)

and the thermal conductivity,

(5–36)

The fits are done for each pair of binary diffusion coefficients in the system.

(5–37)

By default Transport uses third-order polynomial fits (that is, ) and we find that the fitting errors are well within one percent. The fitting procedure must be carried out for the particular system of gases that is present in a given problem. Therefore, the fitting cannot be done "once and for all," but must be done once at the beginning of each new problem.

The viscosity and conductivity are independent of pressure, but the diffusion coefficients depend inversely on pressure. The diffusion coefficient fits are computed at unit pressure; the later evaluation of a diffusion coefficient is obtained by simply dividing the diffusion coefficient as evaluated from the fit by the actual pressure.

Even though the single component conductivities are fit and passed to the Transport Subroutine Library, they are not used in the computation of multicomponent thermal conductivities; they are used only for the evaluation of the mixture-averaged conductivities.