Thermal Conductivity of mixed gases?

I'm working on a model that needs to calculate the approximate thermal conductivity of a mixture of gases. I'm having trouble finding a model which is being used by one of my sources but is not cited and I can't seem to find a source for it. I'm hoping that someone here might recognize the model and identify a source.

The source is an MASc thesis. The formula is given as:
$$k_{gas} = \frac{\sum _i y_{i}\sqrt[3]{M_{i}}k_{0,i}T^{s_{i}}}{\sum _i y_i\sqrt[3]{M_i}} \left( 1 + \left(0.51 T_R ^{-2.26} \right) P_R ^{1+2.5 T_R ^{-6.2}} \right)$$

The first part is a mixing term, the second is pressure dependence. I believe it may be a crude semi-empirical relationship (possibly for monatomic gases)

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I'm currently exploring other models, mainly "Modified Stiel and Thodos" as described in the book, "Properties of Liquids and Gases" 5th Ed by Poling, Prausnitz, and O'Connell.

I was hoping that maybe someone with more fluids experience could help me. The modified Stiel and Thodos model calculates the thermal conductivity of the gas mixture using the Stiel and Thodos model with effective parameters determined by a somewhat complicated system of weightings.

One of the factors I'm having trouble understanding is the compressibility factor. How would I calculate the compressibility factor? Currently I'm using the ideal gas law to calculate the pressure of the system, but that assumes a compressibility factor of 1. Is that a good assumption for mixtures of noble gases at temperatures from 293-1200k and pressures of 0.1-15 MPa?

From what I remember from my thermodynamics classes, noble gases are almost ideal gases except at extreme temperatures and/or pressures, but I don't remember if 15 MPa counts as extreme pressure. Should I be using a different P(n,T,V) model?

I found another source similar to the first, the numbers are very similar, but not to the power. They could be the same if for two typos...

$$k_{gas} = \frac{\sum _i y_{i}\sqrt[3]{M_{i}}k_{0,i}T^{s_{i}}}{\sum _i y_i\sqrt[3]{M_i}} \left( 1 + \left(0.51 T_R ^{-2.26} \right) P_R (12.5 T_R ^{-6.2}) \right)$$