If I run a partially premixed combustion solution and want to post-process NOx, I have the option to use a Mixture fraction or Temperature NOx PDF. Which is better and why?

The Mixture Fraction PDF is better.

1. It uses the same variance as was solved during the combustion calculation, which is superior to the approximate variance transport equation used for the NOX Temperature PDF

2. Both the number and distribution of the variance points in the NOx PDF, and the variance itself, align exactly with what is used in the combustion calculations, so turbulence interaction treatment is consistent across the calculations.