Fluid densities ρ are calculated from pressure p, temperature T, constituent mass fractions yi and molar weights Mi as
$$\rho =\frac{p}{RT\mathop{\sum }^{}\left( {{y}_{i}}/{{M}_{i}} \right)}$$
where R is the universal gas constant. The calculations are coded in appSrc files rhoAir.H and rhoFuel.H.