Mixture fraction statistics in methane-oxygen turbulent combustion for space propulsion

Abstract

Two high-pressure turbulent non-premixed methane-oxygen flames are simulated using the unsteady reactive solver EBI-DNS. The results are used to investigate the statistics of the mixture fraction in the frame of turbulent combustion for space propulsion applications. The capability of the beta distribution to capture the probabilistic behavior of the mixing process is evaluated as well as the validity of extended closure models for the mixture fraction variance. A new closure model for the mixture fraction variance is introduced. The proposed method performs well at sufficiently high Reynolds numbers, although it displays a high error for moderate turbulence. Overall, it is found that the beta distribution struggles to capture extreme mixing events, whose occurrence may be relevant near the mean stoichiometric line at high Reynolds numbers. Furthermore, it is observed that the derivation of the beta distribution from a modeled variance is unstable at certain configurations. When one shape factor approaches unity, the distribution is very sensitive to variance, and modeling errors yield substantial discrepancies in the resulting PDF.