A Hybrid Rans/pdf Approach for Modelling Hot and Diluted Flames

Results of numerical modelling of turbulent non-premixed Methane/Hydrogen (1/1 by volume) flames issuing from a jet in hot coflow (JHC) are presented. The JHC burner is designed to emulate Moderate and Intense Low oxygen Dilution (MILD) combustion regime. A comparison between the measured and computed temperature and species profiles is presented for three flames with different oxygen content in the hot coflow (oxygen mass-fraction of 3%, 6%, and 9%). The composition PDF approach with the EMST mixing model coupled with a finite-volume flow solver was used in the modelling. The performance of three welldocumented chemical kinetics mechanisms (Smooke, ARM, and GRI3.0) is examined. It was found that MILD flames are primarily chemistry controlled; hence selecting an appropriate chemical kinetics mechanism is more crucial than the mixing model. The results also show that sophisticated high-temperature/high-oxygen optimised kinetics mechanisms, do not necessarily improve the predictions accuracy and in most cases may increase the sensitivity of the solution to slight variations in flow conditions to the extend that the flame is lifted or extinguished. Among the three kinetics mechanisms the ARM provided the closest agreement with the experiment for the 3% and 6% O2 flames, particularly in the MILD combustion region. The adequacy of the existing chemical kinetic and combustion models for these flames however remains unresolved.