Modeling turbulent reacting jets issuing into a hot and diluted coflow

Abstract

Numerical modeling of turbulent nonpremixed methane/hydrogen (1/1 by volume) flames issuing from a jet in hot coflow (JHC) is presented. The JHC burner is designed to emulate a moderate and intense low oxygen dilution (MILD) combustion regime. This study is focused on assessing the performance of various turbulence, combustion, and chemical kinetic models in predicting the JHC flames. A comparison between the modeling and experimental data is presented for three flames with different oxygen levels in the hot coflow (oxygen mass fractions of 9, 6, and 3%). Out of three variants, the k-ε turbulence model (standard, renormalization group, and realizable k-ε models), the standard model with a modified dissipation equation constant (Cε1), provided the best agreement with the experiment. Differential diffusion effects are found to have a strong influence on the accuracy of the predictions and therefore should always be accounted for. It was also found that conserved scalar-based models, i.e., the ξ/PDF and flamelet models, are inadequate for modeling JHC flames. The representation of the chemistry in the model was also found to play an important role in accurately predicting flame characteristics. Using detailed chemical kinetics, rather than global or skeletal mechanisms, with the eddy-dissipation concept (EDC) solver was found to improve the accuracy significantly. In general, the EDC model performed reasonably well for the 9% O2 and 6% O2 flames, but not for the 3% O2 case. For the 3% O2 case, the model overpredicted the flame liftoff height. At the 120-mm axial location, the model did not perform well due to the intermittent localized flame extinction. However, overall the EDC model with a detailed kinetic scheme, offers a practical and reasonably accurate tool for predicting the flow and flame characteristics of JHC configurations.