Combustion Characteristics of a Methane Jet Flame in Hot Oxidant Coflow Diluted by H_2O versus the Case by N_2

摘要

This numerical study investigates combustion characteristics of a CH4 jet flame in hot oxidant coflow (JHC) diluted by H2O versus the case by N-2. Both MILD oxy-fuel combustion and traditional combustion are generated by varying the coflow oxygen level from 3% to 85% in volume. To distinguish physical and chemical effects of changing the diluent from N-2 to H2O, the JHC flame diluted by XH2O (artificial H2O without reactivity) is also simulated. It is demonstrated that both the temperature and the dimension of the JHC flame reduce substantially when changing the diluent from N-2 to H2O. Such reductions derive primarily from different fluid properties of N-2. and H2O, i.e., H2O has a lower fluid density and a higher thermal capacity. The change from N-2 to H2O is also found to have an impact more chemically on formation of the main intermediate species (i.e., H, O, CO, OH) and simultaneously to increase the flame lift-off distance due to longer ignition delay. Hence, the MILD combustion is expected to occur more feasibly for the JHC flame diluted by steam than that by nitrogen. The differences noted above lessen as the coflow oxygen is increased. In addition, kinetic oxidation pathways.are analyzed to improve our understanding of the CH4 JHC combustion diluted by N-2, and H2O.