Combustion Mode and Reactivity of Methane Fuel in Hot O ₂ /CO ₂ /N ₂ and O ₂ /H ₂ O/N ₂ Coflows in a Nonpremixed Small‐Scale Tube Burner

The reactivity of methane fuel was numerically studied to compare the effects of CO 2 and H 2 O dilution of hot oxidizer atmospheres on combustion modes and flame characteristics. A nonpremixed small scale burner at 8.6 kW thermal load forming central fuel jet flame in coflow oxidizer atmosphere was employed. The inlet temperature and oxidizer level were assumed to be constant to scrutinize the pure effects of the varying molar fraction of the diluents on flame parameters. The results show that the maximum flame temperature is around the same level of 1665 K for 20% H 2 O and 20% CO 2 cases with flamelet model. H 2 O dilution of the hot oxidizer enhances the combustion stability, while keeping mild behavior, whereas CO 2 dilution maintains an interesting locally varying combustion mode by consuming methane fuel in a longer axial distance and forming temperature jump. The calculated temperature variations show that the varying combustion parameters emerge mainly from the physical effects. It is revealed that the physical effects occurring from CO 2 dilution reduce the calculated Damköhler number (Da). However, H 2 O dilution maintains a more uniform K ( r ) value distribution, which is defined to quantify the degree of reduction in methane fuel reactivity, by eliminating temperature jump and maintaining combustion stability.