Flame Characteristics of CH4/H2 on a Jet-in-Hot-Coflow Burner Diluted by N2, CO2, and H2O

To enhance the knowledge to the combination of oxy-fuel combustion and MILD (moderate or intense low-oxygen dilution) combustion, namely oxy-MILD combustion, the present article examines the effects of different diluents (i.e., N2, CO2, and H2O) on the flame characteristics of CH4/H2 blended fuel in a JHC (jet-in-hot-coflow) burner, which can produce MILD combustion regime in the upstream and conventional combustion regime in the downstream. CFD modeling method considering RANS equations and detailed reaction mechanism (GRI-Mech 2.11) are used for such purpose. Results show that, in MILD combustion regime, peak temperature under oxy-fuel condition (diluted by CO2 and H2O) is reduced as compared to air-fuel condition (diluted by N2) from both physical and chemical effects of the diluents. However, in the conventional combustion regime, chemical effect of the diluents on suppressing the peak temperature is weakened, resulting in the predominance of its physical effect. As for the flame ignition behavior, the dilution with CO2 or H2O causes flame liftoff according to OH appearance. More noticeably, the liftoff distance is even larger in H2O diluted case than CO2 diluted case. On the other hand, by calculating the Damköhler number in the computational domain, MILD combustion region is found well under reaction-controlled status. However, as compared to CO2, H2O plays a weakened role on the reaction-controlled status, suggesting that CO2 might be preferable to H2O in realizing oxy-MILD combustion.