Effects of Radiation Reabsorption on the Flammability Limit and Critical Fuel Concentration of Methane Oxy-Fuel Diffusion Flame.

This study numerically investigates the critical fuel concentration and flammable regions of methane-air and methane oxy-fuel counterflow diffusion flames. The goal is to determine the effects of strain rate, oxidizer composition, and radiative heat transfer models on flame extinction. Calculations were performed using the counterflow diffusion flame with the adiabatic (ADI), optically thin (OTM), and statistical narrow-band (SNB) radiation models at strain rates of 10 s-1, 80 s-1, and 200 s-1. The key findings are as follows: For methane-air flames, radiation reabsorption has a negligible impact. The flammable region decreases with increasing strain rate (SLow > SMid > SHigh) across all models. In O2/CO2 flames, radiation plays a significant role. While the ADI and SNB models maintain the same trend as in air flames, the OTM yields a different order (SMid > SHigh > SLow). Reducing oxygen concentration increases the critical fuel concentration and shrinks the flammable region. When the oxygen concentration is between 0.35 and 0.40, the combustion characteristics of O2/CO2 flames resemble those of conventional air flames. In conclusion, this work highlights the critical influence of radiation modeling and oxidizer composition on oxy-fuel flame extinction limits, providing insights for combustion system design under CO2 dilution.