Numerical study on radiation extinction of non-premixed counterflow CH2F2 flames

Understanding the near-limit combustion characteristics of hydrofluorocarbons (HFCs) is crucial from two perspectives: ensuring the fire safety of wire insulation and improving the understanding of flammability behavior of fluorinated. To elucidate the influence of radiation heat loss from combustion products of HFCs (HF and COF 2 ) on near-limit flame behavior, we numerically investigated non-premixed counterflow CH 2 F 2 flames using the optically thin radiation model. For the 20% fuel-in-N 2 diluted condition, radiation heat losses from HF and COF 2 were additionally considered alongside those of H 2 O, CO 2 , CO and CH 4 . This was found to increase the stretch rate at the radiation extinction limit by 16%, demonstrating the significant impact of radiation losses from HF and COF 2 . In typical hydrocarbon fuels, CO 2 and H 2 O are the dominant products. However, in the case of CH 2 F 2 , CO 2 and HF are the primary products because the standard enthalpy of formation of HF is lower than that of H 2 O. Near the radiation extinction limit, the radiation heat losses from CO 2 and HF accounted for 69% and 24% of the total, respectively, due to their large Planck-mean absorption coefficients (PMACs) and high mole fractions. Additionally, near the radiation extinction limit, the relative importance of radiation heat loss from COF 2 increased despite its lower PMAC than the other species, because its maximum mole fraction increased. Consequently, radiation heat loss from COF 2 had a non-negligible influence on the flame behavior near the radiation extinction limit. Furthermore, including the radiation heat losses from HF and COF 2 increased the minimum fuel concentration from 39% to 43%. Therefore, accounting for radiation heat losses from fluorinated species is critically important for understanding the behavior of CH 2 F 2 non-premixed counterflow flames near the minimum fuel concentration limit. Novelty and significance statement The novelty of this study lies in investigating the near-limit flame behavior of CH 2 F 2 , a representative hydrofluorocarbon and a surrogate for gaseous products from the thermal decomposition of wire insulation materials, by incorporating HF and COF 2 as radiative species. We studied the effects of radiation heat loss from fluorinated species on near-limit flame behavior in non-premixed counterflow CH 2 F 2 flames. To the best of our knowledge, this is the first study to demonstrate that radiation heat loss from fluorinated species increases the stretch rate at the radiation extinction limit and the minimum fuel concentration. Understanding the effects of radiation heat loss on CH 2 F 2 counterflow diffusion flames has significant implications for the fire safety of wire insulation materials, particularly in spacecraft where limiting oxygen concentrations extend beyond ground-based predictions under microgravity conditions. Furthermore, as CH 2 F 2 is a widely used refrigerant (R32), this study provides valuable insights into flammability assessment of fluorinated refrigerants.