Multi-stage oxidation of a CH₂F₂/air mixture examined by weak flames in a micro flow reactor with a controlled temperature profile

Oxidation of CH₂F₂ (R32), a widely used refrigerant, was investigated experimentally and computationally using a micro flow reactor with a controlled temperature profile (MFR). Specifically, we examined weak flames of a stoichiometric CH₂F₂/air mixture in MFR observed at low flow velocities. In the case of the maximum wall temperature of 1300 K, weak flames showed two luminous regions at 1240 and 1290 K, implying two-stage oxidation. Computations also indicated two-stage heat release. Computational weak flame positions also showed good agreement with experimentally obtained results. To assess the validity of chemical kinetics in detail, species measurements of CH₂F₂, CO, CO₂, H₂O, HF, and CF₂O for a stoichiometric CH₂F₂/air weak flame were conducted using Fourier Transform Infrared spectrometry (FTIR). The present species measurements elucidated the CH₂F₂/air weak flame structure at 800–1300 K and remaining intermediates of H₂O and CF₂O at the end of the reactor. Computational results also showed good agreements with measurement, indicating the validity of the computations using the chemical kinetics. For further analysis for CH₂F₂ weak flame at 1300 K, the reaction path and rate-of-production analyses revealed a major reaction at the first-stage heat release: the formation of HF and intermediates, and that at the second stage is the CO₂ formation from CO. Computations also showed intermediates of H₂O and CF₂O remain at the end of the reactor. An additional numerical experiment for a CH₂F₂/air mixture at the maximum wall temperature of 2000 K, demonstrated that the reaction of H₂O and CF₂O proceeded slowly at temperatures higher than 1300 K, with complete oxidation of CH₂F₂/air mixture. In conclusion, results show that the overall reactivity of CH₂F₂ decreases because of the low reactivity intermediates such as CF₂O and H₂O if oxidation occurs in a non-adiabatic system such as MFR. Such specific characteristics of the CH₂F₂/air mixture are concealed in the common zero-dimensional adiabatic simulations. Results demonstrated that the CH₂F₂ reactivity depend strongly on the temperature history during oxidation.