Intermediate species profiles in low pressure premixed flames inhibited by fluoromethanes

Abstract

We have investigated premixed 10 torr methane/oxygen flames containing CH 3F, CH 2F 2, CHF 3, and CF 4. Profiles of temperature and CH∗ chemi-luminescence were acquired, and laser-induced fluorescence (LIF) was used to obtain profiles of the intermediate species H, OH, CH, CF, CHF, CF 2, and CF 2O. The fluoromethanes were added in amounts such that each flame had the same flux of fluorine atoms. In the flames containing CH 3F, CH 2F 2, and CHF 3, the methane flow was adjusted to give an equivalence ratio of 1.07 for all three inhibited flames. The experimental intermediate profiles were compared to predicted profiles calculated from a hydrofluorocarbon kinetic mechanism recently developed at NIST. No fluorinated intermediates were detectable in the CF 4 inhibited flame, indicating that this agent does not react significantly under the flame conditions studied. The temperature profiles, H atom profiles, and OH profiles for the other three fluoromethane inhibited flames are nearly identical, indicating that flames containing different fluorocarbon compounds, but identical proportions of fluorine atoms, have similar structures. The kinetic model correctly predicts the location of the reaction zone in the flames containing CH 2F 2 and CH 3F. In the CHF 3 flame, however, the location of the reaction zone is predicted to be too far above the burner surface, and concentrations of H and OH are too low. The discrepancy appears to be due to pressure dependence and third body efficiencies of the agent thermal decomposition. Furthermore, relative amounts of CF, CH, CF 2, and CHF in the different flames are not very well predicted. In general, partially fluorinated methyl and methylene radicals appear to have a greater than predicted propensity to lose hydrogen atoms rather than fluorine. We propose modifications to the fluorine mechanism to correct the discrepancies observed in the low pressure experiments, while simultaneously achieving good agreement with atmospheric pressure flame speed data in CH 4/air/CHF 3 flames, and, except in rich conditions (φ > 1.25) CH 4/air/CH 2F 2 flames.