Chemical effects of CF3H in extinguishing counterflow CO/air/H2 diffusion flames

The relative importance of introducing CF3H as a fire suppressant with the oxidizer or the fuel stream, and its chemical and thermal effects on the extinction condition of counterflow CO/air/H-2 diffusion flames, are investigated both experimentally and numerically. In experiments, the extinction strain rate is evaluated by measuring the jet exit velocities at extinction and the jet separation distance. In numerical calculations, the potential flow approximation is introduced to describe the outer flow field and the strain rate at extinction is determined from the axial velocity gradient on the oxidizer side. By employing mixture fraction concepts, the shift in flame location within the mixing layer caused by increasing the CF3H mole fraction or switching CF3H from air to fuel stream is shown to affect the measured and predicted extinction strain rates significantly. Flame structure and reaction pathway analyses have been used to identify the rate-controlling reactions and the influence of introducing the suppressing agent from the oxidizer or fuel side on the finite-rate chemistry. Subsequent sensitivity analysis has shown that two alternate CF3-consumption reaction pathways can either increase or decrease the extinction strain rate, similar to the sensitivities found in a recent premixed burning velocity study. Finally, based on the numerical calculations, the overall chemical and thermal effects of CF3H are demonstrated by freezing the CF3H chemistry and by replacing the heat capacity of CF3H with that of N-2 for the frozen case.