Extinction of non-premixed methane-and propane-air counterflow flames inhibited with CF 4, CF 3H and CF 3Br

Abstract

The extinction condition was studied for non-premixed counterflow flames of methane and propane impinging against an air stream with added amounts of CF 4 , CF 3 H, or CF 3 Br. Strain rates near extinction using laser Doppler velocimetry were determined for counterflow methane-air flames, with CF 4 and CF 3 H added to the air stream. The value obtained for the case of methane impinging against an air stream with no agent added, 400±25 s −1 , agrees with the experimental value reported in the literature. For both methane-and propane-air flames, increasing amounts of CF 4 , CF 3 H, or CF 3 Br in the air stream decrease the extinction oxidizer stream mass flow rate (or strain rate) monotonically until, at a given amount of agent, the extinction strain rate rapidly drops. If the molar percentage of agent in the air stream exceeds 2.5% for CF 3 Br and 11% for CF 4 or CF 3 H, non-premixed counterflow methane-air flames cannot be maintained. Counterflow propane-air flames cannot be maintained if agent concentrations in the air stream are greater than 2.7% for CF 3 Br, 11% for CF 3 H, and >14% for CF 4 . For a given amount of agent added to the air stream that does not exceed the limiting agent concentration, the extinction strain rate is comparable for CF 3 H or CF 4 addition in methane-air flames. CF 3 H is more effective than CF 4 in propane-air flames, similar to the reported extinction behavior in heptane-air cup burner studies. CF 3 Br is significantly more effective than CF 4 or CF 3 H in extinguishing methane-or propane-air flames. The net chemical effect of CF 3 H in the inhibition of non-premixed counterflow flames is more pronounced for propane-air flames than for methane-air flames. Explanations for this behavior are discussed briefly.