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The extinction condition was studied for non-premixed counterflow flames of methane and propane impinging against an air stream with added amounts of CF4, CF3H, or CF3Br. Strain rates near extinction using laser Doppler velocimetry were determined for counterflow methane-air flames, with CF4 and CF3H 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 CF4, CF3H, or CF3Br 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 CF3Br and 11% for CF4 or CF3H, 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 CF3Br, 11% for CF3H, and >14% for CF4. 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 CF3H or CF4 addition in methane-air flames. CF3H is more effective than CF4 in propane-air flames, similar to the reported extinction behavior in heptane-air cup burner studies. CF3Br is significantly more effective than CF4 or CF3H in extinguishing methane-or propane-air flames. The net chemical effect of CF3H 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. |
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