Effectiveness of Flame Suppressants on Cool Flames and Hot Flames

While the effectiveness of various flame suppressants such as bromotrifluoromethane and trimethylphosphate on hot flames has been relatively well studied over the years, such suppressants have not been examined in the context of low-temperature cool flames. This investigation solves this issue by exploring the extinction limits of six suppressants on both hot flames and cool flames in the counterflow geometry using n-dodecane as the fuel. In contrast to hot flames, it is found both experimentally and numerically that cool flames are relatively impervious to chemically based suppressants such as bromotrifluoromethane; these suppressants are essentially diluents at low temperatures. Detailed examination of the computed flame structure reveals that the reactions composing the catalytic cycles that interfere with hydrogen radical and hydroxyl radical production in hot flames are orders of magnitude lower in cool flames. Furthermore, mildly flammable suppressants such as trimethylphosphate and 2‑bromo-3,3,3-trifluoropropene are observed to ignite under the conditions necessary to initiate cool flames, which limits measurements of the cool flame extinction limits. This premature oxidation is not predicted by kinetic models describing the suppressant chemistry.