Numerical and experimental studies of extinguishment of cup-burner flames by C6F12O

The extinguishment of propane cup-burner flames by a halon-replacement fire-extinguishing agent C6F12O (Novec 1230) added to coflowing air in normal gravity has been studied computationally and experimentally. The time-dependent, axisymmetric numerical code with a detailed reaction mechanism (up to 141 species and 2206 reactions), molecular diffusive transport, and a radiation model, is used to reveal a unique two-zone flame structure. The peak reactivity spot (i.e., reaction kernel) at the flame base stabilizes a trailing diffusion flame, which is inclined inwardly by a buoyancy-induced entrainment flow. As the volume fraction of the agent in the coflow is increased gradually, the total heat release increases up to three times due to unwanted combustion enhancement by exothermic reactions to form HF and CF2O in the two-zone trailing flame; whereas at the base, the flame-anchoring reaction kernel weakens (the local heat release rate decreases) and eventually the flame blows off. A numerical experiment, in which the C6F12O agent decomposition reactions are turned off, indicates that for addition of inert C6F12O, the maximum flame temperature decreases rapidly due to its large molar heat capacity, and the blow-off extinguishment occurs at ≈1700 K, a value identical to that for inert gases previously studied, while the reaction kernel is still burning vigorously. The calculated minimum extinguishing concentration of C6F12O in a propane flame is 4.12 % (with full chemistry), which nearly coincides with the measured value of 4.17 ± 0.30 %.