Chemical Inhibition of Nonpremixed Flames of Hydrocarbon Fuels with CF₃Br

Abstract

Experimental data are presented for the extinction limits of pilot-stabilised turbulent nonpremixed flames of methane and propane fuels inhibited by Bromotrifluoromethane, CF₃Br commonly known as halon-1301. Compared with thermal inhibitors like N₂ and At it is shown that CF₃Br is far more effective because it acts chemically as well. The halon is mixed either with the fuel or with the coflowing air stream. Results indicate that as the turbulent mixing rates increase, the mass fraction of CF₃Br required to extinguish the flame decreases linearly with jet velocity regardless of whether the inhibitor is added to the fuel or to the air stream.

Using a detailed chemical kinetic mechanism with 34 species and 91 reactions, the structures of laminar counterflow nonpremixed flames of methane inhibited with CF₃Br are calculated. The flames are maintained at a constant, low stretch rate of a = 5s^-1 and the inhibitor is mixed either with the fuel or with the counterflowing air. It is found that when halon is mixed with the fuel, the CF₃Br decomposes in the very rich zone and yields intermediate species like CH₃, Br, CF₃H and CH₂CF₂, These species are not generated when the CF₃Br is mixed with the counterflowing air. This is because the methyl radical, CH₃, which is responsible for the formation of these intermediates, exists only in rich mixtures and does not interact with the halon when the latter issues from the air side. The final combustion products are HF, HBr, CF₂O,Br and Br₂. The peak concentrations of reactive radicals like H, OH and O generally decrease with the addition of CF₃Br and the rate of the decrease is much faster when the halon is mixed with the air stream. The peak laminar flame temperature increases initially with the addition of halon before decreasing again as the flame approaches extinction. Compared with the adiabatic temperature of a corresponding halon-CH₄-air stoichiometric mixture where the CF₃Br is assumed to act like an inert diluent, the laminar flame temperature increases by up to 220K when halon is mixed with the fuel and 160K when halon is mixed with the air. At a stretch rate of a = 5s^-1, the laminar flame extinguishes when the mass fraction of CF₃Br increases to 0.92 (0.55 volume fraction) in the fuel mixture or 0.36 (0.097 volume fraction) in the counterflowing air