Combustion inhibition and enhancement of premixed methane–air flames by halon replacements

Apparent combustion enhancement by some halon replacement fire suppressants (proposed for use in aircraft cargo bays) has been observed in full-scale, constant-volume tests at the FAA. In order to explore the phenomena, laboratory-scale constant-volume combustion experiments were performed. The maximum explosion pressure and burning velocity were measured for methane–air flames with added CF3Br (Halon 1301), C6F12O (Novec 1230), C3H2F3Br (2-BTP), and C2HF5 (HFC-125). The explosion pressure, for initially stoichiometric flames, was increased mildly (up to 11% and 6%) with C6F12O and C2HF5 added at low concentrations, while at lean conditions (Φ = 0.6), it was increased about 50% for added C6F12O, C3H2F3Br, or C2HF5, at agent volume fractions Xa = 0.02, 0.03, and 0.06. The burning velocity for initially stoichiometric flames was always decreased with addition of any of the agents, whereas, for the lean conditions, it increased with added C6F12O or C2HF5 (32% and 14%, at Xa = 0.01 and 0.03). Burning velocities at higher initial pressure (3 bar) and temperature (400 K) showed lower inhibition effectiveness (than at ambient conditions) for the stoichiometric flames, and larger enhancement for the lean flames (and the effect was due primarily to the temperature increase). CF3Br did not increase the explosion pressure or burning velocity for any of the tested conditions. Equilibrium calculations were used to interpret the experiments. The present work is consistent with the FAA results and previous analysis of the full-scale tests.