Oliver Meier

Flame Inhibition by CF3CHCl2 (HCFC-123)

A kinetic model is suggested for hydrocarbon/air flame propagation with addition of hydrochloroflurocarbon (HCFC) fire suppressant, encompassing the combined chemistry of fluorine- and chlorine-containing species. Calculated burning velocities using the kinetic model are in good agreement with available experimental burning velocity data for CF3Cl, CF2Cl2, or CFCl3 added to CO/H2/O2/Ar flames. The agent CF3CHCl2 is more effective than C2HF5, and reaction pathway analysis shows that the inhibition effect of chlorine reactions is greater than that of fluorine. The main reactions of the chlorine inhibition cycle are H+HCl=H2+Cl, OH+HCl=H2O+Cl, Cl+CH4=HCl+CH3, Cl+HCO=HCl+CO, and Cl+CH2O=HCl+HCO. The inhibition effect of CF3CHCl2 is largely the result of competing reactions of chlorine-containing species with hydrogen (and other radical pool) species, decreasing the rate of the chain-branching reaction H+O2, with additional effects from substitution of the reactive chain-branching radicals for less reactive fluorine- and chlorine-containing radicals.

Thermodynamic Analysis of Suppressant-Enhanced Overpressure in the FAA Aerosol Can Simulator

The fire suppressant CF3Br has been banned for most applications but it is still used in come critical applications for which suitable replacements have not yet been found. One such application is the suppression of cargo-bay fires in aircraft. Recently, the agents C2HF5 (pentafluoroethane, HFC-125), and bromotrifluoropropene (C3H2F3Br, 2-BTP) have been evaluated in a mandated Federal Aviation Administration (FAA) test, in which a simulated explosion of an aerosol can must be suppressed by the agent. Unfortunately, unlike CF3Br, either agent, when added at approximately one-half their inerting concentration, created a higher over-pressure in the test chamber than in tests with no agent present (thus failing the test). Similar combustion enhancement has been described in other experiments for certain conditions; however, explanation of the phenomena is lacking. As a first step in understanding this surprising result, the thermodynamics of the chemical systems are examined to predict the over pressure. For all of the cases examined, the over-pressure was predicted well by assuming that the fuel-agent-air ratio is that which produces the peak temperature, or peak CO2. The details of the three chemical systems are examined to provide insight into the anomalous behavior.