Suppression of nonpremixed flames by fluorinated ethanes and propanes

Suppression of methane/air and propane/air nonpremixed counterflow flames, and n-heptane and methanol cup burner flames by fluorinated hydrocarbons was investigated. Four fluorinated ethanes, 10 fluorinated propanes, four bromine- or iodine-containing halons, and the inert agents CF4, SF6, and N-2 were tested in some or all of the flames. Laser Doppler velocimetry (LDV) determinations of peak velocity gradients in the oxidizer flow of the counterflow flames were found to be linearly correlated with the expression for global strain rate derived for plug flow boundary conditions. This correlation was used to estimate strain rate values at extinction. The bromine- or iodine-containing agents are more effective on a molar basis than the fluorinated propanes, followed by the fluorinated ethanes, and finally SF6, CF4, and N-2. Agent effectiveness increases with the number of CF3 groups present in the agent molecular structure. Numerical investigations of the flame speed reduction of methane/air mixtures doped with either CHF2CHF2 or CF3CH2F predict that the latter is the better agent, in accord with experimental observations. Chemical contributions to suppression account for less than 35% of the total suppression offered by fluorinated hydrocarbons not containing bromine or iodine. At strain rates below 100 s(-1), suppression effectiveness rankings in methane and propane counterflow flames are similar to those obtained in n-heptane and methanol cup burner flames. Methanol Barnes are more difficult to extinguish than the alkane flames investigated, particularly with the chemical agent CF3Br. (C) 2000 by The Combustion Institute.