Effect of inhibitor concentration on the inhibition mechanism of fluoromethanes in premixed methane-air flames

The mechanisms of inhibition of premixed methane-air flames in the presence of difluoromethane, trifluoromethane, and tetrafluoromethane are studied. The chemistry of these agents is expected to be similar to that of agents which may be used as replacements for CF3Br. The burning rates of premixed methane-air flames stabilized on a Mache-Hebra nozzle burner are determined using the total area method from a schlieren image of the flame. The three inhibitors are tested over an initial mole fraction from 0 to 8% at nominal values of the fuel-air equivalence ratio, phi, equal to 0.9, 1.0, and 1.1. The measured burning rate reductions are compared with those predicted by numerical solution of the mass, species, and energy conservation equations employing a detailed chemical kinetic mechanism recently developed at the National Institute of Standards and Technology (NIST). Even in this first test of the kinetic mechanism on inhibited hydrocarbon flames, the numerically predicted burning rates are in excellent agreement for CH2F2 and CF4 and within 35% for CF3H. The effects of inhibitor concentration on the decomposition pathway of the inhibitors and on the H, O, and OH radical production and consumption rates are discussed. The modified decomposition pathway and the reduced radical consumption explain the diminishing effectiveness of CF3H and CH2F2 at higher concentrations.