Inhibition of premixed methane-air flames by fluoromethanes

This paper presents the first calculations and measurements of the burning velocity of premixed hydrocarbon flames inhibited by the three one-carbon fluorinated species CH2F2, CF3H, and CF4. The chemistry of these agents is expected to be similar to that of some agents that may be used as replacements for CF3Br, so that studying their behavior in methane flames provides an important first step towards understanding the suppression mechanism of hydrocarbon fires by fluorinated compounds. The burning velocity of premixed methane-air flames stabilized on a Mache-Hebra nozzle burner is determined using the total area method from a schlieren image of the flame. The inhibitors are tested over a range of concentration and fuel-air equivalence ratio, phi. The measured burning velocity reduction caused by addition of the inhibitor is compared with that predicted by numerical solution of the 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 velocity reductions for methane-air flames with val,les of phi of 0.9, 1.0, and 1.1 and inhibitor mole fractions in the unburned gases up to 0.08, are in excellent agreement for CH2F2 and CF4 and within 35% for CF3H. The numerical results indicate that the agents CF3H and CH2F2 are totally consumed in the flame and the burning velocity is reduced primarily by a reduction in the H-atom concentration through reactions leading to HF formation. In contrast, only about 10% of the CF4 is consumed in the main reaction zone and it reduces the burning velocity primarily by lowering the final temperature of the burned gases.