Inhibition of premixed methane-air flames by fluoromethanes

Abstract

This paper presents the first calculations and measurements of the burning velocity of premixed hydrocarbon flames inhibited by the three one-carbon fluorinated species CH 2F 2, CF 3H, and CF 4. The chemistry of these agents is expected to be similar to that of some agents that may be used as replacements for CF 3Br, 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, φ. 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 values of φ 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 CH 2F 2 and CF 4 and within 35% for CF 3H. The numerical results indicate that the agents CF 3H and CH 2F 2 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 CF 4 is consumed in the main reaction zone and it reduces the burning velocity primarily by lowering the final temperature of the burned gases.