Inhibition of non-premixed hydrogen flames by CF 3Br

Abstract

Experimental and numerical studies are performed to elucidate the fundamental chemical mechanisms by which CF 3 Br inhibits non-premixed hydrogen flames. These studies are motivated by previous work, which shows that CF 3 Br and its decomposition products inhibit hydrocarbon flames primarily by reacting directly with the radicals and by depleting radicals in the region where hydrogen and carbon monoxide are oxidized to form water and carbon dioxide. The elementary reaction CF 3 Br+H=CF 3 +HBr plays a prominent role in flame inhibition, but there are considerable uncertainties in the value of the rate parameters for this reaction. In view of these uncertainties, the rate of this reaction is measured directly by employing the flash-photolysis resonance fluorescence (PF-RF) technique and is compared with that predicted using transition state theory based on ab initio calculations. Predicted and measured rate parameters agree well but differ significantly from those used in previous studies. The improved rate parameters are tested by conducting numerical computations and experiments on flames stabilized between two counterflowing streams. The fuel stream is a mixture of hydrogen and nitrogen and the oxidizing stream consists of air and CF 3 Br. The strain rate at extinction is measured and given as a function of the concentration of CF 3 Br in the oxidizing stream. Numerical calculations are performed at conditions identical to those used in the experiments using detailed chemistry. Two different chemical kinetic mechanisms are used. The chemical kinetic mechanism describing the oxidation of the fuel is the same in both mechanisms, but the inhibition chemistries are different. One of the mechanisms has not been tested before on non-premixed flames. One mechanism is found to generally overpredict and the other to underpredict the inhibiting effect of CF 3 Br. The degree of overprediction and underprediction is evaluated using asymptotic theory. The differences between the mechanisms are discussed.