Observations on the kinetics of hydrogen-chlorine flames

Abstract

A detailed study was undertaken of the hydrogen-chlorine flame, in order to obtain reliable burning velocity data and to contribute to the resolution of the question regarding the existence of an energy chain in the propagation reaction. Burning velocities were measured by a schlieren-cone technique for stoichiometric to rich mixtures (140% of stoichiometric hydrogen) diluted with (from) 20% to 50% of nitrogen. The composition regime, accessible to reliable measurement, was circumscribed by the strong diffusional effects encountered in these flames, which sharply limited their range of stability on nozzle burners. The various types of instability observed are illustrated and discussed. A maximum burning velocity of 235 cm/sec was obtained for 120% of stoichiometric hydrogen at the lowest nitrogen dilution. Results found in this work are compared with others reported in the literature, which differ widely among themselves, and reasons for these discrepancies are discussed. The experimental data were analyzed in terms of some current approximate flame theories, and they are shown to be incapable of accounting for the variation of burning velocity of hydrogen-chlorine flames with nitrogen dilution. It appeared that no valid inferences regarding the kinetics of the hydrogen-chlorine flame could be drawn from such analyses of burning velocity data. No evidence was found to support the hypothesis of an energy chain leading to a branching reaction. To obtain further information regarding the mechanisms of reaction in hydrogen-chlorine flames, the effect of additives (CCl 4 , CH 3 Cl, CH 2 Cl 2 , CH 4 , SnCl 4 ) on their burning velocity was examined. Promotional effects were strongest with carbon tetrachloride, as little as 0.1% of this additive produced a 50% increase of burning velocity in the mixture of maximum flame speed, and decreased progressively in passing down the chlorinated methane series. Stannic chloride also exerted a large promotional effect, though its mode of action appeared to be somewhat different from that of the other additives. Vapor-phase chromatographic analysis of the product gases of mixtures containing carbon tetrachloride indicated that the additive was completely decomposed in passing through the flame front. Kinetic schemes to explain the promotional effect are suggested and analyzed. None of them are unequivocally supported by the present work.