Activation energies of global reactions in laminar flame propagation

Abstract

A survey of experimentally determined values for the laminar burning velocity of premixed, laminar hydrocarbon-air and hydrogen-oxygen-nitrogen flames shows considerable scatter between results obtained by different investigators. Within the limits of experimental reproducibility of burning velocities, it is possible to correlate measured burning velocities on the assumption that a single rate-controlling or global reaction exists. Correlation of experimental data by use of a global reaction has been made on the basis of two simplified relations for the laminar burning velocity. The first relation was obtained by the use of an intuitive argument based on the idea that the laminar burning velocity is proportional to the square root of a second order reaction rate, with the rate-controlling reaction step depending on the first power of the initial fuel and oxygen concentrations. For lean mixtures the global activation energy was found to have a value of about 22 Kcals/mole, and for rich mixtures it has a value of roughly 56 Kcals/mole. The second expression for the calculation of laminar burning velocity is based on a theoretical equation derived by Semenov for a thermal mechanism controlling flame propagation in rich hydrocarbon-air mixtures. Application of this relation leads to the conclusion that a global reaction with an activation energy of 87 Kcals/mole correlates rich hydrocarbon-air burning velocities, well within the limits of reproducibility of experimental data. A study of the effect of the concentration of N[subscript 2] in the oxidizing mixture shows an apparent dependence of the global activation energy on the amount of diluent gas. This observation suggests that although good correlation of experimental data has been obtained, by using the concept of a global activation energy, the results are not of fundamental significance but should be regarded simply as useful empirical methods for correlating experimental data. It is possible that additional theoretical work will lead to a modified expression for the laminar burning velocity, which not only permits correlation of experimental data, but also yields a global activation energy which is independent of the concentration of inert diluents.