The structure of flat, counter-flow diffusion flames

Abstract

It is shown that the establishment of a large flat diffusion flame in the counter-flow régime of opposed jets of two gaseous reactants could very considerably extend the range of applicability of flame-kinetics studies by structure analysis. Suitable flames are stabilized and their characteristics and behaviour described. The flow patterns, spectrum , refractive index fields, temperature distributions and gas composition at a few selected points are studied for ethylene flames by methods including the use of thermocouples, sodium line reversal, illuminated particle tracks, interferometry and gas chromatography. The aerodynamic and thermal structures are analyzed to yield the distribution of the rate of heat release per unit volume. The following are among the conclusions: two stagnation points and two planes which particles cannot cross occur when the centres of the reaction and aerodynamic systems are made to coincide—this can be prevented by displacing the plane of stoichiometry from that of aerodynamic symmetry; the radial component of flow velocity is proportional to radial distance in the central parts. Isotherms are parallel to each other and to the flame, away from the edges, but maximum temperatures occur at the periphery, at least for some flames. The heat release profile shows regions where small amounts of heat are absorbed; C 2 H 6 and H 2 appear on the oxidant side of the flame. The most significant observation is that the zone of heat release is about ten times wider than would be expected of the equivalent pre-mixed flame and this makes the method applicable to the study of faster flame reactions.