The influence of geometric nozzle profile on the global properties of a turbulent diffusion flame

Abstract

A detailed comparison of the global performance characteristics of flames issuing from a long pipe and smooth contraction nozzle is presented for the first time. The measurements reveal consistently that the well-established differences in the turbulent source flow propagate far downstream to influence every performance parameter of the flame. This gives further proof to recent findings in non-reacting flows that a widely believed classical theory, arguing that differences in emerging flow do not propagate into the far field, is flawed. Measurements show that the pipe jet has a shorter flame length and smaller projected area than the contraction nozzle, implying a smaller volume and a higher rate of molecular mixing. This is explained by a new analysis of existing cold-flow data, which shows that a segregation parameter, characterising molecular mixing, is closer to zero for the pipe jet, even though the smooth contraction nozzle has a higher rate of mean decay and spread of the scalar field. The radiant fraction and emission indices are also consistent with the pipe jet having higher mixing at the molecular level. New evidence is also presented of differences in the base of the flame, with significant differences in their lift-off behaviour identified.