Nonpremixed combustion in turbulent mixing layers Part 2: Recirculation, mixing and flame stabilization

Further data are presented on the recirculating diffusion flames whose structure and stability were characterized in Part 1 of this study. Special features of the experimental arrangement used included the relative thinness of the boundary layer of the separating air stream and the distributed nature of the gaseous fuel injection into the recirculation zone. These resulted in a situation in which the mixing layer bounding the recirculation zone contained processions of coherent structures with well mixed turbulent cores which ignited individually at some distance from the step to form a lifted flame. A close proportionality was found between the amount of air entrained into the recirculating flow and the lift height of the flame, implying that significant entrainment by the mixing layer occurred only upstream of the ignition point. The amount of the air entrainment and direct Raman measurements both indicated that the mixture within the coherent structures was in general non-stoichiometric at the point of ignition and that its equivalence ratio was a function of the lift height of the flame and the spatial distribution within the reverse flow of the freshly injected fuel. This mixing behaviour and the lift-height characteristics presented in Part 1 are both rationalized in terms of the effect of the lift height on the recirculation ratio of the system and a flame stabilization mechanism which depended upon the temperature distribution created in the reverse-flow region by the mixing between the freshly injected fuel and the hot recirculated fluid. The prospects for exploiting these patterns of behaviour as a possible aid to the control of NOx formation in practical combustion systems are discussed. (C) 2000 by The Combustion Institute.