Nonpremixed combustion in turbulent mixing layers. II - Recirculation, mixing and flame stabilization

Further data are presented on the recirculating diffusion flames whose structure and stability were characterized in Part I 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. The prospects for exploiting these patterns of behavior as a possible aid to the control of NO(x) formation in practical combustion systems are discussed. (Author)