Nonpremixed combustion in turbulent mixing layers Part 1: Flame characteristics

Experiments on recirculating diffusion flames are reported, the focus of the investigation being the interaction between the combustion process and the coherent structures formed naturally in the turbulent mixing layer bounding the recirculation zone. The experimental configuration was that of a rearward-facing step with distributed injection of gaseous fuel into the recirculation zone. Over a wide range of conditions the primary combustion activity was found to be located almost entirely in the mixing layer, commencing at some distance from the step as a lifted flame. In this regime the flame was produced by the quasi-periodic inflamation of coherent structures formed in the non-reacting part of the mixing layer. These were similar to the structures formed in isothermal mixing layers and appeared to have undergone the mixing transition well upstream of the point at which they inflamed. When the lifted flame was stabilized close to the step attachment occurred intermittently over part of the span. The periods during which the flame was attached to the step in this way were characterized by an absence of coherent-structure formation. The average lift height of the flame was sensitive to the chemical composition of the fuel and the pattern of the fuel distribution within the recirculation zone but was also found to be a function of the ratio of the fuel flow rate to the air velocity. Analysis of this behavior suggests that the stabilization mechanism was controlled by mixing-related rather than kinetic factors. (C) 2000 by The Combustion Institute.