Experimental Study of the Flowfield of a Two-Dimensional Premixed Turbulent Flame

Abstract

A turbulent reacting shear layer in a premixed propane/air flow has been studied in a two-dimensional combustor, with a flame stabilized behind a rearward facing streamlined step. Spark shadowgraphs show that in the range of velocities (7.5-22.5 m/s corresponding to Reynolds numbers of 0.5xl0 4-1.5xl04 cm1) and equivalence ratios (0.4-0.7) studied, the mixing layer is dominated by Brown-Roshko type large coherent structures in both reacting and nonreacting flows. High-speed schlieren movies show that these eddies are convected downstream and increase their size and spacing by combustion and coalescence with neighboring eddies. Tracing individual eddies shows that on average eddies accelerate in the reacting shear layer as they move downstream, with the highest acceleration close to the origin of the shear layer. Combustion is confined to those large structures which develop as a result of vortical action of the shear flow. On the average, the reacting eddies have a lower growth rate than nonreacting eddies. A turbulent boundary layer created by means of a tripping wire upstream of the edge of the step virtually eliminates the large coherent structures in the shear layer, while for the case in which the wire could not trigger the transition to turbulence, the large coherent structures dominated the reacting and nonreacting flows.