Instantaneous Flame Anchor Measurements Behind a Rearward-Facing Step

Abstract

The physical mechanisms governing flame anchoring have been examined in an effort to extend the range and maneuverability of compact, low-drag, air-breathing engines. Experiments were performed burning premixed methane and air in a planar dump combustor using reacting-flow particle image velocimetry as the primary diagnostic. Instantaneous two-dimensional images and vector fields were studied to determine changes in the fluid-chemical interactions of the shear layer as flame anchorability became more robust. Conditional averages of combustion products directed toward incoming reactants were evaluated to establish the connection to self-sustained combustion. A lean mixture of methane―air was used as a baseline, and the equivalence ratio and near-field counterflow were varied to affect anchorability. Dilatation was calculated as a marker for heat release and three-dimensionality. Operating points exhibiting a strong flux of products into reactants via a single coherent structure positioned downstream of the step were found to be most stable for flame anchoring. However, a counterflow level equal to 6.2% of the primary stream by mass was found to match the characteristics of a single coherent structure while maintaining multiple structures in the mixing zone, effectively increasing heat release rates at a lower equivalence ratio.