The influence of spanwise nonuniformity on lean blowoff in bluff body stabilized turbulent premixed flames

Abstract

New observations are reported of the processes leading up to lean blowoff (LBO) in a two-dimensional confined bluff body flame by visualizing LBO from a spanwise perspective. Time-resolved OH* chemiluminescence imaging simultaneous with particle image velocimetry are used to characterize the spanwise structure and dynamics. For reacting conditions, newly observed large-scale corner vortex structures (CVS) are present downstream of the bluff body inside the recirculation zone near the span walls. The CVS are orthogonal to the conventional recirculation zone observed in a bluff body flow, and the CVS are absent under non-reacting conditions. As the equivalence ratio is decreased, the vorticity and circulation of the CVS increase, contributing to significant nonuniformity across the spanwise direction both inside the recirculation zone and on the surrounding combustion. Approaching LBO, increased quantities of vanishing combustion near the span walls implies increased quantities of reactants entering and diluting the recirculation zone. Since cold reactant entrainment into the recirculation zone has been suggested as a possibly important LBO contributing factor, the current study suggests that this wake disruption mechanism begins much earlier than previously thought. The CVS are also the final regions of the flow to extinguish before LBO is complete, emphasizing that LBO is a highly three-dimensional process. A bluff body with an aspect ratio three times larger was also investigated, and the LBO process is observed to remain qualitatively similar with significant spanwise nonuniformity and containing the CVS. This paper intends to improve the three-dimensional understanding of two-dimensional bluff body stabilized premixed flames approaching LBO.