Abstract
Direct numerical simulations were conducted to investigate the effect of two parameters, density ratio and laminar flame speed, on the conditions of the onset of local extinction and blow-off of lean premixed flames, stabilized on a meso-scale bluff-body in hydrogen-air and syngas-air mixtures. A total of six simulation cases were considered as isolated comparison of the two parametric effects of the fluid dynamic instability and flame time scale. For all cases under study, the general flame development towards the blow-off limit showed a sequence of five distinct modes, with possible cyclic patterns among the different modes for a range of velocity conditions. The onset of local extinction was observed during the asymmetric vortex shedding and vortex street mode. As the density ratio is decreased, the flow inunder review stability is promoted through the increased sinuous mode, and such behavior was properly scaled by the Strouhal number. Although the blow-off velocity is altered by the fluid dynamic effects, the condition for the onset of local extinction and blow-off was mainly dictated by the competition between flow residence time associated with the lateral flame motion and ignition delay of the local mixtures. Time scale analysis supported the validity of the findings across all the cases investigated. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.