Mechanism of Instabilities in Turbulent Combustion Leading to Flashback

Unstable turbulent flow processes were studied experimentally in a combustion tunnel of a slender rectangular cross section provided with a rearward-facing step to simulate the action of the bluff-body flame holder under planar flow conditions. The flow pattern was recorded by means of high-speed schlieren cinematography taken across side walls that were fitted for this purpose with quartz windows, while the photographs were synchronized with pressure transducer measurements. As the flow rate of fuel was increased toward the lean flashback limit, three modes of instabilities were observed: 1) humming—a significant increase in the amplitude of the vortex pattern in the turbulent mixing zone; 2) buzzing—a large-scale oscillation of the flame up and down across the test section, eventually obliterating the vortex pattern of the mixing zone; 3) chucking—a cyclic reformation of the flame that appears as if it were periodically spilled over the edge of the step. The latter leads to flashback—the actual lifting of the flame from the edge of the step and its propagation upstream, a process that upon a certain penetration into the incoming flow is terminated abruptly, giving rise to an augmented chucking cycle. The mechanism of these phenomena is ascribed to the action of vortices in the recirculation zone and their interactions with the trailing vortex pattern of the turbulent mixing layer behind the step. Simplified configurations of vortices involved in such interactions are illustrated by sketches deduced from a numerical modeling analysis.