Vortex-coupled oscillations of edge diffusion flames in coflowing air with dilution

Abstract

The unsteady characteristics of oscillating methane diffusion flames in coflowing air diluted with CO 2 in earth gravity have been studied experimentally and computationally. The measured frequency of flame flickering due to buoyancy-driven large-scale vortices was bi-modal; it jumped from ≈11 Hz to ≈15 Hz with an increase in the air velocity (at ≈11 cm/s). As CO 2 was added into coflowing air gradually, the base (edge) of the flame detached from the burner rim, oscillated at half the flickering frequency, and blew off eventually. Numerical simulations with full chemistry predicted the internal flame structure and unsteady flame behavior: flame flickering, tip separation, base detachment, oscillation, and blowoff, in good agreement with the experiment. The mechanism of the edge diffusion flame oscillation was due to a cyclic series of events: (1) flame-base detachment and drifting downstream as a result of weakening due to dilution and a momentary increase in the entrainment-flow velocity associated with the vortex evolution, (2) fuel–air mixing in widened, lower-speed, wake space between the flame base and the burner rim, and (3) flame-base propagation through the flammable mixture layer back to the burner rim. A peak reactivity spot (reaction kernel) at the edge diffusion flame controlled the unsteady behavior through its dramatic changes in characteristics from the passively drifting to (premixed-type) propagating phase during a cycle. Because a mixing time of approximately 100 ms was required before propagation was enabled, a subsequent vortex evolved and passed. Thus, the flame-base oscillation was strongly coupled with the buoyancy-driven vortex evolution and the oscillation frequency was locked-in to half the flame-flickering frequency. The results have implications in turbulent flame structure; more specifically, the local extinction–mixing–reignition processes, in that the slow molecular mixing can become rate-limiting and the edge diffusion flame structure can be significantly different, depending on the phase in the process.