Low-Frequency Oscillation of a Non-Premixed Flame on a Bluff-Body Burner

Abstract

The flickering characteristics of a non-premixed flame interacting with von Kármán vortex street on a bluff-body burner were investigated experimentally with varied velocities of the central fuel jet and annular air stream. Flow visualization in terms of a shadowgraph, planar laser-induced fluorescence of hydroxyl, and Mie scattering, which was obtained via PIV used to analyze the flow field, indicated that the flame flickering was caused by periodically growing, toroidal vortices outside the flame surface that were much larger than those observed in buoyant jet flames. In contrast to a weak dependence of the latter on the co-flowing air velocity and the central fuel velocity, the present case was strongly correlated with the von Kármán vortex street shed from the rim of the air stream that produced vortex pairing downstream, and was coupled with buoyancy effects rendered by the pilot flame in the recirculation zone as well as the central jet flame. The flame surface was consequently wrinkled substantially by the strong flame-vortex interaction, leading to frequent local extinction. Furthermore, due to a significant extension of the residence period, the frequency of oscillation was greatly decreased when the velocity of air flow was large enough to introduce a recirculating flame behind the bluff body. It was then increased when the air velocity increased. In contrast, a maximal frequency of oscillation was generated when the fuel velocity was increased, which occurred as the flame mode altered from a recirculation zone flame to a jet-dominated flame, for which the central jet flame was about to penetrate the recirculation zone. This frequency was caused by the intense interactions among the recirculated air, the central jet flame, and small eddies convened around the dome of the recirculation zone. The global behaviors of flame flickers could be further summarized using the Strouhal number and the Reynolds number of fuel jet in a relation of (St)(Re c ) = C, whereas the constant C was modified by the Reynolds number of air flow.