Stabilization of a Premixed Flame by Shear Flow Excitation

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Abstract
The lean flarnmability limit of a premixed flame was extended by forcing the initial shear layer of the jet. It was shown that this extension depends on the forcing frequency and amplitude. The most effective control was obtained using forcing in a Strouhal number range of 0.01 to 0.06 near the most amplified frequency of the shear layer as determined by the jet exit mean velocity profile and initial boundary layer thickness. The small-scale vortices generated in the shear layer by this forcing caused reattachment of the lifted flame and held it at the jet nozzle for equivalence ratio and mean flow rate which were significantly beyond the unforced flarnmability limits. Flame intensity was augmented as well at this forcing. An adverse effect of acoustic forcing was observed when the jet was forced at the preferred mode frequency. For this condition, the generated vortices had length scales which destabilized the flame, causing intermittent flameholding and reduction in heat release. The effect of different excitation conditions on the flame structure was visualized using flash Schlieren photography and PLIF imaging for the reacting flow. It was found that generation of small scale vortices in the initial shear layer provides stable anchoring for the flame, while large scale vortices tend to induce local flame extinction between the vortices and thus destabilized the flame.