Active control of a lifted flame is investigated using a coaxial nozzle with magnetic flap actuators arranged on the inner
periphery of the annular nozzle. Near-field vortical structures of the methane / air coaxial jet are manipulated by
introducing disturbances directly to the initial shear layer. Through the manipulation, we can improve flame stability
and flexibly control the liftoff height. It is found that the large-scale vortical structures play a dominant role in the flame
stabilization, and its spatio-temporal evolution is examined with the aid of PIV and LIF to elucidate the control
mechanism. By introducing flap motion driven with a saw-wave signal, we can force the outer shear layer to roll up into
strong vortices in synchronization with the flaps. When the flapping Strouhal number is unity, the lifted flame is
anchored at x / Do ~ 1.5. The strong vortices induced by the flaps produce a blob of flammable mixture, which has
velocity smaller than the flame speed. The possible stabilization mechanism is that the time period of the premixture
supply is balanced with the consumption time of the premixture at the flame base. On the other hand, when the jet is
manipulated by a square-wave signal, the lifted flame is located stably at x / Do ~ 4, which is downstream of the inner
potential core. It is found that vortical structures in the shear layers break into turbulence close to the nozzle exit. The
possible mechanism of the flame stabilization is that the flame propagating upstream is undisturbed due to the absence
of intermittent passage of large-scale vortices.

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