Active Control of Lifted Flames with Arrayed Micro Actuators

Active control of a lifted flame was achieved using a coaxial nozzle with micro magnetic flap actuators arranged on the inner periphery of the annular nozzle. The spatio-temporal structure of the flame was studied by using a laser-induced fluorescence method and a particle image velocimeter, while the exhaust gas was examined with an FT-IR gas analyzer. Near-field vortical structures in the coaxial jet are manipulated by introducing disturbances directly to the initial shear layer. They were generated by the flap actuators driven with square-or saw-wave signals. The lifted flame modified by the square-wave signal disturbances are stabilized at the downstream of the inner potential core, because the early breakdown of the coherent structures into turbulence is promoted. Mixing enhancement upstream of the flame base shortens the flame length and the emission of nitrogen oxides is decreased. On the other hand, the lifted flame controlled by the saw-wave signal disturbances can be anchored near the nozzle exit by the effect of shedding of large-scale vortices synchronized with the flapping motion. These vortices periodically supply premixture blobs to the flame base in such a way that Damkohler number, which is defined as the ratio between the supply and consumption time spans of the blobs, becomes approximately unity.