Lifted and reattached behaviour of laminar premixed flame under external acoustic excitation

The flame chemiluminescent emission fluctuations and the vortex structure of the lifted jet flame under acoustic excitation were studied in this investigation. By employing high-speed visualization and DFCD (Digital Flame Colour Discrimination) image processing method, the fluctuation of the instantaneous mixture fraction has been found highly correlated with the lifted height variations. It has been observed that during the flame drifting to downstream, there is no obvious shifting on the mixture fraction. However, when the flame travels back to upstream, the fuel mixture has been evidently diluted. In addition, the stabilisation mechanism can be further explained by analysing the velocity fluctuation of the vortices in the shear layer via PIV. Measurements show that, the turbulent stretching at the shear layer generated by the excitation leading to the flame lift-off. On the other hand, the Kelvin-Helmholtz vortices in the unburn part play a key role in preventing flame lift-off. But, the excessive external acoustic excitation leads to blow-off due to over dilution and increased lifted height.