Laminar Flame Dynamics of Coaxial Methane-Air Jets Under Acoustic Forcing*

The present experiments investigate the response of coaxial methane-air laminar jet diffusion flames exposed to transverse forcing within a cylindrical acoustic waveguide. High speed imaging of the forced flame dynamics was analyzed via proper orthogonal decomposition (POD) to study transitions in the combustion process. With increasing amplitude of forcing, the flame's response transitioned from sustained oscillatory combustion (SOC) to periodic lift-off and reattachment (PLOR) and eventual blow-off (BO). In the SOC regime, the flame oscillated primarily at the applied forcing frequency (f_a=332Hz), while during PLOR, the periodic response was dominated by low frequencies corresponding to the much slower cycles of periodic lift-off (f_l=11 to 23 Hz), creating differing POD mode coefficient maps. The effect of different outer to inner jet velocity ratios (R=0, 0.3, and 1) was investigated for different wall thicknesses (δ). It was found that higher R values can aid in flame attachment, although larger absolute velocities can cause the flame to detach and blow-off. A thicker inner tube wall also appeared to promote reattachment, suggesting the likelihood of wake-shear layer interactions with different natural stability characteristics.