Structure of a Controlled Ducted Flame

The structure of self-excited and controlled ducted flames was studied by imaging the CH emission and analyzing the pressure and CH intensity time variation. Self-excited combustion oscillations occur when the flame interacts with the large-scale vortices which are excited in the shear layer by the acoustic forcing at the duct resonance modes. The periodic heat release produced by the combustion inside the vortices further excite the duct acoustics. The fuel to air mixture ratio is shown to have an important effect on the interaction between the flame and vortices. The effect on the flame structure of pressure and CH control systems is described. The transition from controlled conditions lo uncontrolled and vice versa is visualized. The roll-up of coherent structures dominating the flame in the self-excited conditions is disrupted by the controller; the shear layer is forced at the exact phase necessary to cancel the perturbation due to the duct acoustic pressure. This is done more effectively with the pressure controller rather than the CH controller such that the flame structure lacks any organized structure. The Rayleigh integral was forced to be zeroed in the controlled state. It is shown that the suppression is effective when the system operates at a certain range of time delay, gain and filtration. Outside this range vortex roll-up in the shear layer can occur at smaller wavelength, subsequently exciting a high frequency combustion instability.