The Dynamics of Flame Flicker in Conical Premixed Flames: An Experimental and Numerical Study

Flame tip flicker in premixed conical laminar methane/air flames has been investigated using a coupled experimental and numerical approach. A time-dependent, multidimensional, Low Mach number Combustion Simulation (LMCS), an adaptive mesh refinement technique, has been adopted to handle the large range of scales characteristic of buoyantly driven phenomena. These simulations, using a reduced chemical scheme, are compared with experimental measurements made in a 20 mm diameter ring-stabilized flame with an inlet flow of 0.73 m/s and equivalence ratio, φ=0.8. Phase-averaged particle image velocimetry (PIV) provided two-dimensional velocity data. The simulations captured the dynamics of the flame tip oscillation with remarkable fidelity. Time and phase-averaged velocity fields, centerline axial velocities and the flame flicker frequency (10.2 Hz) show good agreement between the numerical and experimental results. The flame tip oscillation is caused by a competition between the pressure fields associated with the predominately radial motion of the burnt gases near the flame front and the rotating vortex motion. The phase-averaged velocity data provided an excellent means for following the evolution of the flow field. Because of the very satisfactory agreement between experiment and simulation a more detailed investigation of the flicker process by means of the much more extensive simulation data was performed. Two types of vortex evolution were observed as alternate vortices were entrained leading to regular variations in the maximum flame tip height. As the inlet flow velocity increases these vortex interactions produce progressively more chaotic flame tip motion. This study shows that a powerful numerical technique coupled with sophisticated experimental diagnostics can lead to significantly improved insight into large-scale combustion/buoyant phenomena.