Dynamics of a Longitudinally Forced, Bluff Body Stabilized Flame

This paper describes an investigation of the response of bluff body stabilized flames to harmonic oscillations. This problem involves two key elements – the excitation of hydrodynamic flow instabilities by acoustic waves, and the response of the flame to these harmonic flow instabilities. In the present work, data were obtained with inlet temperatures from 311 K to 866 K and flow velocities from 38 m/s to 170 m/s. These data show that the flame front response at the acoustic forcing frequency first increases linearly with downstream distance, then peaks and decays. The corresponding phase decreases linearly with axial distance, showing that wrinkles on the flame propagate with a nearly constant convection velocity. These results are compared to those obtained from a theoretical solution of the G-equation excited by a harmonically oscillating, convecting disturbance. This kinematic model shows that the key processes controlling the response are 1) the anchoring of the flame at the bluff body, 2) the excitation of flame-front wrinkles by the oscillating velocity, and 3) flame propagation normal to itself at the local flame speed. The first two process control the growth of the flame response and the last process controls the decay. These predictions are shown to describe the key features of the measured flame response characteristics.