Dynamics of a Transversely Forced, Bluff Body Stabilized Flame

Abstract

This paper describes an experimental and modeling analysis of the response of bluff body stabilized flames to transverse acoustic waves. In these experiments, high speed video of the flame was processed to characterize the flame front response to transverse acoustic excitation at various flow conditions. Unsteady velocity field measurements were obtained with PIV to obtain the corresponding time averaged and fluctuating disturbance field. Data were taken at flow conditions with inlet temperatures between 480K and 755K, and flow velocities of 50 m/s and 100 m/s. Two different modes of acoustic excitation were applied at 450 Hz, corresponding to velocity and pressure nodes/antinodes along the combustor center. The flame front response at first increases almost linearly with downstream distance, exhibits oscillatory behavior, and then decays. The phase of the flame front response at the forcing frequency decays with downstream distance nearly linearly, and corresponds to a convective velocity of disturbances that is slightly less than the mean flow velocity. These results are similar in many respects to prior observations from longitudinally forced flames. These experimental results are compared with predictions from a flame front tracking G-equation. This equation 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 acoustic and vortical velocity, and 3) flame propagation normal to itself at the local flame speed. The validation studies using this model show qualitatively good comparison of the velocity field data and the flame response data measured experimentally. These experiments also show that flame wrinkling is induced by both acoustic and vortical disturbances, whose different propagation velocities leads to interference and oscillatory flame wrinkle amplitude characteristics.