Response of Rod Stabilized Flames to Harmonic Excitation: Shear Layer Rollup and Flame Kinematics

Abstract

The objective of this research is to understand the dynamics of acoustically forced, bluff body flames. Particular attention is focused on the interaction of the flame with the separated shear layer of the bluff body. It is shown that the amplitude of flame front oscillations grows spatially downstream for approximately three convective wavelengths, λ c (given by U o /f, denoting the free stream velocity and forcing frequency, respectively), then reaches a maximum and decays. This flame response depends upon two key processes: the excitation of the flame by shear layer instabilities that are initiated by the acoustic forcing, and the kinematic response of the flame to this forcing. It is suggested that the flame excitation can be parametrized by St θ , the Strouhal number based on the momentum thickness of the shear layer. In turn, kinematic processes are responsible for the basic shape of the flame response. In the bluff body nearfield, the flame response grows with downstream distance due to the flame anchoring “boundary condition”, which prevents significant flame movement near the flame base. Farther downstream, the flame response reaches a maximum and then decays due to the action of flame propagation normal to itself, which acts to smooth out the wrinkles generated by the harmonic flow forcing.