Experimental assessment of the acoustic response of laminar premixed Bunsen flames

The response of a premixed laminar conical flame to periodic perturbation of the flow was analyzed. The idea was used to imitate acoustic flow excitation by an oscillation of the flame anchoring ring. In this case, the flow field is free from the ring vortices and other coherent structure formation typical of the harmonically excited jet pattern. Under this simplified perturbation condition the flame cone kinematics is studied and compared with the flame response to acoustic flow excitation. The heat release rate response transfer function (TF) for a foot-perturbed flame is measured and found to be close to a theoretical TF derived on the basis of the kinematical flame-tracking approach (G-equation) for a uniformly oscillating flow. However, the TF of the jet-perturbed flame deviates strongly from the TF of the anchoring ring-perturbed flame and demonstrates a typical "constant time lag" behavior. A TiO2 smoke tracing of the flow field shows the presence of a convective, sinuous perturbation of the flow core inside the flame cone when the flame is excited by the jet oscillation. In the case of the anchoring-rim-perturbed flame, such a back influence of the perturbed flame on the upstream flow field is weak. Experiments in which the anchoring rim of the flame oscillates in the transverse direction indicate that the flame base in-out radial motion can be a cause of the convective flow perturbation origin. Requirements for the extension of the kinematical type model of the flame response are formulated. (C) 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.