Parametric study of the transfer function of perturbed bunsen flames

The results of Transfer Function (TF) measurements for premixed methane/air laminar Bunsen flames are presented. The frequency dependence of the flame heat release rate (OH chemiluminescence) response to acoustic velocity excitation was studied as a function of the flow parameters (velocity, mixture equivalence ratio, mean velocity profile) and of the burner parameters (tube diameter, burner wall thickness, wall material, tube opening geometry). The heat release TF has a particular dependence on frequency. It exhibits a behavior typical for a system with time delay and for a system with a low-pass filter behavior. Some additional effects lead to a small but weakly frequency dependent part of the TF. The TF can be presented in a universal form with only a few fitting parameters. A relation between the TF time delay, the gas velocity and the flame height is also found. This relation indicates the availability of some convectively (with the mean flow velocity) transported perturbation. The physical mechanism of convective time delay origin is discussed.