Upstream flow dynamics of a laminar premixed conical flame submitted to acoustic modulations
Résumé
This study is concerned with the response of conical flames to acoustic modulations. It deals with the dynamics of the velocity field in the fresh gases feeding the flame. Experiments are carried out to determine the gain and phase shift between the excitation signal and the axial velocity signal. This information, combined with PIV data, is used to identify the propagation mode in the fresh stream. Experiments indicate that three ranges can be defined based on a Strouhal number St involving the burner diameter and the upstream flow velocity. When this number is sufficiently low (St <= 1), the response consists in a convective wave featuring a phase velocity close to that of the mean flow. As St is augmented (1 < S-t < S-tc), where S-tc depends on the flame geometry, the phase difference between the velocity oscillation and the imposed signal nearly vanishes in a finite region adjacent to the burner exhaust indicating that the perturbation propagates at the speed of sound. Further away from the burner, velocity perturbations exhibit convective features again. In the third frequency range, corresponding to higher modulation frequencies (S-t >= S-tc), velocity perturbations are dominated by acoustics in most of the experimental domain. It is shown that this behavior results from the upstream influence of the flame wrinkling. The region of influence may be deduced by considering the velocity potential associated with the flame motion. When this perturbation potential takes large values, the flow is dominated by the convective wave. This suitably reproduces experimental observations.