Dynamics of confined premixed flames submitted to upstream acoustic modulations

Abstract

Dynamics of inverted conical flames anchored on a central bluff-body is investigated in confined and unconfined conditions. The flames are modulated by a driver unit placed upstream. Flame dynamics is strongly coupled to the evolution of large scale structures induced by the modulation and shed from the injector nozzle lip. To study the influence of confinement on the flame motion, the response is measured for different diameters of the duct bounding the flame. This induces changes in the confinement parameter defined as the ratio of the injection duct diameter to the combustor diameter. Natural emission imaging, transfer function measurements and PIV data serve to determine the effect of confinement on the flame response for a set of modulation amplitudes. It is found that when the confinement ratio is low, the time delay between the velocity signal at the burner outlet and heat release perturbations remains essentially constant indicating that the flame is weakly affected by the presence of the wall. As the confinement ratio is increased, the time delay and the gain of the flame transfer function are modified. It is found that interaction with the wall strongly influences the dynamics of the flame tip, the flame wrinkling geometry in the vicinity of the boundary, and to a great extent the amplitude of response. A numerical simulation provides additional information on the fresh stream dynamics inside the tube, and on the dynamical interactions between the flow and the flame.