Stabilization mechanisms of swirling premixed flames with an axial-plus-tangential swirler
Résumé
The stabilization of premixed flames within swirling flows produced by an axial-plus-tangential swirler is investigated in an atmospheric test rig. In this system, flames are stabilized aerodynamically away from the solid elements of the combustor without help of any solid anchoring device. Experiments are reported for lean CH4/air mixtures, eventually also diluted with N2, with injection Reynolds numbers varying from 8 500 to 25 000. Changes of the flame shape are examined with OH* chemiluminescence and OH laser induced fluorescence measurements as a function of the operating conditions. Particle image velocimetry measurements are used to reveal the structure of the velocity field in non-reacting and reacting conditions. It is shown that the axial plus-tangential swirler allows controlling the flame shape and the position of the flame leading edge with respect to the injector outlet. The ratio of the bulk injection velocity over the laminar burning velocity U-b/S-L, the adiabatic flame temperature T-ad and the swirl number S-0 are shown to control the flame shape and its position. It is then shown that the axial flow field produced by the axial-plus-tangential swirler is different from those produced by axial or radial swirlers. It takes here a W-shape profile with three local maxima and two minima. The mean turbulent flame front also takes this W-shape in an axial plane, with two lower positions located slightly off-axis and corresponding to the positions where the axial flow velocity is minimum. It is finally shown that these positions can be inferred from axial flow velocity profiles under non-reacting conditions.