Stabilization Mechanisms of Swirling Premixed Flames With an Axial-Plus-Tangential Swirler

The stabilization of premixed flames within a swirling flow 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 components of the combustor without the 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 8500 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 (PIV) measurements are used to reveal the structure of the velocity field in nonreacting and reacting conditions. It is shown that the axial-plus-tangential swirler allows to easily control 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 S₀ are shown to control the flame shape and its position inside the combustion chamber. It is then shown that the axial velocity field produced by the axial-plus-tangential swirler is different from those produced by purely axial or radial devices. 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 the lowest. It is finally shown that these positions can be inferred from axial flow velocity profiles under nonreacting conditions.