Fully Premixed Hydrogen-Air Swirl Flames: Shapes and Transient Processes

Abstract

Abstract. This article characterizes lean fully premixed hydrogen-air swirled flames obtained experimentally. The laboratory-scale experiment developed prior to acquiring the flame imaging data is described along with the operating conditions, which are mapped on an operating regime map. Two parametric studies are conducted. The first one focuses on the impact of bluff-body diameter (13.6 mm–18 mm) on the flame shape for constant equivalence ratio (0.42) and bulk velocity (4 m s−1) to ensure consistent unstretched laminar flame speed and axial bluff-body bulk flow velocity across the different diameters. By examining the time-averaged chemiluminescence fields of these swirl flames and corresponding flame shapes, the critical role of the inner recirculation zone (IRZ) is identified. An analytical model is derived to link the experimentally observed trend with the change in geometrical swirl number. It is shown that the IRZ cancels out for a decrease in swirl level caused by an increased diameter of the bluff-body impacting the flame shape. It is observed from the schlieren imaging that the turbulence levels are higher, leading to wrinkling of the flame in the case of the bluff-body with the lowest diameter (highest swirl and highest injector velocity). The second parametric study focuses on transient processes for a given fixed geometry. For this study, a bluff-body with a diameter of 15 mm is used, and the air or fuel mass flow rates are varied. Four transient sequences are specifically investigated: statistically steady turbulent swirling flame, flashback flame, lean blowout flame, and lean blowoff flame. These results offer valuable insights into the stabilization of fully premixed hydrogen-air flames.