The effect of diverging angle on flame dynamics of near lean blowout in a centrally staged spray combustor

Abstract

This article focuses on the effect of diverging angle of the pilot stage on swirl spray flame dynamics near lean blowout conditions in a centrally staged lean premixed pre-vaporized combustor. The flame OH* chemiluminescence images, non-reactive flow fields, and spray fields of three diverging angles, P28°, P52°, and P90°, are obtained under elevated temperature and pressure conditions. Results show that the swirl jet angle of the pilot stage and primary recirculation zone increases with increase in the diverging angle. The spray angle also increases with increase in the diverging angle. There are pretty much more droplets distributed downstream of the pilot stage throat in P28°, which is different from the other two cases. The time-averaged flame shapes show that the flame shortens and the stabilized zone changes from the lip recirculation zone and inner shear layer to the central shear layer, as the equivalent ratio of the pilot stage (φp) decreases. The flame axial length decreases with increase in the diverging angle for φp below 2.0. Dynamic mode decomposition is applied to analyze the flame dynamics. The results indicate better hydrodynamic stability near lean blowout conditions at P90°. A bifurcation of characteristic frequency at P28° with decrease in φp indicates that the dominant mode of hydrodynamics instability changes from spiral to vortex shedding. Meanwhile, P52° is always the spiral mode with a higher frequency. These results could help understand the flame dynamics of different diverging angles near lean blowout conditions.