Design rules for the velocity field of vortex breakdown swirl burners

Most gas turbine premix burners without centrebody employ the breakdown of a swirling flow at the transition between the mixing section and the combustor for aerodynamic flame stabilization. As the formation of the desired vortex breakdown pattern depends very sensibly on the shape of the axial and azimuthal velocity profiles in the mixing section, the design of suitable swirlers is typically a cumbersome process and requires an iterative approach consisting of numerical as well as experimental development steps to be iteratively applied until a geometry is found, that provides a spatially as well as temporarily stable vortex breakdown in the primary zone of the combustion chamber without backflow on the centerline of the vortex into the swirler. These difficulties stem from the lack of generally applicable aerodynamic design criteria. The paper attempts to contribute to the development of such design guidelines, which lead quickly to successful swirler designs without need for an excessive number of iterations. For this purpose a family of swirl profiles was generated and the corresponding axial velocity profiles were calculated assuming several radial total pressure distributions. In the next step, the flows were calculated using CFD in order to find out, which velocity profiles produce stable vortex breakdown bubbles at the burner exit. This study reveals that the stable breakdown of the vortex can be achieved for a wide range of velocity distributions, if the radial total pressure distribution is properly selected. However, the radial total pressure distribution in the vortex core is essential for the robustness of the design. Interestingly, velocity profiles with constant total pressure do not show a stable transition of the velocity field at the cross-sectional area change at the entrance of the combustion chamber. In addition, theoretical considerations reveal that an increase of the azimuthal velocity in the vortex core in streamwise direction avoids backflow on the centreline as well as flame flashback. This increase can be achieved using a slightly conical nozzle and introducing a swirl free jet on the centreline upstream of the mixing zone. All effects are explained using the vorticity transport equation.