An Analysis of Unstable Flow Dynamics and Flashback Mechanism Inside a Swirl-Stabilised Lean Burn Combustor

A modern lean burn combustor for propulsion application using a multi-swirler fuel injector system is studied under partially premixed combustion conditions. Combustion induced vortex breakdown (CIVB) plays an important role in establishing the near-field aerodynamic characteristics of lean burn fuel injectors, influencing fuel/air mixing and flame stability. The precise nature of the vortex breakdown can take on several forms mainly consisting of a precessing vortex core (PVC) and the appearance of multiple helical vortices formed in the swirl stream shear layer. In the present study a numerical investigation is carried out with an unsteady Reynolds-averaged Navier Stokes (URANS) solver to capture the evolution of the PVC in the vicinity of the air blast atomizer of the injector and the helical vortex patterns inside the combustor. PVC suppression is captured causing upstream flame propagation and a sudden rise in the temperature near the injector end. The existence of hot-spots is reported near the injector end which can increase NOx production and also cause possible damage to the injector assembly itself. The unsteady dynamics of the PVC, which significantly influences fuel efficiency and emissions, are analyzed for a relatively long period of real time. The variation of velocity along the axis of the combustor confirming upstream flame propagation in to the injector is plotted. Mass weighted average temperatures across different planes in the combustor are monitored as target functions. The influence of different time-step sizes on the prediction of the temperature across these different planes is also presented. For this lean burn combustor using a complex multi-swirler fuel injector system, the results demonstrate the formation of a startup PVC in the vicinity of the air blast atomizer of the injector, its unstable mode of excitation, sustainment and suppression due to CIVB over a period of time and its strong influence on injector near-field aero-thermodynamics.