Vortex breakdown in non-premixed swirling jet flames

Numerical simulations are used to assess the swirl-induced stabilization of low-Mach-number non-premixed axisymmetric swirling jet flames at moderate Reynolds number. Previous experimental investigations have shown that the formation of bubble vortex breakdown, an aerodynamic recirculation region that forms for sufficiently large values of the swirl number S, reduces the velocity at the flame base, and may lead to complex transitions in the structure of the flame. A critical value for the onset of bubble breakdown (S^*_B) is first identified for the isothermal flow and Burke-Schumann flames with infinitely fast chemistry, assuming typical conditions for methane combustion with air. The transition S^*_B is found to be relatively constant as the jet fuel-feed mass fraction Y_F,j is varied. A single-step finite-rate reaction is then considered, leading to flames that lift off the injector for moderate values of the Damköhler number (Da). Increasing values in the prescribed time-dependent inflow swirl S(t) results in increased entrainment and reduced liftoff heights, stabilizing the flame closer to the injector. The onset of bubble vortex breakdown at S^*_B produces a compact flame with enhanced recirculation of hot combustion products.