Computational investigation of the stability of a lifted strongly buoyant jet flame

The present work performs a detailed numerical simulation of a non-premixed turbulent reacting jet at low Reynolds and Froude numbers to investigate the dynamical interaction between the flow field and the embedded flame, with a special focus on the mechanism of flame stabilization. The computations combine a DNS approach for the flow structures, which are fully resolved, with a LES approach for the numerical description of the flame. The results unveil a weak, essentially impeding effect of the flame on the upstream, non-burning flow region. A significant effect is seen in the large-scale motion, which is generated by the periodic formation, growth, and departure of large bulb-shaped low-density structures at the flame base, as it is also seen in experiments. The analysis of the dominant stabilization mechanism of the lifted flame essentially supports the concept of edge-flame propagation. In addition, the buoyancy-driven large-scale flow structures temporarily produce a further strongly destabilizing scenario, where large circumferential sections of the flame base recede deeply downstream, which is shown to be associated with high local values of the scalar dissipation rate. Showing this particular scenario the present work does not only highlight an important stabilization mechanism governed by large three-dimensional (“out-of-the-plane”) structures, it also demonstrates the strong relevance of the scalar dissipation rate even when it is far below the extinction limit.