A numerical study of species transport budgets in a turbulent lifted flame

A turbulent lifted slot-jet flame is studied using direct numerical simulation (DNS). A one-step chemistry model was employed with a mixture-fraction dependent activation energy which can reproduce qualitatively the dependence of laminar burning rate on equivalence ratio that is typical of hydrocarbon fuels. Terms in the transport equation of Favre-averaged product mass fraction were examined to determine their relative contributions and thus help understand the structure of the flame and its mechanism of stabilisation. It was found that the leading edge has features which are expected of a turbulent premixed flame, namely streamwise convection balancing turbulent transport upstream; however transverse mean convection due to entrainment and transverse turbulent transport out of the flame were both significant. Downstream, reaction became a dominant term and the flame has features expected of a non-premixed jet flame. The entrainment flows were found to be significant, and divergence of streamlines around the flame base was found to lead to local mean velocities that were actually in the upstream direction, a feature which may be dynamically important in the flame stabilisation.