A 3D DNS study of the stabilization of a turbulent lifted hydrogen/air jet flame in an autoignitive heated coflow

Direct numerical simulation (DNS) of the near field of a three-dimensional spatially developing turbulent slot-burner lifted jet flame in heated coflow is performed with a detailed hydrogen-air mechanism and mixture averaged transport properties at a jet Reynolds number of 11,000 with over 900 million grid points. The results show that auto-ignition in a fuel-lean mixture immediately upstream of the flame base is the main source of stabilization of the lifted jet flame. Radical chain propagation through H + O₂ + M → HO₂ + M and HO₂ + H → OH + OH is found to facilitate auto-ignition in both fuel-rich and fuel-lean mixtures. Independent of the chemical signature of autoignition, examination of the Damköhler number and key intermediate species behavior near the leading edge of the lifted flame also verify that auto-ignition occurs at the flame base. The flame index shows that both lean premixed and nonpremixed flame modes exist at the flame base but most heat is released from the nonpremixed flame mode. Further downstream, bimodal combustion in the form of rich premixed and nonpremixed flame modes emerges in the flame index space. The DNS of the near field precludes the transition to a fully nonpremixed flame anticipated in the far-field of the jet. Lagrangian tracking of the flame base reveals the passage of coherent jet structures and their correlation with the location of the flame base. In particular, the relative position of the flame base and the coherent jet structure induces a cyclic movement of the flame base both in the transverse and axial directions about a mean stabilization height consistent with Su et al. hypothesis [L.K. Su, O.S. Sun, M.G. Mungal, Combust. Flame 144 (2006) 494-512]. This is determined by Lagrangian tracking of key scalars, heat release rate, and velocity fields at the stabilization point.