Flame base analysis in a turbulent, partially premixed lifted flame

This paper presents an analysis of the flame base of a partially premixed lifted flame using direct numerical simulation (DNS). The chemistry is modelled using a single-step reaction. In most previous studies of turbulent partially premixed flames, a constant activation energy of the reaction is considered when simple chemistry is used. However, this assumption over predicts the laminar flame speed in rich mixtures which probably affects the flame structure. In this paper, the activation energy is modified to mimic the variation of laminar flame speed with the equivalence ratio that occurs for real fuels. In our simulation, a jet of fuel with a high velocity issues into a laminar oxidiser co-flow which has the same temperature as the fuel stream. The flame is stabilised some distance from the nozzle for the conditions presented in this study. Visual inspection of the temperature and reaction rate shows that the maximum reaction rate in a window close to the inflow boundary is a good marker of the flame base. To analyse the structure of the flame base, temperature and mixture fraction are extracted at the flame base. Snapshots of the scalar dissipation rate, vorticity and reaction rate are also investigated in a window including the flame base. The flame index (FI) and average of reaction rate and temperature conditional on the mixture fraction are discussed in detail. It is shown that the flame base exists on the outer boundary of the jet and consists of a shrouding, quasi-laminar lean flame at the outer edge of the jet and a highly turbulent rich flame inside the jet core.