Extinction and reignition in direct numerical simulations of CO/H₂ temporal plane jet flames

Direct numerical simulations of three-dimensional turbulent temporally-evolving plane CO/H₂ jet flames have been performed with skeletal chemistry at Reynolds numbers of up to 9,000 and with up to 500 million grid points (Hawkes, E.R., Sankaran, R., Sutherland, J.C., Chen, J.H., Proc. Combust. Inst. 31 (2007) 1633-1640). In the present paper, the data are analyzed to understand the processes of extinction and reignition observed in the simulations. A measure of extinction based on the amount of stoichiometric surface area having a reacting scalar less than a threshold value is used to characterize extinction. Employing this characterization leads naturally to the appearance of a local displacement speed of 'flame edges' as the primary quantity of interest. Flame edges are defined as the boundaries on the stoichiometric surface between areas having a reacting scalar less than the threshold (extinguished) and those above it (burning). The displacement speed is the speed at which these boundaries move relative to the local flow. The motion of flames edges is studied using a massively parallel analysis tool. The joint probability density function of the local edge flame speed and scalar dissipation rate has been extracted and reveals a transition in character as the simulation progresses. The transition is interpreted in the context of the physical mechanisms of extinction and reignition. Along with evidence of the alignment of the scalar and mixture fraction normal vectors, it indicates that the mechanism of folding by turbulence of burning regions onto extinguished ones is the dominant reignition mechanism for the simulated conditions.