Reignition dynamics in massively parallel direct numerical simulations of CO/H ₂ jet flames

Massively parallel three-dimensional DNS of turbulent temporally evolving nonpremixed plane jet flames have been performed with realistic CO/H ₂ kinetics. Up to 0.5 billion grid points were employed allowing jet Reynolds numbers of 9000 to be attained. Simulations were run on up to 4096 processors and generated more than 30 TB of raw data. New results are presented concerning the extinction and reignition dynamics occurring in these flames. We characterise extinction using a metric based on the total stoichiometric flame surface area having a reacting scalar such as a product or radical species less than a threshold value taken to represent extinction. The motion of edge flames separating extinguished and burning regions of this surface is studied using a massively parallel analysis tool. The joint probability density function of the local edge flame speed relative to the flow and scalar dissipation has been extracted and shows 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 other evidence it indicates that the mechanism of folding by turbulence of burning regions onto extinguished ones is the dominant reignition mechanism. The effect of the choice of the scalar species, scalar cut-off value, and mixture fraction value used to define extinction is discussed.