Local extinction and reignition mechanism in a turbulent lifted flame: A direct numerical simulation study

Local extinction and reignition is studied in a direct numerical simulation (DNS) dataset of a turbulent lifted flame [1]. Extinction holes are identified as regions on the stoichiometric surface which have a product mass fraction less than a critical value. Using this criterion, thirty individual holes are identified and tracked in time. It is observed that large outwardly pushing structures caused compressive strain rates normal to the mixture-fraction iso-surface. These high strain rates caused high dissipation rates and the initiation of the extinction process, leading to the initial creation of holes. Extinction, i.e. hole growth, then occurs in two phases. In the first phase, the edge-propagation velocity is initially negative and the fluid dynamic tangential strain rate on the hole surface is positive, leading to rapid hole growth. Subsequently, in the second phase, local compressive strain rates at the flame edge relax, and the edge-flame propagation velocity switches to positive. However, in this second phase, the hole continues to expand because of positive tangential strain rate on the hole’s surface, which dominates over the healing effect of positive edge-flame propagation velocities. When reignition starts, the edge-propagation velocity is mainly affected by the product-mass fraction displacement speed and shows a dependency on curvature and scalar dissipation rate, similar to what is expected in edge-flame propagation. An analysis of thermal diffusion on the unburned portion of the mixture-fraction iso-surface shows that the edge-flame propagation mechanism dominates turbulent engulfment during reignition in this study.