Direct numerical simulations of diffusion flame extinction at different pressures

Direct numerical simulations (DNS) of ethylene/air diffusion flames in decaying two-dimensional turbulence were performed to investigate flame extinction characteristics at different pressures. A Damkohler number based flame extinction criterion as provided by classical large activation energy asymptotic (AEA) theory is also assessed for its validity in predicting flame extinction induced by strain and heat losses. The DNS code, S3D, solves compressible flow conservation equations using a high order finite difference and explicit time integration schemes. The ethylene/air flame behavior is described by employing a reduced mechanism that accurately describes up to C4 chemistry by the directed relation graph (DRG) method along with stiffness removal. The model configuration is an ethylene fuel strip embedded in ambient air on both sides, which is superimposed by a prescribed decaying turbulent flow field. Due to the high spatial and temporal stiffness associated with the detailed chemistry, a spatial resolution of 8m was used, and the time resolution was varied from 5 to 10 ns. A total physical time of 1 ms was computed in order to observe the temporal evolution of diffusion flame extinction events. The emphasis of this study is on the several flame extinction events observed in the simulations. The effect of pressure on extinction is studied by considering three different pressures: 0.1, 1.0, and 10 atm. To isolate the pressure effects on the turbulence and those on the chemistry, contrived physical parameters were considered by artificially changing the gas transport properties in relation with the pressure change. This methodology allows a consideration of identical turbulent flow fields at different pressure conditions. An extinction criterion based on the local Damkohler number is tested for its validity in predicting various flame extinction events encountered. Results show that, despite the relative simplicity of the AEA flame extinction criterion, it can accurately predict the flame extinction conditions. In particular, radical concentrations near the stoichiometric mixture fraction isocontour follow similar trends at extinction for all pressure conditions considered. A more rigorous mathematical approach based on the chemical explosive mode analysis (CEMA) was used in comparison with the AEA-based diagnostic. It is found that the AEA flame extinction criterion provides identifications of extinction events that are consistent with those provided by CEMA. This study supports the validity of a simple Damkohler number based criterion to predict flame extinction in engineering-level CFD models.