Assessment of the Thickened Flame Model for Large Eddy Simulations of Turbulent Premixed Flames Near Extinction Conditions

Artificially thickened flame (ATF) models are widely adopted as closure models for turbulence-chemistry interactions in large eddy simulations (LES). Most of the existing ATF modeling studies involve one or two-step chemical mechanisms, targeting at strongly burning turbulent premixed flames. Increasing numbers of studies begin to focus on combining ATF models with reduced or detailed chemical mechanisms recently, to account for finite-rate chemistry effects such as auto-ignition or local extinction in turbulent flames. In this study, a hierarchy of laminar and turbulent flames are employed to investigate the effect of the thickening factors near extinction conditions. The suitability of several flame sensors for the detection of the flame fronts is also assessed, and new flame sensors based on minor species that are only available in multi-step chemistry are proposed and evaluated a priori and a posteriori. Preliminary results show that a formyl-radical based flame sensor is able to recover the flame speed under both laminar and turbulent conditions. Sufficiently small thickening factors are required to capture the response of flames to local strain rates.