Bridging the thickened flame and fine-scales frameworks for multi-regime combustion modeling

Large eddy simulation of multi-regime combustion remains challenging due to the coexistence of interacting combustion modes within the same flow. Each regime requires specific modeling assumptions, yet most existing approaches are tailored for single-regime and lack flexibility in mixed-mode configurations. The dynamic version of the thickened flame (TF) model is widely recognized as a state-of-the-art approach for premixed and partially premixed combustion. However, in most common multi-regime implementations, thickening is deactivated in non-premixed regions based on a flame index, leaving turbulence–chemistry interaction (TCI) unmodelled. The resulting finite-rate treatment becomes increasingly sensitive to mesh resolution as the relevant mixing and reaction scales become under-resolved, limiting its applicability at industrial Reynolds numbers. To address these limitations, this study presents a novel unified TF/fine-scales (FS) formalism. The approach employs an FS-based closure to reintroduce TCI in non-premixed regions while remaining fully compatible with TF. A transported premixedness sensor dynamically couples the two models, ensuring smooth and consistent TCI transitions across regimes. Model validation is performed on the canonical Sandia D and E flames, demonstrating improved temperature and species predictions relative to the standard TF deactivation strategy in non-premixed regions.