Assessment of flamelet-based modeling for large eddy simulations of doubly transcritical methane/oxygen flames

Numerical simulations are rapidly emerging as a design tool for modern liquid rocket engine (LRE) injection systems. Yet, experimental validation remains limited by the extreme thermodynamic conditions, particularly for propellant injection under transcritical conditions. Recent 2D DNS-like simulations of doubly transcritical methane/oxygen combustion revealed complex flame–turbulence interactions spanning premixed and non-premixed regimes, exposing the limitations of widely employed non-premixed flamelet models. In this study, using the same splitter-plate configuration, we perform large-eddy simulations (LES) using flamelet-based methods to assess their performance and applicability to practical LRE. Three approaches are compared a posteriori: steady laminar flamelet (SLF) and flamelet progress variable (FPV) using non-premixed flamelets, and flamelet generated manifold (FGM) employing premixed flamelets. The role of LES grid resolution is carefully assessed, from practical LES towards DNS-like resolutions. The resulting flames exhibit significant wrinkling and strong stratification, showing good agreement with the reference high-fidelity simulations. Three flame regimes, identified by a flame index, are observed and further analyzed using heat release rate contributions. Lower LES resolutions lead to reduced temperature peaks and, overall, a larger fraction of the heat release rate arising from the diffusive flame regions is observed. The premixed regime is captured only when a progress variable is introduced, as in FPV and FGM, and is confirmed by the heat release rate partition between regimes. Three-dimensional simulations have also been performed, showing a further reduction in the heat release rate and in the contribution of premixed regions, indicating that the complex flame regimes captured at high resolution are progressively filtered out by grid coarsening and concurrently by 3D turbulence. Additionally, a thermodynamic stability analysis revealed that thermodynamically unstable states are present in all simulations despite the supercritical operating pressure of 100 bar, and are confined to the metastable region, suggesting that spontaneous phase separation remains unlikely. Novelty and significance statement: The investigation of doubly transcritical methane-oxygen flames, relevant to modern liquid rocket engine design, remains experimentally inaccessible, motivating a significant effort in numerical simulations. Recent DNS-like simulations revealed complex flame-turbulence interactions, spanning premixed and non-premixed combustion regimes, exposing the limitations of widely employed flamelet models. The authors, for the first time, provide a systematic evaluation of flamelet-based frameworks under such extreme conditions using both non-premixed and premixed flame archetypes, across a wide range of LES grid resolutions from practical towards DNS-like. Three-dimensional simulations further extend the analysis, providing novel insights into models’ performance and shortcomings. Additionally, a thermodynamic stability analysis reveals the persistence of metastable states despite supercritical operating pressure, with implications for phase separation modeling in high-pressure propulsion systems.