Flamelet-based modeling of doubly transcritical methane/oxygen flames

Large-eddy simulations (LES) of methane oxy-combustion have been investigated in the context of liquid rocket engines coaxial injectors, specifically in a 2D splitter plate configuration. Methane and oxygen are injected under transcritical conditions, characterized by supercritical pressure and subcritical temperature resulting in liquid-like densities and gas-like diffusivities. The effect of flamelet modeling on the flame structure is thoroughly analyzed, and a grid sensitivity study is conducted. Three different flamelet-based combustion models are compared, namely the steady laminar flamelet (SLF) model, the flamelet progress variable (FPV), and the flamelet-generated manifold (FGM). Regardless of the flamelet model used, the resulting flames display significant wrinkling and strong stratification due to the high density of the propellants. The global effect of the grid is to reduce temperature peaks and the integrated heat release rate, while still preserving the characteristic features of the flame. Various flame regimes, identified using a flame index, are observed, aligning with high-fidelity simulations by Monnier et al. (Proc. Combust. Inst., 2024), which are used as a reference. Time-averaged and conditional mean analyses reveal that the FPV and FGM simulations cover a broad range of thermochemical states, from unreacted to equilibrium conditions. As a result, the FPV and FGM simulations exhibit lower temperatures compared to the SLF simulations. Meanwhile, the integrated heat release rate shows values that are consistent with the reference simulations, with notably reduced grid sensitivity.