Characterization of turbulent pseudo-boiling in transcritical and doubly-transcritical oxygen–methane flames

This study investigates the turbulent pseudo-boiling process in transcritical and doubly transcritical LOx-methane flames using large-eddy simulations (LES) under conditions relevant to next-generation liquid rocket engines (LRE). The definition of the turbulent pseudo-boiling rate, based on the concept of displacement speed, is introduced for reacting flows and LES frameworks. A dedicated strategy is developed to identify the effective pseudo-boiling state when using turbulent combustion models, where the actual laminar pseudo-boiling typically occurs at the sub-grid scale. The thermodynamic structure of the pseudo-boiling transition is analyzed in mixture fraction space using laminar flamelet calculations and LES results. Although the pseudo-phase transition of the propellants occurs almost as a pure fluid, the choice of mixing rules for thermodynamic property evaluation is shown to influence the thermal breakup of the jet and the resulting flame morphologies. The LES data are then used, in conjunction with the turbulent pseudo-boiling rate definition, to investigate statistically the heating of the propellants. The analysis reveals that the mass transfer under transcritical conditions is mainly driven by the normal diffusion fluxes induced by the non-premixed flame between the two propellant streams. We found that LOx pseudo-boils at similar rates in both transcritical and doubly-transcritical flames. In contrast, the rates of cryogenic methane in the doubly-transcritical case are significantly lower, not only because of the inherently higher methane thermal inertia but also due to the occurrence of a non-isotropic insulating region influenced by the interplay between thermodynamic conditions and turbulent motions.