Wall heat transfer prediction in CH₄/O₂ and H₂/O₂ rocket thrust chambers using a non-adiabatic flamelet model

The current work presents the extension of the flamelet model for turbulent combustion calculations to account for deviations from adiabatic conditions. The aforementioned extension is expected to significantly improve the prediction of the chemical processes occurring in the vicinity of cooled walls in rocket engine applications. A lower enthalpy level leads to an increase of the recombination reactions, which is of particular interest in the case of methane/oxygen combustion. In the present approach, the flamelet equations are solved in mixture fraction space and the energy equation is replaced by a prescription of the enthalpy profile in order to include non-adiabatic effects. To avoid the over-prediction of the recombination reactions, a local ”freezing” of the chemical reactions is introduced based on the Damkoehler number close to the cold wall boundary. A pre-tabulation of the chemical time-scales in the flamelet tables enables a fast calculation of the Damkoehler number. The model is verified both for CH₄/O₂ and H₂/O₂ using the simulation of a cooled reacting boundary layer. The extended hybrid model is employed for the simulation of a single-element rocket thrust chamber using CH₂/O₂ and H₂/O₂ and is compared to the non-adiabatic and frozen flamelet models. A more accurate wall heat transfer and pressure level prediction is achieved with the hybrid model for both propellant combinations leading to great agreement with the available experimental measurements.