Effects of pressure and inlet temperature on coaxial gaseous methane/liquid oxygen turbulent jet flame under transcritical conditions

This study has investigated numerically turbulent flames of cryogenic oxygen and methane under supercritical pressures relevant to liquid propellant rocket engines. A real-fluid version of the flamelet equations is employed to accommodate simultaneously non-equilibrium chemistry of hydrocarbon fuel and non-ideal thermodynamics in local flame structures while the effect of turbulent fluctuations is accounted for via a presumed probability density functions. The present model reproduced qualitatively well the experimentally observed unique feature of a transcritical flame of coaxial gaseous methane/liquid oxygen injector, which is characterized by sudden flame expansion, abruptly terminated flame tip, and expansion induced flow recirculation. Numerical results reveal that pseudo-boiling phenomena occurred in the transcritical mixing layer between the cryogenic oxygen core and the surrounding hot gas play a crucial role in mixing and combustion processes. It is also found that the transcritical flame structure is drastically affected by elevating the chamber pressure or increasing the oxygen inlet temperature in terms of flame length, sudden expansion angle, and reverse flow strength. Detailed discussions are made for effects of the real-fluid behaviors on the turbulent flame field as well as on the local flame structure in mixture fraction space.