Effects of injection recess in methane turbulent combustion for space propulsion

Direct numerical simulations (DNS) are conducted to analyze the effect of recess in the mixing and combustion performance of gaseous methane—oxygen injection systems. The recess length is varied from 0 to 2 injector diameters to analyze the sensitivity of various physical processes to this geometrical feature. It is found that the injection recess enhances the development of Kelvin-Helmholtz instabilities (KHI), due to the more consistent jet flow disposition at the injection plane, and the higher velocities. A Strouhal number Str R ≈ 0.22 , based on the injector diameter and on the average reacting jet velocity is found for all simulations. In addition, it is found that the enhancement of KHI acts as an injection of turbulent kinetic energy in the large scales, improving macroscale mixing and combustion performance. Finally, the normalized enhancements in combustion performance and averaged hydroxyl emissivity are found to be in good agreement with experimental results in a similar setup.