Study on the recirculation structures in supersonic discrete hole film cooling using hydrocarbon fuel as coolant

Hydrocarbon-fueled scramjet engines operating under supersonic conditions face significant challenges related to aerodynamic heating. The presence of recirculation structures within discrete film cooling holes affects coolant distribution uniformity. In this study, numerical simulations are conducted to investigate the formation mechanisms and influencing factors of recirculation structures within discrete film cooling holes using hydrocarbon fuels as the coolant. The results show that the formation of recirculation vortex structures within discrete film cooling holes is primarily governed by the momentum mismatch between the mainstream flow and the coolant jet. As the jet momentum ratio decreases, intensified shear at the hole exit promotes mainstream entrainment, leading to backflow and the development of strong internal vortices, thereby significantly diminishing film cooling effectiveness. A critical jet momentum ratio determines the onset of recirculation; for macromolecular hydrocarbon fuels such as n-decane, this threshold is observed within the range of I = 0.00141–0.00063. Under supersonic conditions, the elevated momentum and energy of the mainstream lower this threshold (I = 0.00063) compared to subsonic flows (I = 0.00253), indicating that supersonic inflows more readily destabilize the jet. Additionally, the vorticity intensity of the external counter-rotating vortex pair exhibits a nonlinear dependence on the momentum ratio. The diameter of discrete film cooling holes directly influences the distribution of secondary flow intensity within the holes by affecting jet velocity. These findings underscore the importance of optimizing both the jet momentum ratio and hole geometry to suppress detrimental recirculation phenomena, particularly under high-speed inflow conditions with high-density hydrocarbon-fueled injection.