The role of the baroclinic term in fuel/air mixing enhancement

Understanding of physics of supersonic combustion is mandatory for future hypersonic air-breathing propulsion systems. In fact, in order to overcome Mach 5 flight speeds, a supersonic combustion ramjet (SCRJ), where combustion takes place in supersonic conditions is required. In a scramjet, the air entering the combustor is supersonic: this means that in a residence time of about a millisecond air and fuel must mix and react. Hence, mixing in supersonic combustion plays a critical role on the the combustion efficiency and its understanding is critical to properly address a working engine. In this paper a rigorous analysis of the vorticity generation and transport in supersonic flows has been done in order to understand the key parameters to improve mixing and combustion. This 3D LES of the HyShot supersonic combustor performed by means of a fairly dense grid of 50Mnodes showed that the baroclinic term is the primary responsible of vorticity generation. In fact, interactions among the airstream entering the combustor and the H2 crossflow jet, the heat released and the shock waves produce a vorticity rate of order of 105 Hz. This vorticity generation is mainly due to the baroclinic term that creates spanwise vortices just upstream the H2 injection. These vortexes are afterwards tilted and stretched by the vortex stretching in the streamwise direction. LES predicts a very fast and efficient combustion: only 0.2% of hydrogen is found unburned at the combustor exit. Comparison of pressures distribution along the wall centerline at 1.32 ms shows a good agreement, mostly in the first part of combustor, where the grid is much more refined.