The spatial growth of supersonic reacting mixing layers: Effects of combustion mode

Supersonic mixing layer flows are taken in scramjet engines for enhancing mixing and combustion efficiency. The spatial growth of supersonic reacting mixing layers with streams of dilute hydrogen and air has been investigated using direct numerical simulation, with detailed chemical reaction mechanisms. Different inlet air temperatures are considered to trigger auto-ignition of mixtures in mixing layers with Damköhler numbers ranged. By the comparison of auto-ignition sites and vortex shedding positions in the computational domain, four modes are classified as incombustible, premixed combustion, diffusion combustion after premixed one, and diffusion combustion. The spatial growth of supersonic reacting mixing layers, characterized by the vorticity thickness, along the streamwise direction establishes a strong relationship with the combustion mode. For the premixed combustion mode the growth rate increases compared to inert case, but it decreases for the diffusion combustion mode. The difference of combustion mode implies that the spatial growth has to be viewed section by section rather than previous one as a whole. The premixed combustion with strong heat-release within the shear layer causes much dilatation, while the diffusion combustion with relatively weak heat-release suppresses entrainment of the shear interface to an extent so that offsets dilatation influences. These findings provide a new perspective for supersonic reacting mixing layers applied to scramjet engines.