Investigation of Vortex-Driven Mixing in Hydrogen-Ammonia Low Pressure Direct Injection

Hydrogen-ammonia fuels combine the high hydrogen storage density and zero carbon emissions of ammonia with the rapid combustion kinetics of hydrogen, yet their markedly different physicochemical properties pose significant challenges in cylinder mixing. To enhance mixture uniformity and ignition stability, this study employs three-dimensional (3D) CFD simulations to analyze the flow and mixing behavior of hydrogen-ammonia blends under low pressure direct injection (DI) conditions using an auxiliary injection system. The results demonstrate that shear layer instabilities and baroclinic vorticity generation act synergistically to drive vortex formation and trigger a turbulence cascade, thereby strengthening the mixing of fuel and air. In the later stages of injection, the primary vortex ring decays and releases fuel from its core outward, expanding the mixing region and further improving the homogeneity. Applying the <i>Q</i> criterion to identify vortical structures within the jet, the flow field is partitioned into strong, weak, and no vortical regions. Notably, the weak vortex region maintains an equivalence ratio (Φ) near the stoichiometric mixture (Φ ≈ 1.0-1.2) and moderate turbulent kinetic energy throughout the injection, creating ideal conditions for flame kernel development and marking it as the optimal ignition region. These findings provide a theoretical foundation for ignition prediction and mixture optimization in DI hydrogen-ammonia engines.