Under-expanded hollow-cone hydrogen jets in low-pressure direct injection: Shock-vortex dynamics, thermodynamics, and pressure-dependent mixing

Low-pressure direct injection of hydrogen through outward-opening hollow-cone injectors is attractive for maximizing usable tank content, yet even at nominal injection pressures near 20 bars, the annular flow chokes and discharges as an under-expanded jet whose shock-induced expansion, vortex dynamics, and mixing govern in-cylinder performance. Here, a computed-tomography-informed, experimentally constrained compressible computational fluid dynamics framework is developed to resolve the transient dynamics, thermodynamics, and mixing of such jets from a production-representative Bosch hollow-cone injector. A time-resolved inlet mass-flow rate boundary condition derived from measured needle-lift and injected-mass data drives the simulations, which are validated against high-speed schlieren over ambient pressures of 1–5 bar (penetration, projected area, morphology) and against local one-dimensional Raman measurements of hydrogen concentration and temperature. Validation against Raman measurements showed that the predicted hydrogen concentration field was largely unaffected by the selected energy formulation, while noticeable differences were observed in the predicted temperature distribution. The jets are governed by upstream and downstream recirculation and coherent vortex rings whose scale and intensity depend on ambient pressure. Crucially, lower ambient pressure increases penetration and projected area but does not improve mixture homogenization, as large coherent vortical structures promote axial convection, allowing locally rich pockets to persist over longer downstream distances. In contrast, higher ambient pressure produces a more confined jet with a shorter characteristic mixing length, promoting more rapid dilution of the rich hydrogen core and faster homogenization toward lean and near-stoichiometric conditions despite the reduced jet penetration.