Effect of the Injector Pressure-Building Process on Dynamic Gas Flow Characteristics of a Highly Turbulent Underexpanded Hydrogen Jet from a Single-Hole Cylindrical Injector.

High-pressure hydrogen direct injection (DI) technology demonstrates significant potential for high thermal efficiency and ultralow emissions in engines. The hydrogen gas jet at an elevated nozzle pressure ratio (NPR) exhibits turbulent underexpanded jet behavior, manifesting shock wave formations and sequential shock cell structures in the near-nozzle zone. This work investigated transient shock cell evolution and Mach disk parameters during high-pressure hydrogen injection through a single-hole cylindrical injector. The injector inner pressure building-up process was tested, and a three-dimensional large-eddy simulation (LES) model was used to investigate the underexpanded jet. The results show that the inner pressure declines from the hydrogen tank to the injector, undergoing a pressure-building transient process, causing a delay in achieving stabilization. Moreover, shock cell development exhibits distinct transient characteristics. The Mach disk dimension parameters, i.e., cell core length L c, Mach disk width W disk, and Mach disk height H disk, demonstrate phased evolution: an initial growth phase followed by asymptotic stabilization. The turning points of L c, W disk, and H disk depend on the inner pressure-building process. Notably, the constant coefficient C H for H disk estimation requires empirical correction due to transient shock cell behavior. For NPR ≥ 90 of a single-hole injector, our data recommend C H = 0.85-0.9. The shock waves of the underexpanded gas flow induce a lower entrainment ratio within the near-nozzle region (Z/D < 8), resulting in minimal entrainment. Furthermore, this dynamic delay phenomenon becomes particularly pronounced when injection cycles are shorter. It is necessary to consider the dynamic hydrogen jet characteristic for better design and optimization.