Experimental study on the development characteristics of the extremely under-expanded hydrogen jet structure for high pressure direct injection hydrogen engines

This paper investigates the injection process of extremely under-expanded high-pressure hydrogen in an internal combustion engine environment, focusing on transient flow characteristics and jet morphology to systematically reveal spatiotemporal evolution mechanisms under ultra-high pressure ratios. First, tests on a direct-acting injector reveal that, even under choked conditions, injection pressure and energizing time remain the dominant parameters governing the injection rate. Results show that elevating injection pressure increases mass flow rate while significantly accelerating needle valve closure via enhanced internal pressure differentials, thereby shortening the injection duration by up to 11.6%. Second, high-speed schlieren imaging demonstrates that while higher injection pressure enhances spatial expansion, ambient pressure exerts a profound suppressive effect, particularly influencing initial jet tip velocity and cone angle. Because the hydrogen undergoes intense Prandtl-Meyer expansion immediately upon exiting the nozzle, a complex shock wave system forms, causing the near-field cone angle to consistently exceed its far-field cone angle. Finally, this study elucidates three distinct stages of massive Mach disk formation: the emergence of Prandtl-Meyer expansion fans, their intersection within the barrel shock, and their subsequent extension beyond the barrel shock. The pressure ratio is identified as the decisive parameter dictating the shock wave topology transition from a series of shock cells featuring Mach reflection to a single, massive Mach disk, clarifying the dynamic pathway of pressure energy release under extreme conditions.