Turbulent mixing dynamics of under-expanded hydrogen jets in propulsion systems

Underexpanded jets are present in various engineering applications; in recent years, they have gained special attention because of the development of gas-fueled propulsion systems. In these apparatuses, the direct injection of fuels such as hydrogen in innovative low-emission engines' chambers induces turbulent under-expanded jets. In this study, we performed high-fidelity large eddy simulations of under-expanded hydrogen jets to investigate mixing characteristics and provide valuable insights for developing injectors suitable for hydrogen and, more generally, gaseous-fueled propulsion systems. We initially assessed the method's accuracy, evaluating the convergence and uncertainty of the numerical results and validating them against experimental particle image velocimetry and Schlieren data. The simulated jets, the Mach disk dimensions, and the resulting velocity field align closely with the experimental observations. Then, we analyzed the jet structure for pressure ratios of 4 to 25 and examined the effects of the geometrical configuration of the nozzle on the characteristics of the air-fuel mixture obtained. We compared the jets resulting from a round-hole nozzle with annular ones resembling outward-opening injectors.