Numerical characterization of hydrogen under-expanded jets with a focus on Internal Combustion Engines applications

In the context of reducing carbon dioxide (CO2) emissions, hydrogen is gaining momentum as a possible fuel for Internal Combustion Engines (ICEs). In-cylinder direct injections allow for a higher specific power density while enabling different levels of charge stratification. The high-pressure injection leads to the onset of under-expanded jets, characterized by complex patterns of shock waves and expansion fans. In ICEs simulations, such physics needs to be correctly solved to obtain a reliable assessment of the mixture formation. In this paper, the main features of hydrogen under-expanded jets are examined under the conditions typically found in turbocharged engines. Improved correlations are provided for the assessment of the Mach disk height and diameter, up to a Nozzle Pressure Ratio (NPR) equal to 60. The dependency of the hydrogen-air mixing on the cylinder temperature has been analyzed, and the unsteady jet dynamics has been examined by continuously varying the combustion chamber pressure. To the authors' knowledge, no studies of this kind can be found in the literature for the specific case of hydrogen. Lastly, a preliminary investigation of the jet-jet interaction (a Coanda-like effect) is reported. This phenomenon is observable when multi-hole injectors are employed, and it may have a great impact on the mixture formation.