Analysis of Momentum Flux Distribution in Jets Produced by a Hydrogen Direct Injection System

The adoption of hydrogen as carbon-free fuel for internal combustion engines in both transport and off-road applications could offer a significant contribution towards carbon neutrality. In the technical pathway to the conversion of conventional engines operating with liquid fuels to hydrogen, a key role is played by the injection systems. In particular for direct-injected combustion systems, the achievement of an adequate capability to control the gas jets development and the following mixing with air in the combustion chamber is mandatory in order to govern the heat release rate, so to obtain high efficiency levels while limiting the knock tendency and NOx formation. In order to achieve this complex task, the development of effective diagnostic methodologies for a satisfactory characterization of high-pressure gas jets is required in order to obtain a proper matching with the combustion chamber design and air charge flow structure. In the present paper, the jet produced by a prototype hydrogen injector derived from a GDI is experimentally characterized in terms of mean flow rate, plume global development obtained by Schlieren imaging and momentum flux. In particular, the momentum flux technique is applied to characterize both the momentum pertaining to the entire jet and the momentum distribution over planar surfaces cutting the jet, hence gaining a deep insight in the jet internal structure which cannot be obtained by imaging. This approach is of peculiar interest for combustion systems in which hydrogen is injected with complex flow structures, such as when peculiar caps featuring multiple hole (often non uniform in size) are installed on the injector tip so to properly distribute hydrogen in the combustion chamber.