Transient development and structure of supersonic gas jets from narrow-cone-angle pintle-type hydrogen injector

The hydrogen direct-injection engine is one of the promising options for carbon neutrality in the transport sector due to its carbon-free combustion and great potential for efficient energy conversion. Recent hydrogen direct-injection engines adopt narrow-cone-angle pintle-type injectors which generate axially concentrated jets to suppress the abnormal combustion and reduce the heat loss to the cylinder wall. It is important to understand the transient development and structure of gas jets for the successful modeling and optimization of hydrogen direct-injection engines, but limited studies have been performed to characterize hollow-cone gas jets from the narrow-cone-angle pintle-type injector in supersonic flow conditions. The current study investigates supersonic gas jets from a narrow-cone-angle pintle-type hydrogen injector under various engine-like injection and ambient conditions. Jet measurements are performed using near- and far-field high-speed Schlieren imaging. The results showed that a supersonic hollow-cone jet was formed in the near-nozzle region with shock waves inside it. Then, the jet penetration was accelerated axially due to the jet collapse. The increase in the injection pressure led to a more intense and longer duration of the jet acceleration while an increase in the ambient pressure caused a weaker and delayed jet acceleration. In the steady state, the overall jet structure was divided into two regimes: the supersonic hollow-cone-jet region in the near-nozzle region and the turbulent round-jet region downstream. The jet structures appeared almost identical when the ratio of the injection pressure to ambient pressure was the same regardless of the individual injection pressure and ambient condition. Behind mechanisms of the results were discussed thoroughly based on the momentum conservation of the jet and compressible flow theories.