Visualization research on injection characteristics of high-pressure gas jets for natural gas engine

Highlights: • The evolution of high-pressure gas jet macroscopic structure is studied. • Gas injection pressure has little effect on the development of jet tip penetration. • Different empirical formulas of gas jet tip penetration are investigated. • The mixing effect of gas jet is not significantly affected by gas injection pressure. - Abstract: High-pressure gas injection processes with various gas nozzle structures under different nozzle pressure ratio (NPR) were investigated experimentally. Schlieren imaging method was used to investigate macroscopic structure of gas jets. Results showed that increasing the gas injection pressure is unable to improve the jet tip penetration obviously. It is because chocking phenomenon limited gas jet velocity at nozzle exit under high gas injection pressure. The empirical formula for high-pressure gas jet tip penetration was investigated based on the experimental data under various NPRs and orifice diameters. It is found that the experimental data are in good agreement with the empirical formula proposed by Hamzehloo and Aleiferis. The increase of gas injection pressure and orifice diameter could not directly improve the air entrainment effect, nor could it increase the average equivalent ratio of gas jet due to the small gas jet kinetic energy and small density difference between gas jet and the surrounding ambient gas.