Natural gas jet evolution and structural characterization of shockwaves downstream of an injector in a simulated engine environment

Fuel injection controls the Internal Combustion engine's fuel-air mixture quality and combustion performance. Hence, fuel spray characterization helps us to understand the underlying physics behind the processes and evaluate the impact of affecting parameters. In this study, methane jet characteristics are investigated via computational fluid dynamics simulations under different fuel injection pressures (8, 16, and 24 bar) and ambient pressures (1, 2, and 4 bar), resembling conditions prevailing in port and direct-injection application of compressed natural gas fueled engines. The computational domain comprised the injector body of the commercial gas injector located at the top of the constant volume spray chamber. This numerical study used Reynolds-averaged Navier–Stokes equations modeling to simulate the whole injection process and jet evolution in a spray chamber using Converge 3.0 software. The model was validated by comparing the jet tip penetration with experimental data at a fuel injection pressure of 8 bar and an ambient pressure of 1 bar. The results showed the methane jet development and macroscopic structural characteristics under different injection conditions. Increasing the pressure ratio to 16 and 24 led to surface perturbations on the jet periphery due to the development of secondary instabilities. The propensity of choking reduced the rate of increase in jet penetration at higher pressure ratios (>16). The fundamental underlying differences behind shockwave formation and characteristics of subsonic, moderately under-expanded and highly under-expanded jets were analyzed. Furthermore, the discussion was strengthened by studying the variation of Mach disk parameters, thermodynamic parameters, flow field, turbulence intensity, and vorticity characteristics for different categories of jets. Vorticity strength and large-scale coherent structures were observed to diminish with increasing pressure ratio. The energy conversion efficiencies of the injection process were also calculated for different injection conditions. The transfer efficiency deteriorated at a pressure ratio of 4 or lower, while pressure ratios ranging from 6 to 24 resulted in comparable values of 80%–82%.