Numerical modelling of transient under-expanded jets under different ambient thermodynamic conditions with adaptive mesh refinement

The mixing characteristics of highly turbulent under-expanded gaseous fuel jets issued from millimeter-size circular nozzles are important for developing advanced direct injection gaseous-fuelled internal combustion engines. In the present study high-resolution large eddy simulation in conjunction with an adaptive mesh refinement technique was used in order to investigate key mixing characteristics of under-expanded hydrogen and methane jets under various ambient thermodynamics. Penetration rate, volumetric growth and initial transient vortex ring behaviour were investigated under near atmospheric and elevated ambient pressures and temperatures, P∞ ≈ 1 bar and 10 bar, T∞ = 296 K and 600 K, using a nozzle pressure ratio of 10. The conditions corresponded to injection strategies ranging from late intake stroke around inlet valve closure to late compression. It was observed that increasing the ambient temperature at constant pressure resulted in increase in both tip penetration and volumetric growth of the under-expanded jets. It was also found that the effect of diffusivity, ratio of specific heats and ambient density must be considered when scaling volumetric growth of under-expanded jets of different gases and/or when issued into different ambient temperatures. Moreover, substantial differences were observed between the transient jet formation of hydrogen and methane fuels. It was found that the embedded shock structures and supersonic annular shear layers played a significant role in the formation and evolution of the transient preliminary and secondary vortex rings. It was also noted that the evolution of the vortex ring influenced significantly the volumetric growth and hence mixing quality of under-expanded jets.