Large Eddy simulation of near-nozzle shock structure and mixing characteristics of hydrogen jets for direct-injection spark-ignition engines

Abstract

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.

Due to the ever increasing prices of conventional fossil fuels, as well as climate change and sustainability issues, several liquids and gases have been proposed as alternative fuels for internal combustion engines. Hydrogen has been investigated by several researchers as a promising alternative gaseous fuel. In general gaseous fuels are injected either in the intake port of an internal combustion engine or directly into the cylinder. Direct injection of hydrogen offers higher volumetric efficiency and eliminates abnormal combustion phenomena like pre-ignition and backfire. However, due to hydrogen’s low density, direct injection requires high injection pressures to achieve suitable mass flow rates for fast incylinder fuel delivery and mixing. Such pressures typically lead to chocked conditions at the nozzle exit, followed by a turbulent under-expanded jet. Therefore, fundamental understanding of the expansion process and turbulent mixing just after the nozzle exit is necessary in order to design an efficient hydrogen injection system and injection strategies for optimised combustion. In the current study large-eddy simulations were performed to study the effect of different nozzle pressure ratios, namely 10, 30 and 70, on the nearnozzle shock structure and turbulent mixing of underexpanded hydrogen jets. The computational tool was validated against an experimental test case available in the literature. It was found that the simulation methodology captured the nearnozzle shock structure, Mach disk, reflected shocks and turbulent shear layers in good agreement with the experiments. The height and width of the Mach disk and the position of the mixing shear layer were greatly affected by the injection pressure. It was also found that for hydrogen the near-nozzle shock structure and Mach disk need considerably more time to reach an almost steady-state condition in comparison to the time claimed for heavier gases in the literature. It was also seen that during the transient period the dimensions of the Mach disk temporarily reached higher values than the final steady ones. It was also found that not all of the hydrogen jet passed through the Mach disk; hydrogen-air mixing started immediately after the nozzle exit at the boundaries of the jet but the main mixing process started after the Mach disk.