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.
