Numerical Investigation and Experimental Comparison of ECN Spray G at Flash Boiling Conditions 2020-01-0827
Fuel injection is a key process influencing the performance of Gasoline Direct Injection (GDI) Engines. Injecting fuel at elevated temperature can initiate flash boiling which can lead to faster breakup, reduced penetration, and increased spray-cone angle. Thus, it impacts engine efficiency in terms of combustion quality, CO2, NOx and soot emission levels. This research deals with modelling of flash boiling process occurring in gasoline fuel injectors. The flashing mass transfer rate is modelled by the advanced Hertz-Knudsen model considering the deviation from the thermodynamic-equilibrium conditions. The effect of nucleation-site density and its variation with degree of superheat is studied. The model is validated against benchmark test cases and a substantiated comparison with experiment is achieved. It is noticed that immediately after the removal of the rupture disk, a sudden depressurization occurs at the pipe’s exit resulting in the onset of violent evaporation due to flashing which limits the pressure decrease to a value slightly below the saturation pressure. Later, the flash boiling model is applied to investigate a real-size 8-hole GDI injector from Engine Combustion Network (ECN). Preliminary analysis reveals intense generation of vapor due to flashing inside as well as at the nozzle exit, hole to hole interactions in the nozzle sac and appearance of stable string cavitation in multiple nozzle holes. Rate of injection and fuel mass density over radius 2mm from nozzle tip, compared with X-ray spray radiography experiments from Argonne National Laboratory (ANL), show a good agreement. Wide spray angles as in experiments, and spray plume to plume interaction due to asymmetrical flow field leading to nozzle tip wetting, are also observed in the investigation. Finally, an example is given for the application of the model in the context of a 3D CFD in-cylinder flow, spray, combustion and emission simulation supporting engineers in taking design decisions.