Numerical Investigation on GDI Spray under High Injection Pressure up to 100 MPa 2020-01-2108

In recent years, the increase of gasoline fuel injection pressure is a way to improve thermal efficiency and lower engine-out emissions in GDI homogenous combustion concept. The challenge of controlling particulate formation as well in mass and number concentrations imposed by emissions regulations can be pursued improving the mixture preparation process and avoiding mixture inhomogeneity with ultra-high injection pressure values up to 100 MPa. The increase of the fuel injection pressure in GDI homogeneous systems meets the demand for increased injector static flow, while simultaneously improves the spray atomization and mixing characteristics with consequent better combustion performance. Few studies quantify the effects of high injection pressure on transient gasoline spray evolution.

The aim of this work was to simulate with OpenFOAM the spray morphology of a commercial gasoline injected in a constant volume vessel by a prototypal GDI injector. Different operating conditions were considered under very high injection pressure up to 100 MPa. The transient spray evolution in a constant volume vessel was analyzed from an experimental and numerical point of view in different ambient conditions. The resulting development of the jet plumes was assessed, along with the physical effects of injection pressure.

A RANS Eulerian-Lagrangian approach was adopted to couple the gas phase with the liquid jet and a complete validation of atomization and secondary breakup models was performed. Furthermore, different values of ambient pressure were investigated to validate the robustness of the proposed numerical set-up in different ambient conditions.

Experimentally, an optical technique characterized by a hybrid Mie-scattering /shadowgraph approach were adopted registering images on a high-speed C-Mos camera. The spatial distribution and the time-resolved evolution of the free sprays were derived under different ambient conditions along with their characteristics.

Numerical simulations allowed a good reproduction of the fuel penetration and spread in the constant vessel under very high fuel injection pressure, depicting the strong sensitivity of the spray profiles against the ambient conditions and confirming fundamental information on the physics of fuel provided by the experiments.

Under flash-boiling settings, the very high injection pressures induced a loss of the classic mushroom morphology, related to the spray-collapse, because the increased droplet velocities, along the axial direction, become a dominant effect.