Effects of Cavitation and Hydraulic Flip in 3-Hole GDI Injectors

The performance of Gasoline Direct Injection (GDI) engines is governed by multiple physical processes such as the internal nozzle flow and the mixing of the liquid stream with the gaseous ambient environment. A detailed knowledge of these processes even for complex injectors is very important for improving the design and performance of combustion engines all the way to pollutant formation and emissions. However, many processes are still not completely understood, which is partly caused by their restricted experimental accessibility. Thus, high-fidelity simulations can be helpful to obtain further understanding of GDI injectors. In this work, advanced simulation and experimental methods are combined in order to study the spray characteristics of two different 3-hole GDI injectors. First, a simulation approach for computing cavitation and hydraulic flip is presented, which appropriately combines simulations with different levels of abstraction allowing for predictive GDI injector simulations on currently available supercomputers. Next, general resolution requirements and various initial conditions are discussed. Especially the effect of initial bubbles inside the sac on the cavitation formation process is investigated by performing a transient simulation with moving needle. Finally, coupled compressible Large-Eddy Simulations (LES) of the internal nozzle flow with and without models for cavitation and hydraulic flip, Direct Numerical Simulations (DNS) of the resulting primary breakup, and far-field Lagrangian Particle-based LES (LP-LES) are performed in order to achieve a detailed data set of the injection process of two different 3-hole GDI injectors. X-ray measurements of in-nozzle cavitation as well as droplet size distribution obtained by Phase Doppler Particle Analyzer (PDPA) measurements in the far-field are used for validation and understanding the impact of the different effects on the underlying breakup processes. Furthermore, sensitivities between nozzle design features and hydraulic flip are investigated.