Effect of air ingestion at the start of injection process in a diesel injector

The effect of the presence of air in the injector nozzle at the Start of Injection (SOI) in a single-hole high-pressure diesel injector is investigated experimentally and numerically. Experimental measurements are performed using a laser-based backlit imaging technique through a long distance microscope. Numerical investigation of, in- and, near-nozzle fluid dynamics is conducted in an Eulerian framework using a Volume of Fluid interface capturing technique integrated with Large Eddy Simulation (LES) turbulence modelling. Experimental images show transparency in the emerging jet suggesting the presence of air trapped inside the nozzle liquid from the previous injection event. The numerical model provided a clearer insight into the influence of air on the structure and dynamics of an emerging jet at the SOI. A mathematical code is developed to replicate the backlit imaging approach with the numerical results. The virtual images demonstrate a transparent liquid jet emerging into the pressurised spray chamber gas, in improved agreement with the experimental images. The inclusion of air in the nozzle prior to injection in the numerical model also yields improved agreement in the penetration velocity profile of the jet. These results explain how air inclusion inside the nozzle liquid affects the physics of the penetrating jet at the SOI. The air inclusion also provides an explanation for not only the transparency of the emerging jet but also rough interfacial surfaces captured at the very early stages of injection.