End of injection process in a single-hole diesel injector

The end of injection processes in a single-hole high-pressure diesel injector is investigated experimentally and numerically. Experimental measurements are performed using a laser-based backlit imaging technique. Numerical investigation of in- and near-nozzle fluid dynamics is conducted in a Eulerian framework using a volume-of-fluid interface capturing technique integrated with large eddy simulation turbulence modeling. An incompressible noncavitating and a compressible cavitating model are employed to gain a clearer understanding of the nozzle air ingestion mechanism at the end of injection. In the compressible model, a basic cavitation model is allowing liquid fuel to flash to gas at the fuel vapor pressure. The results show that upon needle valve closure, the high-energy core fluid maintains outward flow but the peripheral flow has reversed, as required by continuity, thus ingesting chamber air into the nozzle. The remaining unstable flow forms an asymmetry enhancing the air ingestion. Numerical results of the incompressible noncavitating model show a single bubble of chamber gas remains embedded within the liquid in the nozzle hole only after velocities have largely dissipated. The results of the compressible cavitating model demonstrate how chamber gas is entrained into the sac volume through the air passage previously generated by hydraulic flip. These results provide an explanation of a mechanism for air ingestion at the end of injection recently described using X-ray imaging. This mechanism also provides a possible explanation for the presence of air within the emerging jet of subsequent injections.