Numerical Modelling of the In-Nozzle Flow of a Diesel Injector with Moving Needle during and after the End of a Full Injection Event

Copyright © 2015 SAE International.The design of a Diesel injector is a key factor in achieving higher engine efficiency. The injectors fuel atomisation characteristics are also critical for minimising toxic emissions such as unburnt Hydrocarbons (HC). However, when developing injection systems, the small dimensions of the nozzle render optical experimental investigations very challenging under realistic engine conditions. Therefore, Computational Fluid Dynamics (CFD) can be used instead. For the present work, transient, Volume Of Fluid (VOF), multiphase simulations of the flow inside and immediately downstream of a real-size multi-hole nozzle were performed, during and after the injection event with a small air chamber coupled to the injector downstream of the nozzle exit. A Reynolds Averaged Navier-Stokes (RANS) approach was used to account for turbulence. Grid dependency studies were performed with 200k-1.5M cells. Both k-ε and k-ω SST models were considered in the validation process, with the k-ω SST found to predict better the injectors flow rate. The cavitation models of Schnerr-Sauer and the Zwart-Gerber-Belamri were employed for validation against optical data of cavitation in a simplified nozzle geometry obtained from the literature. The Schnerr-Sauer model was in better agreement with the experiments, hence this model was subsequently employed for the real injector simulations. The motion of the injector needle was modeled by a dynamic grid methodology. An injection pressure of 400 bar was applied at the inlet of the injector. Two outlet pressures were examined, 60 bar and 1 bar. The results showed that the flow was far from steady-state during the injection event and that hysteresis existed between the needle opening and closing phases. This indicated the importance of transient simulations, contrary to widely-used steady state simulations at fixed needle lifts. The two outlet pressures resulted in very different final states of the flow-field in the nozzle. Specifically, the nozzle ended up either full of liquid fuel at the end of injection or full of air after most of the fuel had been ejected into the chamber downstream. These predictions highlighted phenomena that can increase HC emissions due to fuel leakage, as well as processes that may be linked to different formation mechanisms of nozzle deposits