Laurie Goldsworthy

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 an Eulerian framework using a Volume of Fluid interface capturing technique integrated with LES turbulence modeling. An incompressible non-cavitating 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 non-cavitating 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.

In the present study, a new approach for modelling emissions of coke particles or cenospheres from large diesel engines using HFO (Heavy fuel oil) was studied. The model used is based on a multicomponent droplet mass transfer and properties model that uses a continuous thermodynamics approach to model the complex composition of the HFO fuel and the resulting evaporation behavior of the fuel droplets. Cenospheres are modelled as the residue left in the fuel droplets towards the end of the simulation. The mass-transfer and fuel properties models were implemented into a cylinder section model based on the Wärtsilä W20 engine in the CFD-code Star CD v.4.24. Different submodels and corresponding parameters were tuned to match experimental data of cylinder pressures available from Wärtsilä for the studied cases. The results obtained from the present model were compared to experimental results found in the literature. The performance of the model was found to be promising although conclusive validation of the model would require more detailed experimental results about cenosphere emissions from the specific case studied here. According to the results obtained from this model the emissions of cenospheres are a function of both operating conditions and fuel properties. While the droplet evaporation and properties models were used in this study to model cenosphere emissions, the approach could also be used to study the combustion behavior of HFO in a broader sense.