抄録
The spray atomization mechanisms under the common conditions for Diesel engines are very complex and not completely understood. At the same time the detailed spray information is necessary to provide a better control of the combustion process. The main difficulty of numerical spray simulation is the correct representation of the two characteristic zones of the spray: dense near the nozzle and dilute downstream. Combining the advantages of Eulerian and Lagrangian approaches, ELSA model is able to continuously predict the whole spray evolution. In the dense zone the spray and its gaseous environment are presented as an effective single-phase fluid with a highly variable density. To describe the liquid dispersion, transport equations for the liquid mass fraction and for the liquid/gas interface density are solved. The transition to the Lagrangian calculation is applied when the spray is considered to be enough diluted. This complete Eulerian-Lagrangian spray atomization model has been implemented into the computational fluid dynamics code STAR-CD. The model implementation was validated by comparing predicted liquid and vapor penetrations with experimental data reported in literature. Once validated, the ELSA model was coupled with an inside nozzle simulation to study the impact of internal nozzle flow (geometry, cavitation formation) on the spray and its characteristics. Finally, the coupling with the ECFM-CLEH combustion model has been completed in this work. ECFM-CLEH stands for Extended Coherent Flame Model with Combustion Limited by Equilibrium Enthalpy. It simulates the different phases of Diesel combustion i.e. the auto-ignition, the premixed and the diffusion flame. In the premixed phase, combustion is mainly controlled by flame propagation while fuel and air mixing play an important role in the diffusion phase. The auto-ignition is modeled from tabulated fully detailed chemistry. The tabulated TKI (Tabulated Kinetics for Ignition) strategy proposed by IFP in combination with a presumed PDF using the temperature fluctuations for integration is used. Also a special attention has been made to improve the NOx formation considering the extended Zeldovich thermal NO. The set of sub-models presented above has been implemented in STAR-CD. Applications and validations for a full DI Diesel combustion process have been performed for typical automotive DI Diesel engines with variation of EGR level, Injection Timing and injector's nozzle diameter.
