Extension of Lagrangian-Eulerian Spray Modeling: Application to High Pressure Evaporating Diesel Sprays

The Lagrangian-Eulerian approach is commonly used to simulate engine sprays. However typical spray computations are strongly mesh dependent. This is explained by an inadequate space resolution of the strong velocity and vapor concentration gradients. In Diesel sprays for instance, the Eulerian field is not properly computed close to the nozzle exit in the vicinity of the liquid phase. This causes an overestimated diffusion that leads to inaccuracies in the modeling of fuel-air mixing. By now it is not possible to enhance grid resolution since it would violate requested assumptions for the Lagrangian liquid phase description. Besides, a full Eulerian approach with an adapted mesh is not practical at the moment mainly because of prohibitive computer requirements.

Keeping the Lagrangian-Eulerian approach, a new methodology is introduced: the full Lagrangian-Eulerian Coupling (CLE). It consists in retaining vapor and momentum along parcel trajectories as long as the mesh is unsufficient to resolve the steep gradients. Vapor and momentum are gradually released on the mesh following specified diffusion laws.

This model is briefly described. CFD simulations are performed using the KMB code, a modified version of KIVA-II. Comparisons are presented of computed and measured penetrations of liquid and vapor in a high pressure, high temperature Diesel simulation cell. The new CLE model is shown to reproduce properly the evaporating spray structure and penetration while having a very limited mesh sensitivity. DI Diesel engine calculations using a rather crude mesh show a very good agreement of liquid and vapor penetrations. This insures an improved predictivity of the combustion in full load cases.