Numerical Modelling of Diesel Spray Injection and Turbulence Interaction

Licentiatavhandling, 2006

It has been established by many authors that numerical simulations of Diesel sprays, using a Lagrangian description for the liquid phase, are particularly sensitive to the scale of the computational cells compromising the mesh. There have been many suggestions regarding means to reduce the dependency on the mesh by improving the component submodels, but few have addressed the gas phase turbulence modelling. This thesis covers two main topics. The first is the application of cavitation models in order to develop a new primary atomization model for Diesel spray. The second is to investigate the effect of modifying the length scale used in turbulent dispersion models for particles in turbulent flows. The goal of the nozzle flow calculations is to develop a new atomization model, that does not have the drawback of requiring either non-physical parameters or information derived from specific experiments. To validate the cavitation simulations, comparisons with experimental data obtained at AVL were made. The experiments show velocity profiles and pressure contour, and are practically 2D. The results of the cavitaiton simulations do not represent reality to a satisfactory degree yet. Therefore, a new atomization model has not been developed. Since most industrial applications are based on eddy viscosity k-epsilon type models, the spray investigations were limited to this kind of turbulence model. Three versions were tested, the goal was to evalute their effect on the spray behaviour and sensitivity to mesh resolution. This thesis shows that the turbulence model plays a significant role in the sprays' behaviour on grids of different spatial resolution, and a simple and efficient way to reduce the dependency of the mesh resolution, by limiting the turbulent length scale in the liquid core region, is proposed. It is shown that this constraint has a positive effect on the spray behaviour, and reduces grid dependence.