Numerical Modelling of Diesel Spray Injection, Turbulence Interaction and Combustion

Doktorsavhandling, 2008

This thesis covers two main topics. The first is numerical modelling of cavitating diesel injector flows, focusing on describing such flows using a single-phase cavitation model based on a barotropic equation of state together with a homogenous equilibrium assumption. The second topic is Euler-Lagrangian simulations of diesel sprays, focusing on attempts to reduce the high grid/timestep dependencies in numerical simulations of diesel sprays. In addition, the ability of two CFD codes to predict flame lift-off length and ignition delay time, and the advection scheme’s influence on fuel distributions, are considered. A long-term goal was to develop a new atomization model based on calculated flows in injector nozzles, which did not have the drawback of requiring either non-physical parameters or information derived from specific experiments. To validate the cavitation simulations, comparisons were made with experimental data obtained at AVL. The experimental data (which are practically 2D) provide information on velocity profiles and pressure contours. These data were used to validate the code. However, since the code is not stable for diesel-type pressures, no atomization model was developed. The main part of the thesis describes how diesel sprays were simulated using the discrete droplet model (DDM), in which the liquid is described by Lagrangian coordinates and the vapour by an Eulerian approach. The simulations have been used to investigate how the k-ε family of turbulence models influence spray behaviour, and a simple but efficient way to reduce the dependency of the mesh resolution, by limiting the turbulence length scale in the liquid core region, is proposed. This constraint is shown to have a positive effect on the spray behaviour, and to reduce both grid and timestep dependencies. In addition, the ignition delay time and flame lift-off lengths have been investigated, since these two properties are believed to be important for emissions formation. The simulations used a complex chemical mechanism involving 83 species and 338 reactions. The effects of the numerical scheme, the turbulence model and physical parameters (like ambient temperature and oxygen content) on these variables have also been investigated.