One of the major problems in the CFD simulation of Diesel sprays is the incomplete and inadequate specification of initial conditions for the spray droplets. This is mainly a consequence of lack of understanding of the atomization process, which has inhibited the development of sufficiently general and accurate models for use in engine combustion simulations. In this paper a novel CFD approach, combining multiphase volume-of-fluid (VOF) and large eddy simulation (LES) methodologies, is used to perform quasi-direct transient fully three dimensional calculations of the atomization of a high-pressure diesel jet, providing detailed information on the processes and structures in the near nozzle region. The methodology allows separate examination of diverse influences on the breakup process and is expected in due course to provide a detailed picture of the mechanisms that govern the spray formation. It will therefore be a powerful tool for assisting in the development of accurate atomization models for practical applications. Our investigations so far have focused on the performance and initial validation of the methodology, in the absence of cavitation.
The performance studies described herein examined the effects of solution domain extent, mesh resolution and inlet conditions. All are found to have a significant but understandable influences on the spray statistics, including the atomized and unatomized liquid mass distributions, the liquid surface area evolution and the droplet shape and size distributions. The calculated liquid core length and spray angles are found to compare reasonably well with published experimental values. The general morphology of the spray and liquid core are also investigated, including some details of specific flow features believed to be involved in the atomization process. Overall, the simulation suggests that for the conditions investigated, the breakup initialization is fundamentally three dimensional and strongly coupled to the upstream liquid turbulence generated in the nozzle. The disintegration of the liquid core is found to result from non-linear growth of selected initial perturbations through the Kelvin-Helmholtz instability mechanism, which is driven by the inter-phase aerodynamic interaction. Thus the methodology appears to provide a computational link between flow in the injector nozzle and primary break-up of the diesel jet.