Caracterisation des structures complexes de l'ecoulement interne d'un injecteur diesel a levee d'aiguille fixe, utilisant un modele numerique RANS.

摘要

This dissertation aims to improve our understanding of the needle lift influence on the internal flow of single-hole diesel injector. To achieve it, a numerical RANS model was developed. This model is able to predict the dynamic which occurs in diesel injector subjected to realistic injection pressure. Moreover, this model can be adapted for several needle lifts in order to study the influence of this setting. Thus, for a steady injection pressure, it has been concluded that flow coefficients reach a steady value very quickly whatever the needle position. It means that the injected fuel mass rate is the same between a fully opened lift and several intermediate lifts. This study has also identified that the sac volume, upstream the nozzle, is the most complex area for the flow dynamic. Indeed, it was observed that the throttling between the needle and the injector body was responsible of the formation of a high velocity jet which creates several vortex structures in the sac volume. Moreover, this jet can be either stuck against the needle wall or unstuck, depending on the needle position. Creating several vortices in the sac volume. Thereafter, the fluctuating injection pressure influence was studied. Indeed, many previous studies have shown that in realistic injection condition, the pressure in the diesel injector was never steady but it is fluctuating pressure which oscillates around a mean value. Thus, the purpose of the study was to analyse how this unsteady pressure can modify the internal flow behaviour and its vortex structures, in the sac volume. It was concluded that depending on the needle position and the pressure signal frequency, the dynamic response was different. In some case, the initial unstuck jet can be excited by the signal and it is going to sick back against the needle wall. Thus, the flow dynamic is reorganized in the sac volume. A proper orthogonal decomposition (POD) on the sac volume velocity field was done in order to extract coherent structures . From the best our knowledge, this approach was never used to study the flow dynamic behaviour in diesel injector.