Formation du mélange par des stratégies d'injection simple et double en milieu non-évaporatif avec une approche LES

Abstract

Multiple injection strategies are able to reduce the pollutant emissions of internal combustion engines. However, the flow dynamic associated with this practice is still misunderstood. Thus, this thesis’s objective was to improve the understanding of the interaction between the fuel spray and the ambient environment by numerically studying the behavior of multiple injection diesel fuel spray. Fuel spray numerical predictions greatly depend on the model used to represent droplet fragmentation, particularly its calibration. The first part of this thesis consisted in determining, with a Reynolds averaged approach, the optimum calibration for two of the most used fragmentation models : the Reitz & Diwakar and KHRT models. Those calibrations were obtained by proposing a new methodology based on a design of experiment method which allowed reducing the number of simulations. Thus, this method allowed highlighting the relative influence of each constant as well as the coupling between the constants’ effects. Finally, the prédictions obtained with the optimum calibrations showed that the KHRT model provides a more realistic behavior with only calibrating two constants. The restriction of the number of constants also allowed, after an analytical study of the model, to successfully predict the model’s calibration for higher injection pressures with little additional simulations. Given the need to higher details on the Eulerian phase to fulfill the objective, a large eddy simulation method was then chosen. This method being highly influenced by the numerical configuration used, a validation method based on the similarity between a turbulent round jet and a fuel spray was proposed. First, a turbulent round jet with initial conditions extracted from the air entrainment induced by a fuel spray was simulated. This jet was successfully compared with turbulent round jet and fuel spray experimental results. Then, the same numerical configuration was applied to a fuel spray and allowed good predictions of both the Lagrangian and Eulerian phases, particularly of the axial and radial velocity profiles. The validated large eddy simulation numerical configuration was applied to simulate a high pressure simple injection but also a double injection at the same pressure. Those simulations allowed studying the mixing process between the fuel and the Eulerian phase by using 3D and 2D criteria. From the 3D point of view, the Lagrangian phase was analyzed in parallel with the Q criteria applied to the Eulerian phase. Then, the 2D study consisted in visualizing the streamlines of the Eulerian phase with the fuel fraction distribution on the fuel spray midplane. Those visualizations highlighted the creation of an annular recirculation structure by the impulsion given at the injection start, and also the creation of helices-like structures around the jet axis during the rest of the injection. Moreover, the 2D representation also showed the influence of the coherent structures of the Eulerian phase on the fuel diffusion, which is closely related to the mixing process.

Date	17 Jan 2017
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Patrice Seers (Supervisor) & François Garnier (Co-supervisor)