Caractérisation des effets statique et dynamique de l'aiguille sur l'écoulement interne d'un injecteur diesel avec une approche LES

Abstract

Reduction of pollutant emissions is achieved in modern internal combustion engines with multiple injection strategies. However, internal flow characteristics of the injector become highly transient in this context and therefore need to be investigated. The sac-flow dynamics has been reported to influence the nozzle-flow characteristics, however, only few studies have characterized the sac-volume as the in-nozzle flow and cavitation have been the main subjects covered in the literature so far. Thus, this thesis objective was to perform a numerical characterization of needle’s displacement effects on the sac-volume internal flow of a single-hole Diesel fuel injector. Large Eddy Simulation (LES) approach was adopted to gain a better insight into the dynamics of this complex flow. The elaborated model reproduces the monophasic incompressible flow inside the fuel injector at a constant pressure difference with the effect of the axial needle’s displacement. On the one hand, the resolution of near-wall turbulence has been validated with Direct Numerical Simulation (DNS) results in the canonical case of fully developed channel flow. On the other hand, the resolution of large turbulent scales within the sac-volume (far from the wall) has also been verified using an energy spectrum. This model was used to simulate the injector internal flow at partial fixed needle lifts (static model) first, then with needle movement for an injection cycle simulation « opening-closing-opening » (dynamic model). Qualitatively, the sac-volume flow exhibits a generic structure composed of a high-speed fuel jet that separates two dynamic regions. While the vorticity is mainly produced in the needle seat region, the turbulent structures in the sac volume are generated by the instabilities at the jet shear layer. The jet characteristics and the associated instabilities are directly influenced by the needle position. With needle opening, the jet gains in thickness but its kinetic energy decreases and, consequently, shearing at the jet interface and turbulence are reduced in the sac volume too. Quantitatively, the application of Proper Orthogonal Decomposition method (POD) to the LES results of the static model showed that the relative contribution of turbulent kinetic energy (to the total energy) decreases with the needle lift from 55 % to ~10 % only between the lower (6 %) and the higher lift (31 %). The mean kinetic energy is primarily concentrated in the jet fuel (mode 0), while the higher POD modes (1 to 3) decompose the big dynamic region at the jet periphery. A transfer of energy between POD modes was observed depending on the needle lift position. Furthermore, a flow dynamics reconstitution with a reduced-order model has shown that the largest scale of unsteadiness is responsible for a fuel jet oscillation at low frequency (~1 kHz). The intensity of this oscillation was found to decrease with increased needle lift. The transient movement of the needle causes a hysteresis effect which is responsible for the fuel jet detachment from the needle wall in the closing phase, unlike the opening phase and the static model as well. The comparison between static and dynamic models showed similar performance at high needle lifts (beyond 14 %), while the needle movement at low lifts has significant effects on the sac-flow characteristics. Assessment of these effects within the LES framework allowed a better understanding of the needle movement influence onto the dynamics of large coherent structures within the sac-volume.

Date	1 Jun 2018
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Patrice Seers (Supervisor) & Louis Dufresne (Co-supervisor)