NUMERICAL INVESTIGATION ON EFFECTS OF ELLIPTICAL DIESEL NOZZLE ON PRIMARY SPRAY CHARACTERISTICS BY LARGE EDDY SIMULATION (LES)

Fuel-air mixing quality was increased dramatically by using the elliptical orifice diesel nozzle. In this paper, the large eddy simulation (LES) and the volume of fluid (VOF) models were coupled to investigate the mechanisms of spray axis-switching and the breakup process for elliptical orifices with various aspect ratios (1.4 and 1.8), and a circular one with the same cross-section area, at the injection pressure of 120 MPa and backpressure of 3 MPa. The results indicated that the orifice center velocity of the elliptic nozzle was smaller than the circular one, and the elliptical orifice with a larger aspect ratio had the lowest centerline velocity and the smallest centerline mass fraction among three orifices. Moreover, the axis-switching becomes more frequent when the aspect ratio increases, while there was no axis-switching phenomenon for circular spray during the whole injection process. Furthermore, a mushroom-like structure was observed at the tip of the spray for all geometry orifices, and the stream surface fluctuation for the circular spray was smaller than that of the elliptical spray. The liquid core of the circular orifice was longer than that of the elliptical orifice, and the elliptical orifice with the aspect ratio of 1.8 experienced the shortest liquid core. Also, the elliptical orifice with a large aspect ratio of 1.8 had the most turbulent vortex structures downstream of the spray. The main turbulent vortex structures are mainly concentrated in the area of the spray liquid core fracture, which indicated that the turbulence vortex structures could induce the spray breakup. There was a mushroom-like structure for the magnitude of vorticity at the tip of the spray, similar to the liquid mass fraction distribution. The elliptical orifice with the aspect ratio of 1.8 had the most air entrainment vortices along the spray among the three orifices.