Experimental And Numerical Investigation Of Internal Flow And Near-Field Spray Structure Of Nozzles

Knowledge of high-speed liquid jet atomization mechanism is limited at present; especially the disturbance source of the initial spray breakup is not enough understanding, the initial disturbance from the upstream of spray, namely the internal flow of fuel injector nozzle.In this paper, the internal nozzle flow and spray structure of near-field experiments and numerical simulation had been performed.Combined the shadow method and high-speed flash photography, and calculating with open source CFD code for studying of the same region.Based on high-pressure common rail systems for high-speed flash photography device, the device is in a dark environment principle, the use of digital camera shutter B, transient exposure of high-speed flash imaging. Designed and machined the transparent nozzle cap, whose material is acryl.For the purpose of research real scale of the pressure chamber and nozzle, original metal nozzle tip was replaced. In order to clearly observe the internal nozzle flow and the structure of near-field spray, utilizing home-made Ring to match the long-distance microscope and digital camera, near the outlet of the nozzle and spray area for 27 times magnification. Delay control module was developed the difficulties of capturing by a hand-press the shutter effectively, making flash sync time with the objective of shooting time. Experimental on the different structures of the nozzle and varied rail pressure, comparative analysis of different internal flow and near-field spray structure of nozzle. Results show that cavitaion viod and strong turbulence were existed within the nozzle hole, those promote collapse of bubbles near the outlet of the orifice.Distribution of cavitation flow is extremely sensitive on the structure of nozzle, leading to the breakup of near field spray structure diverse directly.Under the operating system of Linux, Simulation of internal flow and near field jet atomizing process of nozzles based on large eddy simulation (LES) and volume-of-fluid (VOF) used open source CFD code-OpenFOAM. LES is employed for solving turbulent field of two-phase flow, and VOF can track the interface of mixed flow. For minisac nozzle, the numerical results reveal that "umbrella" is formed and breakup mechanism at the tip of initial spray, which had a good agreement with experiment. The origin of the jet-string breakup was validated which was the K-H instable wave. Further analysis that initial perturbations of jet interface was effected by both internal turbulent of liquid and boundary relaxation of nozzle exit. For valve over orifice (VCO) nozzle, jet surface had significant asymmetric disturbances, the disturbance contribute to air entrainment, which promoted atomization; added a bubble that slightly larger diameter of hole into nozzle hole, bubble had been squeezed, and found that part of the kinetic energy was absorbed in the process of extrusion, ultimately, intensity of non-axisymmetric of the surface of jet was weaken.