This paper is to present a detailed case study on how the nozzle flow dynamics influences the primary breakup in the spray formation process of diesel injection. The investigation was based on a 3-hole real-application nozzle with highly tapered injection holes using a URANS-LES (Large Eddy Simulation) hybrid approach in combination with the coupled Volume of Fluid (VOF) and Level Set method. High resolution LES was applied to simultaneously resolve the multi-scale nozzle flow dynamics downstream of the needle seat and the primary breakup process in the near-nozzle spray. Phase Contrast X-ray imaging (PCX) was applied to characterize the liquid-gas interfaces in the near-nozzle spray for validation purposes. The results provide detailed information on how the vortex shedding and vortex interactions in the injection hole drives the jet deformation, ligament and droplet formation in the primary breakup process.
Keywords
Primary breakup, Fuel injection, Vortex dynamics, LES, Phase Contrast X-ray imaging
IntroductionClean internal combustion engine technology improvement requires the capability to control and optimise the fuelgas mixing, ignition, and combustion process. However, how to transfer the individual engine requirements on the spray to a specific nozzle design still remains a challenging engineering task. One blocking point is the lack of detailed understanding on the fundamental physics of the primary breakup process. This process involves highly complex multi-phase and multi-scale fluid dynamics phenomena, including turbulence, cavitation and their interaction. A significant number of investigations have been dedicated to the cavitation phenomenon over the last 30 years. As for turbulence, the scales and dynamics of the vortex structures in the nozzle flow need to be understood. Two experimental investigations have reported vortex phenomena in injection nozzles. One is the cavitation visualisation of (1) in a real-size VCO nozzle. The vapour distribution in the injection holes indicated the occurrence of strong swirling vortex structures and vortex shedding. Though the investigation was focused on the in-nozzle flow, the authors proposed that the vortex shedding can impact the jet breakup downstream of the injection hole exit. Another is the string cavitation characterization in a scale-up nozzle (2), which demonstrated that string cavitation is caused by large-scale vortex strings in the sac and injection holes and has a correlation with the fluctuation of the spray dispersion angle. Nevertheless, the vortex structures are expected to be much more complex and have richer scales in real applications due to much higher velocity gradients. It is almost impossible to make detailed experimental characterization of field turbulence and vortex dynamics inside a realsize nozzle due to the small dimensions and high speed of the problem. CFD simulation is advantageous over measurement techniques to gain insight into the nozzle flow dynamics and vortex structures and their impact on the spray as shown in (...
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