City Research Online

Abstract

In the last 20 years the automotive industry has managed an impressive reduction in emissions, in agreement to relevant regulations. One key component in achieving lower emissions is the fuel injector nozzle. This PhD Thesis aims to present an experimentally validated computational fluid dynamics (CFD) method able to better describe the flow developing inside Diesel and gasoline fuel direct injection nozzles. The compressible Navier-Stokes equations are numerically solved considering the vertical motion of the injector’s needle valve. A homogenous three-phase (liquid, vapour and air) barotropic equation of state linking the variation of density and pressure is implemented into the utilised flow solver. This model is able to capture cavitation and its collapse as well as the air entrainment inside the injector taking place in between successive injection events. The flow turbulence is modelled using LES, in order to better understand the details of the physical processes, and URANS, as a suitable method for industry product development time scales and the available CPU resources. For the case of Diesel fuel injection, highspeed visualisations of a transparent nozzle tip replica are used to validate the proposed methodology. The numerical simulations describe the interaction between the vortex flow and cavitation formation that take place simultaneously with air entrainment from the surrounding environment into the injector’s sac volume during the injection and the dwell time between successive injections. The experimentally observed flow phenomena are well predicted by the simulation model, namely, the compression of pre-existing air bubbles inside the injector’s sac volume during the injector opening, cavitation vapour condensation and air suction after the needle closure. In the case of Gasoline direct injection (GDi) nozzles operated with ethanol (E100) fuel, emphasis is put on the prediction of erosion sites. E100 represents a challenge to the durability of the fuel injection system components since it can result in corrosion which is further enhanced by cavitation induced erosion as a result of the collapse of vapour structures. CFD predictions for both the flow development and the locations prone to cavitation erosion inside GDi injectors are reported. The CFD simulations predict the flow structures leading to the observed erosion locations in the nozzle. Three cavitation erosion indices reported in literature are evaluated against new experimental data of erosion damage. Scanning electron microscope erosion images are found to correlate well with the accumulated erosive power predicted by the simulation.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TL Motor vehicles. Aeronautics. Astronautics
Departments:	Doctoral Theses School of Science & Technology > School of Science & Technology Doctoral Theses School of Science & Technology > Engineering > Mechanical Engineering & Aeronautics

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