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Measurement and CFD modelling of diesel engine spray dynamics

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Ghiji, M 2017 , 'Measurement and CFD modelling of diesel engine spray dynamics', PhD thesis, University of Tasmania.

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Abstract

Engine manufacturers are constantly aiming to reduce exhaust gas emissions by optimizing the fuel injection process. Despite the wide use of injectors, the key physics governing the injection process are not yet fully understood. The primary atomisation process of the liquid fuel jet is initiated in the injector nozzle and the region close to the nozzle exit, influences secondary atomisation and ultimately the entire spray dynamics. Computational Fluid Dynamics (CFD) which is used in the design of fuel injectors is subjected to further developments through the inclusion of research findings. The present work focuses on processes in the nozzle and the first several nozzle diameters after the nozzle exit of a sharp edged single-hole solid cone injector. The aim of this research is to gain further insight into the primary atomisation at different stages of the injection process; the Start Of Injection (SOI); the Quasi-Steady stage; and finally the End Of Injection (EOI). Experiments are conducted using back- and side-lit microscopic imaging techniques within a constant volume high pressure spray chamber. The numerical method is based on the Volume Of Fluid (VOF) phase-fraction interface capturing technique, in an Eulerian framework integrated with Large Eddy Simulation (LES) turbulence modelling.
The influences of grid resolution, convection interpolation scheme, temporal integration scheme, compressibility, shock waves, phase changes of the liquid jet, and in-nozzle disturbances on the modelling of jet physics are investigated. The early stage liquid jet leading edge demonstrates an umbrella-shaped structure in the numerical results which is in qualitative agreement with experimental imaging. The existence of vortex before liquid fuel emergence suggests the inclusion of air in the nozzle prior to injection. The development of supercavitation down to the end of the nozzle hole leads to the detachment of the liquid from the nozzle walls, associated with the diminution of boundary layer effects and thus reduced in-nozzle turbulence and increased liquid jet velocity. The numerical and experimental results demonstrate the shock waves occur where the jet velocity at the interface with the surrounding air exceeds the local speed of sound. Analysis of the EOI process shows the mechanism of air ingestion into the nozzle liquid due to the high inertia of liquid jet emerging from the nozzle. Numerical results show not only how a single bubble of chamber gas remaining embedded within the liquid in the nozzle hole, but also show entrainment of air into the sac volume of the injector at the EOI. These results provide further insight into the air ingestion process, an explanation for the presence of gas at the SOP in the experimental images. The existence of large droplets at the EOI is shown, which is potentially a source of unburnt hydrocarbons and detrimental emissions in a real engine.

Item Type: Thesis - PhD
Authors/Creators:Ghiji, M
Keywords: Primary atomisation; In-nozzle flow; Diesel spray; Large eddy simulation; Cavitation; Shock wave; CFD
Copyright Information:

Copyright 2016 the author

Additional Information:

Chapter 2 appears to be the equivalent of a post-print version of an article published as: Ghiji, M., Goldsworthy, L., Brandner, P. A., Garaniya, V., Hield, P., 2016. Numerical and experimental investigation of early stage diesel sprays, Fuel, 175, 274-286

Chapter 3 appears to be the equivalent of a post-print version of an article published as: Ghiji, M., Goldsworthy, L., Brandner, P. A., Garaniya, V., Hield, P., 2017. Analysis of diesel spray dynamics using a compressible Eulerian/VOF/LES model and microscopic shadowgraphy, Fuel, 188, 352-366

Chapter 4 appears to be the equivalent of a post-print version of an article published as: Ghiji, M., Goldsworthy, L., Brandner, P. A., Garaniya, V., Hield, P., 2017. End of injection processes in a single hole diesel injector, Atomization and sprays, 28(1), 23-45

Appendix II has been published as: Ghiji, M., Goldsworthy, L., Garaniya, V., Brandner, P. A., Hield, P., 2016. CFD modelling of primary atomisation of diesel spray, Proceedings of the 19th Australasian Fluid Mechanics Conference, Royal Melbourne Institute of Technology, Melbourne, Australia, 8-11 December 2014, paper 57.

Appendix III has been published as: Ghiji, M., Goldsworthy, L., Garaniya, V., Brandner, P. A., Hield, P., 2016. Effect of air ingestion at the start of injection process in a diesel injector’, Proceedings of the 20th Australasian Fluid Mechanics Conference, University of Western Australia, Perth, Australia, 5-8 December 2016, paper 581

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