Butcher, Adrian James; (2020) Spray Formation and Cavitation of Fuel Injectors with Various Metal and Optical Nozzles. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis addresses the need for fundamental understanding of the mechanisms of fuel spray formation and mixture preparation in direct injection spark ignition (DISI) engines. Fuel injection systems for DISI engines undergo rapid developments in their design and performance, therefore, their spray breakup mechanisms in the physical conditions encountered in DISI engines over a range of operating conditions and injection strategies require continuous attention. In this context, there are sparse data in the literature on spray formation differences between conventionally drilled injectors by spark erosion and latest laser drilled injector nozzles. A comparison was first carried out between the holes of spark eroded and laser-drilled injectors of same nominal type by analysing their in-nozzle geometry and surface roughness under an electron microscope. Then the differences in their spray characteristics under quiescent conditions, as well as in a motoring optical engine, are discussed on the basis of high-speed imaging experiments and image processing methods. Specifically, the spray development mechanism was quantified by spray tip penetration and cone angle data under a range of representative low-load and high-low engine operating conditions (0.5 bar and 1.0 bar absolute, respectively), as well as at low and high injector body temperatures (20 °C and 90 °C) to represent cold and warm engine-head conditions. Droplet sizing was also performed with the spark and laser drilled injectors using Phase Doppler Anemometry in a quiescent chamber and the analysis was extended to include flash boiling conditions (120 °C) and other hydrocarbon and alcohols; iso-octane, ethanol and butanol. This thesis also presents the design and development of a real-size quartz optical nozzle, 200 µm in diameter, suitable for high-temperature applications and also compatible with new fuels such as alcohols. Mass flow of typical real multi-hole injectors was measured, and relevant fluid mechanics dimensionless parameters were derived. Laser and mechanical drilling of the quartz nozzle holes were compared to each other. Abrasive flow machining of the optical nozzles was also performed and analysed by microscopy in comparison to the real injector. Results with a highspeed camera showed successful imaging of microscopic in-nozzle flow and cavitation phenomena, coupled to downstream spray formation, under a variety of conditions including high fuel temperature flash-boiling effects, although undesirable needle movement was an issue and limitation.

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