Van Romunde, R.Z.; (2011) Factors affecting the development of sprays produced by multihole injectors for direct-injection engine applications. Doctoral thesis , UCL (University College London).

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Abstract

The spray form development from a state of the art multi-hole injector for gasoline direct injection internal combustion engines is examined to attempt to determine the thermo-fluid dynamics affecting the spray development. The current state of knowledge regarding spray break-up and the interactivity of the factors on spray form are detailed. The spray under investigation was injected into purposely designed quiescent chambers to decouple the effects of the fluid mechanics on spray development from any in-engine effects. The pressure chambers, experimental apparatus and techniques used to characterise and measure the spray properties are described along with an assessment of any sources of variability in the measurement and analysis methodologies and hardware. Initial spray images of the spray produced by a range of multi-component “retail” fuels as well as single component non-oxygenated and oxygenated hydrocarbons with a range of boiling ranges and points for different injector body (and hence assumed fuel) temperatures and chamber gas pressures are presented. The experimental measurements show the strong interaction between the operational conditions in relation to the fuel properties and the physical spray form. A large amount of deviation from the nominal “ambient” spray form is observed for conditions where the fuel’s bubble point (boiling temperature at given gas pressure) is exceeded by a multiple of 10, termed spray collapse. The dependence of a multi-component fuel on the boiling characteristics of its highest volatility components suggest that it is these components which drive the fuel spray development formation, which is further illustrated by comparing different single component fluids. This suggests that higher volatility fluids are better representatives of full range, multi-component fuels for modelling or other investigative work when a single component fuel is required to be used. The onset of spray collapse was found to be gradual with no sudden “threshold” condition at which collapse occurred, also illustrated by a gradual reduction in measured spray droplet size with increasing injector body temperature and/or reducing gas pressure. The physical factors affecting spray development and break-up, and their effects are examined including the fluid flow inside a real size transparent, optically accessed nozzle, illustrating the effect of cavitation supplying nucleation sites for the subsequent vaporisation of the fuel. The scales of local air turbulence are found to affect the local vapour concentration, and hence vaporisation rate, and hence the interaction of these factors is shown to determine the spray formation.

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