The characterisation of the internal diesel flow and the external spray structure using laser diagnostics

Makri, K. (2018). The characterisation of the internal diesel flow and the external spray structure using laser diagnostics. (Unpublished Doctoral thesis, City, Universtiy of London)

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The advances in Fuel Injection Equipment have increased the injection and combustion efficiency, but have also increased the possibility of failure. Recent studies have identified various types of deposits of different components, such as fuel filters, injector nozzles etc. In this regard, white light scattered from the internal flow structures along with both elastic (Mie signal) and inelastic light (fluorescence) from the external sprays were synchronously captured by two separate high-speed cameras. The part of the present work was based on the experiments conducted by Jeshani Mahesh and post-processed by the author of this dissertation. The analysis performed suggested potential deposit formation mechanisms inside diesel injector nozzles considering the operating conditions of the injection system and the physical properties of the fuels. The observed circumferential bubble motion at the late stages of the needle return and post-injection, has been proven to generate low and high-pressure gradients which govern the bubble movement inside the nozzle passages. In engine conditions, the inward bubble movement inside the passages is believed to be the mechanism for the admission of hot combustion gases inside the nozzle geometry. The reaction of these hot gases with the liquid fuels is believed to produce deposits inside the FIE. The LIF-Mie obtained ratios provided an insight into the external spray drop-sizing and atomisation characteristics. The undertaken analysis revealed a strong link between the spray drop size and the physical properties of the fuels. It was also shown that both the needle lift and the operating conditions played a decisive role in the atomisation process and that an increase in rail pressure led to the formation of smaller droplets, while an increase in viscosity and surface tension led to larger droplets. The size of the spray droplets during the early and late stages of the needle lift was larger in relation to the maximum needle lift, due to the synergy of flow chocking and various types of cavitation (needle cavitation, string cavitation). The final section of this analysis involves the phenomenological study of the emerging sprays based on the LIF spray data. This study that the liquid core of the sprays was destroyed either inside the nozzle passage or in the vicinity of the nozzle exit. The obtained results referred to the LVF of different regions of the sprays as a function fuels’ properties, needle lift and rail pressure.

A similar analysis was based on data obtained from an improved experimental setup. The white light scattering was replaced by LIF to enable a quantitative analysis in terms of Liquid Volume fraction inside the nozzle passage. To the best of the author’s knowledge, such measurements were attempted for the first time and there are no similar results in the available literature. The spray results obtained came to an agreement with the afore-mentioned findings, validating the experimental and processing methodologies. The inelastic scattered light captured from the structures were formed inside a real-size nozzle passage reflected the effects of the fuels’ physical properties, needle lift and rail pressure. The results obtained referred to the relative LVF of the flow inside the nozzle passage. An increase in rail pressure led to lower relative LVF or SFLVF (term introduced for the purposes of the present work), as a result of enhanced cavitation phenomena. Additionally, the SFVLF of lighter fuels was lower compared to heavier fuels, due to intensive cavitation, which reduced the amount of liquid fuel in the hole. The obtained results also confirmed the argument suggesting the spray asymmetry to be associated to the geometric cavitation formed in the vicinity of the nozzle entrance.

Item Type: Thesis (Doctoral)
Subjects: T Technology > TJ Mechanical engineering and machinery
Divisions: City, University of London theses
School of Engineering & Mathematical Sciences > Engineering
City, University of London theses > School of Mathematics, Computer Science and Engineering theses

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