City Research Online

Abstract

Extensive research has been carried out to understand the effects of cavitation in modern diesel injectors. In particular, the deposit formation causing damage to the injection system and injector itself, was linked to the pyrolytic reactions occurring during cavitating bubble collapse of the recirculating diesel fuel in the common rail. While the focus has mainly been on injector nozzles, where the soot was found, little research has been done on the return valve system, which presents a geometry susceptible to hydrodynamic cavitation.

This project consisted of creating optical accessible models of return valve systems in modern diesel injectors in order to carry out different experimental work on cavitation. Two acrylic models were designed with varying the outlet throttle diameter, Ø225μm and Ø245μm, and one fused silica model with modified geometry due to manufacturing limitations. The solenoid mechanism responsible for sealing the return valve was also modelled using a needle regulated manually.

First of all, a characterisation of different fuel mixtures – a paraffinic model diesel fuel and n-hexadecane and n-octane mixtures in 80:20 v/v and 95:5 v/v proportions – was attained utilising a customised low pressure mechanical flow rig and high speed camera. Onset of cavitation was found to occur uniquely at the entrance of the outlet throttle, without affecting the high pressure fuel in the valve control chamber that regulates injection. Low needle lift, large outlet throttle diameter and small saturation vapour pressure of the fuel were all responsible of generating a pressure gradient that would therefore increase the upstream-to-downstream pressure ratios for incipient cavitation.

Moreover a high pressure recirculation rig was setup along an in-situ extinction measurement equipment in order to study the chemical variations of new conventional diesel fuel in the fused silica return model due to hydrodynamic cavitation. Following a spectral attenuation and UV-Vis spectrum analyses, changes were found for a 62:38 v/v proportion of respectively new conventional diesel and previously cavitated at high pressures diesel fuel. In particular, an increase of polycyclic aromatic hydrocarbons (PAH) as a consequence of the presence of particulates from the mixture. Conversely, whilst the UV-Vis measurements for the fresh batch of diesel fuel were inconclusive due to dilution complications, no variations were observed in spectral attenuation and the colouring of the fuel with time. Therefore, the upstream pressures were too low to initiate pyrolytic reaction forming soot formation.

Observations were made of a fluorescent phenomenon resembling a flame occurring in the high pressure recirculation rig, which after a spectral analysis, showed the presence of CH and H2 molecules. It was thought to be an electroluminescence occurrence due to glowing of ionised particles in a fluid turned into plasma.

Finally a new comprehensive soot formation chemical model including paraffins mechanisms of PAH formation was developed and validated against previous experimental and computational data, allowing to study PAHs and aggregates formation in a surrogate diesel fuel cavitating bubble collapse. Simulations were carried out in an isotropic and polytropic temperature and pressure profiles, showing from the reaction pathways the importance of n-paraffins and naphthenes in the formation of mono-aromatics leading to soot formation.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TJ Mechanical engineering and machinery
Departments:	Doctoral Theses School of Science & Technology > School of Science & Technology Doctoral Theses School of Science & Technology > Engineering

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