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Title: Optical characterisation of cavitating flows in diesel fuel injection equipment
Author: Jeshani, Mahesh
Awarding Body: City University London
Current Institution: City, University of London
Date of Award: 2013
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The recent advances in Fuel Injection Equipment (FIE) have led to the identification of deposits found in the fuel filters and injector equipment. The work carried out here identifies the effects of cavitating flows on the physical and chemical properties of diesel fuel in order to try to evaluate the mechanism for deposit formation in FIE equipment using optical techniques to characterise the cavitating flows. Two sets of experiments have been carried out in order to understand the impact of cavitating flow on diesel fuels. The first experiment investigated the effects of sustained cavitating flow using a fuel recirculation rig. Samples of commercial diesel were subjected to forty hours of intense cavitating flow across a diesel injector in a specially designed high-pressure recirculation flow rig. Changes to the optical absorption and scattering properties of the diesel over time were identified by the continuous measurement of spectral attenuation coefficients at 405 nm by means of a simple optical arrangement. Identical diesel samples ~ere maintained at 70°C for forty hours in a heated water bath, in order to distinguish the effects of hydrodynamic cavitation and the regulated temperature on the cavitated diesel samples. The commercial diesel samples subjected to high pressure cavitating flow and heat tests revealed a response to the flow and temperature history that was identified by an increase in the optical attenuation coefficients of the cavitated and heated samples. The contribution of cavitating flow and temperature to the variation in spectral attenuation coefficient was identified. It was hypothesised that the increases observed in the spectral attenuation coefficients of the cavitated commercial diesels were caused by the cavitation affecting the aromatics in the commercial diesel . samples. The fuels were sent for a GC x GC and particle count analysis and results show significant increase in particle number count in the fuels as a result of cavitating flow. An increase in particle count to such high magnitudes was not observed for the heat test samples. Qualitative chemical modelling results of the pyrolysis of fuel vapour cavities during collapse at high pressures and temperatures have shown possible pathways leading to the formation of particulates. The presence of aromatics in diesel fuel was considered to be key species to the formulation of soot particles, however at extreme pressures and temperature paraffins may also have the propensity to breakdown into aromatics and further on to the formation of soot particles as observed by the pathway analysis in the modelling in the appendix. The second study undertaken involved the analysis of the near nozzle external spray dropsizing and atomisation characteristics of fuels with different distillation profiles using LIF-MIE image ratios. The LIF -Mie image ratios were simultaneously captured synchronously with the internal nozzle hole cavitating flow. Internal nozzle flow and sac observations after needle return have led to the conclusions that flow angular momentum is sustained in the sac flow after needle return. This flow was observed to have a high angular momentum which reduced over time. During the end of needle return, bubbles were observed in the sac hole forming as a result of needle cavitation. These bubbles retained the angular momentum of the flow post injection (after needle seal). The vortical motion in the sac lead to regions of high and low pressures in the sac volume and thus resulted in suction and discharge of bubble in the nozzle holes. The bubbles may have a high propensity of containing a mixture of fuel and air vapour whereas the suction and discharge offers a pathway to external gases entering the nozzle holes and sac volume. For operating engine conditions this would be post-combustion exhaust gases re-entering the nozzle holes. The combination of the bubble formation, its vOI1Ical motion due to the angular momentum of the liquid flow, its composition and high temperature, may form ideal conditions for pyrolysis like reactions which may lead to the formation of soot particles and deposits in the nozzle hole, sac and needle. Fuels with different distillation profiles were investigated to observe their external drop sizing distributions at 350 bar injection pressure. Results showed that fuels with lighter fractional compositions which also had lower viscosity produced lower Sauter Mean Diameter (SMD) distributions than fuels with higher distillation fractions and higher viscosity. Whether this is as a consequence of the distillation profile alone and is not influenced by the viscosity differences has not been investigated yet and would form the basis of further investigations and publications.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TA Engineering (General). Civil engineering (General)