Year: 2016
Type: dissertation
Abstract: The low water-diesel interfacial tensions arising in biodiesels pose a problem \nfor fuel filters designed to separate water contamination from diesel fuel. \nSuch filters operate by passing the fuel through a fibrous non-woven \nmaterial with the aim of capturing small water droplets on the fibres and \nholding them while further droplets coalesce with the captured droplets until \nthe droplets are large enough to be carried away from the fibres and \nsubsequently settle out of the fuel by gravity. The coalescence process is \nhowever less effective at lower interfacial tensions. \nThe main purpose of this research is to explore the mechanisms at work in a \ncoalescence filter by developing and applying computer simulations of the \nprocess, and to understand the effects of fibre properties such as wettability, \nsize and separation on the filtration performance. Following a detailed \nreview of the relevant literature, a macroscopic simulation of the flow within \na filter housing is first presented, using finite element analysis via COMSOL \nMultiphysics to establish the main flow patterns through the filter system. \nThe filter medium itself in this model is treated as a continuous porous \nmedium. \nThe flow at the pore/fibre scale is then analysed by means of a multiphase \nlattice Boltzmann method based on the multicomponent Shan-Chen model. \nThe wettability of the fibres is incorporated through specification of a fluid \ndensity at the solid surfaces, allowing easy control of the local contact angle. \nThe code developed is tested against previously published and validated \nfinite volume/volume-of-fluid simulations of free droplet coalescence, with \ngood agreement seen in the predicted dynamics. \nThe interactions between individual water droplets and fibres is explored, in \nparticular to establish critical conditions (flow speed, fibre contact angle, \ndroplet/fibre size, droplet-fibre separation, fuel viscosity etc.) under which \ndroplets carried by the flow can be captured by fibres, and the conditions \nunder which droplets are released from fibres. The results confirm the \ndifficulties in achieving rapid and effective coalescence when the interfacial tension is low, and reveal the sensitivity of the droplet-fibre dynamics to the \ncontact angle on the fibres and the relative size of the droplets and fibres. In \nparticular larger fibres are not effective for small droplets, so small fibres are \nessential in the filtration process. \nAlso investigated are the dynamics of multiple droplets with arrays of fibres \nrepresenting the filtration media. It is found that higher contact angles \n(>120°) lead to lower capture efficiency compared to lower contact angle, \nwhile contact angles less than 106° tend to produce a small variation in \ncapture efficiency and capture most of the droplets at a filter porosity of 0.87. \nIt is concluded that the inlet layer of the filter should have fibres with 78° \ncontact angle and the exit layer fibres with contact angle 106°.
Author: Kirijen Vengadasalam
Language: English
Subfield: Computational Mechanics
Field: Engineering
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W2618104719