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dc.contributor.authorAtkins, Martin Johnen_NZ
dc.date.accessioned2007-11-26T14:50:29Z
dc.date.available2007-11-27T13:01:28Z
dc.date.issued2007en_NZ
dc.identifier.citationAtkins, M. J. (2007). Axial Variations and Entry Effects in a Pressure Screen (Thesis, Doctor of Philosophy (PhD)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/2664en
dc.identifier.urihttps://hdl.handle.net/10289/2664
dc.description.abstractPressure screens are used for contaminant removal and fibre length fractionation in the production of pulp and paper products. Axial variations and entry effects in the screen are known to occur and these variations have not been adequately quantified. This thesis describes a fundamental study of the axial variations of several factors that occur within an industrial pressure screen; namely, pulp consistency, fibre length distribution, rotor pressure pulse, and feed annulus tangential velocity. Axial variations of pulp consistency in the screen annulus and the accept chamber of the screen were studied using an internal radial sampling method. Localised pulp samples were taken and evaluated and common measures of screen performance such as fibre passage ratio and fractionation efficiency were calculated along the screen. Consistency generally increased along the length of the screen although under certain conditions the consistency toward the front of the screen was lower than the feed consistency. A two passage ratio model that incorporated forward and reverse passage ratio was derived to elucidate the flow of both fibre and fluid through the screen and their effects on overall screen performance. The passage of fibre through the screen decreased with screen length which generally had a positive effect on the fractionation efficiency toward the back of the screen. The passage of individual fibre length fractions was also studied and it was found that long fibre had a much lower passage than short fibre which caused the average fibre length in the annulus to increase. Rotor induced pressure pulse variations along the screen length were also investigated. The magnitude of the pressure pulse was significantly lower (up to 40 %) at the rear of the screen. The variation in pressure caused by the rotor is due to a Venturi effect and the shape of the rotor. The relative velocity of the fluid and the rotor, called the slip factor, also directly affects the size of the pressure pulse in the annulus. The slip factor decreases along the length of the screen due to the increase in tangential velocity of the fluid. Pressure pulse data was also used to estimate the instantaneous aperture velocity and back-flush ratio. The instantaneous aperture velocity was calculated to vary considerably from the superficial aperture velocity by up to 5 m/s in the forward direction and 10 m/s in the reverse direction. Computational Fluid Dynamics (CFD) was used to model tangential velocity changes in simplified screen annuli with axial through flow. For a smooth screen rotor the mean tangential velocity increased over the entire length of the annulus without reaching a maximum value. A step and bump rotor were modelled and the shape of the pressure pulses showed good agreement with experimentally measured pulses. The mean tangential velocity and the entrance length were found to be heavily dependant on the screen rotor used.en_NZ
dc.format.mimetypeapplication/pdf
dc.language.isoen
dc.publisherThe University of Waikatoen_NZ
dc.rightsAll items in Research Commons are provided for private study and research purposes and are protected by copyright with all rights reserved unless otherwise indicated.
dc.subjectPressure Screeningen_NZ
dc.subjectPulp Screeningen_NZ
dc.subjectCFDen_NZ
dc.titleAxial Variations and Entry Effects in a Pressure Screenen_NZ
dc.typeThesisen_NZ
thesis.degree.disciplineSchool of Science and Engineeringen_NZ
thesis.degree.grantorUniversity of Waikatoen_NZ
thesis.degree.levelDoctoral
thesis.degree.nameDoctor of Philosophy (PhD)en_NZ
uow.date.accession2007-11-26T14:50:29Zen_NZ
uow.date.available2007-11-27T13:01:28Zen_NZ
uow.identifier.adthttp://adt.waikato.ac.nz/public/adt-uow20071126.145029en_NZ
uow.date.migrated2009-06-14T21:23:34Zen_NZ
pubs.elements-id55732
pubs.place-of-publicationHamilton, New Zealanden_NZ


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