| Summary: | This thesis has examined the factors affecting the passage of fibres through narrow apertures
under conditions similar to pulp pressure screening. This was accomplished by developing
a theoretical model of fibre passage and verifying the predictions of this model
experimentally.
The theoretical model was based on earlier observations of a "wall effect" and a "turn-ing
effect". These factors were represented in the model as a fibre concentration profile
upstream of the aperture, and as the probability of a fibre at a given position entering the
aperture. The near-wall, fibre concentration profile was experimentally determined for
1mm, 2mm and 3mm stiff fibres. Fibre concentration was found to increase linearly from
zero at the wall to the average suspension concentration at a distance approximately 0.3
of a fibre length. It remained constant beyond this point.
For a given initial position and orientation of a fibre upstream-of the slot, the prob-ability
of passage was modeled by theoretically determining the trajectories of individual
fibres at the entry flow of the aperture, including impacts with the aperture wall. To a
first order approximation, fibres passed through the aperture when their centres origi-nated
within the fluid layer that was drawn into the aperture. Using these theoretically
calculated probabilities of passage with the experimentally measured fibre concentration
profiles upstream of the aperture, passage ratios of fibres of different sizes were pre-dicted.
The predictions were compared to experimental measurements of passage ratio.
The theoretical model was found to give good predictions of average passage ratio.
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