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 "turning 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 probability 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 originated 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 predicted. The predictions were compared to experimental measurements of passage ratio. The theoretical model was found to give good predictions of average passage ratio.