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UBC Theses and Dissertations

Modeling of fiber motion in pulp and paper equipment Dong, Suqin

Abstract

This thesis describes a computational tool developed to simulate the fiber motion in pulp and paper equipment with particular emphasis on pulp screens. It is focused on modeling the fiber concentration upstream of a screen slot and examining the effects that different slot shapes have on the passage of fibers through the narrow apertures. This provides insight into the screen performance and has been used in this research to predict the characteristics of the screen. This simulation tool includes three coupled models: the flow model to predict the flow field in the equipment, the fiber model to trace the fiber trajectory in the equipment and the wall model to deal with the case when a fiber touches the equipment wall. The fluid field in the screen was examined using two numerical methods: the Reynolds Averaged Navier-Stokes (RANS) method for the calculation of flow through various slot shapes and Large Eddy Simulation (LES) for the fiber concentration calculation upstream of the screen slot. A three-dimensional flexible fiber model was employed to track the fiber trajectory in the screen. A very general wall model was developed to deal with the fiber-wall interaction. This wall model could be used for the case where a fiber touches a wall of any geometry, while specifying the wall geometry only through the fluid flow calculation routine. The simulated fiber concentration distribution increases linearly near the wall and becomes approximately constant farther from the wall, in reasonable agreement with experimental observations reported elsewhere. Slot shape has the most important influence on fiber passage ratio. For the same kind of fibers, same flow conditions and same overall slot width, a slope-slope slot provides the best passage for the fiber among all the slot shapes tested. For these contour slots, the contour height is also very critical, and a contour depth of 0.5mm seems best for both the step-step contour and slope-slope contour for fiber lengths of 1 to 3 mm. For the contour slots, the upstream side of contour plays no significant role in the fiber behavior; only the downstream side is important.

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