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Doktorsdisputation

Near wall fibre orientation in flowing suspensions


Respondent	Huvudhandledare	Bihandledare	Datum
Allan Carlsson	Daniel Söderberg	Fredrik Lundell	2009-03-27

Opponent
Helge Andersson, NTNU, Trondheim

Betygsnämd
Staffan Toll, Chalmers University of Technology Anna-Karin Tornberg, Kungliga Tekniska Högskolan Tomas Wikström, Metso Paper Inc., Sundsvall

Abstract

This thesis deals with fibre orientation in wall-bounded shear flows. The primary application in mind is papermaking. The study is mainly experimental, but is complemented with theoretical considerations. The main part of the thesis concerns the orientation of slowly settling fibres in a wall-bounded viscous shear flow. This is a flow case not dealt with previously even at small Reynolds numbers. Experiments were conducted using dilute suspensions with fibres having aspect ratios of rp = 7 and 30. It is found that the wall effect on the orientation is small for distances from the wall where the fibre centre is located farther than half a fibre length from the wall. Far from the wall most fibres were oriented close to the flow direction. Closer to the wall than half a fibre length the orientation distribution first shifted to be more isotropic and in the very proximity of the wall the fibres were oriented close to perpendicular to the flow direction, nearly aligned with the vorticity axis. This was most evident for the shorter fibres with rp = 7. Due to the density difference between the fibres and the fluid there is an increased concentration near the wall. Still, a physical mechanism is required in order for a fibre initially oriented close to the flow direction at about half a fibre length from the wall to change its orientation to aligned with the vorticity axis once it has settled down to the wall. A slender body approach is used in order to estimate the effect of wall reflection and repeated wall contacts on the fibre rotation. It is found that the both a wall reflection, due to settling towards the wall, and contact between the fibre end and the wall are expected to rotate the fibre closer to the vorticity axis. A qualitative agreement with the experimental results is found in a numerical study based on the theoretical estimation. In addition an experimental study on fibre orientation in the boundary layers of a headbox is reported. The orientation distribution in planes parallel to the wall is studied. The distribution is found to be more anisotropic closer to the wall, i.e. the fibres tend to be oriented closer to the flow direction near the wall. This trend is observed sufficiently far upstream in the headbox. Farther downstream no significant change in the orientation distribution could be detected for different distances from the wall.
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