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Near wall fibre orientation in flowing suspensions
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. (Linné Flow Centre)
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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 settlingfibres in a wall-bounded viscous shear flow. This is a flow case not dealt withpreviously even at small Reynolds numbers. Experiments were conducted usingdilute suspensions with fibres having aspect ratios of rp ≈ 7 and 30. It is foundthat the wall effect on the orientation is small for distances from the wall wherethe fibre centre is located farther than half a fibre length from the wall. Farfrom the wall most fibres were oriented close to the flow direction. Closer tothe wall than half a fibre length the orientation distribution first shifted to bemore isotropic and in the very proximity of the wall the fibres were orientedclose to perpendicular to the flow direction, nearly aligned with the vorticityaxis. This was most evident for the shorter fibres with rp ≈ 7.Due to the density difference between the fibres and the fluid there is anincreased concentration near the wall. Still, a physical mechanism is requiredin order for a fibre initially oriented close to the flow direction at about half afibre length from the wall to change its orientation to aligned with the vorticityaxis once it has settled down to the wall. A slender body approach is usedin order to estimate the effect of wall reflection and repeated wall contacts onthe fibre rotation. It is found that the both a wall reflection, due to settlingtowards the wall, and contact between the fibre end and the wall are expectedto rotate the fibre closer to the vorticity axis. A qualitative agreement withthe experimental results is found in a numerical study based on the theoreticalestimation.In addition an experimental study on fibre orientation in the boundarylayers of a headbox is reported. The orientation distribution in planes parallelto the wall is studied. The distribution is found to be more anisotropic closerto the wall, i.e. the fibres tend to be oriented closer to the flow direction nearthe wall. This trend is observed sufficiently far upstream in the headbox.Farther downstream no significant change in the orientation distribution couldbe detected for different distances from the wall.

Place, publisher, year, edition, pages
Stockholm: KTH , 2009. , vi, 49 p.
Series
Trita-MEK, ISSN 0348-467X ; 2009:03
Keyword [en]
fluid mechanics, fibre orientation, shear flow, wall effect, fibre suspension, papermaking
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-9995ISBN: 978-91-7415-240-1 (print)OAI: oai:DiVA.org:kth-9995DiVA: diva2:174993
Public defence
2009-03-27, E2, Lindstedsvägen 3, Stockholm, 10:15 (English)
Opponent
Supervisors
Note
QC 20100706Available from: 2009-03-02 Created: 2009-02-26 Last updated: 2010-07-21Bibliographically approved
List of papers
1. Fibre orientation control related to papermaking
Open this publication in new window or tab >>Fibre orientation control related to papermaking
2007 (English)In: Journal of Fluids Engineering - Trancactions of The ASME, ISSN 0098-2202, E-ISSN 1528-901X, Vol. 129, no 4, 457-465 p.Article in journal (Refereed) Published
Abstract [en]

The orientation of fibers suspended in a shear flow flowing over a solid wall has been studied experimentally. The possibility to control this orientation with physical surface modifications, ridges, has also been studied. The fiber suspension was driven by gravity down a slightly inclined glass plate and a CCD-camera was used to capture images of the fibers in the flow. Image analysis based on the concept of steerable filters extracted the position and orientation of the fibers in the plane of the image. From these data, the velocity of the fibers was determined. When viewing the flow from the side, the velocity of the fibers at different heights was measured and found to agree with the theoretical solution for Newtonian flow down an inclined plate. Moving the camera so that the flow was filmed from below, the orientation and velocity of fibers in the plane parallel to the solid surface was determined. The known relationship between the velocity and the wall normal position of the fibers made it possible to determine the height above the plate for each identified fiber. Far away from the wall, the fibers were aligned with the flow direction in both cases. In a region close to the smooth plate surface the fibers oriented themselves perpendicular to the flow direction. This change in orientation did not occur when the surface structure was modified with ridges.

Keyword
simple shear-flow, ellipsoidal particles, viscous-fluid, motion, suspensions
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-6781 (URN)10.1115/1.2436583 (DOI)000245692000011 ()2-s2.0-34248184588 (Scopus ID)
Available from: 2007-02-15 Created: 2007-02-15 Last updated: 2017-12-14Bibliographically approved
2. Orientation of slowly sedimenting fibers in a flowing suspensionnear a plane wall
Open this publication in new window or tab >>Orientation of slowly sedimenting fibers in a flowing suspensionnear a plane wall
2007 (English)In: Svenska Mekanikdagarna, 2007Conference paper, Published paper (Refereed)
Abstract [en]

The effect of a wall on the orientation of slowly sedimenting fibers suspended in a shear flow has been studied experimentally. Experiments were performed at two concentrations with two aspect ratios, rp ≈ 7 and rp ≈ 30, where rp is defined as the fiber length divided by the diameter. For all cases the majority of the fibers were oriented close to parallel to the flow direction for distances farther away from the wall than half a fiber length. As the distance from the wall decreased a change in orientation was observed. At distances from the wall closer than about an eighth of a fiber length a significant amount of the fibers were oriented close to perpendicular to the flow. This was particularly clear for the shorter fibers. Due to the density difference between the fibers and the surrounding fluid the fiber concentration was increased in the near wall region. An increased concentration was found in a limited region close to half a fiber length from the wall. For the shorter fibers a large number of fibers was also detected in the very proximity of the wall.

National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-9999 (URN)
Note
60th Annual Division of Fluid Dynamics Meeting of the American Physical Society, Salt Lake City, UT, USA 2008. QC 20100706Available from: 2009-03-02 Created: 2009-03-02 Last updated: 2010-07-21Bibliographically approved
3. Orbit drift of a slowly settling fibre in a wall-bounded shear flow
Open this publication in new window or tab >>Orbit drift of a slowly settling fibre in a wall-bounded shear flow
(English)Manuscript (Other academic)
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-10000 (URN)
Note

AC and DLK performed theoretical derivations jointly. Numerical computations and writing were performed by AC under supervision of DLK QC 20100706

Available from: 2009-03-02 Created: 2009-03-02 Last updated: 2014-08-12Bibliographically approved
4. Fibre orientation near a wall of a headbox.
Open this publication in new window or tab >>Fibre orientation near a wall of a headbox.
2010 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 25, 204-212 p.Article in journal (Refereed) Published
Abstract [en]

Experimental results on the fibre orientation in a laboratory scale headbox are reported. A steerable filter was used to determine the orientation of bleached unbeaten birch fibres at different distances from one of the inclined walls of the headbox contraction. Due to optical limitations only dilute suspensions were studied. It is shown that the fibre orientation distribution varies with the distance from the wall. Sufficiently far upstream in the headbox a more anisotropic distribution is found closer to the wall as compared to farther away from the wall.

Keyword
Fibre suspension; Orientation; Anisotropy; Shear flow; Boundary layer; Headbox
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-10001 (URN)000279341100009 ()
Note
Uppdaterad från konferensbidrag till artikel 20100706 QC 20100706Available from: 2009-03-02 Created: 2009-03-02 Last updated: 2017-12-13Bibliographically approved
5. Evaluation of a steerable filter for detection of fibres in flowing suspensions
Open this publication in new window or tab >>Evaluation of a steerable filter for detection of fibres in flowing suspensions
(English)Manuscript (Other academic)
Abstract [en]

Steerable filters are concluded to be useful in order to determine the orientation of fibres captured in digital images. The fibre orientation is a key variable in the study of flowing fibre suspensions. Here digital image analysis based on a filter within the class of steerable filters is evaluated for suitability of finding the position and orientation of fibres suspended in flowing suspensions. In sharp images with small noise levels the steerable filter succeeds in determining the orientation of artificially generated fibres with well-defined angles. The influence of reduced image quality on the orientation has been quantified. The effect of unsharpness and noise is studied and the results show that the error in orientation is less than 1◦ for moderate levels. A set of images with fibres suspended in a shear flow is also analyzed. The fibre orientation distribution is determined in the flow-vorticity plane. In this analysis a comparison is also made to a robust, but computationally more expensive, method involving convolutions with an oriented elliptic filter. A good agreement is found when comparing the resulting fibre orientation distributions obtained with the two methods.

National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-10003 (URN)
Note
QC 20100706Available from: 2009-03-02 Created: 2009-03-02 Last updated: 2010-07-21Bibliographically approved

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