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Fluid mechanics of fibre suspensions related to papermaking
KTH, School of Engineering Sciences (SCI), Mechanics.
2005 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

This thesis deals with fluid dynamic mechanisms related to papermaking, specif- ically: the initial dewatering mechanisms during roll-forming and fibre motion in sedimentation and in shear flow.

Pressure and wire position measurements have been conducted in a model resembling the forming zone and the measured pressure distributions are shown to have more complex patterns than the simple model p / T/R (where T is the wire tension and R is the roll radius). It is shown that an increase in wire tension has a similar effect as a decrease in flow-rate on the shape of the pressure distribution. In addition, it is shown that the drainage has a stabilizing effect on the dewatering pressure.

The flow around the forming roll has also been modelled with the assump- tion that the wire is impermeable. A non-linear equation for the position of the wire is derived that clearly shows that the Weber number, We, is an im- portant parameter. The equation is linearized around the trivial solution and has a standing wave solution with a specific wavelength that scales with the We-number.

Motion of non-Brownian fibre settling in a Newtonian fluid at a small but finite Reynolds number has been studied experimentally. Two different regimes of sedimentation were identified. For dilute suspensions, fibres gener- ally fall without flipping and may travel at velocities larger than that of an isolated particle. In the semi-dilute regime we found the settling process to be dominated by large-scale fluctuations. The velocity fluctuations scale with the suspension volume concentrationφ according toφ1/3, which is similar to the findings for settling spheres.

The influence of shear on fibre orientation in the near wall region was studied in cellulose acetate fibre suspensions. At low concentration and low aspect ratio fibres were observed to orient perpendicular to the streamwise direction (named rollers) in the near wall region whereas the orientation further into the suspension was unchanged. As the concentration and aspect ratio increased the fraction of rollers decreased.

Finally, an evaluation of a commercial Ultra Velocity Profiler unit in fibre suspensions are presented. The idea was to determine the velocity and characterise the turbulence from ultra sound echoes from particles in the fluid. However, the spatial and/or temporal resolution of the measurements did not permit turbulence characterisation. These limitations might be possible to overcome and some procedures are proposed and evaluated.

Place, publisher, year, edition, pages
Stockholm: KTH , 2005. , x, 42 p.
Series
Trita-MEK, ISSN 0348-467X ; 2005:1
Keyword [en]
Applied mechanics, applied fluid mechanics, fibre suspensions, roll-forming, sedimentation, shear flow
Keyword [sv]
Teknisk mekanik
National Category
Mechanical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-100OAI: oai:DiVA.org:kth-100DiVA: diva2:6765
Public defence
2005-01-14, Kollegiesalen, Valhallavägen 79, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20101021Available from: 2005-02-02 Created: 2005-02-02 Last updated: 2010-10-21Bibliographically approved
List of papers
1. Experimental studies on dewatering during roll forming of paper
Open this publication in new window or tab >>Experimental studies on dewatering during roll forming of paper
2005 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, Vol. 20, no 2, 205-211 p.Article in journal (Refereed) Published
Abstract [en]

Pressure and wire position measurements have been performed in an experimental facility, the KTH-Former, which intends to model the roll-forming zone of a paper machine. The measured pressure distributions in the forming zone are shown to have more complex patterns than the simple model p=T/R, which normally is referred to as the nominal pressure. It is also shown that an increase in wire tension has a similar effect as a decrease in flow-rate on the shape of the pressure distribution. This is a consequence of that the flow to a large extent is governed by the relation between the dynamic pressure and the nominal pressure. For the case of partial dewatering the suction peak that appears at the roll-wire separation point has a strong influence on the pressure distribution upstream. Finally, it is shown that the drainage has a stabilizing effect on the dewatering pressure.

Keyword
papermaking, forming, roll dewatering, experimental measurements
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-14888 (URN)10.3183/NPPRJ-2005-20-02-p205-211 (DOI)000230271800012 ()2-s2.0-20744447195 (ScopusID)
Note
QC 20100525. Tidigare titel: Experimental studies on partial dewatering during roll forming of paperAvailable from: 2010-08-05 Created: 2010-08-05 Last updated: 2010-10-21Bibliographically approved
2. A theoretical analysis of the flow stability in roll forming of paper
Open this publication in new window or tab >>A theoretical analysis of the flow stability in roll forming of paper
2005 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, Vol. 20, no 2, 212-216 p.Article in journal (Refereed) Published
Abstract [en]

This paper deals with the fundamental mechanisms that control partial roll dewatering in papermaking. The flow around the forming cylinder is modelled in a cylindrical coordinate system and the wire is assumed to be impermeable. The governing equations are reduced based on a discussion where the magnitude of the different terms is estimated. Given this reduced set of equations a non-linear equation for the position of the wire is deduced. This clearly shows that one of the most important parameters is the Weber number, We, which is the non-dimensional number that can be obtained by comparing the effect of wire tension in relation to the momentum of the incoming headbox jet. The characteristics of the non-linear equation are discussed and the equation is linearized around the trivial solution to the equations, which gives that the wire is displaced a constant distance from the roll along the whole wrap. The linear equation has a standing wave solution with a specific wavelength that scales with We. This solution is compared to previously measured profiles regarding the wavelength of the waves.

Keyword
papermaking, forming, roll dewatering, mathematical modelling, gap formers
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-14889 (URN)10.3183/NPPRJ-2005-20-02-p212-216 (DOI)000230271800013 ()2-s2.0-20744454152 (ScopusID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2010-10-21Bibliographically approved
3. Visualization of streaming-like structures during settling of dilute and semi-dilute rigid fibre suspensions
Open this publication in new window or tab >>Visualization of streaming-like structures during settling of dilute and semi-dilute rigid fibre suspensions
2004 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666Article in journal (Other academic) Submitted
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-25426 (URN)
Note
QS 20120313Available from: 2010-10-21 Created: 2010-10-21 Last updated: 2012-03-13Bibliographically approved
4. Influence of shear on fibre orientation in the near wall region
Open this publication in new window or tab >>Influence of shear on fibre orientation in the near wall region
2004 (English)Article in journal (Other academic) Submitted
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-25427 (URN)
Note
QS 20120313Available from: 2010-10-21 Created: 2010-10-21 Last updated: 2012-03-13Bibliographically approved
5. A critical evaluation of ultrasound velocity profiling aiming towards measurements in fibre suspensions
Open this publication in new window or tab >>A critical evaluation of ultrasound velocity profiling aiming towards measurements in fibre suspensions
(English)Manuscript (preprint) (Other academic)
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-25429 (URN)
Note
QC 20101021Available from: 2010-10-21 Created: 2010-10-21 Last updated: 2010-10-21Bibliographically approved

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