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Mechanical modelling of blade forming and drainage of flocculated suspensions
KTH, School of Engineering Sciences (SCI), Mechanics.
2005 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

A method has been developed for flexible modelling of multi-component twin-wire blade formers. Features such as suction devices, loadable blades, curved blades, and partial contact between the blades and the forming fabrics are easily incorporated. New results include a series of calculations demonstrating the non-trivial interaction between the pressure pulses when the blades are positioned successively closer together, the effects of suction on the pressure pulse generated by a blade applied to the opposing wire, and how blades of modest curvature do not necessarily stay in contact with the fabric along their full width and the implications of this on the pressure gradients in the machine direction.

The behaviour of the fibre mats as they experience the first of the blade pulses (after having been formed over a roll) is then considered in detail. Typically, the thickness of the mats decreases during the pulse, which reduces the rate of deposition of new fibres onto the webs. The amount of fibres in the sheets therefore changes marginally. Nevertheless, the resistance to drainage presented by the fibre network is seen to increase significantly due to the low permeability in highly compressed layers of the mat. As a result of the pressure gradients in the machine direction, the shear stresses in the plane of the fibre sheets can attain several hundred Pascal next to the forming fabrics.

Further, a model for sheared consolidation of flocculated suspensions is presented that extends the concept of a concentration dependent yield stress, previously employed in studies of uniaxial consolidation, to comprise flocculated phase shear strength. Rate-dependent viscous stresses are also incorporated. The theory is applied to the problem of combined compression and shearing of a strongly flocculated suspension contained between two plates, one being fixed and acting as a perfectly permeable filter, the other movable and acting as a piston by which the load is applied. Qualitatively, the evolution of the volume fraction of solids exhibits the same behaviour as during uniaxial consolidation without shear. Applying shear is however predicted to increase the rate of the drainage process, due to a reduced load bearing capacity of the flocculated phase, and correspondingly higher pore pressures.

Place, publisher, year, edition, pages
Stockholm: KTH , 2005. , viii, 50 p.
Series
Trita-MEK, ISSN 0348-467X ; 2005:3
Keyword [en]
Applied mechanics, blade forming, pressure distribution, interaction, suction, drainage
Keyword [sv]
Teknisk mekanik
National Category
Mechanical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-141OAI: oai:DiVA.org:kth-141DiVA: diva2:7273
Public defence
2005-03-11, Kollegiesalen, Valhallavägen 79, Stockholm, 14:00
Opponent
Supervisors
Note
QC 20101022Available from: 2005-03-03 Created: 2005-03-03 Last updated: 2010-10-22Bibliographically approved
List of papers
1. A flexible approach for modelling flow in multi-component blade formers
Open this publication in new window or tab >>A flexible approach for modelling flow in multi-component blade formers
2006 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 21, no 1, 73-81 p.Article in journal (Refereed) Published
Abstract [en]

The internal structure of the fibre network constituting a paper is to a dominating extent determined in the forming zone of the paper machine. In this article we present a method for modelling the pressure distributions in blade forming sections, which is commonly considered to be a key quantity of the process. The ambition is to obtain a tool by which the interaction between the different components of blade sections can be studied. It is achieved by defining modules out of which arbitrary sections can be constructed. The modules are solved independently and matched to each other iteratively Keeping the interface between the modules simple provides great flexibility in the modelling. By virtue of a slenderness assumption, the equations governing each module can be reduced to one-dimensional form, hence limiting the computational work and permitting systems of many components to be studied. Dimensionless numbers defining the problem are identified, and the magnitude of the nonlinear effects are estimated. Simulations are presented that illustrate the potential of the method.

Keyword
blade forming, drainage pressure, forming element interaction, computational modules, fuid-solid interaction
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-4967 (URN)10.3183/NPPRJ-2006-21-01-p073-081 (DOI)000236358800012 ()2-s2.0-33645686123 (Scopus ID)
Note
QC 20101021. Uppdaterad från submitted till published (20101021).Available from: 2005-03-03 Created: 2005-03-03 Last updated: 2017-12-05Bibliographically approved
2. Analysis of a model for twin-wire forming
Open this publication in new window or tab >>Analysis of a model for twin-wire forming
2004 (English)In: PROGRESS IN INDUSTRIAL MATHEMATICS AT ECMI 2002    / [ed] Buikis A; Ciegis R; Fitt AD, 2004, Vol. 5, 369-373 p.Conference paper, Published paper (Refereed)
Series
MATHEMATICS IN INDUSTRY, 5
Keyword
2-DIMENSIONAL SIMULATION, SUCTION SHOES, GAP FORMERS
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-25438 (URN)000189413900050 ()3-540-40113-X (ISBN)
Conference
12th Conference of the European-Consortium-for-Mathematics-in-Industry, Jurmala, LATVIA, SEP 10-14, 2002
Note
QC 20101021Available from: 2010-10-21 Created: 2010-10-21 Last updated: 2010-10-21Bibliographically approved
3. Fibre mat behaviour in twin-wire formers
Open this publication in new window or tab >>Fibre mat behaviour in twin-wire formers
2005 (English)Article in journal (Other academic) Submitted
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-4969 (URN)
Note
QS 2010 QS 20120319Available from: 2005-03-03 Created: 2005-03-03 Last updated: 2012-03-19Bibliographically approved
4. Consolidation of sheared, strongly flocculated suspensions
Open this publication in new window or tab >>Consolidation of sheared, strongly flocculated suspensions
2008 (English)In: AIChE Journal, ISSN 0001-1541, E-ISSN 1547-5905, Vol. 54, no 4, 924-939 p.Article in journal (Refereed) Published
Abstract [en]

The concept of a particle concentration dependent yield stress, previously employed in studies of uniaxial consolidation of a flocculated solid phase dispersed in a liquid, is extended to comprise flocculated phase shear strength. The inter-particle stresses are modeled by assuming that the stress state is always located on a yield-surface in stress-space, whose form is adopted from the Cam-clay plasticity theory for the quasistatic consolidation of soil. By treating the time-dependent dewatering of a suspension trapped between a permeable filter and a sliding piston, as well as the asymptotic limit of a cross-flow filtration situation, the differences with respect to the conventional uniaxial models are made apparent, and the effects of the shear stresses on the consolidation process are elucidated. Applying shear is predicted to increase the rate of the drainage process, because of a reduced load bearing capacity of the flocculated phase, and the correspondingly higher pore pressures.

Keyword
filtration, yield stress, shear, piston, cross flow, critical state
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-4970 (URN)10.1002/aic.11419 (DOI)000254217100010 ()2-s2.0-42149103798 (Scopus ID)
Note
QC 20101022. Uppdaterad från submitted till published (20101022).Available from: 2005-03-03 Created: 2005-03-03 Last updated: 2017-12-05Bibliographically approved
5. Influence of viscous stresses on the sheared consolidation of flocculated suspensions
Open this publication in new window or tab >>Influence of viscous stresses on the sheared consolidation of flocculated suspensions
2005 (English)Article in journal (Other academic) Submitted
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-4971 (URN)
Note
QS 2010 QS 20120319Available from: 2005-03-03 Created: 2005-03-03 Last updated: 2012-03-19Bibliographically approved

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Citation style
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