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  • 1.
    Holmqvist, Claes
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Mechanical modelling of blade forming and drainage of flocculated suspensions2005Doctoral 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.

  • 2.
    Holmqvist, Claes
    et al.
    STFI-Packforsk.
    Dahlkild, Anders
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Consolidation of sheared, strongly flocculated suspensions2008In: AIChE Journal, ISSN 0001-1541, E-ISSN 1547-5905, Vol. 54, no 4, p. 924-939Article in journal (Refereed)
    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.

  • 3.
    Holmqvist, Claes
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Dahlkild, Anders
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fibre mat behaviour in twin-wire formers2005Article in journal (Other academic)
  • 4.
    Holmqvist, Claes
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Dahlkild, Anders
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Influence of viscous stresses on the sheared consolidation of flocculated suspensions2005Article in journal (Other academic)
  • 5.
    Holmqvist, Claes
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Dahlkild, Anders
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Norman, Bo
    STFI-Packforsk.
    A flexible approach for modelling flow in multi-component blade formers2006In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 21, no 1, p. 73-81Article in journal (Refereed)
    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.

  • 6.
    Shugai, Galina
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Faxén Laboratory. KTH, Superseded Departments, Mechanics.
    Holmqvist, Claes
    KTH, School of Engineering Sciences (SCI), Centres, Faxén Laboratory. KTH, Superseded Departments, Mechanics.
    Vynnycky, Michael
    KTH, School of Engineering Sciences (SCI), Centres, Faxén Laboratory. KTH, Superseded Departments, Mechanics.
    Analysis of a model for twin-wire forming2004In: PROGRESS IN INDUSTRIAL MATHEMATICS AT ECMI 2002    / [ed] Buikis A; Ciegis R; Fitt AD, 2004, Vol. 5, p. 369-373Conference paper (Refereed)
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