Mechanical modelling of blade forming and drainage of flocculated suspensions

Sammanfattning: 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.