Flow and Rheology of Pulp Suspensions at Medium Consistency
Sammanfattning: Traditionally, development of pulp processing equipment has been based on the accumulation of qualitative knowledge (experience) with respect to the interaction of equipment and pulp suspensions. This way of developing equipment requires several trials and prototypes before the final product and process data can be established.A characteristic of fibre suspensions is the ability to form networks. When examining the ability of the fluid to transfer momentum, the interaction between the fibres must be taken into account. The network strength can be described as the force that the fibre structure can resist before yielding. When exceeding this force, i.e. the yield stress for the suspension, the fluid starts to flow. As the shear rate increases in the suspension, it eventually becomes turbulent. The main feature for the pulp suspension to become turbulent, is the particle interactions to adjust the non-spherical particle to the desired velocity field. This thesis consists of an investigation of the yield stress. Additionally, a new way of determining the point of fluidisation in pulp suspensions based solely on fibre geometry data is presented. The definition of the fluidisation point and the knowledge of the yield stress are used to evaluate the flow field. With the knowledge of the transition point a suggestion to model the pulp suspension is given by adding terms to the Boussinesq approximation in turbulence modelling. The main contributions of this thesis are the findings that the yield stress varies both within a mill as well as at different process conditions, and that the point of fluidisation can be estimated from single fibre properties as the second moment of inertia for each contact point. The yield stress combined with a transition criterion can be used to simulate flow of pulp suspensions at medium consistency. This has been applied to agitation and pressure screening. It has been shown that the flow structure can be captured well but the pressure levels deviate. Further investigations of the boundary layer of pulp suspensions are needed to be able to predict the pressure correctly and further improve turbulence modelling.
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