# Fibre modelling in venturi flow and disc refiner. Implementation and development of models for turbulent fibre suspension flow.

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As a part of a larger project in developing a new technology for increased energy efficiency in the pulp and paper industry a model for turbulent fibre suspension flow was needed. The model was also desired to be suitable for applications alongside additional multiphase flow models to study cavitation in fibre suspension flows. Two models with different levels of complexity were studied. A relatively simple model was the Bingham model where a fibre suspension is described as a non-Newtonian fluid. The model acts through modification of the viscous stresses, as the suspension viscosity is computed as a function of shear rate. A more complex model, denoted the ODF model, was implemented by modifying the viscous stress tensor with additional stresses computed from the orientation distribution of suspended fibres. A model for the orientation distribution function (ODF) was proposed and developed within the thesis work and used to construct explicit expressions for the additional stress tensor components as functions of the flow field with the use of Fourier series. Both fibre models were implemented to Fluent through user-defined functions (UDF) written in C programming language. The models were validated and the best performing parameter setting was identified by performing simulations of turbulent fibre suspension flow and comparing the results to experimental data from literature. Both fibre models resembled experimental data fairly well and had a reasonable computational cost. In comparison the performances of the models were roughly similar. Although both models showed potential the Bingham model gave slightly better resemblance of the experimental data was at the same time fairly simple to implement. In addition the ODF model was judged still needing further and more rigorous study. The Bingham model was therefore identified as the better and more reliable model at the current stage. Further validation of the chosen model, i.e. the Bingham model, was made by using it in an example application. A simplified model of a disc refiner was used and the fibre suspension flow was simulated with the Bingham model. The simulation yielded results to be expected and that resembled results from literature.

**Nyckelord: **Fibre modelling, Fibre suspension rheology, Bingham model, Fibre orientation distribution, Computational fluid dynamics, Energy efficiency, Disc refiner

Publikationen registrerades 2015-06-23. Den ändrades senast 2015-06-23

CPL ID: 218721

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