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UBC Theses and Dissertations

Numerical and experimental investigation of macro-scale mixing applied to pulp fibre suspensions Gómez, Clara


Effective mixing of pulp fibre suspensions is essential for many pulp and paper manufacturing processes. Pulp suspensions display non-Newtonian rheology and possess a yield stress, which complicates mixing. In order to improve our understanding of pulp mixing in agitated vessels, a series of studies was undertaken to assess the suitability of using computational fluid dynamics (CFD) to model these systems. CFD simulations for laboratory-scale and industrial-scale mixing chests were developed with the pulp suspensions treated as Bingham fluids. The computed flow fields were used to determine the dynamic response of the virtual mixers, which was then compared with experimental measurements providing very good agreement under conditions of moderate agitation. Comparison between calculations and measurements of torque and axial force was also good (relative average error of 12% to 24%). The simulation results provided insight into the mixing flow occurring within the systems, showing the formation of caverns around the impeller(s), the location of stagnation regions and the presence of channeling. However, the accuracy of these predictions was limited by the Bingham model used to describe the suspension's rheology and the uncertainty to which the suspension's yield stress could be measured. To assess the degree to which the approximated rheology contributed to the CFD results, the mixing of a model fluid having a well-defined rheology (Newtonian glycerin solution) was extensively investigated in a laboratory-scale vessel using a typical industrial geometry (rectangular chest, side-entering axial flow impeller). The flow fields were measured using particle image velocimetry (PIV) and compared with CFD computations for identical operating conditions. Good agreement was found (avg. 13.1% RMS deviation of local axial velocities) confirming that the approach used in the CFD model was adequate to calculate the complex mixing flow fields for a Newtonian fluid. These results encouraged further research on extending the combined CFD and PIV application on a system more closely representative of pulp suspension agitation. Calculations were then performed to select a suitable non-Newtonian model fluid and appropriate operating conditions to model pulp suspension mixing in the laboratory-scale vessel, with the dimensionless cavern diameter as the mixing criterion for conservation between the two systems.

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