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AGGREGATE BREAKAGE IN LAMINAR COUETTE FLOW

  • Author / Creator
    Cifuentes, Gustavo A.
  • Micron-size particle suspensions are very common in industrial processes such as mineral extraction and wastewater treatment. These particles are difficult to separate from the fluid media due to their size. The formation of aggregates increases the effective size of these particles, increasing the settling velocity of the particles and facilitating separation. Control of the aggregate characteristics, such as size, and structure, influences the efficiency of these industrial processes. This study investigates the effect of shear on the breakage of aggregates. For this purpose, an experimental setup was designed where aggregates were formed and then broken in a Couette cell with a rotating inner cylinder and a transparent outer cylinder, which permits monitoring the aggregates using in situ image analysis.
    The experiments were carried out with 2 µm sulfate latex microspheres in a neutrally buoyant fluid-particle system. The aggregates were grown at a constant shear rate of 17.6 s-1 until they reached their steady-state size. Then, breakage was induced by increasing the rotational velocity of the inner cylinder. Different breakage shear rates within the range of 28.9 s-1 and 86.8 s-1 were tested, which were in the laminar Couette flow regime.
    Two different behaviours were observed for the range of shear rate studied. One range of shear rate produced re-aggregation of the fragments; in the other, only breakage takes place. It was observed that by increasing the breakup shear rate, the aggregate size was proportional to γ^(-p) with p = 0.69 for all the experiments, and p = 0.48 for the region where only breakage takes place. These results are in agreement with previous studies. Measurements of perimeter fractal dimension indicated that aggregate breakup is accompanied by restructuring. The aggregate sizes obtained in the experiments where only breakage was observed were used to estimate the aggregate cohesive force holding the aggregates together, which was found to be 1.94 ±0.11 nN and independent of the aggregate size. This value is in good agreement with the aggregate strength reported in the literature. The value of the aggregate cohesive force obtained in the present study is similar to the theoretical interaction force between two particles, suggesting that, for this system, the aggregate cohesive force is dictated by the attractive force between two primary particles, i.e., between the 2µm latex microspheres. This result suggests that the force required to break the aggregate is related to the force required to break a single chain of particles within the aggregate. More studies of the cohesive force of aggregates of different primary particles sizes and different aggregation mechanisms are needed.

  • Subjects / Keywords
  • Graduation date
    Spring 2022
  • Type of Item
    Thesis
  • Degree
    Master of Science
  • DOI
    https://doi.org/10.7939/r3-zzp7-4t94
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.