Flow visualisation and computational prediction in thickener rake models

Abstract

Although thickener rakes are essential in the transport of bed material to the underflow, few details of the flow resulting from rake action in thickeners have been published. In this investigation, flow visualisation and velocity measurements in a small-scale thickener rake model were undertaken along with computational modelling in order to better understand the flow patterns around rake components and global flow patterns in thickener beds. Experiments were carried out for a range of rake parameters in a small-scale thickener rake model and an optically clear polymeric fluid, Carbopol 980 solution, was used to simulate the sediment bed. It was found that rake blades suck material behind them as well as pushing material in front of them toward the underflow. For inward raking thickeners the overall transport pattern of material from the periphery of the thickener to the underflow was spiral motion. A computational fluid dynamics model was used to compute the flow around the rakes and showed the same transport patterns as in the experiments. Velocity measurements were undertaken in the model using particle image velocimetry. The computational predictions were found to be in good agreement with these measurements, indicating the validity of the computational model in scaling up to full-scale units.