Predicting bidisperse particle settling from mono-sized settling systems

Sedimentation of multi-sized particles is a common phenomenon in mineral applications like tailings management. The mutual interplay of particles in bidisperse flows creates a complex interaction between different-sized particles and fluid in comparison to a mono-sized system. This paper focuses on investigating particle settling in a bidisperse system. To this end, the Volume Penalization Immersed Boundary Method (VP-IBM), as an Eulerian–Lagrangian solver, is employed to provide detailed information about individual particles in a multiparticle system. The solver is first tested for settling cases in an unbounded domain with single and multiple mono-sized particles, and the predictions show a reasonable agreement with existing correlations for their average settling velocity. The validated IB solver is then used to describe the settling behaviour of bidisperse systems with different concentration and diameter ratios. Change in the concentration ratio is provided by replacing some of the faster large particles with the slower small ones, which reduces the hindrance caused by the opposing upward flow and leads to an increase in the settling velocity of both components. The variation in diameter ratio, on the other hand, is made by keeping the large particle diameter constant and reducing the size (and increasing the number) of small particles but maintaining a fixed total concentration. The effect of this change depends on the particle settling Reynolds number. It is found that for a low particle Reynolds number with dominant viscous forces, when the diameter ratio increases, the settling velocity of large particles decreases, while this trend is opposite for a higher Reynolds number flow. Because inertia forces are dominant at higher Reynolds numbers, the influence of the smaller component on the settling velocity of the larger component can be approximated by considering two factors: first, combining fluid and smaller particles to give a pseudo-fluid with a mean density of this mixture, and second, taking into account the role of small particles settling in inducing upward opposing flow. These findings allow a correlation to be derived for the settling velocity of bidisperse systems at high settling Reynolds numbers. The equation relates each size component in bidisperse flow to a counterpart monodisperse suspension for which reliable settling correlations are available.