Wall-retardation effects on particles settling through non-Newtonian fluids in parallel plates

Walls can exert a retardation effect on particles settling in bounded fluid media. In this work, the parallel plate retardation effect was studied for particles falling in non-Newtonian fluids along the centreline of parallel plate ducts. The eccentric effect was also investigated for those particles which approached the wall. For spheres settling in sodium carboxymethylcellulose (CMC) solutions, the variation in wall factors against the size ratio of the sphere’s diameter to the parallel plate wall spacing shows a non-linear trend; the particle settling velocity is independent at small size ratio, and then decreases quickly with increase in size ratio. A new correlation was presented covering a wider range of size ratios (0.02 < λ < 0.83) in the flow region of 0.0011 < Re < 9.75. When particles settle in polyacrylamide solutions, the fluid elasticity reduces the wall-retardation effect and it can be deduced that the drag reduction mechanism of some polyacrylamide solutions may weaken the wall retardation effect. As the spheres settling in the CMC solutions approach the wall, the neighbouring wall exerts no retardation effect at small size ratios (≤ 0.8). Then the settling velocity reduces sharply, while the effect is negligible for polyacrylamide solutions. In comparison with cylinders, the actuating range of the neighbouring wall is smaller for parallel plates.