Wall effects in hard-sphere suspensions in Newtonian fluids

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Title: Wall effects in hard-sphere suspensions in Newtonian fluids
Author: Kaiser, Allen Everett
Abstract: This work investigates the wall effects on the settling velocity of a falling ball in concentrated suspensions . The model suspensions consist of neutrally buoyant spheres in a viscous Newtonian suspending fluid . A number of previous studies have focused on determining the average velocity of the settling particle along the centerline or axis of the containing cylinders . The objective of this study is to determine the wall effects experienced by the falling balls as they settle off center or eccentrically a radial distance b from the centerline in a cylinder of radius R . In the dilute suspension (volume concentration of solids , 0 = 10 % ) , off -center wall effects are indistinguishable from those in pure Newtonian fluids . For the moderately concentrated suspension (0 = 30 % ) , off -center wall effects begin to deviate from Newtonian behavior as the ball approaches the wall {b /R > 0 .5 ) and the ball settles slower than it would in a comparable Newtonian fluid . In a highly concentrated suspension { (p = 50 % ) , the Newtonian region in the center of the cylinder is much smaller {b /R < 0 .3 ) and wall effects are significantly stronger than those found in the moderately concentrated suspension . Additional experiments were performed utilizing on -center drops in suspensions of 100 -niicron particles and suspensions of large , monomodal particles . Off -center wall effects in suspensions of large , monomodal spheres increase in magnitude and area of effect as the suspended particle diameter increases . In additional experiments , the size and /or the density of the falling ball was changed from experiment to experiment to determine if any deviation from on -center Newtonian wall effects could be detected . Again in the dilute cases , the observed wall effects were essentially Newtonian in nature . In the concentrated case , however , it was observed that these suspensions appear to be shear thinning in nature . These results indicate that current suspension constitutive models that assume Newtonian behavior must be reexamined as wall effects are much larger and extend further into the suspension that previously thought
URI: http : / /hdl .handle .net /2346 /10584
Date: 2002-12

Citation

Kaiser, Allen Everett Wall effects in hard-sphere suspensions in Newtonian fluids. Master's thesis, Texas Tech University. Available electronically from http : / /hdl .handle .net /2346 /10584 .

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