Date of Award

1-1-2013

Document Type

Open Access Dissertation

Department

Chemical Engineering

First Advisor

Francis Gadala-Maria

Abstract

In this thesis we investigated through experiment and model, the anomalous normal stress distribution during constant-force squeeze flow of highly concentrated suspensions. Using pressure-sensitive films the normal stress distribution is measured in suspensions of glass spheres in a Newtonian liquid undergoing constant-force squeeze flow. At volume fractions of solids up to 0.55, the normal stress distribution is independent of volume fraction and almost identical to the parabolic pressure distribution expected for Newtonian fluids. However, at higher volume fractions, the normal stresses become an order of magnitude larger near the center and very low beyond that region. At these high volume fractions, the normal stresses decrease in the outer regions and increase in the inner regions as the squeezing proceeds. The normal stress distribution that results when the glass spheres without any fluid are subjected to squeeze flow is very similar to that for suspensions with volume fractions above 0.55, suggesting that the cause for the drastic changes in the normal stress distribution is the jamming of the particles in the suspension.

The drastic changes in the normal stress distribution are explained in terms of the radial flow migration of the liquid phase away from the center of the sample and of the jamming that results from it. Experimental measurements show that changes in the volume fraction of solids due to liquid-phase migration are found to depend on the initial volume fraction of solids, the viscosity of the suspending fluid, and the size of the particles. Under some conditions, the volume fraction of solids remains essentially constant during the squeeze test, indicating that liquid-phase migration does not take place to any significant degree; however, under other conditions, the volume fraction of solids increases throughout the sample as the squeezing proceeds and liquid is expelled from the test region in preference to the solids. In these latter cases the concentration increases are largest toward the center of the samples. Criteria for the occurrence of liquid-phase migration in suspensions undergoing squeeze flow are discussed in terms of dimensionless groups.

Liquid-phase migration is modeled numerically by taking into account the time and position dependence of the rheological properties of the material due to the change in the volume fraction of solids. This is done by coupling the equation of motion for a non-Newtonian material that approximates a Bingham plastic with a continuity equation that includes diffusive flux. The developed model is first validated with experimental data and then used to study the effect of various parameters on pressure-induced phase separation. Changes in the volume fraction of solids within the squeezed suspensions due to liquid-phase migration were found to depend on the degree of slip at the surfaces and on the applied force as well as on the material properties.

Rights

© 2013, Mohsen Nikkhoo

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