טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentRagonis Peleg
SubjectShear-Induced Particle Migration in Viscous Fluids
DepartmentDepartment of Chemical Engineering
Supervisor Professor Emeritus Avinoam Nir
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

Highly concentrated suspensions containing fluids and solid powder are common in many areas such as composites, medical industry, solid rocket fuels et cetera. Production processes of those concentrated suspensions commonly include high shear mixing for homogeneous distribution and injection molding for achieving final configuration.

Since the 1960's researchers encountered unexpected anomaly in suspension flow - flattening of the expected Newtonian flow velocity profile in nonhomogeneous shear flow of mono-modal spherical particles suspensions.

In contrast to diluted suspensions, in high concentrated suspensions many types of inter particle interactions occurs. These include hydrodynamic, electrostatic and surface interactions that become important as surfaces come close together. As suspensions concentration increase, those inter particle interactions become more significant.

Detailed dynamical simulations of such systems that account for each effect would be so computationally expensive that it couldn’t be applied to complex geometries. Hence, researchers started to work on a constitutive model based on essential physics of the suspension - fluxes caused by gradient in local concentration, shear intensity and curvature of streamlines.

All the research so far deals with spherical particles suspensions with mono-modal, Bi-model or finite number of discrete particles sizes distributions.

In this work we expanded the mono, bi or multi modal particles distribution to more realistic case - continuous particle size distribution. In this case the mass and flux balance equations are based on non-dimensional moments of the particles distribution. We chose a case study of Newtonian flow in a tube of a suspension containing normal size distribution spherical particles with total concentration of 0.5 by volume. The case study was solved for two independent approaches - steady state and dynamic model. After solving for the moments profile at steady state or for large number of steps in the dynamic model, it is required to solve the inverse problem of acquiring the particles size distribution. The inverse problem solution is based on the assumption that at every point there is a normal particle size distribution, then we define a small number of parameters and a finite number of moments to solve for the particles size distribution.

When high concentrated solution is sheared two opposite forces - gradient in shear stresses and gradient in concentration, govern the particle migration. When a suspension flow through a tube there are high shear areas at the tube wall and low shear areas at the tube center. In this case gradient of shear stresses will cause the particles to migrate perpendicular to flow direction, from high shear areas to low shear areas. This migration is in contrast to the developing concentration gradient, hence the two forces will determine the steady state inhomogeneous particles concentration profiles. In addition to inhomogeneous particles distribution and particles accumulating at the tube center, for bi, multi or continuous particle size distribution there is also a size separation. The described inhomogeneity concentration and size separation causes changes and inhomogeneity of the effective suspension properties, such a low effective viscosity at the tube wall and high effective viscosity at the center.