Rheological Behavior of Dense Granular Matter

Title

Identifier

d_2009_2013:29a4c4fdfc78:11042

identifier

11309

Creator

Kheiripour Langroudi, Mehrdad,

Contributor

Gabriel I. Tardos | James N. Michaels

Date

2011

Language

English

Publisher

City University of New York.

Subject

Abstract

Research in powder flows is motivated by numerous applications in industrial processes and in geophysics. The division of granular flows into slow or quasi-static and fast or rapid regimes is well documented. The existence of a "transitional regime" where particle mobilization and/or shear are strong enough to dissipate significant energy through particle collisions, but do not completely eliminate the continuous particle enduring contacts, was demonstrated more recently. The basic motivation for our work was to supply relevant experiments that allow the particle bed to dilate freely as shearing is increased and for inter-particle collisions to occur in a dense bed where particle-particle friction is also prevalent and, to more thoroughly study the transitional or intermediate regime of powder flow.;A major objective was to modify a traditional Couette device and using it as a "powder rheometer" by providing continuous powder feed and discharge with a very small flow rate that allows for decrease in bed solid fraction. This decrease enabled the continuously sheared powder bed to translate from the slow, quasi-static regime at low shear rates to the intermediate regime at higher shear rates. Upon reaching steady state, we measured the local normal (with a remote sensor) and shear stresses (back-calculated from the measured torque) inside the shearing zone and generated a reliable continuum model by generalizing the Schaeffer model for quasi-static flow and by introducing additional terms to characterize the transitional regime. The change in solid fraction was then recorded by using a capacitance probe and its influence on the rheology was also studied.;The set of Visco-plastic constitutive equations for a wide variety of particles with different shapes, sizes, surface structure and elasticity (plasticity) obtained from the axial-flow Couette device, were then validated by several theoretical models. Some were in collaboration with DEM simulations from the group of Professor Sundar Sundarasen of Princeton University for flow in a Jenike-type shear cell and more recently with the group of Professor Shankar Subramaniam at the University of Iowa in the Couette geometry. Further comparisons were made with a computational fluid dynamics (CFD) approach developed at the University of Dortmund (FeatFlow) by the group of Professor Stefan Turek. Finally, a continuous model was developed "in house" using commercially available software (FLUENT) that was transformed and adapted to a relatively dense bed and used to model the free surface flow of dry granular matter. We designed a lab scale spheronizer and results from experiments in terms of surface height and shape, solid volume fraction and wall normal stress as a function of rotational speed were compared with those obtained from computations.;Furthermore, we developed a new indirect technique of estimating the thickness of the shear band (number of particles that exist in the active shearing zone) by measuring solid fraction as a function of position and shear rate in the shear gap. Using tracer (colored) particles we found that the flow pattern inside a Couette cell with axial flow is dispersive and mixing increases by increasing the shearing rate. It was also found that the particles inside the shear band move faster axially than those trapped outside of the active shearing zone.;The above studies, performed in different geometries such as plane shear in the Jenike and Couette cells and free surface flows in the centripetal geometry (of the spheronizer) revealed that, using adequate software (even commercially available codes such as FLUENT), reasonably model quite complicated flows with acceptable accuracy as long as the powders are reasonably free flowing. The material in this thesis was partially published in Powder Technology (two papers, denoted Kheiripour Langroudi et al., 2010a and 2010b, in the references) and AIP conference proceeding (one paper, denoted Kheiripour Langroudi et al., 2009) and was partially submitted for publication to two other prestigious journals (also denoted Kheiripour Langroudi et al., 2010c and Kheiripour Langroudi and Tardos, 2010 in the references). The last part of the thesis (Chapter 7) contains the experiments and modeling of the spheronize. To extend the findings to a further geometry, experiments were carried out (and described in the thesis) in emptying a flat-bottom hopper where the discharge orifice is moving across the base at some given velocity. This geometry is the basis for functioning of a tablet filling machine, which is widely used in pharmaceutical industries.

Type

dissertation

Source

2009_2013.csv

degree

Ph.D.

Program

Engineering

Item