(379c) Rheological Behavior of Free Flowing Dense Powders: Use of A Couette Cell to Validate Constitutive Equations for Different Regimes

The rheological behavior of flowing dense granular materials was studied in a controlled-stress cylindrical Couette-type cell by measuring the shear stress-shear rate dependence. The aim of this work is to validate constitutive equations by measuring torque and angular velocity on the rotating cylinder for a variety of materials. We improved the traditional Couette device (that does not allow for axial flow) by providing continuous powder feed and discharge through a conical hopper attached to the lower part of the instrument. The axial flow increases the porosity of the bed and destroys the chains formed by particle-particle interaction in the dense bed. This allows the bulk material to compensate for the increase of inertial forces by dilation and to change the regime of flow from quasi-static to intermediate by increasing shear rate.

In the ?no-axial-flow? (batch experiments) of different size glass particles with relatively high bulk density (1.5 g/cm3), shear stresses increase due to the overburden and packing of particles that results in the formation of force chains and stress build-up. To support this phenomenon, we observed the free surface of granules on the top of the bed that was horizontal at the onset of the experiment and became concave thereafter. Larger particles tended to form more and stronger force chains in shorter times due to the fact that with the same gap (annulus), larger particles have relatively wider shearing zones. Depending on particle size and gap, sometimes torque increased beyond the capability of the sensor, but for the cases that equilibrium is reached, shear stresses are found to be independent of shear rate. Change of regime does not occur because expansion is constrained by the overburden in the no-flow system. The material properties were found to have a significant effect on the rheological properties of bed: by using acrylic powders with a comparatively low bulk density (three times less than glass) dilatation of the bed occurs due to low overburden and compaction rate and by increasing the shear rate the regime of flow changes to ?Intermediate?. Here the powder shows some ?fluid like? behavior that is characterized by a power-law relationship between the shear stress and shear rate [1].

In the ?axial-flow? (continuous experiment), creating a continuous axial flow significantly decreases the torque (shear stress on rotating cylinder) in all glass particles and acrylic powders. Providing enough space for the granules to move, results in breaking the force chains. This small porosity change provides space for the particles to change their position randomly and collide. At low shear rates, shear stresses were found to be independent of shear rate so that the quasi-static regime dominates, while increasing shear rates makes the bed to behave like a fluid and changes the regime to intermediate. The dependency of shear stress to shear rate increases by further increases in shear rates because collisions between particles becomes more important. This has been shown by a jump in the value of the characteristic power-law index for acrylic powders.

Reference

1. G. I. Tardos, S. McNamara, I. Talu, Slow and intermediate flow of a frictional bulk powder in the Couette geometry, Powder Technol. 131 (2003) 23-39