At low shearing rates (or slow flow rates) and high solid fractions, dry, non-aerated powders move in the so-called “frictional” or “quasi-static” regime where frictional forces between particles overwhelmingly control their movement. The main characteristic of these flows is that there is practically no dependence of the shear force in the powder on the applied shear rate. Contrast this with the “rapid-granular” or “inertial” flow regime that takes place at high shearing rates and relatively loose packing or lower solid fractions where particles tumble over each other and the main interaction is by elastic and/or inelastic collisions. The main characteristic of these flows is a quadratic dependence of the shear force on the applied shear rate. Rapid granular flows are usually associated with larger particles that are likely to be free flowing while frictional flows are common to smaller particles both free flowing and cohesive.

The “Intermediate” regime spans the region between the “frictional” and the “rapid-granular” flow regimes and is industrially the more relevant. During this regime, both frictional and collisional forces are important. Recent work in this field showed that a main characteristic of these flows is the presence of fluctuations of both stresses and deformation rates in the powder. This work also showed that, at high enough shear rates (but not as high as to attain the rapid granular regime), powders behave “fluid-like” in the sense that they have a power-law-fluid-type behavior and exhibit a “viscosity”-like term that depends on the pressure and the magnitude of fluctuations.

We present in this talk a theoretical approach that encompasses all these regimes into a continuum theory and show experiments by the present author and others that validate the theoretical findings.

References

1. Tardos, G.I., "A Fluid Mechanics approach to slow, frictional Powder Flows", Powder Technology, 92, pp. 61-74, (1997).

2. Tardos, G.I., M.I. Khan and D.J. Schaeffer, “Forces on a slowly rotating, rough cylinder in a Couette device containing a dry, frictional powder”, Physics of Fluids, vol. 10, no. 2, pp. 335-341, February, (1998).

3. Tardos G.I., S. McNamara and I. Talu, “Slow and Intermediate flow of a frictional bulk powder in the Couette geometry”, Vol. 131, pp. 23-39, Powder Technology, (2003).