Abstract
Traditional powder flow measurement devices, such as shear cells, operate in the quasi-static regime of shear strain rate. The FT4 powder rheometer of Freeman Technology, developed over the last two decades, has provided a clearer differentiation of powder flowability in some instances. This has been attributed to the instrument operating in the dynamic regime of shear strain rates, a feature that has yet to be established. We report an analysis of the dynamic behaviour of a bed of glass beads made cohesive by silanisation and subjected to standard FT4 testing procedure, where a rotating blade is driven into a cylindrical bed, using a combination of experimental measurements and numerical simulations by the Distinct Element Method (DEM). The DEM analysis underestimates the flow energy measured experimentally, although the agreement is improved when sliding friction is increased. The shear stress of the powder in front of the blade is shown to be roughly constant along the radial direction and increasing as the impeller penetrates the bed, suggesting that a characteristic shear stress can be determined for a powder under a given test condition in the FT4. For ease of simulations large beads were used (1.7–2.1mm). Future work will investigate the influence of particle properties and operational conditions on the prevailing stresses and strain rates.
Highlights
Reliable flow of cohesive powders is very difficult to achieve in many particle process operations, such as discharge from hoppers and bins, feeding, and dosing
The device requires a large quantity of powder and gripping of particles is problematic, which results in descent of powder near the walls [8], refinement is needed to establish this as a suitable dynamic flow characterisation instrument
The simulations using a linear elasto-plastic and adhesive contact model underestimated the flow energy measured in FT4 experiments by about 28% at the maximum penetration depth
Summary
Reliable flow of cohesive powders is very difficult to achieve in many particle process operations, such as discharge from hoppers and bins, feeding, and dosing. Suitable designs of hoppers have long been established by shear cell testing [1], where the shear resistance is characterised at a given consolidation stress or state of packing This technique is typically carried out at moderate to high stresses and very low shear deformation rates. Shear cells have been developed that can provide normal stresses lower than 1 kPa [2], alongside this a number of alternative low stress test methods have been developed, including the Sevilla Powder Tester [3], the Raining Bed Method [4], the SSSPIN Tester [5] and the Ball Indentation Method [6] These techniques all operate in the quasi-static regime, and the measurement of flowability under higher strain rates has received less attention in the literature. In this paper we follow a similar approach to analyse the dynamic powder behaviour in the FT4 Powder Rheometer for cohesive particles
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