Analysis of the dynamics of the FT4 powder rheometer

Abstract

a b s t r a c t a r t i c l e i n f o Available online xxxx Keywords: Flowability Dynamic flow Powder rheology DEM Shear stressTraditional 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 (1.7-2.1 mm). Future work will investigate the influence of particle properties and operational conditions on the prevailing stresses and strain rates.

Analysis of powder rheometry of FT4: Effect of air flow

“…It was observed that the flow energy could decrease to 12 mJ if the particle-particle friction coefficient was set to zero. Bharadwaj et al (2010) and Hare et al (2015) also showed that the flow energy was sensitive to the interparticle friction coefficient. Hence, to get a better agreement, it is possible to fine-tune the friction coefficients so that the flow energy is the same in both experimental work and numerical simulation.…”

“…Hence, to get a better agreement, it is possible to fine-tune the friction coefficients so that the flow energy is the same in both experimental work and numerical simulation. This approach has been adopted by Hare et al (2015) in simulating the dynamics of cohesive particles in FT4 rheometer.…”

“…1(c). Similar to the method suggested by Remy et al (2009) and Hare et al (2015 the measurement cell size is about 5-6 particle diameter. Based on the velocity gradient of particles, the strain rate in the measurement cells is given as:…”

“…Therefore, the device can currently be used only for comparative testing, rather than for production process design. Hare et al (2015) simulated the dynamics of FT4 powder rheometer for a bed of glass beads made cohesive by silanisation and plotted the flow energy with the penetration depth. They also analysed the particle stresses in front of the blade and showed the shear stress increased with the penetration depth.…”

Stress and strain rate analysis of the FT4 Powder Rheometer

“…The linear elastic-plastic and adhesive contact model of Pasha et al [3] is used to represent cohesive spheres with a surface energy of 29 mJ/m 2 , in order to correlate with the experimental work of Hare et al [4]. The particle-particle sliding and rolling friction coefficients are set at 0.3 and 0.01, respectively, to best represent the experimental glass beads in their work.…”