(189c) Experimental Investigation of the Existence of the Intermediate Regime in an Aerated-Bed Couette Rheometer

Granular materials represent a large percentage of the materials handled and produced in the chemical and process industries. Such materials are widely used in powder-based unit operations, such as granulation, drying and fluid catalytic processes. Understanding the flow behaviour of those materials is of critical importance as it will help industrial practitioners handle and produce particulates in an efficient and less costly way. Granular materials are widely studied in the quasi-static (slow) and inertial (rapid) regimes. In the slow regime the particle’s stress is independent of the applied shear rate. This regime is characterised by the formation, rotation and breakage of particles force chains. The rapid regime is characterised by instantaneous and binary collisions between particles. A third intermediate regime, which develops between the slow and rapid regimes, has recently become the subject of interest of many numerical and experimental studies. In this regime the particle’s shear stress becomes shear rate dependent as observed in aerated bed configurations [1] when the gas velocity is lower than the minimum fluidisation velocity, U_mf.

The focus of this work is to confirm the existence of the intermediate regime in an aerated virtual Couette rheometer previously developed in our lab [2]. This device is a combination of the FT4 Freeman aeration kit with a new 3D printed cylindrical bladed cell (6 blades). This cell is 70 mm in height and 36 mm in diameter. The experimental procedure is as follows: the cell moves downward through the material placed in the FT4 cup until it reaches a fixed position. The cell is then rotated at different rotational speeds, and the torque, needed to move the aerated powder placed in the cup, is evaluated. The shear stress, t, and shear rate, , are computed from the recorded torque and the rotational speed, respectively. This new configuration was calibrated without aeration using a Newtonian fluid with a known viscosity. It was also tested with aerated and non-aerated glass beads powders in the quasi-static regime [2].

In this study, we expand the results previously obtained [2] to values of the fluidisation ratio U/U_mf below the minimum fluidisation limit (with U being the aeration velocity). Experiments were performed on an aerated glass bead powder with a particle diameter of 0.08 mm and an aeration ratio ranging from 0 to 0.8; the shear rate was increased from 5 to 42 s^-1. The first results showed that, for an aeration ratio lower than 0.6, the shear stress is independent of the shear rate, indicating that the studied powder is in the quasi static regime. For U/U_mf higher than 0.6, the shear stress was found to be independent of the shear rate for shear rates values lower than 25 s^-1. For higher values of the shear rate, the shear stress was found to increase proportionally with the increasing shear rate, which indicates that the studied sample is no longer in the quasi-static regime. These first observations confirm the existence of the intermediate regime in an aerated bed. Further experiments including different particle diameters will be performed to confirm and expand the first measurements and observations.

References

[1] G. I. Tardos, S. McNamara, and I. Talu, ‘Slow and intermediate flow of a frictional bulk powder in the Couette geometry’, Powder Technol., vol. 131, no. 1, pp. 23–39, Mar. 2003, doi: 10.1016/S0032-5910(02)00315-7.

[2] L. A. A. Yahia, T. M. Piepke, R. Barrett, A. Ozel, and R. Ocone, ‘Development of a Virtual Couette Rheometer for Aerated Granular Material’, AIChE J., vol. n/a, no. n/a, p. e16945, doi: 10.1002/aic.16945.