Assessment of a powder rheometer equipped with a cylindrical impeller for the measurement of powder flow properties at low consolidation

A powder rheometer equipped with a cylindrical impeller was studied to assess whether it could be suitable to derive quantitative physical parameters of powder flow properties. The equipment used in was the Powder Cell unit purposely developed by Anton Paar to fit on Anton Paar MCR Rheometers and to carry out torque measurements on aerated and not aerated powders. In this instrument, the state of consolidation of the powder can be adjusted by changing the impeller depth and the aeration rate, and very low consolidation stresses can be achieved. A cylindrical impeller geometry was tested on free flowing and easy flowing powders. A simple model was developed to relate the measured torque with the effective angle of internal friction in the powder measured with a standard powder shear tester. The results indicated the clear relationship between the effective angle of internal friction and the measured torque. The inversion of the calculation procedure highlighted that the Couette system does not always allow estimating the effective angle of internal friction reliably. In fact, small experimental error in the torque measurement may lead to large changes of the estimated effective angle of internal friction.