An experimental/computational approach for examining unconfined cohesive powder flow

This paper describes a new method to quantitatively measure the flow characteristics of unconfined cohesive powders in a rotating drum. Cohesion plays an important role, affecting flow properties/characteristics, mixing rates, and segregation tendencies. The method relies on measuring the change in center of mass of the powder bed as it avalanches in the vessel, using a load cell that is sampled continuously. Filtering and analysis of the signal is done using Fast-Fourier transform into the frequency domain, where noise is eliminated using signal processing methods. The filtered data is transformed back to the time domain by using an inverse Fast-Fourier transform to give quantitative information on the powder flow characteristics. In order to understand the nature of the forces controlling powder flow behavior, a computational model was developed to estimate the relationship between inter-particle cohesive strength and experimental measurements. A "flow index" generated by the method correlates well with the degree of bed expansion (dynamic dilation) of the cohesive powders. The flow index also predicts the dynamics of flow through hoppers. As the flow index increases it becomes increasingly difficult for the powder to flow through the hoppers. (c) 2006 Elsevier B.V. All rights reserved.