Abstract
The flow behaviour of granular materials is relevant for many industrial applications including the pharmaceutical, chemical, consumer goods and food industries. A key issue is the accurate characterisation of these powders under different loading conditions and flow regimes, for example in mixers, pneumatic conveyors and silo filling and discharge. This paper explores the experimental aspects of cohesive powder handling at different compaction levels and flow regimes, namely inertial and quasi-static regimes. So far, laboratory element test set-ups capable of defining the full stress states at very low compaction levels have not been fully explored in literature. In contrast the mechanical behaviour of cohesive powders under relatively high consolidation stress (several kPa upward) can be carefully measured using element tests such as biaxial test, true triaxial and hollow cylinder tests. However in practice these tests are expensive and slow to conduct and are almost never performed for many industrial applications requiring material characterisation. Here we investigate simpler techniques that could be used for filling this important gap with the focus of providing test data for model calibration and simulation validation in line with the spirit of the European Commission funded PARDEM Marie Curie ITN Project. We perform particle and bulk characterisation on limestone powder with 4.7μm and 31.3 μm mean particle size, detergent powder with differences in formulation, cocoa powder with low and high fat content - relevant for different industrial applications. Of particular significance is the 4.7μm limestone powder which is the PARDEM reference powder that have been created and extensively used in a collaborative European PARDEM Project (www.pardem.eu). In the inertial, low consolidation stress regimes - more relevant for powder transport and conveying applications - we present experimental findings on the flowability and avalanching behaviour of the reference material in a rotating drum. On the other hand, in the quasi-static, higher consolidation regime, we perform shear tests with the Edinburgh Powder Tester (EPT), an extended uniaxial tester and the commercially available Freeman FT4 Powder Rheometer. For macroscopic quantities, we report the unconfined yield strength as a function of applied stress. These material characteristics provide important scientific insights for developing innovative solutions for cohesive powder handling problems. From these experiments and for best practice guideline, we highlight subtle issues associated with the experimental setup and measurements. The experiments lead to a rich quantitative description of the flow behaviour and failure properties of the materials which provide the material data for DEM model calibration and validation.