Effect of particle size and cohesion on powder yielding and flow
During storage and transportation processes in the powder processing industry, the material undergoes various modes of deformation and stress conditions, e.g., due to compression or shear. In many applications, it is important to know when powders are yielding, i.e. when they start to flow under shear; in other cases it is necessary to know how much stress is needed to keep them flowing. In this study a large set of shear experiments using different shear devices, namely the Jenike shear tester, the ELE direct shear tester, the Schulze ring shear tester and the FT4 powder rheometer, have been carried out on eight chemically-identical limestone powders of different particle sizes in a wide range of confining stresses. These experiments serve two goals:
i) to test the reproducibility/consistency among different shear devices and testing protocols;
ii) to relate the bulk behaviour to microscopic particle properties, focusing on the effect of particle size and thus inter-particle cohesion.
The experiments show high repeatability for all shear devices, though some of them show more fluctuations than others. All devices provide consistent results, where the FT4 powder rheometer gives lower yield/steady state stress values, due to a different pre-shearing protocol. As expected, the bulk cohesion decreases with increasing particle size (up to 150 μm), due to the decrease of inter-particle cohesion. The bulk friction, characterized in different ways, is following a similar decreasing trend, whereas the bulk density increases with particle size in this range. Interestingly, for samples with particle sizes larger than 150 μm, the bulk cohesion increases slightly, while the bulk friction increases considerably – presumably due to particle interlocking effects – up to magnitudes comparable to those of the finest powders. Furthermore, removing the fines from the coarse powder samples reduces the bulk cohesion and bulk density, but has a negligible effect on the bulk friction. In addition to providing useful insights into the role of microscopically attractive, van der Waals, gravitational and/or compressive forces for the macroscopic bulk powder flow behaviour, the experimental data provide a robust database of cohesive and frictional fine powders for industrially relevant designs such as silos, as well as for calibration and validation of models and computer simulations.
Hao Shi is currently working at MercuryLab B.V. as a Research Engineer, and also a postdoctoral researcher in the MSM (Multi-Scale Mechanics) group at the faculty of engineering technology (ET) chaired by Prof. Stefan Luding at University of Twente, the Netherlands. He completed his BEng degree in refrigeration and air-conditioning engineering at Nanjing Institute of Technology (2011), China. He received his MSc degree in chemical and energy engineering at Otto-von-Guericke University (2013), Magdeburg, Germany, where he worked in the mechanical process engineering group chaired by Prof. Jürgen Tomas. Later, he joined the MSM group in 2014 for his PhD study which is based on the Marie Curie training network “T-MAPPP” and obtained his PhD in 2019. His research interests and expertise range from different processes of cohesive powders to discrete element simulations of granular assemblies with the development of calibration and validation protocols as well as their rheology. He closely collaborates with many other researchers in various institutions such as TU Braunschweig in Germany, UT of Compiègne in France, Université de Liège in Belgium and the University of Edinburgh in UK, etc. He also has strong connections to many industrial partners, e.g. P&G in UK, Nestlé in Switzerland, Freeman Technology in UK, Granutools in Belgium, etc.