Evaluation of drag models for particles and powders with non-uniform size and shape

Abstract One of the key issues with the application of drag models for non-spherical particles is the incorporation of differently shaped particles. Under real world conditions, every single grain in a powder or granule has a unique geometry which strongly affects its drag coefficient. To incorporate the effects of different particle shape factors on their trajectories in the flow field, a drag analysis of crushed coal slag particles with sizes between 400 μm and 850 μm was performed within this work. The particle shapes were examined using brightfield microscopy and an optical particle analyzer. Also, the drop velocities of particles in still air were measured using a “Sony DSC-RX100 IV” standard digital camera at a frame rate of 1000 pictures per second instead of more costly special-purpose high speed camera equipment. A novel particle classification was introduced based on the obtained data. This data was also used as an input for three different drag models from literature. In order to evaluate the practicality of each drag model, a simple test rig in form of an air tunnel separator was built. It basically consisted of a box where freely falling slag particles were deflected by a free stream of air. The particle mass spreading at the bottom of the air flow separator was compared to numerical results. Numerical simulations of the setup were performed using Computational Fluid Dynamics (CFD) and a numerically efficient Euler-Lagrange approach in order to calculate the particle trajectories. It was concluded that the simple particle classification method provided satisfying calculated results. A special drag formulation for crushed coal slag particles was developed on the basis of two drag models from literature. The customized drag model improved the calculated results significantly. It can be added to nearly every commercial CFD code on the market without significant increase on computational cost and the proposed procedure also appeared to be well suited for the determination of drag model characteristics of other particulate materials in many fields of research.