(261e) Investigation into the Role of Holdup on the Gas and Particle Flow Pattern in a Spiral Jet Mill

Both breakage and classification occur within the main chamber of a spiral jet mill. The result is a complex fluid flow field that is difficult to analyse using traditional experimental techniques, whilst underlying mechanisms of particle size reduction remain poorly understood [1, 2]. Using Discrete Element Method (DEM) modelling, coupled with Computational Fluid Dynamics (CFD) the effect of hold-up has on both the fluid and particle phase within the mill. The design of the mill domain is based on the Hosokawa Micron AS-50 spiral jet mill (Runcorn, UK) and five mass loadings were used: 0.4 g, 0.8 g, 1.2 g, 1.6 g & 2.0 g. It was found that as the mass of material held within the bed increases, the average particle velocity decreased, along with the span of particle velocity distribution. As a result, the energy associated with an average collision decreased. It was also found that the bed has its own velocity gradient, and the solid particles travel with a greater velocity towards the surface. The result is that greatest amount of energy transfer takes place where the largest presence of shearing is found. This is located along the bed surface and on the back-face of each jet, as shown in Figure (1). These results are consistent with the experimental work of Kurten and Rumpf [3] and Luczak et al. [4]. Finally, it was found that the fluid velocity surrounding the classifier decreases with increasing particle load. The decrease in fluid velocity is due to damping of the fluid field, as momentum is transferred to stabilise the particle bed.

1. MacDonald, R., et al., The spiral jet mill cut size equation. Powder Technology, 2016. 299: p. 26-40.
2. Bnà, S., et al., Investigation of particle dynamics and classification mechanism in a spiral jet mill through computational fluid dynamics and discrete element methods. Powder Technology, 2020. 364: p. 746-773.
3. Kürten, H. and H. Rumpf, Strömungsverlauf und Zerkleinerungsbedingungen in der Spiralstrahlmuhle. Chemie Ingenieur Technik, 1966. 38(11): p. 1187-1192.
4. Luczak, B., et al., Visualization of flow conditions inside spiral jet mills with different nozzle numbers– Analysis of unloaded and loaded mills and correlation with grinding performance. Powder Technology, 2019. 342: p. 108-117.