(421d) Thermal Modeling of Poorly Flowing Bulk Solids Using Tetrapods Using the DEM

The simulation of poorly flowing (cohesive and strongly interlocking) bulk solids will play a crucial role in the near future, providing a pathway towards efficient utilization of valuable resources and mitigation of environmental impacts. Biomass utilization and different recycling processes, e.g., for batteries or polymers, are examples. Our present contribution focuses on simulating the handling of shredded nickel-metal hydride (Ni-MH) batteries during a typical recycling process: after a shredding step, the material must be conveyed and cooled prior to storage.

The modeling process is challenging due to (i) the cohesive nature of the material (mainly caused by interlocking flakes, fine particles, and liquid bridges), (ii) large differences in the material behavior when stress is applied (ranging from low stress flow to high pressure compaction), and (iii) the low effective heat conductivity of the material. To address these challenges, we propose a novel parcel-based DEM approach [1, 2] using tetrahedral multi-spheres (i.e., “tetrapods”, see Figure 1). Specifically, two key improvements were implemented in the tool LIGGGHTS [3]:

1) Finite intra-tetrapod heat conduction: this feature overcomes the unphysical infinite heat rate transfer problem, present in the classical multi-sphere approach. Our "intra multi-sphere thermal conductivity" model predicts the exchanged heat between individual spheres that constitute the tetrapod. By adjusting this conductivity value, heat transfer within a tetrapod can be tuned to better reflect the presence of air gaps and match the effective bulk conductivity of shredded battery material.

2) Flexible tetrapods: this feature allows individual spheres within a tetrapod to displace towards the center of mass. While rigid tetrapods (i.e., the vanilla multi-sphere implementation) with a simple cohesion model already perform well in low stress scenarios (e.g., a draw-down test, see Figure 2), a compaction process requires this more complex modeling technique. For example, our plastic deformation model of the tetrapods can be used to represent the inherent tendency of the material to remain compacted after the stress has been released.

[1] S. Radl, C. Radeke, J. G. Khinast, and S. Sundaresan, “Parcel-Based Approach For The Simulation Of Gas-Particle Flows,” 8th Interantional Conf. CFD Oil Gas, Metall. Process Ind., no. June, pp. 1–10, 2011.

[2] J. Tausendschön, J. Kolehmainen, S. Sundaresan, and S. Radl, “Coarse graining Euler-Lagrange simulations of cohesive particle fluidization,” Powder Technol., vol. 364, pp. 167–182, 2020, doi: 10.1016/j.powtec.2020.01.056.

[3] C. Kloss, C. Goniva, A. Hager, S. Amberger, and S. Pirker, “Models, algorithms and validation for opensource DEM and CFD-DEM,” Prog. Comput. Fluid Dyn., vol. 12, no. 2–3, pp. 140–152, 2012, doi: 10.1504/PCFD.2012.047457.