Coarse-grain DEM-CFD modelling of dense particle flow in Gas-Solid cyclone

Design, scale-up and operation of cyclone gas-solid separators are mostly based on simplified models and experience. Previous single-phase or one-way coupled simulations are not applicable to cyclones operating with relatively big particles and under high loadings, as used for example in circulating fluidized beds for the polymer and energy industry. Simulations based on the DEM-CFD allow for four-way coupled dense flow, including dissipation, friction, and rotational particle motions. However, the computational cost becomes easily prohibitive. Coarse-graining methods based on lumping smaller particles into parcels or grains have been recently proposed to reduce the number of elements and increase the time-step. Yet, much remains to be characterized in terms of accuracy vs. speed-up. In the present work, a coarse-grain DEM-CFD approach to simulate the two-phase flow in a Stairmand high-efficiency cyclone at gas velocities in the range 10-20 m/s and solid loading in the range 0.1% to 0.5%vol is investigated. In particular, the focus is on the effect that the coarse graining degree (up to 64 particles per coarse grain) exerts on the replicability of the results compared to pure DEM-CFD simulations. It is shown that the macroscopic quantities characterizing the cyclone performances, such as pressure drop, inner vortex length and collection efficiency, are generally maintained even with coarse graining degree up to 64, with an approximation that improves with the increase in the solids loading. However, detailed features of the gas and solids flow (e.g. strand formation) appear significantly affected by the coarse graining degree, already at coarse graining degree 8 and 27.

Automated summary

The design, scale-up and operation of cyclone gas-solid separators are mostly based on simplified models and experience, but previous simulations are not applicable for high-load operations with large particles. The use of DEM-CFD simulations allows for four-way coupled dense flow but is computationally expensive. Coarse-graining methods have been proposed to reduce the computational cost, but more research is needed to understand the trade-off between accuracy and speed-up. This study investigates a coarse-grain DEM-CFD approach for simulating two-phase flow in a cyclone and finds that while macroscopic quantities are maintained, detailed features of the gas and solids flow are affected by the level of coarse graining.