Downer has drawn much attention in both academia and industry for its unique features such as a uniform flow structure and a plug-flow behavior for gas and solids phases, which indicates a great potential as the next-generation reactor technique for ultra-short contact processes. A wealth of experimental studies on the downer has been reported in the literature, while few papers were published on CFD modeling and simulations. In particular, a comprehensive understanding on the particle traveling and clustering phenomena in the downer has not been reached using a theoretical method.

This work aims to present our recent efforts on the study of hydrodynamics and mixing behaviors in downers using a CFD-DEM approach, that is, a discrete element method (DEM) for modeling the particle movement together with an Eulerian model description for gas phase. An up-to-date three-equation linear dash-pot model for DEM is applied to take full account of the particle-particle interactions at the micro-scale. The DEM code has been successfully incorporated with the commercial package of either CFX 4.4 or FLUENT 6.2. The simulations are conducted in a 2-dimensional downer with the width of 10 cm and the height of 10 m to ensure the two-phase flow reaching fully developed. The results clearly show that the gas-solid flow experiences an entrance region, a transit region and a fully developed region in the downer. Interesting phenomena are observed that the particle clustering behaviors are different in these three regions. Loosely agglomerated clusters mostly appear in the fully developed region with some lateral movement. The micro-structure inside the clusters shows that the agglomerated particles move in the same direction as the gravity force so that the clustering phenomena do not cause the backmixing of solids as the riser does. The particle tracking reveals the lateral mixing of particles, which is also a fingerprint of the heat transfer behavior in the downer. More discussions will be addressed in the presentation, involving the detailed hydrodynamics, mixing behavior characterized by the tracking statistics of gas and solids, effect of the inlet configuration, and further discussions based on the fully developed hydrodynamics in downers using periodic boundary conditions in the simulation.