Sink and/or float in gas-fluidized beds

Sinking/floating motions of large solids such as biomass fuel pellets and incombustible wastes in gas-fluidized beds are important in several applications. It has been discussed based on the balance between the buoyant force due to the gas pressure gradient in the beds and the gravity force, characterized by the ratio between the material density of the large object and the bulk density of the bed material (r_object/r_bed). Although this simple description works properly as a first approximation, we observed some deviations from it simultaneously; non-uniformities of the beds make their behaviors complex. This study explores the details of sinking/floating motions using the numerical model we developed for dense gas-solid flows with large size differences based on the DEM-CFD coupling technique. With the help of collaborators, we performed experiments first and then performed the corresponding numerical simulations. We found non-trivial dependencies on gas velocity, object shape, and local concentration; some objects expected to sink float and vice versa. In highly-concentrated cases, we found that some objects having the same mass and volume float, and some others sink. These behaviors cannot be characterized only by the simple averaged picture and the importance of local flow fields around the objects is emphasized. The physical mechanisms based on the local flow fields will be revealed in the presentation.