Flow Behavior Of Multi-size Particles In Dense Fluidized Bed Systems

Flow behavior of multi-size particles in gas-solid fluidized beds has gained wide interests in the field of fluidization engineering. It is also a chief difficulty in numerical modeling. This work aims to build a numerical model for the dense flow of multi-size particles, and study the particle flow, migration and segregation behaviors. Hydrodynamics of the multiphase flow and mechanism of particles motion are also analyzed in this work.The dense flow concerned in this work includes gas phase, medium solid phase and added particles with different properties. Due to the complex of such flows, the modeling work starts from the basic gas-solid flow with the gas phase and medium solid phase only. The present numerical methods are compared in modeling of this basic flow. Their applicability for modeling of multi-size particles are also validated. As proved in the results, these methods can give reliable results for basic dense gas-solid flow in the macro scale, but are still not enough to include the added particles in the modeling.Thus, a TFM-DEM hybrid model is utilized to model the complex multiphase flow involving both dense flow and added particles, and the solid phases have large density and size differences. The TFM is used to model the basic gas-solid flow generated by the gas phase and the medium solid phase, while the DEM is used to model the added particles. Validation of the TFM-DEM hybrid model is performed by comparing the simulation results with experiments conducted on similar conditions. Based on these findings, the flow behavior of the multi-size particles in a dense fluidized bed are numerically studied in detail. Influences of density and size of the added particles, and main operating parameters on the flow behavior are carefully analyzed. The main conclusions obtained includes:(1) The added particles in the dense fluidized bed segregate along the bed height according to their density differences. But the segregation degree decreases with a decrease of particle size. When the added particles are smaller than 3 mm, the averaging particle density distribution is between 1.9 ~ 2.2 g/cm3, and separation of added particles is inefficient.(2) By analyzing the forces exerted on the added particles, it is found that the pressure force acts as a lifting force for the added particles. The upward force controlling motion of the added particles is greater than gravity for the light species and less than gravity for heavy species. This difference results in the segregation of light and heavy particles. The gas drag is negligible while the medium drag has a main contribution to the irregular flow and mixing of added particles. As the size of added particle decreases, these drags show larger weights in particles motion. The fine added particles show similar trajectories to the medium flow, thus they are hard to be separated from the medium.(3) The segregation degree of the added particles are also influenced by the operating parameters. Medium properties and air velocity change the fluidizing flow pattern. Bubble motion and back-mixing of the medium solids are the main reasons of bed disturbance. They have larger influence on the segregation results than changes of bed density. Number of added particles and bed depth have similar influence on the distribution of the added particles. By increasing the number density of added particles in the bed, their segregation degree will be improved.