CFD-DEM study of gas-solid flow regimes in a Wurster type fluidized bed with experimental validation by electrical capacitance tomography

Abstract

Wurster type fluidized bed is widely used for particle coating in many industrial sectors, the gas-solid flow regimes within the fluidized bed is an important issue but rarely discussed in the literature, either numerically or experimentally. In this work, a numerical method that couples computational fluid dynamics with the discrete element method (CFD-DEM) was proposed to investigate the gas-solid flow dynamics in a Wurster type fluidized bed, with a focus on the existing gas-solid flow regimes. Two types of simulation cases, i.e., cold flow cases and the cases accounting for adhesion forces, were carried out. Experimental data by electrical capacitance tomography (ECT) imaging was employed to validate the simulation results. In cold flow cases, two fluidization regimes, including plug, and multi-bubbling were observed in the annulus region of the fluidized bed. It was found that the multi-bubbling regime can further be classified into bottom, localized and global fluidization under different air velocities, and more than 83% of the total particle mass is kept within annulus region. With increase in the inter-particle adhesion force, the flow regime in the annulus region changed from multi-bubbling to single-bubbling, followed by plug flow and eventually defluidization. The multi-bubbling, single-bubbling, and plug flows can be discriminated by cycle time ranges of 0–4, 4–15 and more than 15, respectively. Finally, it was discovered that the particle circulation times was within the ranges of 3–9 s, and the gas-solid flow regimes can be used as an indicator of the circulation time distribution. The methodology and new findings in this study complement the existing techniques and knowledge for the optimization of coating process in a Wurster type fluidized bed.