CFD-PBM Modeling of Hydrodynamics and Bubble Size Distribution in Tapered Bubbling Fluidized Beds

Tapered fluidized reactors are widely used in industry due to their good particle mixing and contact efficiency. However, current numerical studies of tapered beds are heavily reliant on homogeneous models or hybrid drag models. The homogeneous models fail to accurately predict the heterogeneous hydrodynamics and bubble behavior within a coarse grid framework, while the hybrid drag models call for preconceived regional division. In this study, a coupled approach by combining the Eulerian multifluid model, the population balance model, and the energy-minimization multiscale model was applied in tapered bubbling beds to study the multiscale flow structures. The model can achieve the prediction of the bubble size distribution in tapered beds under coarse-grid simulations rather than through the image processing of highly resolved simulation data. The results indicate that the bed expansion ratio, macroscopic hydrodynamics, and bubble characteristics can be well-reproduced by a coupled CFD-PBM approach. With an increase in gas velocity, both the mean size and number density of bubbles increase significantly, while the impact of the taper angle is secondary. The tapered bubbling fluidized beds exhibit superior overall particle mixing efficiency compared to traditional cylindrical ones. Furthermore, a simplified steady-state mathematical model for bubble diameter was further developed based on the coalescence dynamics for Geldart B or D particles, and the predictions show encouraging agreement with the experimental data.