Simulation gas-solid flow in the downer with new structure-based drag model

To improve the accuracy of simulating the heterogeneous flow in a circulating fluidized bed (CFB) downer, a drag model based on the local structure of the gas-solid flow using the multi-scale method has been developed in this work. New stability conditions according to the characteristics of gas-solid flow in the CFB downer have been derived to solve the non-linear model equations for obtaining structure parameters. New structure-based drag coeffidents were incorporated into the two-fluid model (TFM) to simulate the hydrodynamics of the gas-solid flow in the downer. Simulation results showed that the predictions with the new structure-based drag model are more accurate than those with the Wen-Yu drag model. The predictions with the structure-based drag model showed the heterogeneous flow of the gas-solid flow and the cluster phenomenon. The fluctuation of the instantaneous solid fraction of the experiment can be reasonably reproduced by the simulation using the new structure-based drag model, whereas the fluctuation of the instantaneous-solid fraction cannot be captured by the Wen-Yu drag model. The experimental data showed that the radial profiles of the solid fraction are a typical core-annular structure, and the simulated results with new structure-based drag model agreed well with experimental data from different cases. The axial profiles of the simulated pressure gradient, solid concentration and particles velocity distinctly exhibit the axial structure of the gas-solid flow in the downer, which agreed with the experimental results. (C) 2017 Elsevier B.V. All rights reserved.