The difference between specularity coefficient of 1 and no-slip solid phase wall boundary conditions in CFD simulation of gas–solid fluidized beds

• Segregation rates of binary particles were simulated with Eulerian–Eulerian method.
• Differences between φ = 1 and no-slip wall boundary conditions were investigated.
• A layer of large particles is formed at the surface for no-slip boundary condition.
• The averaged wall shear stress is larger for no-slip boundary condition.
• The differences are caused by the nonzero tangential velocity for φ = 1 condition.

When modeling gas–solid fluidized beds with Eulerian–Eulerian approach, the predicted hydrodynamics is significantly affected by the solid phase wall boundary condition. The specularity coefficient φ in Johnson and Jackson wall boundary condition ranges from 0 for perfect specular collisions to 1 for perfectly diffuse collisions. Normally, it is reported that no-slip wall boundary condition is obtained for φ = 1. However, in most reports without applying Johnson and Jackson wall boundary condition, the no-slip wall boundary condition was also claimed to be used for the solid phase analogous to the gas phase. Therefore, the dynamic segregation process of binary particles was simulated to investigate whether φ = 1 is equivalent with no-slip wall boundary condition. The results reveal the significant differences between φ = 1 and no-slip wall boundary condition, which indicate that the no-slip wall boundary condition may not be suitable for describing φ = 1 wall boundary condition.

The instantaneous volume fraction distributions of large particles are significantly different between no-slip and φ = 1 solid phase wall boundary conditions. A layer of large particles is formed at the surface of the bed for the no-slip wall boundary condition due to the larger wall shear stress.Figure optionsDownload as PowerPoint slide