Partic. vol. 6 no. 6 pp. 540-548 (December 2008)
doi: 10.1016/j.partic.2008.07.005

Two-fluid modeling of Geldart A particles in gas-fluidized beds

Mao Ye, Junwu Wang, M.A. van der Hoef^*, J.A.M. Kuipers

M.A.vanderHoef@tnw.utwente.nl

Abstract

We have investigated the effect of cohesion and drag models on the bed hydrodynamics of Geldart A particles based on the two-fluid (TF) model. For a high gas velocity U₀ = 0.03 m/s, we found a transition from the homogeneous fluidization to bubbling fluidization with an increase of the coefficient C₁, which is used to account for the contribution of cohesion to the excess compressibility. Thus cohesion can play a role in the bed expansion of Geldart A particles. Apart from cohesion, we have also investigated the influence of the drag models. When using the Wen and Yu drag correlation with an exponent n= 4.65, we find an under-prediction of the bed expansion at low gas velocities (U₀ = 0.009 m/s). When using a larger exponent (n = 9.6), as reported in experimental studies of gas-fluidization, a much better agreement with the experimental bed expansion is obtained. These findings suggest that at low gas velocity, a scale-down of the commonly used drag model is required. On the other hand, a scale-up of the commonly used drag model is necessary at high gas velocity (U₀ = 0.2 and 0.06 m/s). We therefore conclude that scaling the drag force represent only an ad hoc way of repairing the deficiencies of the TF model, and that a far more detailed study is required into the origin of the failure of the TF model for simulating fluidized beds of fine powders.

Keywords

Gas-fluidized bed; Geldart A particles; Two-fluid model; Cohesive force; Drag correlation