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

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 U 0 = 0 .03 m/s, we found a transition from the homogeneous fluidization to bubbling fluidization with an increase of the coefficient C 1, 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 ( U 0 = 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 ( U 0 = 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.