Mesh size effect on CFD simulation of gas-fluidized Geldart A particles

Abstract

We investigate the effect of mesh size on CFD simulations of a lab-scale sized fluidized bed of Geldart A particles using two-fluid model (TFM), specifically for homogeneous expansion and transition to bubbling. For the first time we have shown a set of lab-scale domain size fine mesh simulations. In this context transient bed voidage profiles have been analyzed in detail. TFM follows the Eulerian–Eulerian approach which has the advantage of being less computationally expensive than Eulerian–Lagrangian approach for an engineering scale. Mesh size has a drastic effect on minimum bubbling velocity. With mesh refining, the observed minimum bubbling velocity approaches its experimental value. On reducing mesh size even up to 1mm×1mm, there was no improvement in capturing homogeneous expansion. Fine mesh simulation revealed void structures and could predict the bubbling transition, though the homogeneous expansion captured was not as much as experimentally observed. Review of several simulations across all the mesh sizes studied, revealed the presence of persisting dilute regions getting triggered around experimental minimum bubbling velocity. Theses persisting dilute regions seem to signal the presence of the bubbling regime. The effect of commonly used drag laws was also studied and it was found that the Gidaspow and Syamlal O'Brien drag laws manifested the dilute region markers at 8mm/s while for the Wen Yu drag law this value was 10mm/s. The effect of frictional stress and wall boundary condition for both phases was qualitatively assessed. Omitting frictional stress or changing no-slip to free-slip boundary condition for gas phase had the effect of delaying minimum bubbling velocity.