Evaluation of drag models for cocurrent and countercurrent gas–solid flows

Gas–solid systems, whether cocurrent or countercurrent, manifest meso-scale heterogeneity to some extent. However, most of the previous modelling efforts, including the study of the effects of meso-scale structures as well as relevant drag closures, are mainly focused on cocurrent up riser flows, leaving cocurrent down and countercurrent systems less explored. In this work, by extending drift flux analysis and experimental validation for cocurrent down and countercurrent flow systems, we show that the most commonly used homogeneous drag models may be successful in a certain range of operating conditions, especially with fine-grid resolution. Fine-grid resolution is indeed helpful for reliable simulation of coexisting states, by predicting concave slip velocity curves. By comparison, the energy minimization multiscale (EMMS) drag model allows the use of much coarser grid resolution and better prediction over wider range of operating conditions. Its predictability of the overall slip velocity as well as the drift flux is nearly grid independent over the entire range of voidage. This fact leads us to the obvious advantage of saving computational time.

► Fine-grid simulation can be used to predict multiple coexisting states in fluidization.
► Fine-grid simulation changes the convex slip velocity curve to concave.
► Drift flux analysis can be used to analyze cocurrent and countercurrent flows.
► Periodic domain simulations may not reproduce bi-stable voidage of risers.