Predictions of undesirable air–sand flow behaviors in a dual fluidized bed cold flow system via a CFD full-loop model

A two-dimensional computational fluid dynamics (CFD) multiphase flow model based on a laboratory-scale dual fluidized bed cold flow system has been developed to study the unsteady flow characteristics in that system. Different sizes of silica sand as multiple solid phases were simulated to thoroughly describe the actual sand flow, and hence enhance the accuracy of the simulation results. The fluidizing air velocity and sand bed height in the riser were found to have strong influences on the results of solid volume fraction, system pressure, and sand circulation rate. It was noteworthy that when the initial sand height and air inlet velocity in the riser were increased over their critical values, an undesirable reverse flow could occur in the system, causing pressure imbalance and unstable system operation. Therefore, the saturated carrying ability of the fluidizing air should be carefully considered to prevent those unexpected phenomena. The proposed CFD model could provide valuable predictions and insights into the sand flow behaviors for the optimization of the design and operation of practical systems.