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

•The unsteady behaviors of air–sand flows in a full-loop DFBG system were studied.•Poly-dispersed phases were applied to thoroughly capture the sand flow features.•Air inlet velocities and initial bed height highly affected the flow characteristics.•The affecting parameters should be properly controlled to avoid undesired phenomena.•The obtained CFD results can provide valuable predictions for system optimization.