3D particle-scale modeling of gas–solids flow and heat transfer in fluidized beds with an immersed tube

A fully 3D CFD–DEM approach has been developed to predict the heat transfer between an immersed tube and packed/fluidized beds. The figure above shows (a) the 3D nature of particle flow and (b) the 3D nature of gas flow. The capability of model in reproducing some typical heat transfer features and the underlying mechanisms are explained in terms of heat transfer modes such as tube–fluid convection and tube–particle conduction. [Display omitted] •A 3D CFD–DEM model with heat transfer in fluidized beds is developed.•A new concept of critical bed thickness is introduced.•Some typical features of hydrodynamics and heat transfer in fluidized beds are reproduced.•The 3D gas–solids flow in fluidized bed is captured.•The predicted maximum heat transfer coefficient is explained in terms of heat transfer modes. In this work, a fully three-dimensional (3D) model of combined computational fluid dynamics and discrete element method (CFD–DEM) is for the first time developed to study the gas–solids flow and heat transfer in fluidized beds with an immersed tube. A critical bed thickness is first determined at which the bed can be regarded as fully 3D. Then the validity of the model using the critical bed thickness is tested both qualitatively and quantitatively. It is shown that the model can successfully reproduce the typical relationship between pressure drop and gas velocity, and flow and heat transfer characteristics such as the four distinct stages of bubble transit through the tube and the peak of heat transfer coefficient between tube and the bed for certain gas velocity (which are however not well-predicted by previous 2D CFD–DEM and 2D CFD–3D DEM models). Finally the results are analyzed to improve the fundamental understanding of the system. It is demonstrated that both the gas and solids phases have 3D flow characteristics including the unique feature of 3D orientations of gas velocity vector field around the bubble. It is predicted that the maximum heat transfer coefficient is a result of the competition between surface–particle conduction and surface–fluid convection. The obtained results should be useful to the development of the fundamental understanding of the flow and heat transfer characteristics in a fluidized bed with immersed tubes.