EMMS-Based Multi-Scale Mass Transfer And Reaction Simulation

A circulating fluidized bed (CFB) embraces complex coupling among flow, heat/mass transfer and reaction kinetics over a wide range of spatial-temporal scales, on which the meso-scale flow structure in the form of clusters or bubbles plays a critical role. The traditional two-fluid model (TFM) is based on local equilibrium assumption and neglects the effects of sub-grid meso-scale structures, thus, it is not suitable for simulating heterogeneous gas-solids flow and reactions. It is necessary to build the meso-scale model based on the physical simplification of real processes. It has proven that the EMMS-based meso-scale drag model can predict properly the flow behaviors in a CFB. However, its grid-dependency still needs to be confirmed, and the effects of the local heterogeneous flow structure on the mass transfer and reaction also need to be studied.In view of these problems, in Chapter 2 we investigated how the slip velocity and mass transfer coefficient vary with grid resolution under different solids volume fraction in a periodic domain with the EMMS/matrix and Gidaspow drag models. For Geldart A particles, the grid-independent slip velocity seems to be achieved under all range of solids volume fractions, investigated with the EMMS/matrix drag model. With the increase of the grid resolution, the mass transfer coefficient decreases and reaches its numerically asymptotic value when the grid scale is about 10 times the particle diameter, no matter if it is for Geldat A or Geldart B particles, which can be due to the more reasonable heterogeneous flow structures captured in the finer grid, investigated with both drag models. And the variation of fine-grid, effective mass transfer factor ? with the solids volume fraction shows the same tendency and magnitude with the heterogeneity index for mass transfer, predicted by the structure-dependent multi-fluid model.In Chapters 3 and 4, the structure-dependent multi-fluid mass transfer model is proposed, to consider the effects of meso-scale heterogeneous flow structure on the mass transfer and reactions. It reduces to the mass transfer model based on conventional TFM model if the local equilibrium or homogeneity is assumed within each grid. By analysing the heterogeneous reaction processes in the cluster-based or bubble-based flow structure, the relationship between subgrid and averaged reactant concentration can be determined. Thus the heterogeneity indexes for reaction and mass transfer can be proposed, which may facilitate the TFM-based reactive simulations with structure-dependent corrections. And it is validated through the two-dimensional (2D) and three-dimensional (3D) simulations of ozone decomposition in the CFB riser and bubbling bed, showing reasonable agreement with the experimental data. The importance of structure-dependent mass transfer modeling grows with reaction rate.In chapter 5, the variation of the heterogeneous flow behavior in a riser caused by online adjusting the solids flow control mechanical valve and changing the length of the suspension section is investigated through virtual experiment, which is characterized by using a moving mechanical valve together with 3D, full-loop simulation for the first time.Finally, we summarized the main conclusions and presented the perspective of this work.