EMMS Based Coarse-graining DDPM And Its Application To Circulating Fluidized Bed Simulation

The flow and reaction processes in a circulating fluidized bed reactor(CFB)are closely coupled with the spatio-temporal multi-scale structure,which is a typical nonlinear non-equilibrium complex system.This kind of multiphase complex system is far from equilibrium state,and it is impossible to capture the real physical process simply by refining computational mesh,thus requiring a reasonable meso-scale model.In the past few decades,a large number of studies have been devoted to simulations of fluidized beds by using Eulerian-Eulerian models,showing that the drag force between gas and solid is very important,and the influence of meso-scale structure must be considered.The energy-minimization multi-scale(EMMS)model reveals the main mechanism of structure formation by analyzing the relationship between different scales through stability conditions.It can accurately capture the flow and reaction characteristics in circulating fluidized beds,and can achieve grid-independent prediction on coarse grids,so it can be applied to the simulation of industrial scale reactors.However,in the case of wide particle size distributions,if multi-fluid simulation is used,the Eulerian-Eulerian model needs to add a set of equations for each quasi-fluid of a particle phase,which requires more computational time and resources,and makes the coupling convergence more difficult.The Eulerian-Lagrangian model treats the discrete phase by Newton's second law to obtain the information of displacement,velocity etc.,and the use of discrete method to simulate solid phase directly is more suitable for the study of wide particle size distributions.However,if the information of each particle is tracked directly,the computational scale is limited due to the frequent and large number of particle collision searches.Coarsening method not only reduces the size of calculated particles,but also retains the characteristics of discrete particles tracking.Thus it is more suitable for large-scale reactor with wide particle size distribution.The coarse-grained discrete simulation method is still in development,and the published literature on simulations of large-scale reactors mainly focuses on the Geldart B system.For Geldart A system,the influence of mesoscale structure on drag force,grid dependency and the selection of coarsening parameters need further study.In consideration of the above problems,in this thesis,the simulations of fluidized bed reactors from two-dimensional to three-dimensional full loop scale were carried out,and the results were further compared with the results of the Eulerian-Eulerian method.The researches revealed that the influence of mesoscale structure on drag force should also be considered in the coarse-grained Eulerian-Lagrangian model.The key model parameters,such as coarse graining ratio,particle collisions were analyzed systematically,which provided the basis for the selection of model and parameter for the simulation of flow and reactions in large-scale fluidized bed reactors.In Chapter 1,the basic information of fluidization,particle classification and model classification are introduced,and the research contents and objectives are also introduced.In Chapter 2,the governing equations,drag coefficients and coarsening parameters of Dense Discrete Phase Model(DDPM),one of the coarse-grained Eulerian-Lagrangian models,are introduced.In Chapter 3,simulation with DDPM in two-dimensional circulating fluidized bed riser is introduced.The effects of drag model and mesh size on Geldart A and B particle systems are studied.Similar to the results of the Eulerian-Eulerian model,the DDPM with the EMMS drag can obtain mesh-independent results for the Geldart A granular system with coarse grids,and the simulation results are in good agreement with the experimental results.However,there exist a large gap between the simulation results by using the conventional uniform drag and the experimental results.For Geldart B granular systems,the simulations using both the EMMS and uniform drag models can reach grid-independent results,and the results are in good agreement with the experiment.On this basis of the previous chapter,in Chapter 4,3D DDPM simulations are carried out on the riser of a pilot-scale circulating fluidized bed.The selection of coarsening parameters in the model is studied.The simulation results using the EMMS drag are closer to the experimental than those of the uniform drag in coarser grids.Coarse-graining ratio(dcl/dp),a key parameter of the DDPM model,is correlated with mesoscale structure-the diameter of clusters(obtained using EMMS).The results show that,the simulations done with the coarse-graining ratio(dcl/dp = 65-125)within the range of the cluster diameters can give reasonable results with the experimental data.Based on this,the DDPM model which is coupled with the EMMS drag and with coarse-graining ratio calculated by the EMMS model is called the EMMS based DDPM model.In addition,the simulation results of Eulerian-Eulerian model and coarse-grained Eulerian-Lagrangian model are compared,and the results show that the effect of mesoscale structures should be considered in both models.In Chapter 5,3D DDPM simulations are carried out on the full-loop of a pilot-scale circulating fluidized bed,and the effects of the drag coefficient,the key parameters of particle interaction(the particle-particle restitution coefficient and particle-wall restitution coefficient)and particle size distribution are systematically studied.The results further proved that the EMMS based DDPM simulations can obtain reasonable results in a full-loop CFB.In addition,the results showed that the particle-wall restitution coefficient(epw)has little effect on the hydrodynamic behavior;for the particle-particle restitution coefficient(epp),when it is in the non-ideal range,i.e.epp=0.1-0.9,the simulation results agree well with the experimental data.For the case of narrow particle size distribution,reasonable axial distribution,radial velocity and concentration distribution can be obtained by using both particle size distribution and average particle size,though the simulation with considering of particle size distribution allows detailed description of the trajectories of different particles.On this basis,in Chapter 6,the EMMS based DDPM model is applied to the simulation of combustion reaction in an industrial scale circulating fluidized bed.The complex flow,heat transfer and reaction characteristics in the furnace were successfully captured,thus providing a foundation for the design,optimization and pollutant removal of a CFB boiler.Chapter 7 summarizes the main achievements of this paper,and discusses the application prospects and further research directions of the EMMS based DDPM model.