Discrete Element Modeling And Characteristic Research For The Flow And Heat Transfer Of Rod-like Particles

As the main form of biomass,understanding the flow,mixing,and heat transfer behaviors of rod-like particles is the basis for the development of new technology and the optimization of the existing processes.However,the flow,mixing,and heat transfer characteristics of rod-like particles are very complex because of the orientation of the rod-like particles.To better understand the characteristics of rod-like particles is therefore helpful to control and optimize the related industrial processes.Nevertheless,researches on the mechanism of the flow,mixing,and heat transfer behaviors of rod-like particles in fluidized bed,drum,and other equipments are still insufficient.Therefore,investigating the particulate systems containing rod-like particles is not only of academic interest,but is also of significance for industrial applications.Along with the rapid development of computer technology,numerical simulation has been attractive in investigating the particulate systems.And among these numerous numerical simulation methods,Discrete Element Method(DEM)has become a widely used mathematical model.This research mainly focuses on the existing technical problems in non-spherical DEM,and then puts forward solutions accordingly.Subsequently,the established DEM model is used to numerically investigate the particulate systems involving rod-like particles.The main content of this thesis is as follows:(1)Developing a robust and efficient computing solver for non-spherical DEM and coupled CFD-DEM model based on the super-ellipsoid particle model.Within the framework of DEM,particles are described by super-ellipsoid model,and an efficient algorithm to detect the contact between the super-ellipsoid particles is developed.In addition,the establishment of an efficient algorithm for the contact between the super-ellipsoid particle and the complex geometric boundary of the device is also achieved.In CFD-DEM,an algorithm for searching particles in an unstructured CFD mesh is developed,and the voidage fraction of the CFD cell and the drag force between the super-ellipsoid particle and the fluid are calculated by dividing the particle into more than 100 parts according to latitude and longitude,to realize the two-way coupling between the fluid and super-ellipsoid particles.Subsequently,the corresponding expertiments and simulations are carried out to validate the accuracy and efficiency of the established model.The validation results indicate that the DEM model can simulate the flow of rod-like particles with rather high simulation accuracy and computation efficiency.(2)Establishing the heat transfer model for rod-like particles,including the conductive heat transfer model between the area-contacted super-ellipsoid particles and the convective heat transfer model between the fluid and rod-like particles.According to Hertz contact theory and differential geometry,the conductive heat transfer model for super-ellipsoid particles is developed,and the related algorithms are modified to improve the stability of the model;the convective heat transfer model for a single rod-like particle is established through the IBM-LBM(Immersed Boundary Method-Lattice Boltzmann Method)fully resolved simulation,and then the model is modified by quantifying the influence of the voidage fraction so as to it can be used in the multiple interacting particle systems.Subsequently,the heat transfer models are incorporated into DEM/CFD-DEM model to simulate the heat transfer behaviors of rod-like particles,and the related expertiments and simulations are conducted to validate the accuracy and stability of the established model.The validation results imply that the current model has rather high accuracy and stability to simulate the heat transfer behaviors of rod-like particles.(3)Investigating the flow,mixing,and heat transfer behaviors of rod-like particles by the established DEM/CFD-DEM model to obtain the flow,mixing and heat transfer characteristics of rod-like particles.Taking the rotating drum and fluidized bed as the research objects,the numerical investigations of the flow,mixing,and heat transfer characteristics of rod-like particles by DEM/CFD-DEM are carried out.According to the simulation results,the interaction mechanism among the flow characteristics of rod-like particles,the energy dissipation,and the gas are clarified.The research results indicate that the rod-like particles possess apparent preferred orientation.The operating conditions and particle shape have important influence on the flow,mixing and heat transfer characteristics of particles,and the effects of these factors are rather complicated.For example,how the particle aspect ratio affects the fluidization,mixing and heat transfer characteristcs of particles in the fluidized bed is generally related to the fluidization velocity.This work should lay the theoretical basis for the development of new process and new equipment involving rod-like particles in energy and chemical industries.Sytheszing the above,a DEM model for rod-like particles is developed in this paper,and the investigations of the flow,mixing and heat transfer characteristics of rod-like particles are subsequently carried out for the design,optimization,and control of the related industrial processes,such as the drying of biomass particles in rotating drum and the combustion in fluidized bed.