Research On Kinetic Theory Of Cohesive Particle And Numerical Simulation Of Gas-solid Flow

Flow of particles plays an important role in both natural phenonena and industrial processes.According to Geldart classification of powders,particles are divided into four types among which C type particles have nano-scale or micron-scale diameter.This type of particles is named as superfine particle.The interaction between particles increases with the increase of surface area of particle which leads particles forming agglomerate easily.So the C kind particles is also called cohesive particle.Agglomeration will influence the industrial operation,decrease the production efficiency,debase the quantity of products,even cause an industrial accident.The challenge of research about cohesive particle is to reveal agglomeration mechanism and rules.That is very important to truly understand agglomeration process and flow pattern of agglomerates.With the development of computer technology,numerical simulation has already become one of the most promising approaches to study the gas-solid multiphase flow.To establish a accurate,reasonable,impeccable mathematical model is the key to perform the calculation.On the bssis of kinetic theory of granular flow,the model for cohesive particle flow is established.The devivation of solid viscosity,solid pressure are is described in detail.The population balance equation is coupled with modle to describe the cohesive particle number in the system.The diameter of agglomerate is calculated according to the change of partile number.The kinetic theory of granular flow for cohesive particle and two-fluid modle is perfected.In order to improve the computing accuracy of the mathematicla model,it is necessary to take transformation caused by collision between particles into account.The concept of configurational temperature based on fluctuation of transformation is put forward.The mathematicla model is corrected by generalized granular temperature.The calculating results show that calculaton accuracy has been improved.The solid pressure increases with the increase of agglomerate concentration which is consistent with data based on Johnson-Jackson model.And solid velocity agree well with experiment data.The flow behavior of cohesive particle in bubbling fluidized bed is investigated by means of CFD model for cohesive particle.The simulation results show that diameter of agglomerate increases first and reaches the steady value.And agglomerate diameter decreases with the increase of height of bubbling bed.Three main region can be found in the bed,they are big size agglomerae congregate area,small size agglomerate congregated ared and transition area.Comparing the results under different gas velocity,the range of diameter can be narrowzed with high gas velocity.The simulated results of concentration agree well with experimental data.And concentration of agglomerate is lower in the center that that near the wall.The simulation of cohesive particle flow in internally circulating fluidized is carried out.The value of agglomerate diameter is very different between agglomeration chamber and breakage chamber.This pressure is the driven force for circulating between chambers caused by the difference of diameter.The flow behavior of cohesive particle in spouted fluidized bed is investigated by means of CFD model for cohesive particle.The results show that steady state can be achieved in spouted fluidized bed under the effect of cohesive force.The shape of flow region is consistent with typic structure but the boundary of spout area becomes unclear.The area of jet region increases with the increase of spout gas velocity with a nearly linear relationship.Diameter of cohesive agglomerate can be controlled by adding non-cohesive particles.The increase of mass fraction or diameter of non-cohesive particle will lead the decrease of diameter of agglomerate.The flow behavior of cohesive particle in circulating fluidized bed is investigated by means of CFD model for cohesive particle.The distribution of concentration,diameter,granular temperature,solid pressure is studied.The results show that concentration has a core-annular flow pattern in radial direction.The simulation results agree well with experimental data.Diameter of agglomerate increases with increase of cohesive fore,but decrease with increase of gas velocity.