Numerical Simulation Of The Hydrodynamics And Heat Transfer Characteristics Of Fluidized Bed Based On Double-Grid Method

Fluidized bed with its wide adaptability of fuel,environmental protection superior performance,the simple preparation system and many other advantages are widely used in industrial fields.However,due to the complexity of the fluidization process,the interaction between two phases in the bed makes the microscopic reaction process of the fluidized bed is not fully understood.In recent years,with the development of numerical simulation accelerating,the accuracy of numerical simulation method for predicting dense gas-solid two-phase flow system is also increasing.This paper combines the discrete element method with computational fluid dynamics and developed the program by FORTRAN.The optimization of the gas-solid double-grid program is realized based on the previous program.The gas-solid flow characteristics,heat transfer characteristics and heat storage characteristics in the fluidized bed are studied.This study present a double-grid model,which is designed to exist instant mapping relationship between two independent grids.Including coarse grids are used to calculate the particle phase,fine grids are used to calculate the gas phase,and to achieve phases momentum and energy exchange by transmiting grids data at the same time.In order to verify the effectiveness of the double-grid model,three grid models are used to obtain the voidage profiles within bed,the particles flow sequence diagram,the particle velocity distribution,the gas velocity distribution,bed voidage contours plots and some of the results are compared with experiment.Then compare the general single grid model's calculating time with the double-grid.Results show that the double-grid model is feasible for improving the simulation accuracy of the gas-solid flow.The double-grid model can perform well in improving the caculation accuracy of CFD-DEM numerical simulation.The present results indicate that the double-grid model can achieve coupling calculation between the coarse grids and the fine grids,and the caculating increasment is not much.When the particle phase and the porosity are caculated,double-grid models not only meet the high spatial resolution requirements for the gas phase calculation within the fine meshes,but also follow the theoretical system for each mesh is greater than 3 times of the particle diameter in all directions.The simulation method can effectively solving the contradiction for different requirements of physical computing scale with CFD-DEM.Secondly,the flow characteristics of gas-solid is studied on the basis of the double-grid program,and the mixing index is introduced to analyze the mixing quality of the particles.The flow sequence diagram of particles,the change of the bed height,the axial and radial mixing sequence diagram of particles,the distribution of the bed mixing index,the distribution of the mixing index when the parameters changing are obtained.The results show that with the increasing of inlet gas velocities or particle sizes,the bed expansion height bed is increasing,but there is no linear relationship.The process of particle mixing in the bed is affected by the ability of particle circulation capacity and particle diffusion.To change the inlet gas velocity,particle elasticity coefficient or number of nozzles will not only affect the mixing speed of particles,but also change the mixing quality of the whole bed particles.Finally,a comprehensive gas-solid heat transfer model is established,then the characteristics of the heat transfer and heat storage are analyzed.The results show that with the deepening of the particle motion cycle,the collision heat transfer coefficient between particles,convective heat transfer coefficient between particles and gas,the radiation heat transfer coefficient between particles and bed are fluctuating at a certain value respectively.The temperature of the solid phase is increasing and the temperature distribution of the solid phase tends to be uniform.The simulation results of the heat storage characteristics of the fluidized bed show that the fluidized bed has a high heat transfer rate between the carrier fluid and the heat storage medium,meanwhile,it has a uniform temperature distribution along the bed.Under this condition,the heat storage efficiency of fluidized bed increases when the inlet gas velocity is improved.With the increasing of inlet air temperature,the heat storage efficiency will increase first and then remain basically unchanged.