Numerical Simulation Of Gas-Solid Two Phase Flow In A 2D Spouted Bed With Immersed Tubes

The dense gas-solid two-phase flow has a wide range of applications in the fields of chemical engineering,agriculture,metallurgy,energy and so on.The research on the flow and heat transfer mechanism is of great significance not only in theoretical research,but also for practical applications.With the rapid development of computer technology,it has become possible to use numerical simulation methods to study complex systems in depth.This method can provide a good guarantee both in computational speed and computational accuracy.Based on the Eulerian-Lagrangian framework,numerical simulation of dense gas-solid two-phase flow is carried out by coupling computational fluid dynamics(CFD)with discrete element method(DEM).The calculation of the gas phase is carried out on the grid scale,and the calculation of the solid phase tracks the trajectory of each particle on the particle scale.Fortran language programming was adopted,and the immersed tubes in a two-dimensional spouted bed were described using body-fitted grid.The effects of immersed tubes on the gas-solid two-phase flow pattern and heat transfer in a spouted bed were also studied.The module of wall erosion calculation was added into the program to predict the circumferential distribution of the erosion of immersed tubes.In addition,the particle mixing ability in bed was also studied at the microscopic level,using the rich information of particles obtained by discrete element method.Firstly,the influence of immersed tubes on the particle movement and mixing in the spouted bed was investigated on the particle scale based on the CFD-DEM method.The particle relative circulation scale(L/S)of the particles reflecting the particle mixing characteristic in the micro-scale was defined.The changes of the average height of particles and the average vertical velocity of particles versus time were obtained and analyzed under two conditions.The results show that the existence of immersed tubes weaken the moving ability of particles in the bed,deteriorates the mixing ability and hinders the particle movement and mixing process.However,it also implies that the required time to establish a stable spout fluidization process in the spouted bed was correspondingly shortened;the particle relative circulation scale(L/S)proposed in the paper can reflect the ability of circulating of particles in the bed,the larger the L/S,the better the mixing characteristic of the particles in the bed.Secondly,based on the CFD-DEM method,the influence of immersed tubes on the heat transfer characteristics of gas-solid two-phase was investigated from the particle scale.Different roles which thermal convection and heat conduction played in the heat transfer process in the bed were statistically analyzed.The variation of average particle temperature,particle temperature variance and particle-gas convection heat transfer coefficient versus time were obtained.The results show that the existence of immersed tubes increase the contact time between particles and high temperature high speed gas,increase the average temperature of particles in the bed,and strengthen the convective heat transfer between particles and fluid.Also,the existence of immersed tubes complicate the geometry of the spouted bed,increase the probability of collision between the particles,extend the duration of collisions between particles due to particles' weaker moving ability.Thus,the heat exchange between the particles is more complete,thermal conduction between particles is enhanced,and the particles temperature variance decreases,which makes the bed temperature distribution more uniform.Finally,the erosion model proposed by Finnie et al.was added into the program to predict and analyze the erosion distribution along the circumference of the immersed tubes.The rose diagram of the erosion distribution of immersed tubes along the circumference was obtained.The results show that the erosion of immersed tubes results from the high speed collision between particles and the tubes,and the erosion amount changes greatly along the circumferential direction.Erosion near the center spout below the tubes(about 165 °)is the severest due to the higher particle velocity,while erosion near the wall(about 270 °)is relatively slighter due to the smaller particle velocity.