Numerical Simulation Of Hydrodynamics And Heat Transfer Characteristics Of Wet And Dry Particle In A Three-dimensional Spouted Bed

Gas-solid spouted bed is widely used in efficient combustion,pharmaceutical coating,particle granulation and other industrial production field because of its excellent heat and mass transfer characteristics.However,due to the complexity of the strong multi-scale nonlinear interphase mechanism of dense two-phase flow in a spouted bed,he microscopic reaction process of the fluidized bed is not fully understood.In recent years,with the development of computer and numerical simulation technology,numerical simulation which was used to simulate the dense gas-solid two-phase flow system became more effectively and accurately.In this paper,based on the framework which combined the discrete element method and computational fluid dynamics,gas-solid flow model in three-dimensional spouted bed was built up and programmed by Fortran language.Besides,the liquid bridge force was introduced and the gas-solid flow characteristics,heat transfer characteristics of the dry and wet particles in the spouted bed are studied respectively.First of all,the self-compiled program was used to simulate the flow characteristics of two-phase flow in the spouted bed.Based on this,the mixing characteristics of the particles in the bed and the influencing factors were studied.The Lacey mixing index was introduced to quantitatively analyze the degrees of mixing in the whole bed and characteristic regions of the bed.The mixing sequence diagram of particles,the velocity distributions of gas and particles,the distribution of the bed mixing index and the distribution of the mixing index when the parameters changing are obtained.The results show that the energy contained in particles is mainly the gravitational potential energy.The tendencies of gas and particles velocity distribution are similar,but the values differ greatly.The particle velocities of the different sizes are similar except in the annulus area.The speed of the particles' mixing in the whole bed was depended by the one in the annulus area.Change the superficial gas velocity and particle density ratio will cause the whole bed particle mixing quality changes and the superficial gas velocity has a greater impact on the particle mixing index.Secondly,the wet particle flow characteristics in the spouted bed are simulated after considering the liquid bridge force caused by the particle-carrying liquid film.The results show that the flow state of wet particles in the spouted bed is similar to that of the dry particles in the same condition,and no obvious agglomeration occurs.In the whole bed range,the collision force of the particles are much greater than the liquid bridge force,the particles tend to be dispersed and cannot form a stable agglomeration.The velocities of particles decrease in the entire bed caused by the liquid bridge force.Besides,the liquid bridge has a greater influence on small particles than the large ones,which reduces the difference of velocity between wet particles with different diameters.The liquid bridge volume has a more significant influence on the mixing of particles,compared with the ratio of particles densities and the superficial gas velocity.Finally,the gas-solid heat transfer model in the spouted bed was established and the heat transfer characteristics in the spouted bed were simulated.The results show that the temperature of the particles increases rapidly and the temperature distribution of the particles gradually becomes uniform.However the temperature of particles in the dead zone is significantly lower than that of other particles because they are not involved in the particle circulation.The trend of large particles temperature is linear and the one of small particles is parabolic and the temperature of small particles higher than the temperature of large particles in the fluidized bed gas-solid phase heat transfer process,The convective heat transfer coefficient between gas and solid is close to the heat transfer coefficient between particles,but the value of convective heat transfer is much larger than the one between the particles.The collision heat transfer between the particles and the wall and the radiation heat transfer have limited influence to heat transfer in the spouted bed.