Investigation On Spherocylindrical Wet Particle Gas-solid Two-phase Flow And Heat Transfer Characteristics In Bubbling Fluidized Beds

Bubbling fluidized bed can provide sufficient momentum and energy exchange space for particle systems,and is frequently used in energy,food,and chemical industries due to its advantages of uniform temperature and excellent heat transfer efficiency.However,the particle systems involved in nature and industrial production are typically unconventional particle systems with irregular particle shapes and liquids between particles.Because of this,unconventional particle systems in the fluidized bed exhibit a very distinct mesoscale structural evolution process from conventional spherical particle systems.Additionally,the properties of particles and liquids significantly affect the multiscale flow and heat transfer characteristics of the gas-solid two-phase in fluidized beds.Consequently,this thesis conducts numerical simulations and experimental researches on the characteristics of gas-solid two-phase flow and heat transfer of spherocylindrical wet particles in bubbling fluidized beds.A numerical simulation model of unconventional particle gas-solid two-phase flow and heat transfer was developed based on the CFD-DEM model.The collision process between non-spherical particles was described by the combined spherical element model and soft sphere collision model,and a liquid bridge force model appropriate for characterizing non-spherical particles was constructed based on the liquid bridge force model of spherical particles.Incorporating the effect of liquid on the heat transfer process between particles,the multilayer heat conduction model considering liquid film and gas film suitable for non-spherical wet particle systems was further developed and the CFD-DEM model of flow and heat transfer of unconventional particles was further improved on the basis of "particle-gas filmparticle" heat conduction model.Experimental studies on the flow characteristics of spherocylindrical dry particles in bubbling fluidized beds under cold conditions were carried out using PIV technology,and the model’s accuracy was verified.The flow characteristics of spherocylindrical one-component and spherical and spherocylindrical mixed twocomponent particle systems were investigated using the established non-spherical dry particle CFD-DEM model.The effects of particle shape and pulsating airflow on particle flow characteristics were analyzed in one-component systems.The effects of gas velocity and particle aspect ratio on particle flow behavior and mesoscale structure evolution characteristics in a two-component system were investigated.The results show that increasing the volume fraction of spherocylindrical particles increases the size and frequency of bubbles,as well as the gas-solid contact efficiency.Experimental studies on the flow characteristics of spherocylindrical wet particles in bubbling fluidized beds were carried out using the PIV technology,and the improved non-spherical wet particle CFD-DEM model was validated.Furthermore,the improved non-spherical wet particle CFD-DEM model was applied to investigate the evolution law of bubble structure in spherocylindrical wet particle jetting bubbling fluidized bed and uniform air distribution bubbling fluidized bed systems.The research results show that there is a competition and coordination mechanism among contact force,liquid bridge force and drag force in the process of bubble evolution.Through factorial analysis,it was found that there is an interaction between particle shape and viscous liquid on the translational kinetic energy and rotational kinetic energy of particles,and this interaction is more pronounced in the intermedia phase and the emulsion phase regions.Based on infrared thermography,experimental studies on the heat transfer characteristics of spherocylindrical wet particles in bubbling fluidized beds were carried out,and used to verify the accuracy of the improved multilayer heat conduction model of non-spherical wet particles.The heat transfer characteristics of spherocylindrical dry particle and spherocylindrical wet particle system were investigated by CFD-DEM numerical simulation,and the effects of particle shape and liquid properties on the heat transfer characteristics of spherocylindrical wet particle system were explored.The research results show that using the multilayer heat conduction model considering liquid film and gas film in the spherocylindrical wet particle system effectively reduces the temperature standard deviation of the particles by 22.1%,indicating that the application of the improved multilayer heat conduction model is more effective.The multilayer heat conduction model produces a more uniform temperature distribution of the particles in the fluidized bed.Furthermore,as the particle aspect ratio increases,the heat transfer amount increases.Agglomeration structure forms between particles as liquid content and viscosity increase,enhancing heat conduction between wet particles while weakening convective heat transfer between wet particles and fluid.