Experimental And Numerical Study On The Flow Behavior Of Flexible Filamentous Particles In The Entrained-flow Bed

The entrained-flow bed is widely used in various industrial processes,due to the large gas-solid contact area,high heat and mass transfer efficiency,and convenient material transportation.Flexible filamentous particles(such as chemical fibers,pasture,cut-tobacco,cut-straw,wood fibers,etc.)are a kind of common industrial processing particles,which are soft,anisotropy,easily bending and deforming and have slender shapes.Therefore,the deformation,rotation and orientation distribution of the flexible filamentous particles and the forces on them in the entrained-flow bed are extremely complex and they would interact with each other and gas phase in the entrained-flow bed and then form the so-called clusters,leading to the poor equipment performance.The movements of the flexible filamentous particles forms a complex gas-solid two-phase flow system.In the literatures,the research on the flow characteristics of such kind of particles are not in-depth.The movements of filamentous particles,the formation,development,coalescence and fragmentation of clusters are not fully understood,which leads to the basic research cannot meet the needs of the industrial requirements.Therefore,this thesis systematically studied the flow characteristics of flexible filamentous particles in the entrained-flow bed through experiments and simulations.In the experiment part,a three-dimensional visualized entrained-flow bed platform and a highspeed camera system are established to observe and explore the microscopic trajectory and macroscopic flow behavior of filamentous particles in the entrained-flow bed.Based on the threedimensional entrained-flow bed system,the methods for measuring the three-dimensional timeaveraged particles concentration field(3D time-averaged particle concentration reconstruction methods in a single view and a dual view)and the Flexible Filamentous Particle Tracer Velocimetry are proposed to study the trajectory,concentration and velocity of these special flexible filamentous particles in the entrained-flow bed riser.By regression analysis,the empirical correlations of particle translation and non-translation movement ratio are also established.In addition,the critical stable fluidization velocity of flexible filamentous particles and the velocity curve of the critical stable fludization are also obtained.Then,this work puts forward to a new cluster identification algorithm,which helps to get profound researches on the cluster size distribution,cluster shape,and other cluster properties.According to the morphology and evolution process of different particle clusters,5 types of typical cluster structures in the central region of the riser(stripe-shaped cluster,inverted U shape cluster,saddle-shaped cluster,irregular shape cluster and micro cluster)and 4 types of typical cluster structure at the neighboring wall region of the riser(vertical strip-shaped cluster,cluster extending from the neighboring wall region to the center region of the riser,spherecial cluster and composited cluster)are proposed,and their formation and evolution mechanism as well as the characteristics are also described.Then,the effects of the particle properties and the operating conditions on the cluster size,shape,and internal solid volume fraction distribution are also emphasized with the single-factor experimental analysis.It is concluded that the particle moisture content is found significantly affects the particle flexibility and viscosity.Both particle length and particle moisture content have positive relationship with the average size of clusters and the amounts of large clusters.In addition,the average internal particle volume fraction of cluster decreases first and then keeps stable with the increment of the cluster sizes.In order to obtain information which is difficult to obtain in the experiments,this paper carries out numerical simulations at the same time.Different methods are used in this paper to model the gas and solid phases.The gas phase is solved by the computational fluid dynamics solver,and the particle is described by the flexible filamentous particle chain model.The two-way coupling numerical iteration scheme is used,and the flow behavior of flexible filamentous in the entrained bed riser at the different operating conditions(superficial gas velocity and particle mass flow rate)are simulated.In order to accurately describe the flexibility of flexible filamentous particles,a calibration method of spring constants of the flexible filamentous particle chain model is proposed.The spring constants are calibrated against experimental datum,thus,this model can accurately describe the bending and deformation of flexible filamentous particles.The numerical simulations of the movement,dynamic characteristics and residence time of the particle phase are validated.When the superficial gas velocity is small,the simulated value of the particle concentration at the bottom of the riser is slightly smaller than the experimential value.Nevertheless,this model can still give the more realistic and reliable axial particle concentration distribution,velocity distribution and the probability distribution of particle residence time.Besides the double chain cut tobacco particle model,the RTD of spherical particles under the same operating conditions is also calculated.It is found that the probability distribution function of particle resistance time and mean residence time of spherical particle model is much smaller then the experiment results.If the spherical particle model is used to simulate the residence time distribution of the flexible filamentous particle flow,it will cause larger errors,which further highlights the superiority of the flexible filamentous particle chain model.The uniformity of the end product of the entrained bed process can be inferred by the particle residence time distribution(RTD).Generally speaking,a narrow RTD indicates a more consistent product quality attributes(drying and mixing uniformity,coating consistency,and etc.),while a wide RTD suggests high variability.It is found that the probability density curve shows a bimodal sharp with a small superficial gas velocity(3.82 m/s).The particles form a ring-core structure in the entrained-flow bed,where the second peak value is caused by the horizontal movement of the particles.Also,the spread of the probability of residence time decreases with the increase of superficial gas velocity.The transition point of the acceleration zone and the fully developed zone is obtained by analyzing the minimum residence time of particles.The minimum residence time increases proportionally with the riser height in the fully developed zone.