Research On The Characteristics And Adjustment Of The Heterogeneous Structures In Bubbling Fluidized Beds

Fluidized beds are potential reactors for non-blast furnace ironmaking,due to various advantages such as efficient heat and mass transfer and good reaction performance.However,the formation of heterogeneous structures during fluidization hinders these advantages of fluidized beds by generating many problems,such as short-circuiting problem of gas trench flow caused by over-large bubble and over-fast bubble rise rate,resulting in low utilization rate of reducing gas.And particle segregation and delamination caused by inappropriate particle size distribution and superficial gaseous velocity.This leads to uneven heat transfer and reaction in the bed,and reduces the pre-reduction effect of iron ore powder.It is urgent to improve the heterogeneous structure of the bed through a certain control method.Therefore,the study on the heterogeneous structures formed during fluidization is important for the industrial applications of fluidized bed.In the thesis,a systematic study on the characteristics of heterogeneous structures inside a bubbling fluidized bed is carried out by physical experiments combining with numerical simulations,and the regulation method of the heterogeneous structure characteristics inside the bubbling fluidized bed is further explored,such that a scientific supports can be provided for the design and operation of fluidized bed reactors.The main research contents and conclusions are listed as follows:(1)The fluidization experiments were conducted under typical conditions with glass beads to study the effects of apparent gas velocity,mass ratio and other factors on the heterogeneous structure in the bed.In the fully fluidized state,the bubbles are distributed diffusely,and the bubble motion cycle can be divided into two stages,the initial stage of bubble generation and growth in the lower part of the bed periodically,in the second stage of bubble rise to the middle of the bed,the bubbles expand sharply,and the bed shows obvious heterogeneous,the reason is that the upper part of the bed is more strongly influenced by the airflow disturbance,and at the same time the bubble aggregation and more intense,resulting in the later stage of bubble motion on the bed The reason is that the upper part of the bed is more strongly influenced by the airflow disturbance,while the bubble aggregation is more intense,which leads to a greater influence on the heterogeneous of the bed at the later stage of bubble movement.Under the specific proportioning conditions,the particle size of B type is less than 300?m,and the particle size ratio of B:D is less than 0.56.Particle segregation gradually appears in the bed as the apparent gas velocity decreases,and obvious stratification occurs when the apparent gas velocity is lower than 0.278m/s.At this time,the lower part of the bed is in a static state and the upper part of the bed is in a fluidized state.The reason is that the apparent gas velocity is between the two types of particles u_mf,the B type particles are fluidized,and the D type particles are stationary or critical state,and the two types of particles are separated under the action of bubble tail vortex,which is manifested as the phenomenon of D type particles deviating at the bottom of the bed,and with the circulation of particles in the bed,the degree of deviation is increasing,which finally leads to particle stratification.(2)Based on the Eulerian-Lagrangian reference system modeling,the fluidization process of a 2D bubbling bed was investigated using CFD-DEM method simulations.The effectiveness of the CFD-DEM method is verified by comparing the results of two-fluid simulations and experimental results in the literature under the same conditions.Typical experimental conditions are simulated to study the effects of the velocity distribution of the gas phase in the bed and the opening rate of the distribution plate on the heterogeneous in the bed.The results show that the periodic motion of bubbles is accompanied by the fluctuation of velocity field in the bed,and the behavior of bubbles in the bed after complete fluidization is consistent with the experiments,the bubble tail vortex plays the role of converging airflow,which makes the velocity distribution of gas-solid phase in the upper part of the bed uneven,the disturbance is intense,the local particle aggregation is high,and the bed heterogeneous is strong;the size of bubbles in the lower part of the bed is small,the effect of tail vortex on airflow is limited,the velocity distribution is uniform,and the bed heterogeneous is weak;decreasing the The velocity distribution range is reduced by 29.4%compared with that without the opening rate of distribution plate.(3)Based on the Eulerian-Lagrangian reference system,CFD-DEM method was used to simulate and study the fluidization process of 3D bubble bed,and compare the difference of heterogeneous structure characteristics between 2D bed and 3D bed.It is shown that the heterogeneous structural characteristics of 3D bed and 2D bed show the same pattern with similar particle circulation,but the convergence effect of bubble wake on the gas flow is stronger in 2D bed because of the obvious sidewall effect of 2D bed,which restricts the lateral movement of gas flow.The degree of influence of apparent gas velocity,particle size of type B particles and distribution plate opening ratio on the heterogeneous structure was studied.By adjusting the above factors,the bubble size can be reduced by more than 35%and the heterogeneous in the bed can be improved;in the two-component system of mixing type B and D particles,the apparent gas velocity is controlled at 1.5?2 u_mf of type D particles and the mass ratio of type B and D particles is controlled above 0.56,which can effectively avoid particle segregation and delamination.Through the experimental and simulation study of 2D bubbling fluidized bed and3D bubbling fluidized bed,the understanding of the heterogeneous structures in the bed is enhanced,and a theoretical basis is provided for the control method of the heterogeneous structure of the fluidized metallurgical process.