Discrete Element Model Simulation And High-speed Photographic Experimental Study Of Dense Gas-solid Two-phase Transient Flow In Fluidized Bed

Fluidized bed technology is widely used in coal combustion,catalytic reaction,material drying and other energy chemical and food processing industries.Understanding the gas-solid two-phase flow mechanism within a fluidized bed is the basis for optimizing the design of fluidized beds and improving combustion efficiency.Under the joint training program of Jiangsu University and Washington University in St.Louis,the author have carried out a series of research work on fluidized bed gas-solid two-phase transient flow based on theoretical analysis,experimental measurements and numerical simulations.This work was funded by Consortium for Clean Coal Utilization and the Jiangsu Provincial Advantage Discipline Program of "Power Engineering and Engineering Thermophysics".The main research content and creative results achieved in this paper are shown as below.(1)A high-speed photographic experiment bench of fluidized bed was designed to measure the gas-solid two-phase transient flow during fluidized bed startup.It was found that changes in bed height affect the shape of the bubbles when the inlet flow is constant,but have less effect on bubble area and bubble equivalent diameter.The inlet flow rate is the main determinant of the flow pattern.The higher the inlet flow rate,the greater the energy transferred by the gas to the particles within the bed.The larger the bed height and bubble area and the longer it is maintained and the later the break-up time.In the design of the fluidized bed,attention must be paid to the rational selection of the inlet flow rate.(2)The appropriate grid spacing was determined by grid-independence analysis,and the difference between the numerical simulation results of the two-fluid model(TFM)and the discrete element model(DEM)was investigated in conjunction with experimental measurements.It was found that TFM is only suitable for large-scale macroscopic analysis and cannot accurately predict the bubble fragmentation and stratification phenomena.Relatively speaking,the results predicted by DEM are more similar to experiments and more suitable for the study of transient flow mechanisms within fluidized beds.Numerical simulations were performed on each of the six widely used drag models and compared with the experimental results.It was found that the Gidaspow model has significant advantages in predicting bed height and bubble equivalent diameter,and is in good agreement with the experiments in terms of bubble morphology and pressure fluctuations,allowing more accurate prediction of dense gas-solid two-phase transient flows.(3)The effect of inlet flow on the flow pattern within the fluidized bed were investigated,and the transition of flow pattern with increasing flow rate was analyzed.Bubble flow morphology was shown in the initial stage,then transitioning to a state in which bubble flow morphology is the dominant nodal surge morphology,and finally presenting a state in which bubble flow morphology and turbulent flow morphology co-exist.The pressure fluctuations at different height positions of the fluidized bed were measured and the intrinsic relationship between pressure fluctuations and the flow pattern transition process was analyzed.It was found that the fluctuating characteristics of the bed pressure and the change in the number of bubble break-ups can be used as important indicators,to judge the transformation of the bubble flow morphology into turbulent flow morphology.The discriminant equation for the internal flow pattern of the fluidized bed is established.When the discriminant equation is positive that the fluidized bed is in turbulent fluidization.When the discriminant formula is negative,the fluidized bed is in bubbling fluidization.(4)Numerical simulations and experimental measurements of gas-solid two-phase transients within fluidized beds at different inlet locations were carried out,and the transient flow characteristics were compared in terms of bed height,bubble area and other fluidization parameters,as well as gas-phase pressure field and velocity vectors.It was found that the position of the inlet in the fluidized bed has a strong influence on the morphology and evolutionary mechanism of the bubbles,and that the closer to the right wall,the more the bubble's morph centricity slopes to the left.At the same time,the moment when the bubble tilts also varies with the inlet position,and the further away the inlet is from the wall,the more delayed the moment when the bubble tilts.The location of the inlet has a strong influence on the manner in which the gas escapes,the area of escape,and the particles ejection.The closer the inlet location is to the center of the bottom of the fluidized bed,the more intense the particle ejection is and the larger the area covered.The inlet position determines the energy transfer in the fluidized bed,and as the inlet position moves away from the wall,the degree to which particles are constrained by the wall during kinetic energy transfer and displacement gradually decreases.In this case,more and more particles can obtain effective kinetic energy and undergo displacement.(5)The effect of dual inlets on the gas-solid two-phase transient flow characteristics within the fluidized bed was investigated using discrete element simulations.The particle transient distribution,particle velocity vector,gas-phase pressure,velocity streamline,and gas-phase pressure distribution were compared for different inlet spacing schemes.It was found that dual inlets can significantly reduce the area of local solid phase dead zones in fluidized beds compared to single inlet.When the two inlets are spaced far apart,the bubbles in the fluidized bed are brought closer to the middle by the influence of the walls on both sides,the top of the bed appears concave,and the phenomenon of particle ejection accompanies the bubble rupture.When the two inlets are spaced appropriately,a critical state of independent development of the two bubbles is presented.When the spacing between the two inlets is further reduced,the bubbles appear to merge.The pressure in the bed is greater,the closer to the inlet.The pressure gradient decreases from the center of the bubble to the left and right sides of the bed.When the two inlets are spaced closer together,the bubbles tilt to the left and right sides of the bed as they progress upward.Pressure fluctuations in the fluidized bed arise at the inlet and gradually decay as they propagate toward the outlet.A reasonable inlet arrangement allows for a more uniform pressure distribution and high gas-solid contact efficiency.