Simulation And Experiment Of The Burden Descent And The Raceway Formation In A Blast Furnace

The main iron-making equipment,blast furnace(BF)is a complex high-temperature vertical moving-bed reactor.The burden descent and raceway formation in BF are two crucial particle dynamics factors influencing the stability and efficiency of BF.In this thesis,a physical model experiment and a DEM/CFD-DEM coupled numerical simulation were used to study the descending motion of the burden and the formation mechanism of the raceway in the BF,and the problem of dimensionality with the model design can be solved.First of all,a three-dimensional(3D)semi-circular cold experimental model was established,and the "symmetrical-plane cutting method" was proposed to observe the internal real structure of the solid burden flow.Combining with DEM numerical simulation,the characteristics of the solid flow in the BF were studied.The experimental and simulation results show that the plug flow is the main form in the process of solid flow,whereas,convergence flow occurs only below the lower part of bosh.In order to determine the boundary between plug flow and convergence flow,Wilcox-Swailes coefficient(Uws)is introduced to describe the uniformity of granular velocity distribution,where Uws≥0.65 is the plug flow,and Uws<0.65 is the convergence flow.Further parameter analysis shows that there is little effect on the solid flow pattern by changing the bosh angle,while significant influence by the size of the raceway.The particle velocity distribution of mixed burdens is more uniform than that of layered burdens.Shape of deadman shape can be affected by size of the raceway and the discharging speed.Secondly,a 3D optical cold experimental research platform was established for the formation mechanism of the tuyere raceway,and a method for measuring the pneumatic cavity in packed bed by laser attenuation was developed to realize the non-contact measurement of the size,shape and position of the real 3D cavity.The experimental results show that a relatively stable ellipsoidal cavity can be formed in the burden layer with moderate amount of air blowing in the tuyere.The size of the ellipsoidal cavity is positively correlated with the volume of air blowing and negatively correlated with the diameter of the tuyere,while there is little effect by changing the insertion depth of the nozzle.Changing the angle and diameter of tuyere affects the position and shape of the ellipsoid void.The orthogonal experimental analysis of the parameters reveals that the structure and operation parameters show different influences on the shape of the raceway.The blast volume is the decisive factor for the size of the raceway,while the blast angle dominates the depth of the raceway and the tuyometer diameter has a greater effect on the height of the raceway.Then,five 3D models of the raceway formation with different thickness were constructed.The influences of different model thickness on the raceway shape,size and formation process were analyzed by using different observation methods of thin and thick models.The research method adopted cold experiment and CFD-DEM coupled simulation.The experiment and simulation have got basically the same results and can verify each other.In the models with different thickness,a relatively stable raceway can be formed within a certain range of blast volume.The raceway area of the thin model is mainly feather-like void extending upwards,while the raceway formed by the thick model is ellipsoidal.Under the condition of the same blast volume,the raceway size decreases with the increase of the model thickness.When the model thickness increases to more than 3 times of the tuyere diameter,the raceway size basically stabilizes.The raceway size in the slot model(the model thickness=the tuyere diameter)is 3-5 times deeper and higher than the thick model.The phenomena of dynamic circular zone and "false boundary" in thin model were found in experiments,and the motion of particles around the raceway was discussed by CFD-DEM simulation.The numerical simulation shows that the raceway formation has a process:starts from becoming larger,then gradually becoming smaller,and tending to dynamic stability at last.The thick model is more likely to form a stable raceway,which takes about half as long as the thin model.The workload of numerical simulation increases exponentially with the increase of the model thickness,a more appropriate select is about 3 times of the tuyere diameter(S3).S3 model can ensure the authenticity of the results and have high computational efficiency.In this thesis,the 3D particle bed model with symmetrical structure is used in both the BF burden descent and raceway experimental equipment,which avoids the"wall effect" error and the false result caused by the reduction of model dimension,and makes the experimental results more authentic and reliable.The numerical simulation method was introduced to analyze the experimental details and related parameters,and the motion behavior of the granular bed and the formation mechanism of the dynamic and stable morphology were discussed.The guidance suggestions for simplification of the geometric dimension of the numerical modeling of the granular bed were put forward.