Experimental And Simulation Of Segregation During Flowing Process For Multiple Solid Particles

The solid particles are easily seen either in nature or in human life. One can use Newton's laws of motion to accurately describe the state of motion for one single particle at a certain time while it becomes much more complicated and indescribable when lots of particles gather together and flow under gravity. As a result, the complex patterns of particle behaviors will lead to a series of frequently problems, such as blockage, segregation, adhesion and attrition. Particle segregation, which will cause the interruption of the production or render the final products that do not meet the necessary standards of product quality, is one of the most difficult problems demanding urgent solution in many industrial production. For instance, in the metallurgical industry, segregation of particles will give rise to the uneven distribution of the burden inside the blast furnace, resulting in the imbalance of gas flow and finally affects the smooth operation of blast furnace.This study aims to find the proper method to characterize particle segregation and explore the inherent laws of segregation. The segregation characteristics for multiple solid particles under steady-state conditions and unsteady-state conditions are both discussed. The main conclusions are as followings:(1) For mono particle, small particles flow fast and continuously with stable stream outline while large particles flow slowly and intermittently and often block easily because of arching at the outlet with irregular stream outline when falling from the same vertical height. The discharge time for mono particles is 1275ms(3mm particles)?1843ms(6mm particles) and 3155ms(9mm particles), respectively, which indicates that he discharge time increases nonlinearly as the particle size increases. After the finish of particle flowing, heap height for 3mm particles is the largest(3.85cm) within a small fluctuation of 0.25 cm. The heap surface is steep and smooth and the angle of the particle accumulation is approximately 42 °. The heap height for 9mm particles is about 2.70 cm but with a larger fluctuation(more than 0.5cm). The heap surface of 9mm particle accumulation is relatively flat and irregular with a smaller angle(approximately 25 °).(2) For binary particle, the surface of the particle heap is between this two kinds of mono particle. And with the increase of mass fraction of small particles, the heap height gradually increased. When binary particles mixed, with the increase of mass fraction of small particles, the discharge time decreases with a nonlinear tendency, resulting from that small particles can improve the condition of particle flow and reduce the probability of arch structure formed by large particles at the outlet of the hopper. When binary particles packed in two layers, there is a specific cut-off point of the discharge time curve. In the vicinity of this point, the discharge time is almost the same whether smaller particles is in the upper or lower part.(3) Under the condition of steady-state accumulation, the distribution of particle segregation is substantially symmetrical on both sides of the packing, but it has different characteristics for the bottom part, the intermediate part and the top part of the packing.(1) With the initial mass fraction of small particles increases under the same particle combination, positive segregation(caused by agglomeration of small particles) gradually increased and expands from the bottom of the heap to the upper part of the heap while negative segregation(caused by agglomeration of large particle) gradually reduced and shrinks from the heap surface to the edge of hopper wall. For 3mm and 7mm particles, when initial mass fraction of 3mm particles increases from 0.3 to 0.7, the degree of positive segregation increases from 16.0% to 38.5%, while the degree of negative segregation decreases from 38.3% to 10.4%. The overall uniformity of particle heap increases slightly.(2) With the increase of particle size difference under the same initial mass ratio of particles, the degree of positive and negative segregation increases which indicates that the particle segregation becomes serious. In addition, the overall uniformity for the three particle combinations(3mm and 5mm, 3mm and 6mm and 3mm and 7mm) are 64.9%, 55.8% and 41.5%, respectively.(4) Under the condition of unsteady-state flow, particle segregation gradually rises during the discharging process, and negative segregation occurs in the center part of hopper while positive segregation occurs on the edge of the hopper.(1) For the same particle combination, the initial mass ratio of particles has a significant impact on the negative segregation in the center part of the hopper during the middle and end of discharging process and also has some effect on the secondary positive segregation(see Table 3.5) on the edge of the hopper. With the increase of initial mass fraction of large particles, on the one hand, the degree of secondary negative segregation increase rapidly; on the other hand, the distribution of primary negative segregation has different characteristics. For 3mm and 6mm particles, when initial mass fraction of 6mm particles increased from 0.3 to 0.7, the degree of secondary negative segregation in the central region of the hopper jumped from 5% to 40% with an increase of 8 times; while on the edge region the degree of secondary positive segregation decreases from 30% to 10%.(2) For the same initial mass ratio of particles, different particle combinations have some influence on the uniformity during the middle and end of discharging process. Moreover, the greater of the particle size difference is, the lower of particle uniformity is. And the increase of the particle size will increase the degree of secondary negative segregation during the middle and end stage of discharging. The degree of uniformity for the three kinds of particle combinations are 60%(3mm and 5mm), 50%(3mm and 6mm) and 40%(3mm and 7mm) on the central part of the hopper during the beginning of discharging process.