Discrete particle simulation of granular flows in hoppers and blast furnace charging systems

This work examines the fundamental behaviour of granular flow in hoppers and blast furnace discharging system. Although extensive researches have been carried out for many years, most of them are empirical, they are limited by numbers of constraining assumptions, and even the most basic flow mechanisms of granular materials are not well understood. In recent years, the researches of granular flow mechanisms have been developing and improving rapidly worldwide, which benefit from the powerful computer technology and the appearance of discrete particle simulation technique. Discrete element method (DEM) has been used increasingly in particulate research in the past three decades, which can provide micro-dynamic information of particles which is extremely difficult to be obtained by physical experiments. Despite all these contributions, the fundamental mechanism of granular flow still has not been fully understood and the information obtained from experiment and simulation is still scrappy, resulting in lacking of coherence. In this project, the mechanism of granular flow in hopper is studied using DEM simulation, the micro variables are analysed in steady and unsteady flows, such as velocity, force, coordination number and porosity. The micro information generated by DEM simulation is used to analyse the macro dynamics of granular flow by using a proper averaging method. The results show that the distribution of macro variables such as velocity, mass density and stress are highly related to that of micro variables. These analyses should be useful for a more comprehensive understanding of the complex dynamic behaviour of the granular flow. In addition, these analyses are applied to a series of hoppers with different sizes in order to study the size effects on hopper flow. Hopper size relative to particle size indeed has some effects, however, the ratio between particle-packed bed height and hopper length plays an important role. By a proper value, the micro and macro dynamic information can be well obtained from a small hopper. The previous studies on the granular flow in hoppers are mainly focused on the steady flow stage. However, the dynamic behaviour of the granular flow in a hopper in the early stage before the steady state stage is formed, has not been well understood. The start up stage happens in a very short time period, and is therefore difficult to detect by physical experiments. However, very fewer studies are found related to the start up stage. In this work, the DEM simulation method is applied to study the granular flow in a hopper at the start up stage. The discharge rate, velocity changes and the force propagation are studied, and the outlet size effects are examined. In the start-up stage before a steady state is reached, the discharge rate firstly increases dramatically until its maximum, and then decreases to a constant.After the outlet opening, particles change the state from static to flowing and a curve surface is formed between these two particle states. A force arch on which the particles have very small forces is formed below the switch curve. The switch curve and force arch propagate into the bed with the discharge of particles, and reach the top of the bed when a steady state is achieved.There are no significant effects of outlet sizes on the propagation velocity of the switch curve, however, the vertical force at the switch curve inclines with the increase of the outlet sizes, but decrease with the propagation. The simulation and analyzing methods are then applied to the granular flow in industrial processes. First, the P-W discharge system is selected to study the micro and macro dynamics of granular flow. The results show the different trend with that in a cuboid hopper with flat bottom. Secondly, the discharge rate of granular flow is investigated at different gate openings in the material bins of blast furnace system. The factors which may influence the flow rate are analysed, including rolling friction coefficient, sliding friction coefficient and damping coefficient. The results show that there is a critical gate opening beyond which, further increase in the gate opening will not result in any change of the flow rate. However, this can be solved by changing the location of the material gate to the bin neck area. Lastly, a full model of charging process simulator of blast furnace was developed in this work, which is an important step to decrease the gap between simulation and the real industrial process. Some micro-dynamic variables are analysed during charging process, such as velocity and contact force, a stagnant zone is found in discharge funnel, particles in this area keep stable until all particles are discharged from the material bin. Size segregations of coke and ore particles are studied in each part of the blast furnace. These are not only spatially-distributed but also exist in the discharging process. After filling the material bin, larger particles are more likely to distribute at the side wall with opposite direction of charging particles, smaller particles are more likely concentrate on the other side of the wall. Smaller ore particles discharge slower from discharge funnel through the DCD gate because many are located at stagnant zone