**1. Introduction**

The lumpy zone of the blast furnace (BF) is composed of layers of piled burden formed naturally during charging. Two significant variables characterize the properties of the layers: The angle of repose and porosity distribution, which reflect the external shape and internal structure, respectively. The former reflects the stability and surface profile of the piles. The latter is a direct reflection of the permeability of the burden, which is closely connected to the gas flow resistance and heat exchange efficiency between the burden and gas in the blast furnace. Therefore, an improved understanding of the formation mechanism and internal state of a pile is important when measures are to be taken to improve the efficiency of the conditions in the upper part of the blast furnace. The coordination number (CN) is an important parameter reflecting the internal structure of the granular pile, which is closely related to porosity. However, it is difficult to gain a deep understanding of the flow and packing of granular materials by experimental methods, due to the complex behavior of granular materials in bulk systems [1,2]. Therefore, numerical simulation has become an interesting and viable option, and, in particular, the discrete element method (DEM). This method can provide estimates of the position, velocity and stress information of each particle in a granular system.

The angle of repose is a fundamental property of a pile, which usually reflects the liquidity potential of it. By simulation, it has been found that the angle of repose is related to DEM parameters, such as the rolling and static friction coefficient [3]. Elperin et al. [4] and Coetzee et al. [5] revealed that

the angle of repose is positively correlated with the friction coefficient, but when the friction coefficient increases to a certain value, the angle of repose does not any longer increase or instead even decreases, due to the collapse of the pile. Alizadeh et al. [6] also found that the angle of repose is strongly affected by the particle shape. Furthermore, the particle size [7,8] and packing method [9] also influence the angle of repose.

Porosity is closely related to the permeability of the packed bed. Thus, by adjusting the porosity in the BF can help control the gas distribution, and thus, the heat transfer and reactions in the lumpy zone. It has been observed that the gas permeability of the stock column will deteriorate rapidly when the porosity is reduced to or below 0.3. It is easier to measure the porosity of particles in a container either in an experiment or in simulation than in the real process. Zou and Yu [10] presented experimental research on porosity by using a cylindrical container and found that the initial porosity of a pile is strongly dependent on both particle shape and packing method. Another simulation work by the same authors [11] concluded that particle size also influenced the porosity. However, there are only a few publications on the porosity distribution in a three-dimensional particle pile, which is of interest for the distribution of gas flow in industrial applications, such as in reactors, moving and fluidized beds.

Iron oxide particles are used as the main raw material in the blast furnace. Still, many papers on the simulation of gas-solid two-phase flow in the blast furnace have not considered the radial distribution of porosity, even though the porosity distribution is known to be non-uniform. Therefore, it is an important aspect to consider. The present work studied the effects of DEM parameters and packing method on the properties of pellet piles, with the aim to provide insights that can be used in modeling and further research on the porosity distribution of burden in the blast furnace.

Packing density, as the opposite of porosity, has also been studied by many investigators. Most studies focus on packing density of spherical [12–14] and non-spherical particles [15–20] in a container by dense or loose packing [15]. However, as it is hard to measure the porosity distribution of a conical pellet heap, there are still few publications in this field, due to the anisotropic properties of granular materials, the complexity of pile structure and the opaque mechanism by which the packing evolves in three dimensions [1]. DEM has become a viable choice for studies on the properties of granular piles, and therefore this modeling method has become popular in blast furnace investigations [21–24].

The present work introduces several novel aspects, including the treatment of the porosity distribution in the pile and the fact that industrial-scale pellets are studied, as well as the 1:10 scale-charging system used. The paper studies the angle of repose and porosity distribution of a pile of iron oxide pellet by experimental and numerical methods. Section 2 introduces the experimental work, including the methods and apparatus. The simulation theories and conditions are presented in Section 3. In Section 4, the angle of repose and porosity distribution of a pellet pile are studied to validate the DEM physical parameters determined by the discharging method. The effects of packing method on the angle of repose are also discussed. Finally, the conclusions of the work are proposed.
