Abstract:

The cylindrical particles were characterized by a super-quadric model under the discrete element method (DEM) framework and the effects of aspect ratio (AR) and rotating speed on the mixing and diffusion characteristics of cylindrical particles in a rotating drum were studied. The results showed that enlarging AR increased the inter-locking and packing effects, leading to a higher elevation height and steeper inclination angle of free surface. Three mixing mechanisms were illuminated in the rotating drum: ①macro-scale convection mixing induced by particle internal circulation; ②meso-scale shear mixing induced by active-passive layers relative motion; ③micro-scale diffusion mixing induced by particle diffusion. Under the given operating parameters, cylindrical particles with the AR of 2 had the largest time-averaged mixing index while cylindrical particles with the AR of 4 had the smallest time-averaged mixing index. The primary factor influencing the mixing rate was rotating speed. Increasing the rotating speed enhanced the kinematic of cylindrical particles, and slightly increased the axial diffusion coefficients. The axis of cylindrical particles in the active layer tended to have the orientation in the range of 0°—30° while cylindrical particles in the drum preferred to be perpendicular to the drum axis. Computational costs for the cylindrical particles using the super-quadric model were higher than that for the spherical particles and increasing the AR increased computational costs.

Key words: mixing, numerical simulation, diffusion, non-spherical particles

摘要：

在离散单元法（DEM）框架下基于超二次曲面模型描述非球形颗粒，探讨了不同颗粒纵横比（AR）和转鼓转速对圆柱形颗粒混合性能及扩散特性的影响。结果表明：增大AR使得圆柱形颗粒的互锁和堆积效应增强，抬升高度更高，具有更陡的床面倾角。转鼓内混合存在三种机制：颗粒内循环导致的宏观对流混合、主动-被动区相对运动导致的介观剪切混合、颗粒扩散导致的微观扩散混合。在给定的研究参数范围内，AR为2时，圆柱形颗粒的时均混合指数最高；AR为4时，圆柱形颗粒的时均混合指数最低。转鼓转速是影响圆柱形颗粒混合速率的首要因素。增大转速加剧颗粒运动并缓慢增大颗粒的轴向时均扩散系数。主动区内的圆柱形颗粒和水平方向的夹角集中在0°~30°，转鼓内圆柱形颗粒倾向于和转鼓轴线方向垂直。基于超二次曲面方法的圆柱形颗粒计算耗时高于球形颗粒，增大AR进一步增加计算耗时。

关键词: 混合, 数值模拟, 扩散, 非球形颗粒