湿颗粒聚团碰撞解聚过程的离散元法模拟

基于线性接触模型、库仑滑移接触模型以及平行黏结三种接触模型的组合, 利用离散元法对包衣结构的湿颗粒聚团与壁面碰撞解聚的物理过程进行了数值模拟, 研究了碰撞过程中湿颗粒聚团解聚模式、解聚过程中聚团内各颗粒的速度变化以及颗粒间液桥断裂的规律, 分析了聚团的碰撞速度、黏附小颗粒的重力以及中心大颗粒的旋转对聚团解聚的影响. 研究发现: 聚团的碰撞解聚呈现出碰撞式、重力-碰撞式和剪切-碰撞式三种解聚模式. 湿颗粒聚团与壁面的碰撞打破了聚团内颗粒速度的一致性, 颗粒间出现相对运动而使颗粒间的液桥发生拉伸断裂. 液桥的断裂由聚团的碰撞点向外、由底部向上、由内层向外扩展. 聚团内液桥的断裂经历了缓慢断裂、快速断裂和完全断裂三个阶段. 碰撞速度越大, 黏附的小颗粒质量越大、大颗粒的转速越高, 湿颗粒聚团的缓慢断裂阶段越短暂且解聚程度越高. 模拟结果和实验符合.

Discrete element simulation of impact disaggregation for wet granule agglomerate

Based on the combination of linear contact model, Coulomb slip contact model and parallel bond contact model, a discret element model (DEM) of wet granule agglomerates with coating structure is constructed. Disaggregation processes of wet agglomerates in impacting to a horizontal plate are performed by applying particle flow code (PFC). Three failure patterns are obtained corresponding to those in experiment. The variation of velocities and rupture characteristics of liquid bridge in disaggregation process are investigated. Effects of impact velocity, gravity of adhered granules, and rotation of core granule are analyzed. DEM simulations show that there are three disaggregation patterns in the coating structure of agglomerates: impact disaggregation, gravity-impact disaggregation and shear-impact disaggregation, depending on the size of primary particles and the rotation of the core granules. With the enlargement of size, gravity plays an increasingly important role and the impact disaggregation pattern shifts to gravity-impact disaggregation. The rotation of core can generate a shear force to separate the fine and disaggregation pattern to turn to shear-impact disaggregation. Impacting results in a heterogeneous distribution of granule velocities and a tendency of relative movement in agglomerates. Relative movement will bring about the stretch of liquid bridge between granules. If the maximum separation distance of wet granules exceeds the rupture distance of liquid bridge, disaggregation happens. The ruptures of liquid bridge start from impact point and expand to outward, from bottom to up, from inside to outside in coating agglomeration. It is found that the rupture of liquid bridge needs time for accumulation and goes through three stages termed as slow rupture stage, quick rupture stage and entire rupture stage. With the increase of impact velocity, particle gravity, and rotating speed of core granules, disaggregation processes of wet granule agglomerates become fast and thorough. Impact velocity plays a primary role in disaggregation. DEM simulations are consistent with the experimental results.