受碾区域内颗粒轴向流动特性的离散元模拟

Granular axial flow characteristics in a grinding area studied by discrete element method

为探讨受碾状态颗粒的稳定流动,在碾辊轴与筛筒组成的受碾区域内,建立了轴向运动的颗粒流离散元物理模型.研究结果表明:受碾区域内各颗粒沿轴向运动能力的差异造成了颗粒流密度不均匀;颗粒与筛筒间的静摩擦系数影响颗粒轴向流动的形态、速率及集散程度,受碾区域内单层颗粒的轴向均方偏差与流动时间的平方正相关,属于"super"扩散;整体分析受碾区域发现,颗粒的轴向平均速度沿轴向坐标逐渐增大,而颗粒的三轴合成平均速度沿轴向坐标逐渐降低;受碾区域内各轴向位置处颗粒运动的剧烈程度不同,沿轴向坐标颗粒的波动速度平方呈现先增大后降低而后又增大的趋势;单颗粒的碰撞总能量损失能谱也表明了颗粒运动程度不同,即轴向流动时在受碾区域的前半段碰撞剧烈,能量损失多,在后半段碰撞程度弱,能量损失较少.通过对受碾区域内颗粒流动的数值模拟分析,明晰了颗粒在受碾条件下稳定流动特性,有益于碾磨工业对产品品质控制及设备参数优化的研究.

Granular grinding is one of the most important unit operations used in a wide variety of industries.Examples can be found in the food industry,for instance,rice processing,etc..The performance of grinding can be characterized by the particle flow process.Thus in order to study the stable flow process of particles during grinding,we must establish a discrete element model（DEM） of granular axial flow in the grinding area between the grinding roller and the screen drum.DEM is a numerical method used for modelling the mechanical behaviour of granular materials.When DEM is used in grinding,the particle motion is controlled by contact models that are governed by physical laws.Using EDEM software,the process of grinding can be simulated and analyzed.The simulation system chooses continuous feeding;after a period of time,it reaches a steady flow.Research results show that the uneven distribution of particle flow density（PFD） is caused by the axial movement difference of particles in the grinding area.The form,flow rate and distribution of granular axial flow are influenced by static friction coefficient difference between particles and screen drum.Axial mean square deviation of single particles in the grinding area is positively correlated with the square of time,which follows a ＂super＂ diffusive behavior defined by some studies.By an overall consideration of the grinding area,we find that the axial average velocities increase,however,the average velocities that are synthesized by three-axis velocities gradually decrease along the axial direction.This is because in a different axial position with different PFI,the PFI plays the key role in energy transfer.More energy will be transferred between high PFI particles that may cause high particle velocity.We also find that the fluctuation velocity square of particles presents the trend of first increasing then decreasing and finally increasing along the axial direction.The difference between PFIs is also elucidated by the total energy dissipation in each collisional energy level for a single particle.Results show that the single particle can endure intenser collision,more energy loss in anterior half segment than those in the second half of the grinding area.As mentioned above,the particle flow was analyzed in terms of particle flow intensity,particle velocity,collision energy,collision number,and so on.Some experimental results confirm the validity of the simulation.The simulation reflects the stable flow characteristics of particles in the grinding area and provides bases and references for further studying the product quality control and grinding equipment parameters optimization.