颗粒聚团结构对曳力特性影响的数值模拟

Effect of agglomerate structure on drag force by numerical simulation

黏性颗粒多以聚团形式存在于气固两相系统中,流体施加于聚团的曳力对两相流动及传热传质起着至关重要的作用,而聚团的不规则、分形结构增加了曳力特性的复杂性。基于黏性离散单元方法生成不同分形结构的聚团,利用计算流体力学方法(CFD)直接求解分形聚团内部多孔结构的气流流动,得到了气体流过聚团时的周围与内部流场,研究了低Reynolds数(Re=0.1~10)条件下聚团结构特征对曳力的影响。结果表明:聚团的疏密程度显著影响聚团整体流场分布,多孔疏松结构增强了聚团的渗透性,使其与流体接触面积增加,所受曳力增加。分析不同结构聚团的曳力系数发现:除了聚团孔隙率、分形维数等结构参数的影响,气体流经聚团的方向也影响聚团曳力系数。在此基础上,综合考虑聚团分形维数、聚团与气流的夹角方向、Reynolds数拟合得到聚团曳力系数关联式。

Viscous particles mostly exist in the gas-solid two-phase system in the form of agglomerates,and the drag force exerted by the fluid on the agglomerates plays a crucial role in the two-phase flow and heat and mass transfer.However,the irregular and porous structure of the agglomerate result in complexities of the drag characteristics.In this study,a series of fractal-like agglomerates are generated by discrete element method(DEM)with a cohesive contact model,and the flow field including the external and interior flow through the agglomerate is resolved by computational fluid dynamic(CFD).The effect of agglomerate structure on drag characteristics at low Reynolds number flows(Re=0.1—10)is investigated.The simulation results show that the flow through the agglomerate is greatly influenced by the agglomerate structure.The permeability of agglomerate is enhanced by increasing agglomerate porosity,resulting in an increase of the contact surface between the fluid and agglomerate,thus increasing the drag force.The drag coefficient for agglomerates with different structures are calculated,which indicates that the drag coefficient of non-spherical agglomerates is influenced not only by structure parameters(size,porosity,fractal dimension)but also by the orientation between the agglomerate and gas flow direction.Finally,a correlation of the drag force coefficients based on the current simulations is proposed for porous agglomerates with fractal-like shapes.