流场涡旋中颗粒碰撞黏附机制的CFD-DEM(XDLVO)模拟

Mechanism of the particle collision and adhesion in flow field vortex using CFD-DEM(XDLVO)simulation

混凝装备中流场的涡旋特征是影响煤泥水等微细颗粒絮凝效果的关键因素之一。为解析流场涡旋影响颗粒混凝碰撞与黏附机制,以圆柱绕流产生的涡街为代表性涡旋流场,采用CFD方法模拟了流速和圆柱直径对流场涡旋强度和尺度的影响;引入XDLVO理论辅助描述离散元方法中颗粒间的相互作用力,对粒径为25~100μm颗粒在上述流场中的碰撞与黏附过程进行CFD-DEM模拟,并试验验证。结果表明,在流速0.06~0.12 m/s、圆柱直径2~6 mm、雷诺数为120~720条件下,圆柱绕流场中的涡旋半径(r)与圆柱直径(D)的关系近似为r=0.133 3D+0.421 4,与流速无显著相关性;涡旋中心的最大涡量与流速成正比,涡旋强度与圆柱直径的2次方正相关。CFD-DEM(XDLVO)模拟方法获得的黏附聚集体的特征参数与试验结果吻合较好,说明其可较准确地描述圆柱绕流场中颗粒的碰撞与黏附过程。对流场涡旋分布特征和聚集体在流场分区中分布特征的分析表明,流场中的涡旋通过惯性离心力使颗粒远离涡旋中心向涡旋周边的黏性剪切区富集,在改变颗粒运动方向的同时,增加了颗粒在黏性区的局部浓度,从而提高了颗粒的碰撞概率,有效促进了颗粒的黏附并大。当涡旋尺度为物料粒径的10倍左右时最有利于颗粒的碰撞黏附,因此可根据应用场景的物料粒度来优化设计绕流圆柱直径,使其达到最好的黏附效果。

The vortex characteristics of the flow field in the coagulation equipment are one of the key factors affecting the agglomeration efficiency of fine particles such as coal slurry and water.To analyze the impact of vortex flow fields on the collision and adhesion of particles,in this study,the vortex street generated by the flow around a cylinder was chosen as a representative vortex flow field.Computational Fluid Dynamics(CFD)was used to simulate the influence of flow velocity and cylinder diameter on the strength and scale of the vortex in the flow field.Additionally,the Extended Derjaguin-Landau-Verwey-Overbeek(XDLVO)theory was introduced to describe the interaction forces between particles in the Discrete Element Method(DEM),and the CFD-DEM simulations of the collision and adhesion processes of particles with diameters ranging from 25−100μm in the aforementioned flow field were conducted,followed by experimental validation.The results indicated that within the range of flow velocities of 0.06−0.12 m/s,cylinder diameters of 2−6 mm,and Reynolds numbers of 120−720,the relationship between the vortex radius(r)and the cylinder diameter(D)in the cylinder flow field was approximately r=0.1333D+0.4214,but with no significant correlation with the flow velocity.The maximum vorticity at the vortex center was directly proportional to the flow velocity,and the vortex intensity was positively correlated with the square of cylinder diameter.The characteristic parameters of the adhesive aggregates obtained from the CFD-DEM(XDLVO)simulation method were in good agreement with the experimental results,indicating its ability to accurately describe the collision and adhesion processes of particles in the cylinder flow field.Analysis of the vortex distribution characteristics in the flow field and the distribution of aggregates in the flow field zones revealed that the vortices in the flow field enriched the particles away from the vortex center towards the viscous shear zone through inertial centrifugal force.This altered the direction of particle motion and increased the local concentration of particles in the viscous zone,thereby enhancing the probability of particle collisions and effectively promoting particle adhesion and agglomeration.When the vortex scale was approximately 10 times the particle size,it was most conducive to particle collision and adhesion.Therefore,the design of the flow-around cylinder diameter can be optimized based on the particle size of the application scenario to achieve the best adhesion effect.