流化床微观混合过程的定量评价

Quantitative Evaluation of Micro-mixing Processes in Fluidized Beds

将离散单元法与计算流体力学相结合,对流化床内物料微观混合过程进行了研究。给出了在水平布风板均匀布风、倾斜布风板非均匀布风两种情况下的示踪颗粒场变化过程,并引入Ashton指数,以定量描述混合发展程度。模拟结果表明:在均匀布风情况下,床内气泡横向运动受到限制,横向混合以扩散方式为主;而对于非均匀布风流化床,床内存在较大的横向颗粒浓度梯度,初始阶段对流混合作用起主要作用,且混合速度较为迅速;随后局部剪切作用使混合进一步细化。

By combining the discrete element method （DEM） with computer fluid dynamics （CFD）, and introducing Ashton indices, quantitative micro mixing mechanism of materials in a fluidized bed was studied under different air distribution conditions. Analysis of the particles＇ vector field and mixing indices show that in case of even air distribution, transverse mixing proceeds mainly by diffusion, because lateral motion of air bubbles is hampered, and sufficient mixing can only be attained in 4.8 s; whereas under uneven air distribution conditions, convective mixing gets proounced even at the start and adequate mixing results can be achieved witin 2 s. Subsequent mixing is enhanced by local shearing. The mixing time grading agrees well with published data. Statistics of particsel flux show that there exists a regular corrective circulation of particles in fluidized beds under uneven air distribution conditions, while under even conditions lateral motion capability of particles is but feeble. Figs 5, table 1 and refs 10.