Abstract:

The flow pattern of a fluidized bed is a key factor for heat transfer and new reactor design.The flow pattern of the gas-solid fluidized bed was experimentally measured by a novel acoustic emission（AE） technique.The acoustic energy generated by the collision of solids particles on the reactor wall or in the local space is a reflection of the speed and frequency of collision, and consequently the flow pattern of particles.The flow pattern of polyethylene particles with the average size 460 μm in a φ 150 mm fluidized bed in the range of superficial gas velocity from 0.3 m·s^-1 to 0.7 m·s^-1 was multi-circulation pattern with main-circulation zone, sub-circulation zone and stagnant zone.The multi-circulation flow pattern would transit to the single circulation flow pattern if superficial gas velocity exceeded 0.8 m·s^-1 or average particle size decreased to below 365 μm.The height of stagnant zone remained unchanged with the increase of static bed height.It was also found that the type of the distributor had an evident effect on flow pattern.The single circulation flow pattern in the fluidized bed with a perforated stainless steel plate distributor would change to the multi-circulation flow pattern for the same particles and the same fluidized bed with a cone-shaped distributor.An empirical equation calculating the height of stagnant zone was presented and the experimental data could be correlated with the empirical equation with fairly good accuracy.

Key words:

声发射, 能量, 气固流化床, 流动模式, 聚乙烯颗粒

摘要： 颗粒在气固流化床壁面区域（或局部空间区域）碰撞产生的声波能量反映了颗粒的碰撞速度和频率（活跃程度），从中可以揭示流化床内颗粒的流动混合模式。通过在φ150mm流化床冷模装置中，对聚乙烯颗粒－空气体系进行流态化实验，运用声发射技术测得声能量沿气固流化床的轴向分布，继而获得了颗粒的流动模式，并发现其与颗粒粒径、表观气速和分布板形式密切相关。对于颗粒粒径为460 μm的聚乙烯颗粒，当表观气速在0.3～0.7 m·s^-1内，其对应的流动模式为带有滞留区的双循环流动模式。如果气速增大到0.8 m·s^-1以上时，流动模式将转化为无滞留区的单循环流动模式。而当颗粒平均粒径降为365μm，对应的双循环流动模式蜕化为单循环模式，壁面不存在滞留区。进一步发现，滞留区位置与静床高无关。研究同时发现，颗粒的流动模式和分布板形式密切相关，对于在多孔平板分布板下为单循环流动模式的小粒径颗粒，在锥帽式分布板下，则在稍高气速时表现为双循环流动模式。

关键词:

声发射, 能量, 气固流化床, 流动模式, 聚乙烯颗粒