摘要
本实验以原生纳米级的SiO_2、ZnO、TiO_2以及Al_2O_3为物料，首先系统地在直径为4cm的玻璃流化床中考察了它们的流态化行为，结果发现四种纳米颗粒在刚接通气源时都形成活塞，活塞破裂后有沟流和固定床现象。增大气速后除纳米ZnO外其他三种纳米颗粒都出现聚团分层现象：上部为聚团鼓泡流态化(ABF)的小聚团，下部为未实现流态化的大聚团和固定床。通过在流化床中加入平均粒径2mm铸铁颗粒作为磁响应性物质后，铸铁颗粒在由电磁线圈和交流电源产生的交变磁场作用下在分布板上发生强烈的无序振动，可以有效破碎纳米SiO_2在较低气速下形成的大聚团和沟流，抑制鼓泡和扬析的发生。
实现聚团散式流态化(APF)的纳米SiO_2具有很明显的液体性质，生成的小聚团随流化气体在床层内循环流动。针对其他三种纳米颗粒在添加大颗粒磁场流化床中改善效果不明显的缺点，实验中尝试以纳米SiO_2为流化介质，通过减小待流化物料与流化介质之间的密度差来提高其流化性能。结果发现，一定质量分数的添加纳米颗粒能与流态化的纳米SiO_2形成共同流态化的混合系统，其中纳米ZnO的最大添加质量分数在90％以上。混合物的APF操作气速范围由添加纳米颗粒的质量分数和磁场强度决定，添加质量分数的增大使APF区域减小。
最后，分别应用Matsuda等提出的模型和Richardson-Zaki方程对纳米SiO_2聚团与最小流化速度和混合纳米颗粒流态化的散式化程度进行了验证，提出了预测混合纳米颗粒流态化的终端流速模型，为更大规模的混合纳米颗粒的流态化研究及其应用打下了良好的基础。
The fluidization behavior of SiO_2, ZnO, TiO_2 and Al_2O_3nanoparticles in glass pip with diameter of 4 mm is investigated. Thereexisted fixed bed, plug formation, and channeling at the low superficialgas velocity. There exist two areas even at high gas velocities for SiO_2,TiO_2 and Al_2O_3 nanoparticles in traditional fluidized bed, smallagglomerate bubbling fluidized (ABF) area at the upper and bigagglomerate de-fluidized area at the bottom. By adding cast iron particleswith the average size of 2mm into the fluidized bed as magnets, bigagglomerates and channeling formed by SiO_2 nanoparticles at lowsuperficial gas velocity were broken up then, the bubbling and elutriationwere also restrained because cast iron particles vibrated violently abovethe distributor under oscillating magnetic filed engendered by a magneticstator and AC current.
The agglomerate particle fluidized (APF) of SiO_2 nanoparticlesshowed liquid-like properties and the agglomerates circulated in thefluidized bed just like fluid. When the fluidization ability of ZnO, TiO_2and Al_2O_3 nanoparticles was poor in a traditional fluidized bed or even inthe magnetic fluidized bed with adding coarse magnets, the fluidizedSiO_2 nanoparticles were used as the fluidized medium in order to improvetheir fluidization ability. It was found that homogeneous mixtures formedwhen ZnO nano-particles were added in the fluidized bed of SiO_2nanoparticles, and the maximum mass fraction of nano-ZnO that canform homogenous mixtures is more than 90%. The stable operation areaof the APF was determined by the mass fraction of additivenano-particles and magnetic field intensity, and the stable operation areadecreased with increasing the mass fraction of additive nano-particles.
Finally, the Matsuda's model and the Richardson-Zaki equation wereused to check the relation between the agglomerate size and the minimumfluidization superficial gas velocity of SiO_2 nanoparticles and theexpansion degree of nanoparticle mixtures, respectively. Model forestimating the terminal velocity of the mixture formed by APF and ABFnano-particles is proposed for further research in mixed nano-particles and its application in the future.
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