摘要
1 前言快速流态化床的理论及应用研究,近年来取得了迅速的发展,其中有关团聚体的存在、形成机理及其对流动、传递和反应的影响日益成为人们议论的中心。Yerushalmi,Kwauk 等人〔1〕〔2〕〔3〕〔4〕均根据快速床中气固滑移速度大大超过单个颗粒终端速度的实验事实,推断快速床中有颗粒的团聚体(Cluster)存在,进而又提出了种种假说和模型来预测空隙率的轴向及经向分布,团聚体的大小等;Grace 等人〔5〕则从流体动力学的角度,根据空隙率波的增长速率,予见快速床中可能形成颗粒的团聚体;Geldart 等人〔6〕则对快速床中团聚体的存在保持异议,他们认为快速床中高的滑移速度可以简单地用气固流系统中的径向不均匀性来解释,因为快速床由稀的中心区与浓的边壁区组成,计算出来的高的平均滑移速度是无意义的,只能引向错误的结论,用团聚体的存在来解释快速床中的高速度是不必要的。由于团聚现象本身的复杂性及实验手段的局限,所有关于团聚体的理论的正确性,都无从验证。
Whether the particles in a fast fluidized bed exist as clusters is still a matter in dispute among the present fast fluidized bed researches.In this work the micrographs in a 10m high,90mm ID,FCC-air fast fluidized bed were taken by using a videocamera with a special probe,which consists of a lens-optical fiber image transmitter and an optical fiber flash light transmi- tter,inserting into the bed in order to confirm the clusters in existence. These micrographs,which were taken under different gas velocities and solid circulating rates,at different radial and axial positions in the bed,clearly show that there are two phases in the fast fluidized bed.a dispersion phase and a cluster phase;the solid particles in the dispersionphase exist individually while in the cluster phase particles are clustering,the shape of clusters is irregular and their size is not uniform;the frequency of clusters displayed on the micrographs increases along the,radial direction,which implies that the bed voidage decrease from the centre to the wall,the shape of clusters transforms roughly from strand at the centre into sphericity near the wall; the mean diameter of clusters also appears to increase along the radial dire- ction.Although the present probe is too small to take a integrated micrograph ted of the cluster larger than 3mm in diameter,the micrographs filled with cluster phase indicate that the clusters larger than 3mm diameter are defin- itely present. The micrographs taken at the upper section of the bed are similar to those at the lower section of the bed.The radial variations of size,shape and concentration of clusters at the upper section of the bed are found to be similar to those at the lower section of the bed except that the frequency of clusters displayed on the micrographs at the lower section of the bed is higher than that at the upper section of the bed.
出处
《化工冶金》
CSCD
北大核心
1990年第1期68-73,共6页
关键词
流化床
两相结构
微观显示
Fast fluidized bed
Two-phase structure
Cluster
Mierograph transducer
Micro—visualization
