粘附性颗粒流态化聚团尺寸的预测模型

在大量实验的基础上,通过对细颗粒碰撞聚团过程进行受力分析,提出了一种流化床中粘附性颗粒聚团和破碎的力学模型．通过模型计算,求出了不同类型粘附性颗粒流态化时聚团的大小．模型预测结果与实验测定基本一致．     

细颗粒振动流态化研究进展

概括了细颗粒在振动流化床中的流态化实验研究和理论模型,着重介绍了振动场对细颗粒流化行为的影响,综述了振动场中粘性颗粒的运动,有利流化的振动参数以及聚团尺寸测量和计算的研究现状。     

粘性SiC颗粒聚团流态化特性

对不同粒径的°SiC粘性颗粒的流态化实验表明,颗粒粒径对流化性能有较大影响,颗粒粒径越小,颗粒间粘附力越大,其流化性能越差;提出了粘性颗粒自然聚团数Ae_n和流态化聚团数Ae_f,用来表征颗粒的流化性能;指出了应开展粘性颗粒聚团流态化的研究。     

超细粉流化机理和团聚现象的探讨

在直径60mm的流化床中，以SiO2和TiO2超细粉为原料，考察了粉体物性、粉体填充状态和空气湿度等因素对超细粉流化行为和聚团性质的影响，结果表明：超细粉的流态化经历了活塞流、过渡区和聚团流化3个阶段；聚团的流化行为与大颗粒相似；初始填充状态对超细粉流化行为和聚团大小有重要影响，松填充有利于减小聚团尺寸和减少颗粒夹带，提高流化质量；并结合粉体层受力分析，对超细粉的流化机理和团聚现象进行了探讨。     

声场流化床中超细颗粒聚团受力与尺寸

在内径40 mm的流化床中,采用平均粒径为7.4 mm的超细铁矿颗粒进行声场流态化实验. 结果显示,聚团尺寸随声压级增大逐渐减小,在固定声压级的条件下存在最优声波频率,本实验为130 Hz. 由铁矿颗粒声场流态化中聚团受力分析提出聚团受力平衡模型,当促进聚团破碎的力和促进聚团形成的力相等时,计算出一定频率不同声压级下的聚团尺寸,在频率130 Hz、声压120.5 db下,根据模型计算得到的聚团直径为384 mm,而通过最小流化速度计算值为367 mm,二者较接近.     

粒径及声场对SiO2超细颗粒流化行为的影响

采用内径为56 mm的玻璃管流化床,考察了平均粒径分别为5~10 nm(1#), 0.5 mm(2#)及10 mm(3#)的SiO2超细颗粒在无声场及声场存在下的流化行为. 无声场时,1#和2#颗粒可在较高的气速下形成稳定聚团,单位质量颗粒团间作用力与原生颗粒相比显著下降,因而可实现稳定的聚团流化,3#颗粒因颗粒间粘性力较大,无法实现稳定流化. 40~60 Hz的声场对3种超细颗粒的流化行为均可起到一定的改善作用,在此频率范围外,声场的作用不明显. 提高声压级,可以使1#和2#颗粒团发生一定程度的破碎,聚团尺寸减小,最小流化速度降低. 在实验范围内,添加声场无法使3#颗粒实现稳定流化.     

基于气泡和聚团的结构模型及其应用

综合考虑鼓泡流化床内气泡及聚团对床内细颗粒流动的影响,建立基于气泡和聚团的结构曳力模型及结构参数模型,同时,借助计算流体软件预测细颗粒在鼓泡床中流动状态。首先,基于细颗粒在鼓泡流化床的流动状态,在介观尺度上将床内气固流动结构划分为3个子结构,即气泡相、相间相及乳化相(聚团相)。然后,综合考虑细颗粒鼓泡流化床中气泡和聚团对气固流动的影响,根据力平衡、质量守恒建立基于气泡和聚团的结构参数模型及结构曳力模型。通过对结构参数模型封闭求解,得到11个结构参数值(f_b,U_b,d_b,U_(gb),f_i,U_(gi),ε_i,f_e,U_(ge),ε_e,d_c)。对结构参数计算结果进行分析,结构参数模型能够很好反映床内流动情况及床内各结构参数之间的关系,并能有效地预测颗粒聚团直径。此外借用非均匀因子,耦合结构曳力模型及结构参数模型到欧拉双流体模型对气固在床内流动行为进行数值模拟。模拟结果表明,使用基于气泡和聚团的结构曳力模型能够较好地预测细颗粒在鼓泡流化床中的流动行为。在对模拟结果中颗粒径向浓度比较时,可以发现,相对比基于气泡模型的结构曳力模型,使用基于气泡和聚团的结构曳力模型的模拟结果与实验结果更一致。     

颗粒聚团结构对曳力特性影响的数值模拟

黏性颗粒多以聚团形式存在于气固两相系统中，流体施加于聚团的曳力对两相流动及传热传质起着至关重要的作用，而聚团的不规则、分形结构增加了曳力特性的复杂性。基于黏性离散单元方法生成不同分形结构的聚团，利用计算流体力学方法（CFD）直接求解分形聚团内部多孔结构的气流流动，得到了气体流过聚团时的周围与内部流场，研究了低Reynolds数（Re=0.1～10）条件下聚团结构特征对曳力的影响。结果表明：聚团的疏密程度显著影响聚团整体流场分布，多孔疏松结构增强了聚团的渗透性，使其与流体接触面积增加，所受曳力增加。分析不同结构聚团的曳力系数发现：除了聚团孔隙率、分形维数等结构参数的影响，气体流经聚团的方向也影响聚团曳力系数。在此基础上，综合考虑聚团分形维数、聚团与气流的夹角方向、Reynolds数拟合得到聚团曳力系数关联式。     

粘附性颗粒流态化研究进展

主要阐述了粘附性颗粒流态化特性（初始流化速度、床膨胀、塌落）及聚团流化过程,提出了粘附性颗粒流态化的机理是聚团流化．并对近年来国内外在预测聚团大小、床层结构模型、改善粘附性颗粒流化性能方面的研究进展进行了总结．     

密相流化床中介尺度流动结构的流体力学特性研究

在一套流化床冷模实验装置中对黄沙颗粒和黄沙-硅微粉 （20 μm）混合颗粒进行实验。测量固含率时间序列信号并进行统计分析,提出并建立复杂光纤脉动信号的解耦方法,实现稠密气固流中介尺度流动结构的准确识别。基于统计矩一致性原理提出气泡阈值的计算方法,通过遍历法确定气泡阈值。对气泡阈值变化规律进行分析,发现加入细颗粒有助于改善流化质量,随表观气速的增加,气泡阈值减小。对气泡、乳化和聚团三相的相分率进行统计,发现在黄沙颗粒中加入少量（5%,质量分数）细颗粒能够显著改善流化质量,细颗粒添加量过多时（10%）,对流化质量的改善将减弱。对气泡的流体力学特性进行分析,发现加入10%硅微粉后,气泡弦长增大,频率降低,速度略有降低。对颗粒聚团流体力学特性进行分析,发现随硅微粉含量增加,表观气速对聚团速度的影响减弱,聚团弦长略有减小。加入5%硅微粉后,颗粒聚团的出现频率较小且径向上分布均一。加入10%硅微粉后,聚团频率有所增大,说明加入过多硅微粉会促进聚团的形成。     

微细粉体在振动流化床中团聚行为的研究

本文研究了微细粉体在振动流化床中的团聚行为和振动参数对团聚物尺寸的影响。观察了床层流化现象,并发展了团聚物的测量方法。提出了Ｃ类粉体振动流化的团聚机理。     

CFD–DEM modeling of gas fluidization of fine ellipsoidal particles

Particle characteristics are important factors affecting gas fluidization. In this work, the effects of both particle size and shape on fluidization in different flow regimes are studied using the combined computational fluid dynamic–discrete element method approach. The results are first analyzed in terms of flow patterns and fluidization parameters such as pressure drop, minimum fluidization, and bubbling velocities. The results show that with particle size decreasing, agglomerates can be formed for fine ellipsoidal particles. In particular, “chain phenomenon,” a special agglomerate phenomenon exists in expanded and fluidized beds for fine prolate particles, which is caused by the van der Waals force. The minimum fluidization velocity increases exponentially with the increase of particle size, and for a given size, it shows a “W” shape with aspect ratio. A correlation is established to describe the dependence of minimum fluidization velocity on particle size and shape. Ellipsoids have much higher minimum bubbling velocities and fluidization index than spheres. © 2015 American Institute of Chemical Engineers AIChE J, 62: 62–77, 2016     

Prediction of minimum fluidization velocity for vibrated fluidized bed

The prediction of minimum fluidization velocity for vibrated fluidized bed was performed. The Geldart group A and C particles were used as the fluidizing particles. The method based on Ergun equation was used to predict the minimum fluidization velocity. The calculated results were compared with the experimental data.The calculated results of minimum fluidization velocity are in good agreement with experimental data for Geldart group A particles. For group C particles, the difference between the calculated results and experimental data is large because of the formation of agglomerates. In this case, the determination of agglomerate diameter is considered to be necessary to predict the minimum fluidization velocity.     

气固流态化中颗粒介尺度结构的动力学研究

气固流化床中的团聚等介尺度结构严重影响气固的整体流化特性、传热、传质和反应效率,因此介尺度结构的研究至关重要。简要回顾颗粒介尺度结构的定义与分类,然后从流体-颗粒、颗粒-颗粒之间的力和非弹性碰撞相互作用出发,总结分析了颗粒介尺度结构形成原因和动力学演化过程的研究现状和发展动态。重点关注细颗粒与超细纳米颗粒形成的介尺度结构,即结块,并针对结块形成原因,讨论了颗粒间范德华力计算以及结块的力平衡理论模型,提出了细颗粒介尺度结构进一步的研究方向,即研究带有颗粒-颗粒作用力的介尺度结构的动态演化规律,为精准调控介尺度结构提供理论依据。     

Application of discrete element method simulation for studying fluidization of nanoparticle agglomerates

The usefulness of discrete element method simulation for studying fluidization of nanoparticle agglomerates is explored. Nanoparticle agglomerates were simulated by using solid particles of equivalent sizes and densities. Validity of the present simulation was assessed through comparisons of simulation results and experimental observations of bed expansion, characteristic fluidization behaviour, and dense‐bed settling. The simulation was then used to investigate initial bed expansion and bed uniformity under particulate fluidization conditions. The role of inter‐agglomerate interparticle force in fluidization of nanoparticle agglomerates was examined. A stability analysis originally developed for addressing the transition from particulate to bubbling fluidization for conventional particles was used for predicting the start of bubbling in fluidized beds of nanoparticle agglomerates.     

Agglomerate Sizes of Binary Nanoparticle Mixtures in a Vibro‐Fluidized Bed

Agglomerate size is one of the key factors influencing the fluidization behavior of nanoparticles. The effect of fluidization time, superficial gas velocity, and vibration frequency on agglomerate sizes for different binary mixtures of nanoparticles at the top of the bed was investigated in a vibrated fluidized bed (VFB). The agglomerate sizes decreased and fluidization quality was significantly enhanced owing to introduction of vibration energy. The Richardson‐Zaki equation combined with Stokes' law permitted the prediction of mean agglomerate sizes. Experimental and estimated results indicated that vibration led to a smaller agglomerate size. The mean predicted agglomerate sizes were in agreement with those determined experimentally in the VFB.     

Parametric study of fine particle fluidization under mechanical vibration

Investigations into the effects of vibration on fluidization of fine particles (4.8-216 μm average in size) show that the fluidization quality of fine particles can be enhanced under mechanical vibration, leading to larger bed pressure drops at low superficial gas velocities and lower values of u_mf. The effectiveness of vibration on improving fluidization is strongly dependent on the properties (Geldart particle type, size-distribution and shape) of the primary particles used and the vibration parameters (frequency, amplitude and angle) applied. The possible roles of mechanical vibration in fine particle fluidization have been studied with respect to bed voidage, pressure drop, agglomeration, and tensile strength of particle bed. Vibration is found to significantly reduce both the average size and the segregation of agglomerates in the bed, thus improving the fluidization quality of cohesive particles. Also, vibration can dramatically reduce the tensile strength of the particle bed. Obviously, vibration is an effective means to overcome the interparticle forces of fine powders in fluidization and enhance their fluidization quality.