内构件对高密度提升管内气体混合行为的影响

在Φ18 6mm的提升管内 ,以空气和FCC颗粒为实验对象 ,采用气体稳态示踪技术对高密度提升管内气体混合规律研究的结果表明 :引入钝体的高密度提升管内 ,远离构件区域的气体在径向上的混合仍符合气体的拟均相二维扩散模型 ,颗粒的引入使得气体的径向扩散变差 .钝体的引入 ,使得床内流体产生强烈的绕流 ,径向分布变得均匀 .模拟结果表明 ,气体的径向扩散系数Er达到了 1.0× 10 ³cm² ·s^-1,比提升管 30cm² ·s^-1高出一个半量级 ,可很好地改善高密度提升管内气体的径向混合行为     

多段分级转化流化床提升管局部固体体积分数分布

在射流流化床与提升管耦合的多段分级转化流化床冷态实验装置上,采用PC6D型颗粒体积分数测量仪,系统研究了提升管中局部固体体积分数径向分布及其轴向发展规律。结果表明:固体体积分数的径向分布为中心区固体体积分数较稀、分布均匀,边壁区为体积分数高、分布陡峭的环核流动结构。提升管中径向颗粒体积分数的轴向发展为:提升管底部,中心区和边壁区固体体积分数随轴向高度的增加而减小,边壁区固体体积分数的减小趋势明显高于中心区;提升管的发展段,边壁区固体体积分数随高度的增加而减小,而中心区固体体积分数几乎不变。提升管所有截面上各径向位置的固体体积分数随表观气速ur,g的减小或固体循环量G S的增加而增大,边壁区固体体积分数受操作条件影响的敏感程度明显高于中心区和过渡区。拟合得到了升管不同径向区域内固体体积分数与截面平均固体体积分数的关系式,误差分析表明该表达式的计算值和实验值吻合较好。     

气固循环床提升管内的局部颗粒浓度及流动发展

采用反射式光纤浓度探头对f100mm×15.1m循环床提升管8个轴向截面上11个径向位置的局部颗粒浓度进行了测量, 分析研究了颗粒浓度径向分布的不均匀性及其沿轴向的发展变化。结果表明:提升管内气固两相流的发展并不同步,而是一个由核心区向边壁区逐渐扩展,并最终达到总体充分发展的过程,该过程主要受边壁区发展过程所控制;相对于核心区,边壁区的发展不仅显著缓慢,而且受操作条件的影响也较显著。实验还发现:在颗粒加速段,无因次颗粒浓度的径向分布不具有相似性,不仅与径向位置有关,而且还与床层截面高度有关。     

多段分级转化流化床提升管速度场的研究

针对流化床煤气化过程中需要长气固接触时间和高固体浓度,开发了耦合灰熔聚流化床和提升管的多段分级转化流化床。为了研究多段分级转化流化床提升管中局部颗粒速度的径向、轴向分布,在不同的操作条件下,采用PV-6型颗粒速度测量仪在冷态实验装置中系统测定提升管内局部颗粒速度。实验结果表明:提升管中任何径向、轴向位置的颗粒速度随着操作气速的增大而增大,随循环量的增加而减小。操作条件对中心区颗粒速度变化的影响明显高于边壁区。颗粒的加速首先发生在提升管中心区域,然后向边壁区域扩展。颗粒速度径向分布的不均匀性沿轴向逐渐增大,并且受操作气速影响比较大。     

提升管-下行床耦合反应器内颗粒混合行为

引　言循环流化床中气固两相的流动有两种不同的方式 :提升管中为气固并流上行的逆重力场运动 ,下行床中为气固并流下行的顺重力场运动 .其差异表现为提升管内颗粒浓度、速度以及气体速度在径向上严重的不均匀 ,颗粒浓度概率密度分布以及速度的瞬时信号都表明了颗粒团 -空穴两相结构的存在[1] ,一些研究[2 ,3] 还发现提升管中颗粒的停留时间分布 (ＲＴＤ)曲线存在较大的拖尾甚至出现双峰 ,研究者认为提升管内存在弥散颗粒和颗粒团两种不同的混合机理 ;下行床则比提升管大大改善 ,气固速度、颗粒浓度沿径向分布要均匀得多 ,颗粒的ＲＴＤ曲线…     

提升管内颗粒浓度梯度力的特性

由于Kutta-Joukowski横向力与浓度梯度力的共同作用，提升管内颗粒沿径向在边壁大量聚集并形成稳定的环?核结构。根据实验数据，分析了颗粒浓度梯度的径向分布特征，考察了不同操作条件下浓度梯度力系数K的分布特性。由Kutta-Joukowski横向力与浓度梯度的关系，提出了浓度梯度力的表达式Fρ=K(dρ/dr)A及浓度梯度力系数K的表达式K=[?ρg(νg?νp)(?v/?r)r]/?dρ/dr+(d2ρ/dr2)r?。提升管内颗粒群受到的浓度梯度力与浓度梯度力系数K有关。浓度梯度力系数K在提升管中心处为0，沿提升管径向呈“N”型分布，随表观气速增加而增加；提升管内充分发展段K的数值明显大于提升管加速区和出口约束区，总结了浓度梯度力系数K的经验关联式。     

环形截面提升管内颗粒的运动行为

采用PDPA(phase Doppler particle analyzer system)及磷光颗粒示踪技术对环形截面提升管中颗粒的运动行为进行了研究。与传统提升管相比，环形截面提升管中颗粒速度分布的均匀性有所改善，其最高速度与最低速度之差变小。环形截面提升管最大速度值出现在相对径向位置φ=0.3-0．4处。与传统提升管相似，环形截面提升管内颗粒的轴向返混较严重，停留时间分布曲线存在较明显的拖尾，其中颗粒的轴向Peclet数与传统提升管也处于同一数量级范围。提升管床结构的改变并未显著改变其中气固流动的微观相结构，稀相与密相颗粒团微观两相仍然存在，这种微观相结构是造成颗粒严重返混的决定性原因：稀相中的颗粒与密相颗粒团中的颗粒分别造成了颗粒停留时间分布曲线的前峰与拖尾峰。     

流动方向对循环流化床中颗粒混合行为的影响

对循环流化床提升管及下行床两种不同气固流动方式对颗粒混合行为的影响进行了较为深入的对比分析，发现在影响循环流化床颗粒混合的众多因素（如操作条件、床层直径、颗粒性质及床层内构件等）中，气固流动方向是影响颗粒轴向混合的最主要因素．当气固流动为顺重力场时（下行床），颗粒的轴向混合很小，流型接近平推流；当气固流动为逆重力场的提升管时，轴向颗粒混合将成倍增大，颗粒流动远离平推流流动．分析表明，下行床中颗粒混合仅为单一的弥散颗粒扩散，而提升管中则存在着两种颗粒混合机制：弥散颗粒扩散及颗粒团扩散．弥散颗粒的扩散基本以平推流的形式通过循环流化床，提升管中大量的颗粒轴向返混归因于颗粒团的严重返混并由此形成了提升管中颗粒停留时间的双峰分布．     

管型结构对提升管流动特性的影响

采用边壁补气模拟重油催化裂化提升管反应装置中气体的膨胀行为，通过比较直管型提升管和锥型提升管中的气–固流动行为，研究了锥形提升管结构对油气膨胀所带来的流动特征变化的适应与改善. 实验结果表明，相对于直管型提升管，锥型提升管对流化气量的变化有较好的适应能力，且能有效地改善床层内的颗粒速度、空隙率的径向分布以及压力的轴向分布.     

多段分级转化流化床提升管中颗粒聚团特性的研究

采用PC6D型颗粒浓度测量仪,在多段分级转化流化床冷态实验装置上,测量了提升管中局部固体浓度,对颗粒聚团特性进行了实验研究。结果表明:径向上絮状物的时间分率Fc和絮状物内的颗粒浓度εsc在床层中心区(r/R0.6)较低,而在边壁区(r/R0.8)相对较高。提升管所有截面上各径向位置的Fc和εsc均随表观气速urg的减小和/或固体循环量Gs的增加而增加,轴向上操作条件对底部加速段Fc、εsc的影响明显高于对上部充分发展段Fc、εsc的影响;径向上操作条件对中心区Fc、εsc的影响显著高于对边壁区Fc、εsc的影响。在絮状物颗粒浓度0到εsmf范围内,得出絮状物内的颗粒浓度εsc与该处相应的局部颗粒浓度εs的变化关系式为εsc=3.33εs0.8。     

内构件存在时提升管内流动及颗粒混合行为研究

he hydrodynamics and solids mixing behavior in a riser with blunt internals are studied. A uniform radial distribution for solids fraction and particle velocity achieves near the internals. The turbulent velocity of particles near the wall increases with the addition of the internals, with the lateral solids mixing enhanced significantly. Probability density distribution of particle velocity is bimodal in the riser with internals, which is similar to that in the conventional riser, indicating that no significant difference in the micro flow structure exists between the riser with internals and the conventional riser. At the same time, the axial solids mixing behavior changes insignificantly with the addition of internals. These results indicate that the micro flow structure in the riser is very stable, which changes insignificantly with the change of the bed structure.     

Mixing behavior of wide-size-distribution particles in a FCC riser

A comparison study on the axial and lateral mixing of wide-size-distribution (WSD) particles with normal-size-distribution (NSD) FCC particles is presented. The fines smaller than 20 μm contribute 24.5% in volume in the WSD particles. Phosphor tracer method is used. The axial backmixing of the WSD is slightly lower than the NSD. However, the lateral solids Peclet number increases linearly with the increase of the content of fines smaller than 20 μm because of the serious agglomeration of the fines, which leads to very poor lateral mixing for the WSD particles. And the solids lateral mixing for the WSD particles decreases significantly with increasing solids fraction. These results show that the particle size and the particle-size-distribution is a very important factor that controls the lateral solids mixing and hence the heat and mass transfer in lateral direction, which should be taken into full consideration in the riser reactor. And the NSD may be a good particle size distribution for the FCC riser, the lateral solids mixing of which is satisfactory in the reactor.     

双颗粒提升管中细颗粒的混合行为研究(Ⅱ)──弥散颗粒的轴径向混合行为

利用磷光颗粒示踪技术，使用点源示踪的方法在不同的径向位置测得反映颗粒径向扩散行为的停留时间分布曲线，并对弥散细颗粒的径向混合行为进行了分析．提出二维扩散模型描述所测量的停留时间分布曲线特征．实验发现弥散细颗粒的径向扩散Peclet数随粗颗粒加入量的增加而增加，随细颗粒浓度的增大而增大，而几乎不随气速变化．给出了一个与实验数据吻合较好并可适用于单颗粒提升管的关联式．     

Axial and lateral dispersion of fine particles in a binary-solid riser

Axial and lateral mixing of fine particles in a binary-solid riser have been investigated using a phosphor tracer method. The measured bimodal residence time distribution (RTD) demonstrated two types of axial dispersions of the fines: the dispersion of discrete particles and that of clusters. A proposed one-dimensional, bimodal dispersion model describes the bimodal RTDs very well. The axial Peclet number of the fines is not sensitive to the fraction of coarse particles, gas velocity and solids circulation rate. Lateral solids dispersion was determined by measuring the solids RTD at different radii using a point source tracer injection. A two-dimensional dispersion model describes the measured RTDs satisfactorily. Lateral solids mixing decreased as coarse particles were added into the riser. Correlations of the axial and lateral Peclet numbers obtained fit the experimental data well.     

Mixing of a lateral gas stream in a two-dimensional riser of a circulating fluidized bed

A cold model of a circulating fluidized bed having a two-dimensional riser, with a 12 × 120 mm section and a 6.4 m height, was equipped with a device to inject a lateral gas stream along the riser. The apparatus was operated under conditions ranging from those characteristic of combustors to those of gas-conversion processes. Flow structures in the interaction region between the rising gas—solids suspension and the lateral gas stream were studied by means of a motion analysis system. Three main configurations were identified. The ratio between the momentum of the lateral gas stream and that of the rising suspension was found to be the parameter able to discriminate among the three configurations. A satisfactory agreement was found with mixing data obtained using a cylindrical riser.     

Experimental study of the effect of internals on optimizing gas-solid flow in a circulating fluidized bed

The solids concentration in a circulating fluidized bed of flue gas desulfurization (CFB-FGD) is low, and the gas-solid mixing needs to be strengthened to improve the reaction efficiency. The addition of internals in a square CFB-FGD is studied here. Different sizes of wedge-shaped internals were set in the middle of the riser on opposite sides of the bed, at same height or at different heights. By comparing the concentration distributions, we found that positioning the internals at different heights is more effective in increasing the density of the center region. By observing the radial distribution of the fractal dimension, we found that a strong degree of turbulence can improve the gas-solid mixing. A larger upper/smaller lower arrangement of the internals with a moderate apparent gas velocity provided the best flow field, with increased concentration gradients and improved gas-solid mixing.     

磷光颗粒示踪技术在循环流态化中的应用

<正> 1 引言 固体颗粒的停留时间分布(Residence Time Distribution简称RTD)在循环流态化研究中十分重要,它对于了解其两相流动特性、反应器的模拟计算和工程设计是必不可少的,对于流态化干燥过程及传热行为的研究也很重要。流化床中RTD的研究已有大量文献报道,发展了诸如染色颗粒、盐颗粒、磁性颗粒、放射性颗粒及热(冷)颗粒等众多的示踪方法,但是由于固体颗粒示踪本身存在着示踪剂的注入、在线检测、残留及示踪颗粒与床体物料一致性、示踪过程对床内流场的干扰等一系列技术上的困难,不但使实验操作繁琐,而且实验结果的可靠性、重复性均不理想,特别是在颗粒运动速度较快的循环流化床中,其颗粒示踪的难度更大。为了解决上述难题,本文参考了Jin Yong等和Yu Zhiqing及M.Kwauk在常规流化床中曾采用的磷光示踪方法,在循环流化床条件下进行了新的探索。     

双颗粒提升管中细颗粒的混合行为研究(Ⅰ)──轴向混合行为

利用磷光颗粒示踪技术对双颗粒提升管中细颗粒的轴向混合行为进行了研究测得反映细颗粒轴向扩散的停留时间分布曲线呈双峰分布，这表明细颗粒存在两种类型的扩散形式：弥散颗粒扩散和颗粒团扩散本文提出的一维两组分扩散模型可较好地描述停留时间分布曲线的双峰分布.实验结果表明细颗粒的两种轴向混合行为有较大的差别，而总包Peclet数几乎不随操作条件及床内粗细颗粒比例变化     

内构件对高密度提升管内流体力学及径向气体混合的影响

Effect of bluff internals on the hydrodynamics and lateral gas mixing was studied in a 0.186m ID high-density riser. With the bluff internals, the extremely non-uniform radial profiles of solid fraction and particle velocity become flat and the dense downflow layer near the wall disappears, indicating the significant enhancement of solid turbulence introduced by the internals. The fluctuation velocity and solid fraction transient signal analysis indicates a significant increase in fluctuation intensity near the wall region. The length influenced by the internals on the flow structure is about 1 meter. The lateral gas dispersion coefficient increases significantly as the bluff internals exist in the riser.