EFFECTS OF SECONDARY AIR INJECTION ON GAS-SOLID FLOW BEHAVIOR IN CIRCULATING FLUIDIZED BEDS

Effects of secondary air injection on the hydrodynamics such as solid holdup and gas-solid flow behavior were investigated in a circulating fluidized bed. The gas velocity in the riser, the ratio of secondary air velocity to that of primary air, and the solid circulating rate were chosen as operating variables. Fluid cracking catalyst(FCC) with a density of 1840 kg/m³ and a mean diameter of 74 um was employed as the solid phase. The secondary air was fed to the riser radially or tangentially at the wall of the column. Pressure drop fluctuations in the riser were measured and analyzed by adopting the stochastic method to characterize the effects of secondary air injection on the gas-solid flow behavior in the bed.

It has been found that the injection of secondary air into the riser can increase the solid holdup in the riser considerably, and that the tangential injection of secondary air is more effective for the increasing the solid holdup than the radial injection. However, the gas-solid flow behavior has been found to become less persistent with the injection of secondary air; the resultant flow behavior is more complex when the air is injected tangentially than radially. The solid holdups in the primary as well as secondary zones of the riser have been well correlated in terms of not only operating variables but also fractal dimension of the pressure fluctuations.     

CFD SIMULATION OF THE GAS-SOLID FLOW IN THE RISER OF A CIRCULATING FLUIDIZED BED WITH SECONDARY AIR INJECTION

A comprehensive investigation was carried out to study hydrodynamics aspects of secondary air injection in circulating fluidized beds. This article presents modeling and results of computational fluid dynamics simulations of gas-solid flow in the riser section of a laboratory-scale (ID = 0.23 m, height = 7.6 m) circulating fluidized bed with a radial secondary air injector. The gas-solid flow model is based on the two-fluid (Eulerian-Eulerian) approach, where both gas and solids phases are treated as interpenetrating continua. A granular kinetic theory model is used to describe the solids phase stresses. The simulation results are compared with measured pressure drop and axial particle velocity profiles; reasonable agreement is obtained. Qualitatively, excellent agreement is obtained in predicting the increase in solids volume fraction below secondary air ports, the accumulation of solids around the center of the riser due to momentum of secondary air jets, and the absence of the solids down-flow near the wall above the secondary air injection ports, which are the prominent features of secondary air injection observed in the experiments.     

CFD SIMULATION OF THE GAS-SOLID FLOW IN THE RISER OF A CIRCULATING FLUIDIZED BED WITH SECONDARY AIR INJECTION

A comprehensive investigation was carried out to study hydrodynamics aspects of secondary air injection in circulating fluidized beds. This article presents modeling and results of computational fluid dynamics simulations of gas-solid flow in the riser section of a laboratory-scale (ID = 0.23 m, height = 7.6 m) circulating fluidized bed with a radial secondary air injector. The gas-solid flow model is based on the two-fluid (Eulerian-Eulerian) approach, where both gas and solids phases are treated as interpenetrating continua. A granular kinetic theory model is used to describe the solids phase stresses. The simulation results are compared with measured pressure drop and axial particle velocity profiles; reasonable agreement is obtained. Qualitatively, excellent agreement is obtained in predicting the increase in solids volume fraction below secondary air ports, the accumulation of solids around the center of the riser due to momentum of secondary air jets, and the absence of the solids down-flow near the wall above the secondary air injection ports, which are the prominent features of secondary air injection observed in the experiments.     

循环流化床壁面的几何结构对床层气固流动特性的影响

采用反射式光导纤维探头对平壁和膜式壁面的循环流化床内的局部瞬态颗粒浓度进行测量,并应用时间序列分析方法对两种不同壁面几何结构的床内的动态特性进行分析.研究结果表明,循环流化床内存在明显的时空结构不均匀性,壁面的几何形状对壁面附近的气固流动特性存在显著的影响．     

AN EXPERIMENTAL STUDY OF LIQUID-PARTICLE FLOW IN CIRCULATING FLUIDIZED BED

A liquid-solid circulating fluidized bed (LSCFB) is operated at high liquid velocity, where particle entrainment is highly significant and between the conventional liquid fluidized bed and the dilute phase liquid transport regimes. LSCFB has potential applications in the fields of food processing, biochemical processing, and petrochemical and metallurgical processing. It is well known that the flow characteristics in a liquid-solid circulating fluidized bed are different from those of a conventional liquid-solid fluidized bed. The limited studies available in literature do not provide complete understanding of the flow structure in this typical regime.

In the present work, experiments were carried out in a 0.0762 m ID and 3 m height laboratory-scale liquid-solid circulating fluidized bed apparatus by using various solid particles and tap water as fluidizing medium. In the experimental setup, two distributors (specially designed) were used to monitor solid circulation rate in the riser. The effects of operating parameters, i.e., primary liquid flow rate in the riser (Up), solid circulation rate (G_s), and particle diameter (dp), were analyzed from the experimental data. Finally, a correlation was developed from the experimental data to estimate average solid holdup in the riser, and it was compared with present experimental and available data in the literature. They agree well with a maximum root-mean-square deviation of 7.83%.     

ＣＦＢ稀相段直长对颗粒内循环流动规律的影响

采用光纤探头测定Ｄ１m射流循环流化床（半圆形）稀相区颗粒速度、颗粒浓度和颗粒流通量的径向分布规律,并考察了操作条件及其轴向的影响,实验结果表明,在循环流化床（ＣＦＢ）中,颗粒流动在床层径向有较大的不均匀性,并呈明显的内循环流动结构,进而考察了影响其流动规律的,以期进一步理解循环流化床颗料流动机理,强化和改善反应器的设计操作。     

气固流化床流型特性及其识别的复杂性研究

运用复杂性C2、涨落复杂性Cf及Lempel-Ziv复杂性C(n)等复杂性参数对气固流化床压力脉动信号进行分析,研究它们随流化床操作气速增大历经不同流型的变化趋势并将结果作了比较,进一步探讨了流化床流型特性的内在规律性,研究结果表明,在起始流化致鼓泡态转变的过程中,气－固体系会进行一种所谓的“重构”现象,并证实了气泡的存在是影响压力脉动信号复杂性的重要因素,同时实验显示复杂性参数能明确地指示固定床,鼓泡流化及湍动流化等不同流型之间的转变过程,为流型识别提供了新思路。     

循环流化床燃烧数学模型及试验研究

利用循环流化床内气－固两相流动基础方面的研究成果, 提出床内气固浓－淡流动模型, 建立适用不同结构参数的循环流化床燃烧模型, 考虑了床内气体、固体颗粒的返混、循环过程以及煤燃烧、污染气体的生成和分解、颗粒磨损等过程． 在循环流化床燃烧试验台上进行实验研究, 模型仿真结果和实验数据吻合良好, 表明气固两相浓－淡流动模型所建立的循环流动床燃烧系统模型可以正确地模拟循环流化床的燃烧过程．     

循环流化床下行管内气固两相流流体力学模型与数值模拟

从流体力学和湍流理论出发，将ｋ－ε模型运用到气固两相流动体系中，建立了气固两相湍流模型。模型计算得出的轴径向颗粒浓度及速度与实测值相吻合，为下行管的设计放大提供了理论依据。     

灰熔聚流化床气化炉内气固两相流的数值模拟

应用欧拉双流体模型模拟了某化肥厂现运行的灰熔聚流化床煤气化炉(用ICC表示)内的气固两相流动行为,得出了所模拟ICC的合理流化气速与喷动气速的速度范围及匹配关系:流化气速不能太小,否则布风板区域会出现死区;流化气速也不能太大,否则将失去ICC的设计运行特点.当流化气速一定时,随喷动气速的增加,搅动混合增强,但过大的喷动气速会使床内的流动结构出现腾涌,不利于ICC的高效安全运行.     

高速气固流化床局部瞬态行为混沌分析

在内径 1 86mm、高 9m的提升管中 ,通过改变操作气速获得湍动流化床、快速流化床和稀相输送典型的操作状态 ,使用光纤密度探头在轴向不同截面的不同径向位置处测得密度脉动时间序列 ,以反映不同流型下 ,不同浓、稀相区的瞬态动力学行为 .利用确定性混沌理论分析了局部密度脉动信息 ,以Kolmogorov熵定量地表征系统的动力学特征 .实验结果表明 ,Kolmogorov熵随流型的变化能正确地反映相应流型的特点 ,可用来描述不同流型下的流动结构 .在忽略壁面附近行为的条件下 ,对 3种流型、不同的浓稀相区 ,局部熵值随局部固含率变化可分为两个区 :当局部固含率高于 0 0 5时 ,熵几乎不随局部固含率的变化而变化 ;当局部固含率低于 0 0 5时 ,熵随局部固含率的减小而急剧增大 .这说明系统的混沌特征与局部固含率关系密切 ,局部固含率对系统局部动态行为起控制作用 .     

气固流化床压力脉动信号的混沌特征分析

对气固流化床内压力脉动时间序列进行分析,研究了流化床内的混沌特性,分别利用李雅普诺夫指数、相对分散度、分形维数和混沌度来定量描述稳定性、复杂性,自由度和系统状态数,证明四个混沌特征参数的相关性是好的,得到了混沌特性随着气速和床层高度的变化趋势,并且提出气泡行为是流化床内产生混沌运动的重要原因。     

循环流化床内颗粒运动与换热分析

对循环流化床锅炉床体内部贴近壁面的颗粒团运动和受力特点进行了分析,探讨颗粒团沿壁面下滑贴壁时间的计算方法,由此发展了床体内部颗粒-气体对流换热传热系数的理论计算模型和方法.与现有文献的实验数据资料比较表明,计算的结果较好地反映了实际情况.     

三相循环流化床中气泡大小及其分布的实验研究

用光纤探头技术对三相循环流化床中的气泡大小及其分布进行了系统研究 ,实验测定了操作条件对气泡大小及其分布的影响规律 .实验结果表明 ,三相循环流化床中气泡的大小分布可用对数正态分布表征 ,在实验条件下气泡平均直径在床中心区域较小且沿半径方向由中心向边壁逐渐增大 ,并随表观气速的增大而减小 ,随固含率的增大而增大 ,表观液速对气泡平均直径的影响较小     

大型循环流化床流动结构分析

采用双光路光纤密度探头和激光多普勒测速仪测量了内径418mm,高18m的大型循环流化床提升管和下行床中的瞬态颗粒浓度信号和颗粒速度信号.对瞬态颗粒浓度和颗粒速度的概率密度分布分析表明,下行床中存在着和提升管中不同的微观流动结构,在提升管内流动结构存在明显的两相：即颗粒团相和空穴相,两相的固含率分别为接近1-ε_mf和0.01～0.02.而在下行床中,虽然在边壁也存在着颗粒的团聚行为,但不能形成稳定的、固含接近于起始流化状态固含值的颗粒团相.这种流动结构的区别揭示了下行床中气固顺重力场运动和提升管逆重力场运动在流动机制上的差异.     

液固循环流化床两相流动模型

引　言流化床换热器具有防、除垢和强化传热等优点 ,在化工、食品、海水淡化、废水处理等领域具有广阔的应用前景[1].目前 ,流化床换热器历经散式流化床、内循环流化床 ,已发展到外循环流化床换热器[2 ],它要求在较稀的颗粒浓度 (颗粒浓度小于 5% )、较高的流速 ( 1～ 3ｍ·ｓ- 1)下操作 .流化床换热器中液体流动及颗粒运动状态的研究对流化床换热器的设计和操作具有重要意义 ,但人们对循环流化床换热器中颗粒运动情况的研究还很缺乏 .考虑到循环流化床换热器中的每根换热管都可作为一个独立的循环流化床对待[3].本文试图建立一滑移速度模型…     

液固循环流化床中的颗粒速度场

利用超声多普勒测速仪（UDV）对液固循环流化床中颗粒流场进行了系统的实验研究.实验测定了不同轴向位置处的颗粒速度分布,得到了较完整的颗粒速度场,分析了入口区对流场分布的影响,考察了表观液速和颗粒循环速率等操作条件对颗粒速度场分布特点的影响,并利用各影响因素之间的相互关系对流型和速度的变化进行了分析.     

并流下行循环流化床反应器气固传质模型

建立了并流下行循环流化床反应器气固吸附传质模型,并采用稳态示踪的实验研究方法,测定了下行床中气相示踪剂浓度的轴向分布,进而利用Marquardt多元非线性回归方法求得了模型参数即轴向扩散系数和传质系数的值.实验结果的回归得到了模型参数与操作条件的经验关联式.该模型能很好的预计传质过程,包括轴向扩散系数和传质扩散系数的值以反应器中组分浓度的轴向分布,因此可用于描述并流下行循环流化床反应器的气固吸附传质过程.     

循环流化床中颗粒团聚物性质的PDPA测量

提出了一种利用相位多普勒粒子分析仪（PDPA）来测量气固循环流化床中颗粒团聚物性质的方法,并运用此方法初步考察了操作条件对循环流化床稀相区中颗粒团聚物性质的影响.在本实验操作条件下,颗粒团聚物的时间分率、频率、内部空隙率以及轴向速度等性质都存在轴径向的不均匀分布,具有较明显的环核特征;固体循环速率对颗粒团聚物性质径向分布影响不大,表观气速的变化可引起其轴向分布规律发生显著改变,但其径向的环核特性仍然存在.     

循环流化床锅炉结焦浅析

结合循环流化床锅炉近几年的实际运行情况,从点火、运行中结焦原因、现象及预防处理等方面对流化床锅炉结焦问题进行讨论与总结。