流化床内压力脉动信号的检测与分析——复合探头在流化床测量中的开发应用

设制了将压力探头与电导探头置于同一轴以测量同一点的电导率信号和压力脉动信号的压力电导复合探头,并在二维床(1000×380×19 mm)内,用活性炭作流化粒子、空气为流化介质,钢瓶 N_2产生气泡源进行实测,通过对单个气泡,气泡串和操作态等复合探头信号进行分析,初步得到了压力脉动信号的性质和传播规律,对气泡特性有了新的认识,并对气泡信号与压力脉动信号间的联系作了分析,为进一步定量研究打下了基础。     

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     

沸腾转化催化剂床内气泡特性研究与工业反应器的模拟和优化

在直径190mm的有机玻璃塔内,对填装0.27～0.79mm的以球形硅胶为载体的钒催化剂(B类粒子)流化床进行了实验研究。用直射式光纤探头连接微机采样和分析。结果表明,气泡直径和上升速度与A类粒子流化床获得的关联式基本相符。但气泡分率随床高增加而明显增加。根据实验结果,确定了B类粒子流化床反应器的模型参数。用两相平推流模型与本研究所获得的气泡参数及反应动力学方程相结合,对直径为1500mm的工业反应器进行了模拟,得到令人满意的结果,并进一步优化了工业操作条件。     

三相循环流化床中气泡上升速度的实验研究

开发了一种新型的光纤探头多相流气泡测试系统,可用于气－液两相和气－液－固三相体系中气泡参数的测定。应用此系统研究了三相循环流环化床中不同径同位置气泡的上升速度分布,气泡上升速度均值的径向布以及操作条件对这现任中分布的影响。     

气固摆动流化床的气泡运动特性

气泡现象是气固流态化的基本现象之一，了解摆动床气泡特性对推动流态化技术的海上应用具有重要意义。采用二维床及B类玻璃球颗粒的摆动床，通过对图像进行分析，研究了气泡在摆动过程中不同时间段的上升特点；提出了“热区图”方法，发现在较高表观气速时存在气泡聚集方向转换“滞后”于床体姿态转换的现象，原因为颗粒所受科氏力使其向摆动反方向聚集，抑制气泡聚集方向的转换；对IOP曲线的分析结果表明，气泡在床内分布位置、气泡平均尺寸及气泡生成与破裂频率等时均特征介于倾斜床与直立床之间；操作气速增加会使摆动床内气泡平均尺寸及气泡生成破裂频率都增加；摆动幅值增加只会使气泡生成与破裂频率明显增加；摆动周期对两者的影响均不明显。     

细颗粒流化床的特性

在二维流化床内,进行了粗颗粒和细颗粒的流化实验,测定了气泡尺寸和气泡数。观察了细颗粒床内的湍动状态。在φ100与φ280流化床内,进行了气体两相分配实验。细颗粒流化床浓相空隙率大干初始流化床的空隙率。并对细颗粒流化床的规模放大问题进行了初步探讨。     

二维流化床光反应器中的光强分布及波动规律

考察了外部平行光源照射下液-固及气-液-固二维流化床中流体流动状况对床层光强分布和波动规律的影响. 采用朗伯-比尔定律建立了两相及三相流化床中光强分布的数学模型，分析了液含率、气含率及气、固相吸光系数对床层光强分布的影响，并采用自制光纤光强传感器测定了床层中的光强分布和波动信号. 二维流化床中床层光强沿径向呈指数衰减，随着液含率和气含率增大而增大，理论计算与实验结果相吻合. 对光强波动信号的功率谱分析表明，液-固流化床中功率谱密度由低频至高频呈规律性衰减，气-液-固三相流化床中由于受气泡的影响功率谱密度出现起伏，在5~12 Hz处出现谱峰，与气泡的产生频率一致. 上述模型可推广应用于其他光源和不同结构流化床光反应器的数学描述.     

双组分颗粒鼓泡流化床内气泡形状参数

在二维双组分鼓泡床实验装置上,采用高速摄像技术,对床内气泡的形状特性进行了研究,考察了不同形状气泡在床内的轴径向分布,探索了颗粒组成和操作气速对气泡形状的影响。结果表明:不同形状的气泡在鼓泡床内呈正态分布,球形度较好的气泡主要分布于床层底部和壁面附近,而细长的气泡则主要集中于床层中心区域。随着气体速率的增加,气泡的球形度和宽纵比降低,气泡形状趋于细长和不规则;随着重组分增加,气泡的球形度增大而宽纵比减小。双组分颗粒鼓泡流化床内气泡球形度的概率密度较单组分的分布更宽,而宽纵比的概率密度分布与添加的颗粒密度有关。     

流化床密相区颗粒扩散系数的CFD数值预测

应用离散颗粒模型直观获得颗粒运动情况,并从单个颗粒和气泡作用的角度分析颗粒运动和混合,证实气泡在床层中上升、在床层表面爆破以及气泡上升引起的乳化相下沉运动对颗粒混合起关键作用。应用基于颗粒动理学的双流体模型系统地对床宽分别为0.2、0.4、0.8 m的二维流化床在鼓泡区和湍动区的气固两相流动行为进行数值模拟。受离散颗粒模型启发,在双流体模型计算结果基础上,引入理想示踪粒子技术计算床内平均颗粒扩散系数。计算结果表明,颗粒横向扩散系数(D_x)总体上随流化风速增大而增大,但受床体尺寸影响较大;颗粒轴向扩散系数随流化风速增大而增大,受床体尺寸影响较弱。文献报道的密相区颗粒横向扩散系数分布在10^-4～10^-1 m²·s^-1数量级。本文提出的计算方法在数量级上与文献实验结果吻合,表明在大尺寸流化床且高流化风速下,颗粒横向扩散系数远大于小尺寸鼓泡流化床,为不同研究者实验结果的分歧提供了理论依据,也为预测大型流化床内颗粒扩散速率提供了放大策略。     

振动流化床与浸没水平管平均传热特性理论分析与实验研究

在二维流化床(240mm×80mm)中,以平均粒径dp为1.83mm的玻璃珠为物料,研究了振动流化床与浸没水平管间传热规律;考察了流化数、振动频率、床高、水平管管径等因素对平均传热系数的影响。采用自制探头对浸没加热管束和振动流化床层间平均传热系数进行实验测定,利用颗粒团模型,建立了振动流化床层与浸没水平管间平均传热模型,并对平均传热系数的理论预测值与实验测定值进行了比较。结果表明:计算值与实验值吻合较好,误差在±15%范围内。在较高流化数、低振动频率时,实验值处于理论值上方;随着振动频率、管径增大,平均传热系数实验值逐渐趋于理论预测值甚至低于理论预测值。结果可为带浸没水平管的振动流化床设计和研究提供参考。     

Bubble measurement in three-phase fluidized beds using a u-shaped optical fiber

A photometric technique is being developed to study bubble behaviour in three-phase fluidized beds constituted by fine particles. A custom made U-shaped single core silica fiber (400 μm diameter) is used as the bubble sensor. A helium-neon laser beam is introduced at one end of the U probe and the variation of the light intensity, occurring when a gas bubble contacts the sensor, is detected at the other end of the fiber. The velocity and axial length (cord) of the bubbles were measured by four such probes in a two dimensional bed. This technique could be applied to study the bubble behaviour and to provide experimental data for estimating the performance of reactors with severe operating conditions of high temperature and pressure.     

Effect of bed size on hydrodynamics in 3‐D gas–solid fluidized beds

It is well known that hydrodynamics observed in large scale gas–solid fluidized beds are different from those observed in smaller scale beds. In this article, an efficient two‐fluid model based on kinetic theory of granular flow is applied, with the goal to highlight and investigate hydrodynamics differences between three‐dimensional fluidized beds of diameter 0.10, 0.15, 0.30, 0.60, and 1.0 m, focusing on the bubble and solids flow characteristics in the bubbling regime. Results for the 0.30 m diameter bed are compared with experimental results from the literature. The bubble size evolution closely follows a correlation proposed by Werther for small beds, and a correlation proposed by Darton for sufficiently large beds. The bubble size increases as the bed diameter is increased from 0.10 to 0.30 m, and remains approximately constant for bed diameters from 0.30 to 1.0 m. Concurrently, an increase in bubble rise velocity is observed, with a much high bubble rise velocity in the largest bed of diameter 1.0 m due to gulf stream circulations. The dynamics in shallow and deep beds is predicted to be different, with marked differences in bubble size and solids circulation patterns. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1492–1506, 2015     

Observation of 3‐D Structure of Bubbles in a Fluidized Catalyst Bed

The interface area between the bubble and emulsion phases in a fluidized catalyst bed is one of the important parameters used to analyze and design the fluidized bed reactor. We used a fast‐scanning X‐ray CT system to observe the bubble shape and structure. We then obtained the transient 2‐dimensional cross sectional gas‐phase distribution in a fluidized catalyst bed. Using image‐processing techniques, pseudo 3‐dimensional images of the bubbles were reconstructed. The bubble structure was studied based on the 3‐dimensional images and the previously obtained results in a 2‐dimensional fluidized bed. It was found that the bubble shape was not spherical but complicated, and that the bubbles ascending in a fluidized catalyst bed consisted of some smaller bubbles.     

微型流化床内气体返混

微型流化床基础和应用在近几年受到越来越多的关注。针对微型流化床对气固反应分析的应用要求,利用脉冲示踪法研究了内径10 mm和21 mm两种尺寸微型流化床中的气体返混特性,具体考察了管内径、颗粒静床高度、床料颗粒粒径和气体表观流速对气体返混程度的影响。结果表明:随着床内径、颗粒静床高度和表观气速的减小和床料颗粒粒径的增大,气体在床内的返混程度减小。使用粒径约270 μm粗颗粒时,两种床径的浅层微型流化床中的气体返混程度都较小,对应的Peclet数在27以上,证明了床内气体流动接近平推流,从而为利用微型流化床最小化气体返混对反应测试的影响,获得近本征反应动力学参数提供了流动特性的保障。     

Distribution of the gas flow in fluidized beds with a slugging behaviour

Capacitance probe measurements of the visible bubble flow rate have been made in a pressurized fluidized bed burning coal. The bed, of 0.3 × 0.3 m cross-section, was operated at pressures between 1.0 and 2.0 MPa and at temperatures between 750 and 900°C. The fluidizing velocity was 0.95 m/s and the mean particle diameter was 0.9 mm. Based on the experimental results, a model of the gas distribution between the bubble phase and the particulate phase in fluidized beds with a slugging behaviour was developed. The model accounts for the lack of bubble flow obtained if the two-phase theory is employed. In order to verify the model, simultaneous measurements of the visible bubble flow rate and of the gas flow rate through the bubbles were carried out in a bed of similar geometry but operating at ambient conditions. In this bed the fludizing velocity was varied between 1.6 and 2.7 m/s and the mean particle diameter was 1.0 mm. The through-flow of gas was measured with the aid of pressure probes. Evaluation of the experimental results using the model showed that this gas through-flow in the bubble phase subsequently increases the superficial gas velocity in the particulate phase between the vertically aligned bubbles (slugs), and that this gas velocity in excess of the incipient fluidization velocity is responsible for the large deviation from the two-phase theory. The associated increase of the particulate phase voidage was calculated via the Ergun equation.     

Maximum entropy estimation of the bubble size distribution in fluidized beds

This work presents a new methodology, based on the maximum entropy method, to obtain bubble characteristics in fluidized beds. The probability distributions (PDF) of bubble pierced length and velocity are obtained applying the maximum entropy principle to experimental measurements. In addition, the bubble diameter distribution has been inferred from experimental pierced length measurements. This method is applied to characterize bubbles in fluidized beds for the first time and the most general bubble geometry, a truncated spheroid, is considered. The distance between probes, s, which is the minimum pierced length that is possible to measure accurately using intrusive probes, has been introduced as a constraint in the derivation of the size distribution equation.The maximum entropy method is applied to experimental measurements of bubble characteristics carried out using optical and pressure probes in a three-dimensional fluidized bed of Geldart B particles. Results on bubble size obtained from pressure and optical probes are very similar, although optical probes provide more local information and can be used at any position in the bed. The maximum entropy principle has been found to be a simple method that offers many advantages over other methods applied before for size distribution modeling in fluidized beds.     

基于EMMS模型的气固鼓泡床的模拟及气泡特性的分析

基于EMMS曳力模型,采用双流体的方法对气固鼓泡床内的气固流动特性进行模拟,建立基于图像处理气泡特性的分析方法,重点研究了不同表观气速下气泡在床层内分布特性,包括气泡平均当量直径、气泡速度和气泡球形度的轴向分布,以及气泡的生命周期。研究结果表明,小气泡多集中在床层底部和壁面区域,而大气泡多集中在床层中间区域。随着表观气速的增加,床层高度不断增加,气泡的球形度降低,气泡的大小、出现频率、上升速度以及生命周期均增加;然而,当表观气速增大到一定程度,继续增加气速对气泡的上升速度影响不大。     

RADIAL DISPERSION AND BUBBLE CHARACTERISTICS IN THREE-PHASE FLUIDIZED BEDS

The effects of gas and liquid velocities, liquid viscosity and particle size on the radial dispersion coefficient of liquid phase (D_r) and the bubble properties in three-phase fluidized beds have been determined. A new flow regime map based on the drift flux theory in three-phase fluidized beds has been proposed.

In three-phase fluidized beds, D, increases with increasing gas velocity in the bubble coalescing and in the slug flow regimes, but it decreases in the bubble disintegrating regime. The coefficient exhibits a maximum value in the bed of small particles with increasing liquid velocity at lower gas velocities. However, it increases with increasing liquid velocity at higher gas velocities. In two and three-phase fluidized beds of larger particles (6,8 mm), D_r exhibits a maximum value with an increase in liquid viscosity at lower gas velocities, but it increases at higher gas velocities. The mean bubble chord length and its rising velocity increase with increasing gas velocity and liquid viscosity. However, the bubble chord length decreases with an increase in liquid velocity and it exhibits a maximum value with increasing particle size in the bed. The radial dispersion coefficients in the bubble coalescing and disintegrating regimes of three-phase fluidized beds in terms of the Peclet number in the present and previous studies have been well represented by the correlations based on the concept of isotropic turbulence theory.     

Measurement of Solid Circulation Patterns in Liquid—Solid and Gas—Liquid—Solid Fluidized Beds

Solid circulation patterns were determined by measuring collisions between tracer particles and cylindrical probes in liquid—solid and gas—liquid—solid fluidized beds. Special probes were used to obtain two‐ and three‐dimensional views of particle motion. Circulation patterns for 3 and 5 mm glass beads were studied. Mixing cells, which had been formed at low liquid velocities, grew in size and eventually merged as the liquid velocity increased. The flow patterns of smaller particles having the same density as bed particles and particles lighter than bed particles (graphite particles) with the same size were also measured.     

流化床内部构件评选

在φ109mm流态化床里评选内部构件之后,在φ360mm流态化床里,通过邻二甲苯氧化制苯酐反应,以苯酐收率作为检验指标,对四种内部构件再次作了评选。获得的结果与以前是相同的,即斜片挡板效果最好,多孔板次之,垂直管束最不理想。但横向挡板与垂直管束联用时,效果与间隔小的横向挡板相当。回归方程指出,垂直管束构成的床层对温度和气速的敏感性较差,说明两者的气-固流动形态是不一样的。