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。     

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。     

基于DEM模拟的气固鼓泡床内流场间歇性及颗粒相干结构的分析

采用计算流体力学和离散元方法（CFD-DEM）对气固鼓泡床流动行为进行模拟研究,并基于模拟结果分析鼓泡床内气泡和颗粒微观运动特性。对颗粒速度的脉动能谱进行分析,发现鼓泡床流场中存在间歇性。通过对比鼓泡床不同轴、径向位置的颗粒脉动速度的平坦因子,发现鼓泡床内不同位置的流场间歇性不同,随着床层高度的增加,流场的间歇性减弱;在径向上,过渡区的流场间歇性明显大于边壁区和中心区。进一步采用连续小波分析方法揭示了相干结构（颗粒涡团）的分布以及演化过程,并分析了不同尺度下相干结构（颗粒涡团）的分布与鼓泡床内颗粒与气泡运动的关系。     

基于DEM模拟的气固鼓泡床内流场间歇性及颗粒相干结构的分析

采用计算流体力学和离散元方法(CFD-DEM)对气固鼓泡床流动行为进行模拟研究,并基于模拟结果分析鼓泡床内气泡和颗粒微观运动特性。对颗粒速度的脉动能谱进行分析,发现鼓泡床流场中存在间歇性。通过对比鼓泡床不同轴、径向位置的颗粒脉动速度的平坦因子,发现鼓泡床内不同位置的流场间歇性不同,随着床层高度的增加,流场的间歇性减弱;在径向上,过渡区的流场间歇性明显大于边壁区和中心区。进一步采用连续小波分析方法揭示了相干结构(颗粒涡团)的分布以及演化过程,并分析了不同尺度下相干结构(颗粒涡团)的分布与鼓泡床内颗粒与气泡运动的关系。     

静电效应对有无埋管气固鼓泡床内气泡特性的影响分析

采用基于双流体模型(TFM)耦合静电模型的方法,研究颗粒的静电对有无埋管气固鼓泡床内气固流动特性和气泡特性的影响。首先在无静电场存在的条件下,利用双流体模型对自由鼓泡床和埋管鼓泡床内的流动情况进行模拟并与实验结果进行对比;进一步耦合静电模型,考察静电对自由鼓泡床和埋管鼓泡床内床层的整体性质和气泡特性的影响。研究结果表明,在无静电场条件下采用双流体模型能较好地预测自由鼓泡床和埋管鼓泡床内的气固流动状况以及气泡的平均直径和气泡的上升速度。埋管的存在使鼓泡床内气固流动发生强烈扰动,并使气泡的平均直径和气泡的上升速度均呈振荡分布。静电的存在对自由鼓泡床和埋管鼓泡床内床层的平均固含率影响不大,但对气泡分布规律影响较大,使得自由鼓泡床内气泡数目减少,而埋管鼓泡床下部区域的气泡分布比较集中,上部有大气泡出现。     

流化床还原铁精矿粉动力学初算

采用鼓泡床模型对流化床还原铁(精)矿粉动力学进行了初步的研究、计算和讨论.结果表明,流化床内的气泡直径随密相床层高度的增加而增加,气泡-气泡晕交换系数(Kbc)b及气泡晕一密相交换系数(Kbc)b则随密相床层高度的增加而降低;(Kbc)b＞(Kce)b;小粒度颗粒的还原速度随密相床层高度的增加变化不大,大粒度颗粒还原到一定程度后还原速度会降低,而且明显慢于小粒度颗粒的还原速度.     

鼓泡床内气泡直径的分布

气泡的直径分布是决定鼓泡床反应器气液接触面积的重要参数，其轴向分布对鼓泡床反应器的设计和性能优化具有重要意义。喷嘴作为气体分布器的最小单元，对气体分布器的分布性能起决定性因素。本文考察了不同孔径喷嘴对鼓泡床内气泡分散的影响，结果表明，对于在孔口产生的小于最大稳态气泡直径ds的气泡，将稳定存在于床层内，而直径大于ds的气泡，将迅速破碎，最后形成近乎一致的气泡粒度分布，喷嘴的影响距离在21cm左右。     

基于LBM-DEM的鼓泡床内气泡-颗粒动力学数值模拟

将修正后的格子Boltzmann方法（LBM）与离散单元法（DEM）相结合,建立LBM-DEM四向耦合模型对单口射流鼓泡床中气泡运动进行模拟。其中,流体相采用格子Boltzmann方法中经典的D2Q9模型,颗粒相求解采用离散单元软球模型,颗粒曳力求解采用Gidaspow模型,流固耦合基于牛顿第三定律。应用Fortran语言编程对上述模型进行求解,模拟得到了鼓泡床内气泡演化过程,并与相关实验进行对比,有效验证了当前模型的准确性。同时,分析了床层内颗粒速度、颗粒体积分数以及能量分布。结果表明：颗粒时均速度分布不仅能体现颗粒运动强弱,也可以反映气泡运动过程;床内空隙率与颗粒体积分数分布在预测床层膨胀高度上具有高度的一致性;初始堆积效应使得床内颗粒势能始终大于颗粒动能;随颗粒密度增加,势能增大,动能逐渐减小。     

基于LBM-DEM的鼓泡床内气泡-颗粒动力学数值模拟

将修正后的格子Boltzmann方法 (LBM)与离散单元法(DEM)相结合,建立LBM-DEM四向耦合模型对单口射流鼓泡床中气泡运动进行模拟。其中,流体相采用格子Boltzmann方法中经典的D2Q9模型,颗粒相求解采用离散单元软球模型,颗粒曳力求解采用Gidaspow模型,流固耦合基于牛顿第三定律。应用Fortran语言编程对上述模型进行求解,模拟得到了鼓泡床内气泡演化过程,并与相关实验进行对比,有效验证了当前模型的准确性。同时,分析了床层内颗粒速度、颗粒体积分数以及能量分布。结果表明:颗粒时均速度分布不仅能体现颗粒运动强弱,也可以反映气泡运动过程;床内空隙率与颗粒体积分数分布在预测床层膨胀高度上具有高度的一致性;初始堆积效应使得床内颗粒势能始终大于颗粒动能;随颗粒密度增加,势能增大,动能逐渐减小。     

加压二维鼓泡床气固流动特性的数值模拟

采用欧拉双流体模型模拟了加压下二维鼓泡床内的气固流动特性,结果表明:在相同的表观气速下,加压使气泡体积分数增大,气泡相与乳化相间的分解越发明显,气固两相流动、混合剧烈;同时,床层中上部颗粒轴向速度的径向分布不均匀性增强:中心区颗粒速度增加,近壁区下降;随着操作压力变大,流化床膨胀高度增大,相应地,整体气含率增大,床层下部的颗粒浓度减小,而上部颗粒浓度增加,固含率在轴向上的分布更均匀;床层压力波动主要由两种成分构成:低频率高幅值和低幅值高频率成分.压力脉动强度随床高的增加呈现先增大后迅速减小的趋势;此外,加压下床层压力脉动强度变大,即床压波动更加剧烈;而且加压下颗粒拟温度增大,即颗粒速度脉动增强.     

高密度浓相流化床中气泡的兼并与分裂特性

利用先进的高速动态分析系统对二维床中气泡的行为进行了研究,通过对所拍摄图象的分析处理．得到了不同介质流化床内形成的气泡形状、大小、聚并及分裂的基本规律和特点．实验研究表明．气泡的兼并主要是两气泡问的合并、被合并气泡总是从气泡的尾涡区曳入气泡;气泡分裂主要发生在操作气速较大或大气泡中,是由于其顶部粒子流(或“剪切流”)的侵入造成的;操作气速较低,粒度、密度较大粒子形成的流化床更易于造成气泡的湮灭。     

Novel phenomenological discrete bubble model of freely bubbling dense gas–solid fluidized beds: Application to two‐dimensional beds

A phenomenological discrete bubble model has been developed for freely bubbling dense gas–solid fluidized beds and validated for a pseudo‐two‐dimensional fluidized bed. In this model, bubbles are treated as distinct elements and their trajectories are tracked by integrating Newton's equation of motion. The effect of bubble–bubble interactions was taken into account via a modification of the bubble velocity. The emulsion phase velocity was obtained as a superposition of the motion induced by individual bubbles, taking into account bubble–bubble interaction. This novel model predicts the bubble size evolution and the pattern of emulsion phase circulation satisfactorily. Moreover, the effects of the superficial gas velocity, bubble–bubble interactions, initial bubble diameter, and the bed aspect ratio have been carefully investigated. The simulation results indicate that bubble–bubble interactions have profound influence on both the bubble and emulsion phase characteristics. Furthermore, this novel model may become a valuable tool in the design and optimization of fluidized‐bed reactors. © 2012 American Institute of Chemical Engineers AIChE J, 2012     

射流流化床内流体力学行为的模拟研究

以流化床应用为代表的气固流动系统是许多化工过程中的重要组成部分 ,CFD( computational fluid dynamics)方法能够为其优化设计和放大提供所需要的信息 .本文采用双欧拉模型 ,与 Gidaspow等的实验结果进行了对比 ,模拟了二维射流流化床内气泡的形成规律 ,得到了带锥型分布器的流化床内瞬时空隙率和气固相速度分布等流体力学参数 .对锥型分布器流化床的实验结果表明 ,模拟得到的气泡的形状与实验现象相接近 .     

The bubbling behavior of cohesive particles in the 2D fluidized beds

The present work focuses on a fully statistical analysis of bubbling behavior in the two-dimensional (2D) fluidized beds with cohesive particles. Various significant bubble properties such as bubble size, rising velocity, aspect ratio, bed expansion and bubble hold-up, etc., were investigated. An equation for bubble diameter is developed, , and the observed bubbles are generally smaller than the ones generated in the beds with A or B type powders. Both the average bubble size and rising velocity initially increase with the elevation above the distributor and keep constant beyond certain heights. The bubbles exhibit oblong with the most density aspect ratio (β) equal to 0.7. In addition, the bubble rising velocity coefficient ranges from 0.8 to 1.5. Two core-annular flows form in the large diameter, shallow fluidized bed used in this experiment.     

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.     

Effect of mixing state of binary particles on bubble behavior in 2D fluidized bed

In this study, a thin 2D fluidized bed was used to investigate the effect of mixing state of the binary particles on bubble behavior through the analysis of images captured by a high-speed digital camera. Experimental results show that the mixing index increases gradually with increasing gas velocity and the binary particles are in different mixing states though they are in the steady fluidization state. The maximal bubble number is near the interface of the bed when the binary particles are in the segregation state, whereas the maximal bubble number is at the bottom when the binary particles are in the well mixing state. The small bubbles are position at the bottom and are adjacent to the bed wall, while the large bubbles are mainly located in the central regions of the bed. The average bubble diameter shows the different variation trends with the different mixing states of the binary particles. The correlations estimating bubble diameter according to the mixing state of the binary particles are developed, and the computing value agrees well with the experimental data.     

Simulation and experimental studies on fluidization properties in a pressurized jetting fluidized bed

The influence of pressure on the bubble size and average bed voidage has been investigated experimentally and computationally in a circular three-dimensional cold-flow model of pressurized jetting fluidized bed of 0.2 m i.d. and 0.6 m in height with a central jet and a conical distributor, which roughly stands for the ash-agglomerating fluidized bed coal gasifier. The pressurized average bed voidage and bubble size in the jetting fluidized bed were investigated by using electrical capacitance tomography (ECT) technique. The time-averaged cross-sectional solids concentration distribution in the fluidized bed was recorded. The influence of pressure on the size of bubble and the average bed voidage in a pressurized fluidized bed was studied. Both experimental and theoretical results clearly indicate that there is, at the lower pressure, a small initial increase in bubble size decided by voidage and then a decrease with a further increase in pressure, which proves the conclusion of Cai et.al. [P. Cai, M. Schiavetti, G. De Michele, G.C. Grazzini, M. Miccio, Quantitative estimation of bubble size in PFBC, Powder Technology 80 (1994) 99-109]. At higher pressure, bubbles become smaller and smaller because of splitting. The average bed voidage increases gradually with the pressure at the same gas velocity. However, there is a disagreement between the experimental results and simulation results in the average bed voidage at the higher gas velocity, especially at the higher pressure. It suggests that the increase in density of gas with pressure may result in the drag increase and the drag model needs to be improved and revised at higher pressure.     

Distribution of bubble properties in a gas-liquid-solid fluidized bed

The behavior of bubbles in a cocurrent gas-liquid-solid fluidized bed was investigated in a column of 76.2 mm ID in this study. The particles used were glass beads of 3 and 6 mm and a binary mixture of these particles. A novel dual electrical resistivity probe system was developed and utilized to obtain bubble properties including bubble size and rise velocity. The distributions of the bubble properties in the gas-liquid-solid fluidized bed were evaluated for three flow regimes: the dispersed bubble flow regime; the coalesced bubble flow regime; and the slug flow regime.     

EFFECT OF DISTRIBUTOR ON BUBBLE SIZE IN A CYLINDRICAL FLUIDIZED BED AT AN ELEVATED TEMPERATURE†

The effects of temperature and distributor on bubble diameter were investigated using a cylindrical fluidized bed of 147 mm in diameter. Three perforated distributors having different holes in diameter and the same ratio of holes to bed area were used. Eruption diameters of bubbles were measured using a high speed video-camera system under the following conditions: bed temperature = 300 and 600 K, bed particles = spherical glass beads of 272 μm in average size, excess gas velocity = 1-4 cm/s, and static bed height equals; 10-42 cm. The bubble diameter at 600 K was larger than that at 300 K. The difference became smaller with increasing the static bed height and with increasing the excess gas velocity. The distributor with larger holes gave larger bubbles. The effect of hole diameter of the distributor on the bubble diameter became insignificant with increasing the static bed height and with increasing the excess gas velocity.