振动流化床中双组分颗粒流化特性的研究

本文研究了内径为１４８ｍｍ振动圆柱床中等密度和不等密度的双组分颗粒流化特性,考察了不同振动强度对双组分颗粒的床层空隙率、最小流化速度及相图的影响,给出了床层空隙率和最小流化速度的计算式,此计算值与实验值基本相符,且对振动流化床的实际操作和工程设计起到一定的指导作用。     

双粒度非球形颗粒的散式流化特性

用河砂的几种筛分配制成多种粒径比的双粒度颗粒,在流化床设备中以水为流化介质考察了双粒度混合非球形颗粒的流化过程。讨论了流化形式转换点的空隙率,双粒度非球形颗粒床层的空隙率、压力降、床层高度和各临界速度的计算方法,并与单粒度颗粒加以对比。     

宽筛分非正态分布颗粒物料的流态化特性

对宽筛分、粒度非正态分布颗粒物料的基本流态化特性进行了实验和理论研究 ,探讨了颗粒群的填充现象对临界空隙率及起始流化速度的影响 ,描述了颗粒群的离析现象在床层阻力特性曲线上的表征 ,分析了气泡运动与颗粒的轴向混合。     

石英砂二元混合颗粒初始流化过程的研究

在一套Φ300 mm×3000 mm的有机玻璃冷模流化床实验装置上,考察了石英砂二元混合颗粒的初始流化特性。所有二元混合颗粒均由颗粒密度相同但粒径不同的A、B、C、D四类颗粒组分两两按照一定质量分数混合而成。采用FXC-Ⅱ/32型压力巡检仪测得了不同轴向位置床层的压降曲线,得到了不同二元混合颗粒的起始流化过程特性曲线和起始流化速度。实验结果表明,两种颗粒组分平均粒径大小和差异及其组分质量分率对二元混合颗粒的起始流化特性具有显著影响。A类颗粒加入可显著改善B、C和D类颗粒的流化质量;C类颗粒加入量过大会使混合颗粒在流化过程中出现严重的沟流现象;当浮升组分(小颗粒)质量分数为0.4时,组分粒径差异较大的二元混合颗粒在流化过程中最容易发生完全分级现象;对于粒径差别较大的二元颗粒组分,床层最小流化速度随小颗粒组分的增多而下降,而对于具有较强颗粒间作用力组分的二元颗粒组分,床层最小流化速度则随小颗粒组分的增多而增大。根据实验数据对等密度BD二元混合颗粒的起始流化速度预测公式进行了修正,发现实验值与计算值吻合较好。     

宽筛分颗粒流化特性实验研究

通过实验,比较系统地研究了气固流化床内,不同筛分组合的颗粒在流化状态时的气泡特征、轴向固含量和颗粒粒径分布、床层空隙率等特征。实验结果显示:宽筛分物系中的细颗粒,一方面可提高气泡聚并破碎频率,从而抑制大气泡的产生;另一方面可使颗粒整体流化高度增加,提高轴向固含量分布的均匀性及流化状态的稳定性。实验结果也表明,在表观气速一定的条件下,若逐渐增大颗粒粒径,则流化床空隙率逐步减小,细颗粒可有效地提高床层空隙率。     

大颗粒振动流化床颗粒临界流化速度研究

建立了带内置水平管的振动流化床,并以颗粒状磷酸一铵为实验物料对带内置水平管的大颗粒振动流化床颗粒临界流化速度进行了实验研究。结果表明,振动的引入对颗粒临界流化速度有着明显的降低作用;同样床层条件下振动频率和振幅越大,床层临界流化速度越低;振动对小粒径颗粒的影响稍强于大粒径颗粒;由实验数据拟合出用于预测该类流化床临界流化速度的经验公式,公式与实验数据吻合良好。     

基于图像法喷动床内空隙率研究

利用图像二值化方法处理图片,并采用Matlab编程实现空隙率的测量,从微观层次分析空隙率对喷动流化床内流动状态的影响。研究了颗粒粒径、喷口数量和表观气速对空隙率分布的影响。结果表明,增大表观气速使床层底部的稀相区增大,床层膨胀高度增加,空隙率也随之增加。增大颗粒粒径会增大最小临界流化速率,所以在其达到流化状态后流化床内颗粒粒径增大,床内喷动区空隙率减小比较明显。在相同条件下,与单喷口相比双喷口在床层底部附近存在合并射流,同时颗粒能到达的高度显著增加,底部两侧停滞区面积减小。结合对空隙率的分析,提出一种新的表征流化床内流化状态的参数(流化指数),可以直观地表示流化床内颗粒的流化状态。     

不同粒径固体颗粒的悬浮速度计算及测试

分析讨论了不同粒径固体颗粒群的料浆流体力学特性，并在模型反应器中对其悬浮速度进行了实验测量。结果表明，以粒径范围为判据判定颗粒周围流场的流态，可使悬浮速度的测量、研究变得更简单、准确。研究采用悬浮分级模型，提高了悬浮速度的分析计算精度     

大差异双组分混合颗粒的最小流化特性

在一套f260 mm′2000 mm的有机玻璃实验装置中,对大差异双组分混合颗粒的最小流化特性进行了实验研究,得到了混合颗粒的流化曲线,由此给出了其起始流化速度、最小流化速度、临界分离速度、完全流化速度等特征速度. 实验结果表明,流化过程可分为4个阶段,即完全流化、大小颗粒分离、大颗粒静止小颗粒流化、固定床阶段,对应混合颗粒的3个状态：完全混合、部分混合部分分离、完全分离状态;混合颗粒的特征速度随小颗粒质量分率的增加而减小,且在小颗粒质量分率达到0.4~0.5后其减小的趋势减缓;混合颗粒的固定床阶段和完全流化阶段的床层空隙率及混合颗粒的体积收缩比在小颗粒质量分率为0.4时达到极值.     

流化床内垂直向上射流脉动特性的试验研究

本文利用图像处理技术,得到了二维流化床内垂直向上射流深度的时间序列。基于Ｒ／Ｓ分析方法,得到了表征射流脉动强弱的Ｈｕｒｓｔ指数和分数维。研究了流化数、射流初始速度、颗粒平均粒径及静态床层高度对射流脉动特性的影响。结果表明：垂直向上射流的脉动随射流初始速度、流化数、静态床层高度以及颗粒平均粒径的增加而减弱。     

液－固喷动流化床的膨胀特性和液体混合

在φ１０４ｍｍ×１５００ｍｍ的液－固喷动流化床内，对２种不同孔径的喷嘴分别进行喷动流化，利用轴向扩散柱塞流模型得到了液体轴向扩散系数。讨论了流体速度、床层空隙率、颗粒物性和喷嘴孔径等因素对液体轴向扩散系数的影响，并得到了轴向扩散系数的关联式。另外，还研究了床层的膨胀特性，得到了床层空隙率的关联式。     

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.     

Distributor effects near the bottom region of turbulent fluidized beds

The distributor plate effects on the hydrodynamic characteristics of turbulent fluidized beds are investigated by obtaining measurements of pressure and radial voidage profiles in a column diameter of 0.29 m with Group A particles using bubble bubble-cap or perforated plate distributors. Distributor pressure drop measurements between the two distributors are compared with the theoretical estimations while the influence of the mass inventory is studied. Comparison is established for the transition velocity from bubbling to turbulent regime, U_c, deduced from the pressure fluctuations in the bed using gauge pressure measurements. The effect of the distributor on the flow structure near the bottom region of the bed is studied using differential and gauge pressure transducers located at different axial positions along the bed. The radial voidage profile in the bed is also measured using optical fiber probes, which provide local measurements of the voidage at different heights above the distributor. The distributor plate has a significant effect on the bed hydrodynamics. Owing to the jetting caused by the perforated plate distributor, earlier onset of the transition to the turbulent fluidization flow regime was observed. Moreover, increased carry over for the perforated plate compared with the bubble caps has been confirmed. The results have highlighted the influence of the distributor plate on the fluidized bed hydrodynamics which has consequences in terms of comparing experimental and simulation results between different distributor plates.     

Intensification of mass transfer in liquid fluidized beds with inert particles

The influence of inert particles on liquid/solid mass transfer is studied in fluidized beds by using a binary-mixture of solids of differing size and density. The addition of inert particles of higher density and smaller diameter, e.g. glass beads, exerts remarkable effects on mass transfer coefficients in comparison to that of mono-component active particles at the same liquid velocity. The extent of the effect on liquid–solid mass transfer coefficients increases with an increasing fraction of the small inert particles in the mixture. The liquid–solid mass transfer coefficients for binary-mixtures are well correlated in terms of dimensionless groups and the voidage parameter.     

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…     

流化床与固定床耦合反应器中的颗粒磨损

在直径50mm的冷模流化床与固定床耦合反应器中,考察了活性炭颗粒在130～150℃及不同气速下的磨损情况,得到了不同气速下固定床中圆柱状颗粒的磨损率随时间的变化关系,同时分析了滞留在流化床、固定床及袋滤器中的细颗粒在不同气速下的粒径分布与质量分布.结果表明,颗粒在该耦合反应器中磨损严重,在0.212～0.424m/s气速下,固定床中颗粒质量损失可达3%～4%,流化床中颗粒平均粒径由200μm降至100μm以下.     

Axial voidage profile in a cold model circulating fluidized bed

The axial voidage profile was measured in a cold model circulating fluidized bed (0.38 m in diameter and 9.1 m in height) of sand particles as bed materials. Voidage in the riser column increases along the height above the distributor plate with increasing the gas velocity. However, it decreases with an increase in solid circulation rate in the bed. Model correlations to predict the solid circulation rale and the axial voidage profile in the bed are proposed.     

边界条件和结构尺寸对提升管内气固流动特性的影响

引言提升管是非均匀结构显著的气固两相流动体系,其流动特性主要表现为轴向空隙率的S形分布、径向的"环-核"结构以及团聚物的生成和破碎等。近年来,有关数值计算方法的研究增多,其中双流体模型的应用最为广泛,颗粒动理学则是目前封闭控制方程的最合理有效的方法。     

EFFECTS OF PARTICLE CHARACTERISTICS ON THE PERFORMANCE OF A FAST FLUIDIZED BED REACTOR

A heterogeneous model for the fast fluidized bed reactor which carries out a gas-solid non catalytic reaction is presented. The hydrodynamics of the fast fluidized bed is characterized by the model of Kwauk et al. (1985) which assumes the existence of two phases; a dense phase and a dilute pneumatic transport phase. For a given solid flowrate, the length of the reactor occupied by each phase depends on gas velocity, particle diameter and density and average voidage within the reactor. The gas-solid reaction is assumed to follow the shrinking core model. The solids are assumed to be completely backmixed in the dense phase and move in plug How in the dilute pneumatic transport phase. The gas phase is assumed to be in plug flow in both phases

For given gas and solid flowrates, the transition from the dense phase flow to the fast fluidized bed (containing two regions) as functions of particle size and density is determined using the model of Kwauk et al. (1985). The numerical solution of the governing mass balance equations show that for given solid and gas flowrates, (and average voidage) the gas phase conversion shows an unusual behavior with respect to particle diameter and density. Such behavior is resulted from the effects of particle diameter and density on the reactor volume occupied by each phase and the effect of particle diameter on the apparent reaction rate. The numerical results show that a fast fluidized bed gives the best conversion at large particle density and for the particle diameter which results the fast fluidized bed to be operated near the pure dense phase flow.