基于CFD-DEM的柱形颗粒固定床反应器流场特性数值模拟

采用计算流体动力学-离散单元耦合法(CFD-DEM),研究了柱形颗粒随机堆积固定床床层的流场流动特性。建立更接近柱形催化剂实体形状的颗粒模型,采用颗粒随机堆积法建立了固定床床层,通过对床层内部的流动特性数值模拟,得到床层内部空隙分布云图及压力场和速度场的分布规律。改变管径和粒径的比值(N,简称直径比),建立N为3,5,10和20的反应器固定床床层,分析了直径比对流场特性的影响。结果表明,当N为5时,床层堆积较为密实均匀,压降较大,速率变化幅度较小,故此管径比下固定床反应器内柱状催化剂床层有较好的流场特性。     

小管径比固定床中圆柱颗粒床层内气体流动特性

为了更好地探究气体在小管径比固定床内圆柱形颗粒床层中的流动特性,采用离散元法(DEM)生成严格的圆柱形颗粒床层,并对圆柱颗粒-颗粒及颗粒-壁面之间的接触进行"桥"处理。在验证模拟可行性的基础之上,运用计算流体力学(CFD)对空气在圆柱形颗粒床层中的流动过程进行数值模拟,通过提取固定床整体空隙率、径向空隙率及流场分布可知:小管径比固定床中的壁效应明显,近壁区空隙率存在较大幅度的振荡,速度场中存在局部不均匀性。在靠近壁面和填充松散处气体会形成沟流,某些情况下,在颗粒尾流中会出现停滞或回流的区域。     

基于气泡EMMS模型的鼓泡床两相流动CFD模拟

采用两步法将气泡EMMS模型拓展到计算网格层次,用于模拟鼓泡床两相流动,改进的EMMS曳力模型不仅与空隙率有关,且与网格内的速度直接关联.求解是在操作条件下利用宏观稳定性约束条件寻优得到乳化相内空隙率关系式,再用计算网格层次上的微观守恒方程结合乳化相空隙率关系式封闭求解其余结构参数及对应的曳力系数,最后借助用户自定义函数将气泡EMMS曳力嵌入双流体模型中,对含Geldart A类颗粒的鼓泡床内气固两相流动行为进行模拟与验证.结果表明,该模型能较好地模拟鼓泡床内的非均匀流动结构,准确捕捉到上稀下浓的轴向颗粒浓度分布特征,轴向颗粒浓度分布结果与实验值较接近,平均相对偏差为6.4%.床层径向上颗粒分布均呈明显的环-核结构.在不同床层高度(0.6,0.8和1.1 m)处的径向颗粒浓度分布与实验值基本吻合,仅在底部(0.4 m)略低,相对偏差小于14%.而Gidaspow曳力模型在4个床层高度与实验数据差距甚远,相对偏差可达91%.     

管式固定床反应器柱状颗粒床层流体流动模拟与实验研究

针对管式固定床反应器内管束数量多、规模大等特点,选取单个管束作为特征结构。对装填不同直径柱状颗粒的管束,采用程序坐标定位的方法,建立柱状颗粒床层的物理模型。采用DEM与CFD联合数值仿真方法,探究反应管内径与柱状颗粒的等比表面积球当量直径之比(管径比Di/dp)对柱状颗粒床层流体流动的影响,并建立单管固定床反应器试验台,采用差压测试方法进行实验研究。结果表明,当Di/dp由5.37增至12.75时,床层空隙率和流体分布均匀性均得到改善,壁面效应的影响由床层中心减弱到管壁。基于数值模拟及实验结果对Di/dp=12.75的柱状颗粒床层进行床层压降Ergun公式常系数修正,CFD模拟计算的结果与拟合公式吻合较好。研究结果能为固定床反应器压降预测提供参考。     

倒置液固流化床内液固两相流动特性的数值模拟

基于欧拉-欧拉双流体模型,数值模拟倒置液固流化床内液固两相流动行为.数值模拟预测了床内颗粒的速度、浓度分布以及空隙率的变化.研究结果表明颗粒在床内分布呈现非均匀分布,床内形成局部高空隙率的流体团;随着床层高度增加,颗粒轴向速度增大:数值模拟床内空隙率与Renganthan等的实验结果相吻合.     

固定床内压降的CFD分析

采用CFD法分析填充单一粒径和不同粒径颗粒固定床内的压降。分别采用PFC3D和Fluent软件建立固定床模型和模拟计算压力场,床层内颗粒流动雷诺数Re_p介于1~2 200之间。将床层空隙率、压降等模拟结果分别与已发表文献中的半经验公式的计算结果进行了比较,发现当Re_p小于120左右时压降的模拟结果与半经验公式计算结果基本吻合;而当Re_p较高时,二者之间的偏差较大;大颗粒使床层内空隙率分布的峰位置向固定床中心移动,第1个峰的位置与固定床壁面之间的距离和颗粒的质量平均直径（d₄₃）值基本相同;采用d₄₃代替半经验公式中的粒径参数,得到的压降计算结果与模拟结果更加吻合。     

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.     

基于两相两区模型的气固流化床DEM模拟

气固流化床DEM模拟中,通常采用面积加权平均法计算局部空隙率,为了考虑网格中显著非均匀结构对局部空隙率的影响,提出一个计算局部空隙率的两相两区模型。该模型将网格中的非均匀结构虚拟划分为稀相和密相,将实际网格区域划分为稀区和密区,并采用时空关联性原理识别稀区和密区;模型还采用自适应方法计算网格中颗粒分布的非均匀度;将非均匀度作为非均匀结构的影响权重计算密区颗粒的局部空隙率。模拟了鼓泡流化床,模拟结果表明:与传统的DEM相比,基于两相两区模型的DEM能更好地模拟气泡形态,且能捕捉气泡冒出床层和气泡破裂的复杂现象。     

新型催化裂解快速流化床内颗粒浓度分布实验研究

催化裂解反应器是石油深度加工的重要反应器,采用实验方法对新型快速床催化裂解反应器内气固两相流动特性进行了研究,测量了床层内颗粒浓度分布,考察了气体流量对床层轴向和径向上颗粒浓度分布的影响。实验结果表明,床层轴向上颗粒浓度呈现下部稠密上部稀疏的分布规律;当气体流量较低时轴向颗粒浓度呈S形分布,高气量下呈现指数函数形分布,即反应器上部区域的颗粒浓度分布影响较小;床层径向颗粒浓度分布呈现中心稀、边壁浓的特征,且增大空气流量,径向分布趋于均匀。在一定操作条件下,与传统提升管相比,新型快速床颗粒浓度显著提高。     

颗粒旋转对鼓泡流化床内气固两相流动特性影响的数值模拟

运用考虑颗粒自旋转流动对颗粒碰撞能量交换和耗散影响的颗粒动理学方法,建立鼓泡流化床气固两相Euler-Euler双流体模型,数值模拟流化床内气体颗粒两相流动特性。分析表明,颗粒平动温度与旋转温度之比是法向和切向颗粒弹性恢复系数和摩擦系数的函数。与不考虑颗粒旋转效应计算结果相比,考虑颗粒旋转效应后床内较容易形成气泡,颗粒自旋转运动将导致床内非均匀结构更明显。并且床层平均空隙率和床层膨胀高度增加,床中心区域颗粒轴向速度提高,床内颗粒平动温度下降。考虑颗粒旋转效应后预测的颗粒轴向速度和颗粒脉动速度与文献实验结果基本吻合。考虑颗粒旋转效应后获得的气泡直径更接近于前人经验关联式。     

喷动流化床流动特性数值模拟

为了捕捉喷动流化床中微观层次上的颗粒运动信息,建立了基于CFD的二维非稳态喷动流化床欧拉-欧拉两相流模型。分析了不同流化气速对喷动流化床气固流动特性的影响,即不同工况下的炉内压力降、颗粒浓度、床内空隙率分布、气体速度分布和固体颗粒速度分布。数值模拟研究结果表明:随流化气速的增大,压降和炉内平均空隙率逐渐增大,密相床层高度逐渐增加,沿着轴向方向的气体流量增大,喷动气的射流深度逐渐增加,同时射流半径也逐渐增加。     

大孔树脂膨胀床的膨胀特性及液相轴向混合性能

以AB-8大孔树脂为固相,对膨胀床的膨胀特性及流体混合性能进行了研究。结果表明,不同直径范围的颗粒的膨胀性能可用Richardson-Zaki方程描述。在一定的膨胀比下,液含率随着轴向高度的增加而增加,存在较明显的不均匀分布。在一定的固定床高度下,液体轴向分散系数随着表观液速的增加而增加,轴向分散系数在0.5×10-5~5×10-5m2/s。     

A 3-zone model for protein adsorption kinetics in expanded beds

The adsorption behavior of expanded beds is more complex than that of fixed beds, since the adsorbent particle size, local bed voidage and liquid axial dispersion will vary axially with expanded height. Models applicable to fixed beds maybe not adequately describe the hydrodynamic and adsorption behavior in expanded beds. In this paper, a 3-zone model is developed, in which the model equations are written for the bottom zone, middle zone, and top zone of the column, respectively, and the model parameters, such as the adsorbent particle diameter, bed voidage and liquid axial dispersion coefficient, are zonal values. In-bed breakthrough curves are predicted by the 3-zone model, and tested against literature data for lysozyme adsorption on Streamline SP in an expanded bed.Model parametric sensitivity is analyzed. The effects of film mass transfer resistance, liquid axial dispersion and adsorbent axial dispersion on the breakthrough curves are weaker than that of protein intraparticle diffusion resistance for stable expanded beds. Adsorbent particle size axial distribution and bed voidage axial variation significantly affect in-bed breakthrough curves, therefore, model parameters should not be assigned uniform values over the whole column; instead the model should account for the adsorbent particle size axial distribution and bed voidage axial variation.     

Particle-resolved simulation of packed beds by non-body conforming locally refined orthogonal hexahedral mesh

The traditional fixed-bed reactor design is usually not suitable for the low tube-to-particle diameter ratios (N=D/d < 8) where the local phenomena of channeling near the wall and backflow in the bed are dominant. The recent "solid particle" meshing method is too complicated for mesh generation, especially for non-spherical particles in large random packed beds, which seriously hinders its development. In this work, a novel high-fidelity mesh model is proposed for simulation of fixed bed reactors by combining the immersed boundary and adaptive meshing methods. This method is suitable for different shapes of particles, which ingeniously avoids handling the complex "contact point" problem. Several packed beds with two different shapes of particles are investigated with this model, and the local flow in the bed is simulated without geometrical simplification. The predicted pressure drop across the fixed bed and heat transfer of the single particle are in good agreement with the corresponding empirical relations. Compared with spherical particles, the packed bed packing with pentaphyllous particles has lower pressure drop and better heat/mass transfer performance, and it shows that this method can be used for the screening of particle shapes in a fixed bed.     

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。     

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.     

一个气体在液体中溶解度的关联式

基于两个假设,本文提出了一个气体在液体中的溶解度随温度、压力变化的关联式。用该式关联H_2-H_2O、N_2-H_2O、He-H_2O、C_2H_5-H_2O体系以及外推至低压,都得到满意的结果。     

60米高循环流化床内物料浓度分布的冷态试验

利用电厂循环流化床锅炉现有的结构和设备, 搭建提升管高度60m、内径400mm的超高循环流化床冷态实验台, 重点研究了流化风速和颗粒密度对提升管内轴向和径向空隙率分布的影响。实验结果表明:空隙率分布形式与流化风速和物料密度密切相关, 对于一定的床料高度, 在底部密相区一直有床料堆积的情况下, 随着流化风速的增加, 提升管底部密相区空隙率增大, 上部稀相区的空隙率减小并且其在径向的分布变得更加不均匀;在一定的流化风速下, 密度较小的物料将更多的被带入上部稀相区, 上部稀相区的空隙率减小, 其在径向分布将变得更加不均匀。     

湍动流化床过渡段固含率分布特征的实验及数值模拟

在湍动流化床中,过渡段对于包括甲醇制烯烃在内的气固催化快反应有着重要的作用。采用PV6D反射型光纤探针对内径95 mm的湍动流化床内过渡段的固含率分布和脉动参数进行了测量,分别考察了表观气速和静床高的影响,并采用修正的基于颗粒动力学的三段曳力双流体模型进行模拟。实验表明,湍动流化床过渡段中固含率的轴向分布呈现S型和指数型两种类型,固含率轴向与径向分布都在过渡段内出现最大梯度,表明过渡段中固体浓度分布比稀相段和密相段更不均匀。表观气速和静床高的变化将导致S型和指数型分布的相互转变,并且对过渡段底部与壁面附近的固体高浓度区影响最为显著。局部固含率脉动概率密度分布表明,在静床高较小时,随着气速的增大,床层下部气含率最大值位置将从中心区移动至环隙区,呈现气含率的双峰型分布。本文提出的修正三段曳力模型考虑了颗粒团聚的影响,对过渡段中分布板影响区之外的固含率分布均能较好地模拟。     

Research and development of advanced structured packing in a rotating packed bed

As the core component of the rotating packing bed, packing is a place for efficient gas–liquid mixing and mass transfer. In this paper, a 3D structured packing composed of a mesh structure and a support structure was designed. The mesh structure is a ring-shaped mesh surrounded by triangular meshes, which is stable in structure and can achieve a high degree of dispersion and aggregation of the liquid phase. The support structure is composed of ring-shaped structural units arranged at a certain a...