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。     

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。     

固定床流体流动特征数值模拟

为了研究固定床边壁效应、固定床床层数的变化以及颗粒的填充倾斜角度等参数对床内流体流动状况的影响,基于Ergun方程建立了轴对称多孔介质数学模型。同时对床内流体流动状况进行了研究:在床高确定的情况下,随着床层数的增加,压强降减少;随着颗粒填充倾斜角度的增加,压强降也减少,速度径向分布不均;在固定床边壁附近,气体速度明显增大。计算结果与实验值的比较表明模型能有效地描述固定床压强降和床内流体流动状况。     

烯烃催化裂解反应器局部颗粒堆积结构的颗粒解析模拟

烯烃催化裂解固定床工艺中的反应过程对压力敏感,深入研究催化剂堆积颗粒结构中的流动及压力分布对优化固定床结构及操作参数有重要意义。颗粒解析模拟方法广泛用于固定床内堆积结构的模拟,可以准确描述堆积结构中的流体力学行为,但对于复杂堆积结构网格生成困难。采用基于多孔介质模型的浸入边界法(PMM-IBM)结合网格自适应,实现了对固定床堆积结构的颗粒解析模拟,既解决了网格划分困难的问题,又节省了计算资源。采用网格自适应技术后,与均匀网格相比,堆积结构的网格总数减少大约80%。通过与贴体网格法的单颗粒表面受力分析对比,确定了此浸入边界法的关键模拟参数。随后模拟预测了三种床层与颗粒直径比值条件下堆积结构的空隙率及其内部的压力及流动分布。研究表明,堆积结构空隙中的局部轴向速度的最大值可以达到入口速度的10倍以上,轴向平均速度的径向分布与轴向平均空隙率分布一致,均成震荡衰减趋势。除此之外,预测的床层压降与Reichelt经验关联式结果较为吻合。在此基础上,耦合单颗粒内扩散和烯烃裂解的主反应,预测了反应物随孔径和孔隙率的变化,为进一步考虑外流场的变化奠定了方法基础。     

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。     

CFD方法在固定床反应器传热研究中的应用

固定床反应器是一种常见的化学和生化反应器,由于内部结构十分复杂,固定床内的局部流动和传热过程研究一直是一个颇具挑战性的问题。有效参数法是长期以来固定床传热研究的常用方法,但是由于对固定床内温度场缺乏准确的了解,目前为止许多基于有效参数法的传热模型的普适性仍然存在问题。计算流体力学（CFD）的数值模拟方法是近年来应用于固定床流动和传热研究的一种新的研究方法,它通过数值方法求解流动和传递的微分方程组而获得流场和温度场。CFD数值模拟方法能够提供精确的局部流动和传递信息如速度分布、压力分布、温度分布、组分浓度分布等。本文综述和分析了CFD方法在固定床反应器的流动和传热研究领域的最新进展,讨论了CFD方法在该领域的应用前景。     

固定床中丝状颗粒的传热传质特性

目前对于固定床中丝状填充颗粒传热传质特性的认识仍处于初始阶段。为了能够从颗粒尺度的微观层面揭示丝状颗粒与气体、颗粒与颗粒之间的热、质传递机理,建立了一种丝状颗粒传热传质数学模型,之后将离散单元法与计算流体力学相结合,对实验中较难获得的床层局部流动及传递信息进行了数值模拟,并着重分析比较了气流入口温度以及表观气速等关键因素对固定床中丝状颗粒温度和含水率变化的影响规律。通过模拟结果与实验数据的对比,验证了所建模型的可行性。研究结果表明:同一时刻,固定床中颗粒温度大体上是随着床层高度的增加而降低,含水率则是随着床层高度的增加而升高;气流入口温度对于固定床中丝状颗粒平均温度的提升起着主导作用,而颗粒的传质速度则受表观气速的影响更为明显。     

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。     

水冷螺旋输送机内颗粒的流动特性

水冷螺旋输送机内颗粒的运动对其换热效率起决定性作用。为揭示应用于兰炭行业的水冷螺旋输送机内颗粒的流动特性,本文基于离散单元法(DEM)建立了水冷螺旋输送机的计算模型,从颗粒群力链的角度分析了螺旋转速、填充系数、螺距以及螺旋轴直径对其内部颗粒流动的影响。结果表明:壳体附近颗粒的运动速度因边界抑制作用有所降低。填充系数小于80%时,颗粒的运动以滑移为主;填充系数大于80%时,颗粒的运动以旋转为主。满填充状态下,螺距与螺旋直径之比小于2/3时,颗粒的运动以旋转为主;螺距与螺旋直径之比大于2/3时,颗粒的轴向运动逐渐占据主导地位。质量流率随螺旋转速和螺距的增加而增加,随螺旋轴直径的增加而降低。质量流率对高于80%填充系数的敏感程度高于低填充系数的敏感程度。     

填充沸石的大型离子交换柱内流场模拟

针对沸石法海水富钾工程化放大中的关键问题,采用Fluent中多孔介质模型对填充沸石的大型离子交换柱内流场进行了模拟研究。主要考察了沸石的孔隙率和颗粒直径、海水进口速度、沸石柱长径比以及柱内形成的沸石层凹坑等条件对离子交换柱内流体流动的影响,结果发现：孔隙率和颗粒直径直接关系到沸石层内流体的速度分布,海水进口速度、沸石柱长径比等条件对沸石层内的流体流动并无显著影响;而在孔隙率和颗粒直径确定的条件下,由于海水冲击形成的凹坑对沸石层内流体影响较大,会在凹坑两侧形成滞留区,进而影响沸石对海水中钾的吸附。根据模拟结果优化了填充材料、设备结构以及工艺等参数,为沸石法海水富钾关键装备进一步扩大规模提供了理论支撑。     

小直径比固定床壁效应的CFD分析

引言固定床作为反应器、分离器、换热器等单元设备广泛应用于化工、能源、环境等许多领域。因用途的不同固定床的管径-粒径比(D/d)的范围很宽(1～1000的数量级)。大热负荷的填充床设备,如强放热的化学反应器、核反应堆的冷却壁管列等,常采用较小的管径-粒径比,以利于热量通过     

不同圆球复合无序堆积床内流动传热数值分析

圆球堆积床内孔隙分布影响其内部流场及温度场分布, 且小管径-球径比堆积床由于壁面限制, 内部孔隙率变化剧烈, 其内部流动和传热不均匀现象明显。针对D/d_p为3的圆球无序堆积床构建了3种非等直径圆球复合堆积结构:径向分层复合堆积、轴向分层复合堆积以及随机复合堆积结构, 并采用DEM-CFD方法建模计算, 从径向及整体角度分析比较不同复合堆积床内流动换热特性及其流场和温度场分布的均匀性。结果表明:孔隙率及孔隙大小分布共同影响堆积床内流场和温度场分布;相对于单一等直径圆球堆积, 采用复合堆积结构能使堆积床内部孔隙率分布更均匀, 其内部流场和温度场分布也更为均匀;对于D/d_p为3的堆积通道, 径向分层堆积结构对于提高整体流动换热性能及改善内部流动换热均匀性都有显著效果。     

Experimental and numerical investigation of structure and hydrodynamics in packed beds of spherical particles

In chemical industry, flows often occur in nontransparent equipment, for example in steel pipelines and vessels. Magnetic resonance imaging is a suitable approach to visualize the flow, which cannot be performed with classical optical techniques, and obtain quantitative data in such cases. It is therefore a unique tool to noninvasively study whole‐field porosity and velocity distributions in opaque single‐phase porous media flow. In this article, experimental results obtained with this technique, applied to the study of structure and hydrodynamics in packed beds of spherical particles, are shown and compared with detailed computational fluid dynamics simulations performed with an in‐house numerical code based on an immersed boundary method‐direct numerical simulation approach. Pressure drop and the radial profiles of porosity and axial velocity of the fluid for three packed beds of spheres with different sizes were evaluated, both experimentally and numerically, in order to compare the two approaches. © 2018 The Authors AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers AIChE J, 64: 1896–1907, 2018     

A study of local liquid/solid mass transfer in packed beds under trickling and induced pulsing flow

Induced pulsing flow (by cyclic liquid feeding) in packed beds, operated in the trickling flow regime, is studied as a method of overall improvement of catalytic reactor operation. In this paper results are reported of experiments aimed at determining local and global liquid/solid mass transfer rates, mainly for the so-called fast mode of ON-OFF periodic liquid feeding, with frequencies of order 0.1 Hz. Such mass transfer data for the fast mode of induced pulsing are not available in the literature. Uniform 6 mm glass spheres and alumina cylindrical extrudates, of 1.5 mm diameter and a narrow distribution of lengths, are employed in the tests. For completeness, results are also reported for single-phase (liquid) and trickling flow through the same packed beds. A well-known electrochemical technique is employed to measure instantaneous local mass transfer coefficients by means of quite a few probes distributed throughout the bed. The hydrodynamic characteristics under the above conditions, reported in companion papers, are helpful in interpreting the new mass transfer data.There is a wide spread of the time-averaged local mass transfer rates, in all cases tested, apparently due to packing and flow non-uniformities. This spread is much smaller in the case of packed uniform spheres. In general, the benefits of cyclic liquid feeding are more evident in the packed bed of spheres than in that of cylindrical extrudates; for instance, with increasing mean liquid rate, induced pulsing tends to reduce the spread of local mass transfer coefficients, which suggests that more uniform fluids distribution is promoted. The imposed liquid pulses are reflected in the observed periodic variation of local mass transfer coefficients; the latter appear to decay along the bed in the same manner as the liquid pulses. Other trends of local mass transfer rates are identified and discussed in relation to measured variation of liquid holdup, under the same conditions. For packed spheres, the measured global mass transfer rates are in fair agreement with literature correlations obtained for the trickling flow regime, unlike the case of packed extrudates where significant deviation is observed.     

An experimental model for visual studies of turbulent flow in porous materials

A two-dimensional experimental model has been developed to simulate turbulent flow in porous materials. It uses cross flow through a matrix of cylinders between parallel plexiglass plates. Matrix geometry was chosen to approximate a bed of packed spheres. Pressure loss and axial dispersion in several models have been measured over a wide range of flow rates in the non-Darcy region. Photographs have been taken of flow patterns developed in the transition from laminar to fully turbulent flow. All typical characteristics of flow through beds of packed spheres were exhibited; however, both pressure loss and dispersion differed somewhat when compared to similar three-dimensional flow.     

COMPUTATION OF THREE-DIMENSIONAL FLOW AND HEAT TRANSFER IN GAS AGITATED REACTORS

Equations have been written, and a computational scheme developed, for predicting flow fields and heat transfer phenomena associated with asymmetric gas-driven flows in systems of cylindrical geometry. Cases considered include axisymmetric gas injection wherein the gas-liquid plume is either of constant radius, or expands in a cone-like manner towards the surface. It is, thereby, shown that the model compares reliably with existing axisymmetric computations. Non-central gas flows are then considered and resulting 3-D liquid flow patterns computed. Finally, the temperature fields in an initially stagnant and thermally stratified liquid resulting from the local introduction of gas on the bottom surface is illustrated     

Flow of viscoelastic polymer solutions in mixed beds of particles

New extensive pressure drop-flow rate data are reported for the creeping flow of well characterised viscoelastic polymer solutions through packed beds of mixed size spheres and of spheres and cylinders. It is established that the effective mean surface diameter is adequate in correlating the pressure loss data in flow through packed beds of mixed size and shape. A distinct correlation exists between the excess pressure loss observed with viscoelastic fluids and the Weissenberg number.     

Pore scale inertial flow simulations in 3‐D smooth and rough sphere packs using lattice Boltzmann method

Pore‐scale inertial flows in periodic body centered cubic (BCC) arrays of smooth and rough sphere packs were simulated using lattice Boltzmann method. Computed velocity fields were visualized and averaged to calculate macroscopic flow parameters characteristic of porous media such as permeability, tortuosity, and β factor as well as the transition Reynolds number values and compared well with established correlations. Furthermore, hemispherical depositions on the smooth spheres in the regular BCC array were used to calculate roughness induced changes in macroscopic flow parameters. As the next step toward simulating inertia dominated flow in natural porous media, simulations were validated for low Reynolds number flow in a three‐dimensional (3‐D) CT image of irregular pack of uniform diameter spheres. This work aims to define 3‐D canonical studies for roughness induced inertial flows in porous media and to assess the capability of LBM for simultaneous prediction of absolute permeability and β factor. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4858–4870, 2013