基于EMMS介尺度模型的双分散鼓泡流化床的模拟

双分散气固鼓泡流化床中颗粒通常具有不同粒径或密度,导致产生颗粒偏析等现象,影响传递和反应行为。颗粒分离和混合与气泡运动密不可分,其中相间曳力起关键作用。最近Ahmad等提出了一种基于气泡结构的双分散介尺度曳力模型,能成功预测双分散鼓泡流化床的床层膨胀系数。本研究耦合该曳力模型与连续介质方法,模拟了两种不同的双分散鼓泡流化床,通过分析不同流化状态下的气泡运动、颗粒浓度比的轴向分布等参数,进一步检验模型的适用性。研究表明,当双分散颗粒处于完全流化状态时,耦合双分散介尺度曳力模型可合理预测不同颗粒的分离现象;而其处于过渡流化状态时,新曳力模型和传统模型均无法获得合理结果,此时调节固固曳力可改进模拟结果。     

基于EMMS介尺度模型的双分散鼓泡流化床的模拟

双分散气固鼓泡流化床中颗粒通常具有不同粒径或密度,导致产生颗粒偏析等现象,影响传递和反应行为。颗粒分离和混合与气泡运动密不可分,其中相间曳力起关键作用。最近Ahmad等提出了一种基于气泡结构的双分散介尺度曳力模型,能成功预测双分散鼓泡流化床的床层膨胀系数。本研究耦合该曳力模型与连续介质方法,模拟了两种不同的双分散鼓泡流化床,通过分析不同流化状态下的气泡运动、颗粒浓度比的轴向分布等参数,进一步检验模型的适用性。研究表明,当双分散颗粒处于完全流化状态时,耦合双分散介尺度曳力模型可合理预测不同颗粒的分离现象;而其处于过渡流化状态时,新曳力模型和传统模型均无法获得合理结果,此时调节固固曳力可改进模拟结果。     

分区曳力模型的2D鼓泡流化床流动特性模拟

曳力对两相流动特性具有很大影响,由于颗粒团聚的影响,传统曳力模型不能准确预测气固间曳力。为了提高两相流动特性的模拟精度,文中综合考虑不同曳力模型在特定浓度的曳力系数特性,分区确定曳力模型,并在模型断点处使用光滑函数,基于CFD开源平台OpenFOAM耦合TFM模型对2D鼓泡流化床进行流动特性模拟。结果表明:分区曳力模型床层膨胀率与局部压降预测平均相对误差分别比Gidapow模型降低15%和12.5%,同时可以正确表征气泡融合以及周围颗粒速度方向,宏观流动状态呈现出鼓泡流化床的内循环特性,表明分区曳力模型对鼓泡流化床流动特性模拟具有良好的适用性。     

锥形分布板射流流化床CFD模拟及参数分析

采用双流体模型结合颗粒动力学理论,对锥形分布板射流流化床内气固流动行为进行了三维的计算流体力学(CFD)模拟研究,系统分析了曳力模型、恢复系数和颗粒间摩擦力对射流流化床膨胀高度和气泡动力学行为的影响.结果表明,Syamlal-O'Brien和Gidaspow曳力模型低估了床内实际曳力,从而导致模拟膨胀高度低于实验值,而Modified Syamlal-O'Brien曳力模型能更好地预测床内实际曳力,计算结果与实验值吻合得较好;恢复系数对于射流流化床内气泡动力学行为有着重要影响,气泡的大小、上升速率和产生频率均随着恢复系数的增加而减小;颗粒间的摩擦力也是影响床层膨胀高度和床内气泡产生的重要因素.     

bubble-based EMMS/PFB模型的建立及在加压流化床浓相段的应用

基于多尺度分解和能量消耗分析方法,结合压力下锥形分布板射流床气泡直径关联式,建立了一个适于加压流化床（PFB）的能量最小多尺度模型--bubble-based EMMS/PFB模型。应用此模型模拟一个二维加压射流床,分析了操作压力、位置高度、空隙率及剩余速度对非均匀因子的影响。通过模拟结果与实验数据的对比,发现该模型相比于Gidaspow模型,能够更准确地模拟加压射流床内颗粒浓度的分布状态及颗粒靠近壁面处的速度变化;将这种曳力模型应用到流化床浓相段的模拟,预测了床内颗粒浓度瞬时分布及沿轴向的时均值分布、颗粒的速度分布等流动行为,使流化床浓相段的气固流动行为可视化,对流化床的设计、放大有一定的指导作用。     

B类颗粒在鼓泡流化床中流动特性的数值模拟

采用欧拉双流体模型对鼓泡流化床中气-固两相流动行为进行数值模拟。模拟结果表明,采用结构曳力模型能够较好地预测B类颗粒在鼓泡流化床中的流动行为。对比初始流态化颗粒浓度图和完全流态化颗粒浓度分布图,可以发现结构曳力模型能够较好地展现鼓泡流化床中气泡的运动特性。当比较不同曳力模型下的模拟结果时,结构曳力模型比传统曳力模型能够更好地预测颗粒的径向分布。     

bubble-based EMMS/PFB模型的建立及在加压流化床浓相段的应用

基于多尺度分解和能量消耗分析方法,结合压力下锥形分布板射流床气泡直径关联式,建立了一个适于加压流化床(PFB)的能量最小多尺度模型——bubble-based EMMS/PFB模型。应用此模型模拟一个二维加压射流床,分析了操作压力、位置高度、空隙率及剩余速度对非均匀因子的影响。通过模拟结果与实验数据的对比,发现该模型相比于Gidaspow模型,能够更准确地模拟加压射流床内颗粒浓度的分布状态及颗粒靠近壁面处的速度变化;将这种曳力模型应用到流化床浓相段的模拟,预测了床内颗粒浓度瞬时分布及沿轴向的时均值分布、颗粒的速度分布等流动行为,使流化床浓相段的气固流动行为可视化,对流化床的设计、放大有一定的指导作用。     

网格尺寸对循环流化床内气固两相流动的影响

将基于能量最小多尺度方法(EMMS)的曳力模型耦合到双流体模型中,并针对循环流化床内的气固两流动进行了模拟研究。采用全滑移壁面边界条件处理颗粒相,考察了3种网格尺度对轴向空隙率和出口颗粒循环量等气固流动特性的影响。计算结果表明,应用EMMS曳力模型处理相间作用力,同时在采用全滑移壁面边界条件处理颗粒相时,双流体模型能够正确预测轴向空隙率分布。采用网格尺寸为2.325 mm×20 mm时,模拟结果和实测数据吻合较好,表明在循环流化床的数值模拟中选择恰当的网格尺度是极为重要的。     

颗粒相壁面条件对非球形颗粒流动影响的数值模拟

采用双流体模型对设置竖直隔板的气固密相流化床中非球形颗粒的运动进行了模拟,颗粒形状的影响由相间曳力模型考虑,重点考察壁面处颗粒边界条件的影响。同时进行了实验室规模三维流化床的流化实验,以验证模型的有效性。通过压降轴向分布、颗粒浓度径向分布以及物料出口处颗粒质量流率功率谱估计等定量分析,结果表明：对不设置内构件的自由床,壁面反射系数对系统宏观流动特性影响较小,而对壁面处局部颗粒运动影响较大;对壁面面积大幅增加的内构件床,壁面反射系数可显著改变气体和颗粒的运动特征,取值需控制在适当范围内。     

颗粒相壁面条件对非球形颗粒流动影响的数值模拟

采用双流体模型对设置竖直隔板的气固密相流化床中非球形颗粒的运动进行了模拟,颗粒形状的影响由相间曳力模型考虑,重点考察壁面处颗粒边界条件的影响。同时进行了实验室规模三维流化床的流化实验,以验证模型的有效性。通过压降轴向分布、颗粒浓度径向分布以及物料出口处颗粒质量流率功率谱估计等定量分析,结果表明:对不设置内构件的自由床,壁面反射系数对系统宏观流动特性影响较小,而对壁面处局部颗粒运动影响较大;对壁面面积大幅增加的内构件床,壁面反射系数可显著改变气体和颗粒的运动特征,取值需控制在适当范围内。     

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。     

Particle‐resolved direct numerical simulation of gas–solid dynamics in experimental fluidized beds

Particle‐resolved direct numerical simulations (PR‐DNS) of a simplified experimental shallow fluidized bed and a laboratory bubbling fluidized bed are performed by using immersed boundary method coupled with a soft‐sphere model. Detailed information on gas flow and individual particles’ motion are obtained and analyzed to study the gas–solid dynamics. For the shallow bed, the successful predictions of particle coherent oscillation and bed expansion and contraction indicate all scales of motion in the flow are well captured by the PD‐DNS. For the bubbling bed, the PR‐DNS predicted time averaged particle velocities show a better agreement with experimental measurements than those of the computational fluid dynamics coupled with discrete element models (CFD‐DEM), which further validates the predictive capability of the developed PR‐DNS. Analysis of the PR‐DNS drag force shows that the prevailing CFD‐DEM drag correlations underestimate the particle drag force in fluidized beds. The particle mobility effect on drag correlation needs further investigation. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1917–1932, 2016     

Experiment and computational fluid dynamics investigation of biochar elutriation in fluidized bed

In fluidized bed biomass fast pyrolysis, the biomass is converted to biochar and elutriated. The elutriation rate is a key parameter in reactor designs and operations. This research presents a video-based continuous measurement of biochar elutriation rate in a fluidized bed with sands and biomass as bed materials. The fluidized bed is simulated with the computational fluid dynamics—coarse-grained discrete element method (CFD-CGDEM) in MFiX. The fluidization behavior of nonspherical sands can be more accurately captured when a rolling friction model is used. The predicted elutriation rate is close to the experimental measurement when the particle size distributions are considered and the filtered drag with a shape correction is used. These results validated the accuracy of the MFiX-based CFD framework for the prediction of biochar elutriations in the fluidized bed biomass fast pyrolysis reactor.     

A bubble-based EMMS model for gas–solid bubbling fluidization

An EMMS/bubbling model for gas–solid bubbling fluidized bed was proposed based on the energy-minimization multi-scale (EMMS) method (Li and Kwauk, 1994). In this new model, the meso-scale structure was characterized with bubbles in place of clusters of the original EMMS method. Accordingly, the bubbling fluidized bed was resolved into the suspending and the energy-dissipation sub-systems over three sub-phases, i.e., the emulsion phase, the bubble phase and their inter-phase in-between. A stability condition of minimization of the energy consumption for suspending particles (N_s→min) was proposed, to close the hydrodynamic equations on these sub-phases. This bubble-based EMMS model has been validated and found in agreement with experimental data available in literature. Further, the unsteady-state version of the model was used to calculate the drag coefficient for two-fluid model (TFM). It was found that TFM simulation with EMMS/bubbling drag coefficient allows using coarser grid than that with homogeneous drag coefficient, resulting in both good predictability and scalability.     

CFD modeling and validation of the turbulent fluidized bed of FCC particles

An experimental and computational study is presented on the hydrodynamic characteristics of FCC particles in a turbulent fluidized bed. Based on the Eulerian/Eulerian model, a computational fluid dynamics (CFD) model incorporating a modified gas‐solid drag model has been presented, and the model parameters are examined by using a commercial CFD software package (FLUENT 6.2.16). Relative to other drag models, the modified one gives a reasonable hydrodynamic prediction in comparison with experimental data. The hydrodynamics show more sensitive to the coefficient of restitution than to the flow models and kinetics theories. Experimental and numerical results indicate that there exist two different coexisting regions in the turbulent fluidized bed: a bottom dense, bubbling region and a dilute, dispersed flow region. At low‐gas velocity, solid‐volume fractions show high near the wall region, and low in the center of the bed. Increasing gas velocity aggravates the turbulent disorder in the turbulent fluidized bed, resulting in an irregularity of the radial particle concentration profile. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Computationally efficient incorporation of microkinetics into resolved-particle CFD simulations of fixed-bed reactors

A method is developed to couple microkinetics with fluid flow in a fixed bed and transport inside the catalyst particles using computational fluid dynamics. Initially, the microkinetics model is solved for a wide range of different temperatures, and partial pressures. Next, reaction rates are mapped into quadratic multivariate splines. Splines coefficients are then imported into our user-defined function, and consequently the reaction rates are evaluated at each iteration simultaneously with the CFD simulations. This method has been applied to our solid particle model to investigate the effects of fluid flow, transport and elementary reaction steps on each other for ethylene and methanol partial oxidations. Reaction rates of all elementary steps as well as species surfaces sites and compositions are evaluated inside the particle. The suggested method can couple complex reaction mechanisms with detailed CFD simulations without increasing the computational time compared with global kinetics methods.     

A CFD‐PBM‐PMLM integrated model for the gas–solid flow fields in fluidized bed polymerization reactors

Although the use of computational fluid dynamics (CFD) model coupled with population balance (CFD‐PBM) is becoming a common approach for simulating gas–solid flows in polydisperse fluidized bed polymerization reactors, a number of issues still remain. One major issue is the absence of modeling the growth of a single polymeric particle. In this work a polymeric multilayer model (PMLM) was applied to describe the growth of a single particle under the intraparticle transfer limitations. The PMLM was solved together with a PBM (i.e. PBM‐PMLM) to predict the dynamic evolution of particle size distribution (PSD). In addition, a CFD model based on the Eulerian‐Eulerian two‐fluid model, coupled with PBM‐PMLM (CFD‐PBM‐PMLM), has been implemented to describe the gas–solid flow field in fluidized bed polymerization reactors. The CFD‐PBM‐PMLM model has been validated by comparing simulation results with some classical experimental data. Five cases including fluid dynamics coupled purely continuous PSD, pure particle growth, pure particle aggregation, pure particle breakage, and flow dynamics coupled with all the above factors were carried out to examine the model. The results showed that the CFD‐PBM‐PMLM model describes well the behavior of the gas–solid flow fields in polydisperse fluidized bed polymerization reactors. The results also showed that the intraparticle mass transfer limitation is an important factor in affecting the reactor flow fields. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1717–1732, 2012     

Geldart A类颗粒节涌床气固流动特性的实验及模拟

针对气固节涌床,在实验基础上,基于欧拉?欧拉双流体模型结合颗粒动力学理论,考虑Geldart A类颗粒聚团对气固间曳力的影响,采用修正后的Gidaspow曳力模型对气固节涌床进行数值模拟。结果表明,通过与实验结果及经验公式进行对比,修正的模型可准确合理地模拟流化床内节涌特性。表观气速0.09 m/s≤Ug≤0.39 m/s时,床层内部压力脉动标准偏差随表观气速增加而增加,流型由鼓泡转变为节涌直至节涌程度最大,床内气固流动主要受轴对称栓运动特性影响,床内压降、床层膨胀比、气栓平均上升速度、最大轴对称栓长度随表观气速增加而增加,最大轴对称栓产生位置随表观气速增加而降低;Ug>0.39 m/s后,床内压力脉动标准偏差随表观气速增加而降低,节涌程度降低至向湍动流态化流型转变,床内气固流动主要受壁面栓运动特性影响,增加表观气速,节涌床内压降变化幅度较小,气栓平均上升速度增加幅度加大,床层膨胀比及最大轴对称栓长度降低,最大轴对称栓产生的位置略有升高。     

Experimental and numerical research for fluidization behaviors in a gas–solid acoustic fluidized bed

The effects of sound assistance on fluidization behaviors were systematically investigated in a gas–solid acoustic fluidized bed. A model modified from Syamlal–O'Brien drag model was established. The original solid momentum equation was developed and an acoustic model was also proposed. The radial particle volume fraction, axial root‐mean‐square of bed pressure drop, granular temperature, and particle velocity in gas–solid acoustic fluidized bed were simulated using computational fluid dynamics (CFD) code Fluent 6.2. The results showed that radial particle volume fraction increased using modified drag model compared with that using the original one. Radial particle volume fraction was revealed as a parabolic concentration profile. Axial particle volume fraction decreased with the increasing bed height. The granular temperature increased with increasing sound pressure level. It showed that simulation values using CFD code Fluent 6.2 were in agreement with the experimental data. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

CFD study of solids concentration in a fluidised-bed coater with variation of atomisation air pressure

The solids volume fraction inside a tapered fluidised bed coater was simulated with the use of an Eulerian computational fluid dynamics (CFD) model with atomisation nozzle sub-model. The drag force, describing momentum transfer between the gas and solid phases was calculated using the drag model proposed by [1]. In order to account for the particle size distribution of the fluidised solid materials, a 4-phase Eulerian model was used. The model-predicted results for different atomisation air pressures were verified using published experimental data [2]. It was shown that the model proved to be highly sensitive to changes in the fluidisation air flow rate with regard to the model-predicted solids volume distribution.