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。     

梯度磁场下气固流化床中磁颗粒运动数值模拟

在外加竖直方向梯度磁场的气固鼓泡流化床中,考虑铁磁颗粒受到的梯度磁场力和颗粒间磁感应力,对气相采用流体力学方法(CFD),颗粒相采用离散元法(DEM),建立二维磁鼓泡流化床数学模型,模拟不同磁场强度下全磁颗粒圆形床料的气固流动过程,分析了不同磁场强度对磁流化床中气泡生长、颗粒运动、床层压降和磁颗粒受力的影响。研究结果表明在沿高度磁感应强度递减的梯度磁场中,磁颗粒在颗粒间磁感应力的作用下凝聚成链,破坏了大气泡的形成。随着磁场强度增加,颗粒扩散系数减小,颗粒间磁感应力和梯度磁场力增大;气体与颗粒相间作用力先减小、后增加;而颗粒接触力先增加、后减小。     

基于气泡和聚团的结构模型及其应用

综合考虑鼓泡流化床内气泡及聚团对床内细颗粒流动的影响,建立基于气泡和聚团的结构曳力模型及结构参数模型,同时,借助计算流体软件预测细颗粒在鼓泡床中流动状态。首先,基于细颗粒在鼓泡流化床的流动状态,在介观尺度上将床内气固流动结构划分为3个子结构,即气泡相、相间相及乳化相(聚团相)。然后,综合考虑细颗粒鼓泡流化床中气泡和聚团对气固流动的影响,根据力平衡、质量守恒建立基于气泡和聚团的结构参数模型及结构曳力模型。通过对结构参数模型封闭求解,得到11个结构参数值(f_b,U_b,d_b,U_(gb),f_i,U_(gi),ε_i,f_e,U_(ge),ε_e,d_c)。对结构参数计算结果进行分析,结构参数模型能够很好反映床内流动情况及床内各结构参数之间的关系,并能有效地预测颗粒聚团直径。此外借用非均匀因子,耦合结构曳力模型及结构参数模型到欧拉双流体模型对气固在床内流动行为进行数值模拟。模拟结果表明,使用基于气泡和聚团的结构曳力模型能够较好地预测细颗粒在鼓泡流化床中的流动行为。在对模拟结果中颗粒径向浓度比较时,可以发现,相对比基于气泡模型的结构曳力模型,使用基于气泡和聚团的结构曳力模型的模拟结果与实验结果更一致。     

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。     

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。     

气固鼓泡床结构双流体模型及其模拟验证

气固鼓泡流化床因具有较好的传热传质特性已被广泛应用于工业生产,而气泡这类非均匀结构普遍存在于流化床中,它显著影响流化床内动量、能量和质量传递以及化学反应过程。合理描述非均匀结构与三传一反的定量关系对提高连续介质模型模拟的准确性至关重要。结构双流体模型在控制方程及本构关系方面均考虑气固系统内非均匀特性的影响,是一种逻辑自洽、完备地考虑了介尺度结构的全新连续介质模型;本研究拓展了结构双流体模型应用于鼓泡流化床的数值模拟,在构造控制方程时将系统划分成颗粒主导的乳化相和气体主导的气泡相这两类相互渗透的连续流体,同时构造本构关系时涉及的气泡直径、乳化相固含率及黏度等均考虑非均匀结构影响。模拟结果表明,结构双流体模型可成功预测鼓泡床系统内的气固流动特性,同时确定气泡直径影响稀/密相相间相互作用,对模拟结果影响显著。     

外加电场辅助流化床的研究进展

流化床反应器中静电的存在严重影响着反应器的长周期操作,因此综述了外加电场对流化的影响。总结了外加电场流化床的研究历史,分析了外加电场对气泡行为和颗粒运动的影响以及作用机理,介绍了外加电场辅助纳米流化的最新进展。外加电场有可能实现破碎聚团、强化纳米流化的目的,该结果可为外加电场流化床的发展提供指导。     

循环流化床中颗粒聚团特性的模拟

考虑到循环流化床中分散颗粒和颗粒聚团同时存在的多尺度结构,确定了密相和稀相加速度与计算网格局部参数之间的关系,建立了多尺度曳力消耗能量最小的稳定性条件,基于双变量极值理论,构建了考虑颗粒团聚效应的多尺度气固相间曳力模型。结合双流体模型,对循环流化床内气固流动特性以及颗粒聚团特性进行了模拟研究。通过与实验值比较,考虑颗粒聚团影响的计算模型可以更好地贴近实验结果,颗粒聚团直径随颗粒浓度增大呈现先增大后减小的分布趋势,气体和颗粒的加速度在模拟中与重力加速度同处一个数量级,求解过程中不能被忽略。     

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     

Effective particle diameters for simulating fluidization of non‐spherical particles: CFD‐DEM models vs. MRI measurements

Computational fluid dynamics—discrete element method (CFD‐DEM) simulations were conducted and compared with magnetic resonance imaging (MRI) measurements (Boyce, Rice, and Ozel et al., Phys Rev Fluids. 2016;1(7):074201) of gas and particle motion in a three‐dimensional cylindrical bubbling fluidized bed. Experimental particles had a kidney‐bean‐like shape, while particles were simulated as being spherical; to account for non‐sphericity, “effective” diameters were introduced to calculate drag and void fraction, such that the void fraction at minimum fluidization (ε_mf) and the minimum fluidization velocity (U_mf) in the simulations matched experimental values. With the use of effective diameters, similar bubbling patterns were seen in experiments and simulations, and the simulation predictions matched measurements of average gas and particle velocity in bubbling and emulsion regions low in the bed. Simulations which did not employ effective diameters were found to produce vastly different bubbling patterns when different drag laws were used. Both MRI results and CFD‐DEM simulations agreed with classic analytical theory for gas flow and bubble motion in bubbling fluidized beds. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2555–2568, 2017     

Experimental and numerical study on a bubbling fluidized bed with wet particles

As liquid bridge between particles acts an important role in the particle system, it is of considerable significance to analyze the flow hydrodynamics of wet particles in fluidized beds, which will improve the reactor design and process optimization. Thus, experimental and numerical investigations on wet particles in a bubbling fluidized bed are conducted in current work. On experimental side, particle image velocimetry (PIV) technology is employed with a designed bubbling fluidized bed. The silicone oil is used in this work because it is nonvolatile and transparent. On numerical side, a modified discrete element method (DEM) numerical method is developed by compositing an additional liquid‐bridge module into the traditional soft‐sphere interaction model. Most of the physical parameters are chosen to correspond to the experimental settings. Good agreements of particle velocity are found between the DEM simulation and PIV measurement. The performance of different liquid contents and superficial gas velocities are examined. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1970–1985, 2016     

Heat transfer within dynamically structured bubbling fluidized beds subject to vibration: A two-fluid modeling study

Bubbling fluidized beds are often used to achieve a uniform particle temperature distribution in industrial processes involving gas and particles. However, the chaotic bubble dynamics pose significant challenges in scale-up. Recent work (Guo et al., 2021, PNAS 118, e2108647118) has shown that using vibration can structure the bubbling pattern to a highly predictable manner with the characteristic bubble properties independent of system width, opening opportunities to address key issues associated with conventional bubbling fluidized beds. Herein, using two-fluid modeling simulations, we studied heat transfer characteristics within the dynamically structured bubbling fluidized bed and compared to unstructured bubbling fluidized beds and packed beds. Simulations show that the structured bubbling fluidized bed can achieve the most uniform particle temperature distribution because it can achieve the best particle mixing while maintaining a global heat transfer coefficient similar to that of a freely bubbling fluidized bed.     

Heterogeneity analysis of gas-solid flow hydrodynamics in a pilot-scale fluidized bed reactor

Mesoscale drag model is of crucial significance for the reliability and accuracy in coarse-grid Eulerian-Eulerian two-fluid model (TFM) simulations of gas-solid flow hydrodynamics in fluidized bed reactors. Although numerous mesoscale drag models have been reported in the literature, a systematic comparison of their prediction capability from the perspective of heterogeneity analysis is still lacking. In this study, in order to investigate the effect of several typical drag models on the hydrodynamic behaviors, the nonuniformity analysis and the sensitivity to material properties, extensive coarse-grid TFM simulations of a bubbling pilot-scale fluidized bed reactor are carried out. The results demonstrate that the mesoscale drag models outperform the empirical drag model in terms of nonuniformity due to the consideration of the influence of the mesoscale structures on the drag force in the bubbling region. Furthermore, the results reveal that our previously developed three-marker gradient-based drag model considering the solid concentration gradient exhibits satisfactory performance in predicting the bubbling flow hydrodynamics. Besides, the material-property-dependent drag model considering the explicit effect of material properties on drag corrections is most sensitive to the particle diameter. This work provides guideline for possible future improvements of mesoscale models to simulate gas-solid flow more accurately and universally.     

Effect of a dispersed immiscible liquid phase on the hydrodynamics of a bubble column and ebullated bed

Secondary undesired reactions in ebullated bed resid hydroprocessors can generate an additional dispersed liquid phase, referred as mesophase, which is denser and more viscous than the continuous liquid phase and affects the operation and transport phenomena of the fluidized bed. This study investigates the effect of a dispersed immiscible liquid phase on the overall phase holdups, bubble properties, and fluidization behavior in a bubble column and ebullated bed. The experimental system consisted of biodiesel as the continuous liquid phase, glycerol as the dispersed liquid phase, 1.3 mm diameter glass beads, and nitrogen. The addition of dispersed glycerol reduced the gas holdups in the bubble column for the studied gas and liquid superficial velocities. Dynamic gas disengagement profiles reveal a rise in the large bubble population and reductions to the small and micro bubble holdups when increasing the glycerol concentration. Liquid–liquid–solid bed expansions at various liquid flowrates confirm particle agglomeration in the presence of a more viscous dispersed liquid phase. Overall phase holdups in a gas–liquid–liquid–solid ebullated bed were obtained while varying the gas and liquid flowrates as well as the glycerol concentration. A coalesced bubble flow regime was observed in the bed region without glycerol whereas the addition of glycerol resulted in the dispersed bubble flow regime due to particle clustering and a greater apparent particle size. The resulting bubble flow regime increased the bed and freeboard region gas holdups due to enhanced bubble break-up. Observations of the fluidized bed behavior following the addition of the dispersed glycerol are also discussed.     

介尺度视角下干法重介流态化分选过程强化

煤炭在保障我国能源安全中具有重要作用。干法重介流态化分选是煤炭分选加工领域的重要组成部分,有助于推动我国煤炭资源的高效洁净利用。流态化分选密度的稳定调控是实现高效分选的必要条件,调控的核心是如何削弱气泡扰动作用,关键是理解流态化分选过程中介尺度结构的演变及调控机制。从介尺度视角分析了气固流态化干法分选调控过程的关键科学问题,梳理了单元干法分选设备以及系统放大过程中介尺度结构的研究进展,分析了介尺度结构的演变规律,提出了介尺度结构的精准调控策略,对干法流态化的工业推广应用及煤炭的分选提质具有重要意义。