循环流化床内颗粒的加速作用及气固两相间的动量交换

根据对气固两相流动特性的分析,表明在循环流化床提升管内,颗粒的加速运动及气固两相间的动量交换对床层各项参数具有明显的影响。由于颗粒聚集,减小了气固两相间的相互作用,使曳力系数小于单颗粒标准曳力系数(C_D/C_(DS)<1.0)。从而使颗粒具有较长的加速段,其加速压降在总压降中所占的分率不可忽略。考虑颗粒加速作用及由于颗粒聚集使气固两相相互作用力减小的事实后,用一维轴向稳态流动的模型可以获得截面平均空隙率、平均颗粒速度以及曳力系数等参数及其轴向分布规律,并与实验直接测定结果吻合很好。通过对大量数据的分析,表明C_D/C_(DS)可用下述经验关联式表示:C_D/C_(DS)=1.68(ε)~(0.253)(Re_(?)/Re_t)~(-1.213)(d_P/D)~(0.105)     

微裂缝内液固两相流动特性研究

应用欧拉-拉格朗日模型,对微裂缝中液-固两相流动过程进行数值模拟。在数学模型中,液相采用连续相模型,颗粒当做离散分散在连续相中,并考虑了相间耦合作用。对于物理模型,建立了二维直微裂缝模型,研究裂缝的液相粘度、液相速度和颗粒密度等因素对裂缝中两相流动的影响。模拟得到了微通道内颗粒瞬时分布状态、颗粒运动轨迹、颗粒停留时间、颗粒轴向速度分布、液相轴向速度分布等参数的变化规律,定性的揭示了储层裂缝内液固两相流动过程。     

提升管内气固流动行为的数值模拟

应用计算流体力学软件Fluent,对空气为连续相、固相为催化裂化反应催化剂的循环流化床提升管内的气固流动行为进行模拟。采用用户自定义函数引入颗粒与壁面的恢复系数和颗粒的镜面反射系数,对颗粒在边壁处的部分滑移运动进行描述。采用不同的计算动力学模型及参数,数值模拟了径向颗粒浓度、轴向床层压降的空间分布,以及用以描述颗粒脉动动能的颗粒温度与固含率的关系,并与文献报道的实验和数值模拟结果进行对比分析。结果表明,选取的颗粒动力学理论模型及参数、颗粒部分滑移边界条件及气固曳力模型,可计算得到合理的颗粒轴向及径向分布,验证了提升管中存在典型的径向环核流动结构和轴向压降分布。进一步分析表明固含率显著影响颗粒温度,当固含率为0.05~0.1,颗粒温度存在转折区。     

基于不同亚格子尺度过滤模型下提升管内颗粒流动的数值模拟

固相亚格子尺度过滤模型是在高精度的网格下,系统地过滤了基于结合颗粒动理学的双流体模型的模拟结果而得到的曳力和固相应力等本构关系的计算模型。今分别采用固相亚格子尺度过滤模型(Filtered Model I)、壁面修正固相亚格子尺度过滤模型(Filtered Model II)和改进的固相亚格子尺度过滤模型(Filtered Model III)模拟NETL/PSRI挑战问题中的提升管内的颗粒流动特性,得到了时均气体压力梯度和时均轴向颗粒速度等分布。亚格子尺度模型和均匀流动模型(Huilin-Gidaspow model)的研究结果相比,改进的固相亚格子尺度过滤模型(Filtered Model III)与实验值更接近,尤其是对于高颗粒浓度流动。壁面修正可以提高压力梯度,从时均轴向颗粒速度分布曲线,可以看出在提升管内颗粒流动结构呈现更为明显的环核流动结构;同时,研究了提升管内气体压缩性、壁面修正和计算网格对模拟结果的影响,分析表明气体的可压缩性对提升管内轴向气体压力梯度有影响,在模拟计算时考虑气体的压缩性,可以提高计算精度。     

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。     

旋流燃烧室内气体-颗粒两相湍流流动的数值模拟

综合应用代数Reynolds应力模型和流体相脉动速度大小和方向均具有随机性的颗粒相随机轨道模型,对旋流燃烧室内有直流射流与旋转射流相互作用的气-固两相湍流流动进行了数值模拟.得到的气相轴向与切向速度和轴向脉动速度均方根值分布以及颗粒相轴向总质量流通量和轴向与切向速度分布与实验基本相符合,并比对气相湍流采用k-ε模型的相应计算结果有较明显的改进.     

气－固并流上行系统的环／核内循环流动结构模型

提出了一个概念合理、预测性能好的两通道模型，描述垂直向上气固流动系统的环／核内循环流动结构。基于气固相互作用最少及质量、动量守恒原理，确定了气固两相流动状态、计算结果（如预测核心区半径、核心区及环形区内空隙率、气体速度、颗粒速度及其轴向分布等）合理，且与实验结果有良好的一致性。该模型还可以用于预测在两通道内的曳力系数，气、固返混以及两区间质量交换速率等，对反应器设计具有一定参考价值。     

循环流化床多组分颗粒气固两相流动模型和数值模拟

基于稠密气体分子运动论和颗粒动力学,考虑多组分颗粒中颗粒组分与颗粒组分、颗粒组分内颗粒之间的相互作用以及气体与颗粒之间的相互作用,提出多组分颗粒非等温颗粒气固两相流动模型.以颗粒压力、径向分布函数、黏度、颗粒碰撞耗散等耦合各颗粒组分间和颗粒间的相间作用.采用大涡模拟方法模拟气相湍流流动.提出了多组分颗粒的径向分布函数计算方法.对循环流化床上升管中双组分颗粒气固两相流动特性进行了数值模拟,模拟结果揭示了上升管中双组分颗粒气固两相流动的环-核流动结构,得到了平均颗粒粒径的轴向和径向分布规律,计算结果与文献中实验结果相吻合.     

下行床气固两相流动计算流体力学模拟

基于颗粒相动力学理论 ,对层流机制表达的气固两相流体力学模型采用Reynolds平均的方法获得气固两相流的湍流模型描述 .其中 ,气相湍流行为以k -ε模型描述 ;颗粒相的碰撞行为以颗粒流的动力学模型表达 ;而湍流行为以kp 模型描述 .因此建立的k -ε -Θ -kp 模型综合考虑了气相和颗粒相的湍流运动以及颗粒的碰撞行为 .依据此模型建立了三维流体力学求解程序并对下行床气固两相流动行为进行了模型预测 .讨论了恢复系数的选取及壁效应假设 ,从机理上分析并考察了 3种模型的预测能力 .针对内径 1 40mm、高 7m的下行床冷态设备 ,在较宽的操作范围内 ,对比了详细的颗粒浓度和速度径向分布以及轴向参数分布 ,并对下行床的放大行为进行了预测 .     

气—固并流上行系统的环／核内循环流动结构模型

提出了一个概念台理，预测性能好的两通道模型，描述垂直向上气固流动系统的环／核内循环流动结构。基于气固相互作用最少及质量，动量守恒原理，确定了气固两相流动状态，计算结果（如预测核心区半径，核心区及环形区内空隙率，气体速度，颗粒速度及其轴向分布等）合理，且与实验结果有良好的一致性。该模型还可以用于预测在两通道内的曳力系数，气，固返混以及两区间质量交换速度率等，对反应器设计具有一定参考价值。     

快速流态化气固两相间的动量交换

本文根据一维定常态流动模型,对快速流化床内气固两相间的相互作用及其机理进行了研究.结果表明,在快速流态化条件下,颗粒总是趋于聚集,以减小气固两相间的相互作用力,从而使曳力系数c_D小于单颗粒标准曳力系数c_(DS)(c_D/c_(DS)<1.0).c_D/c_(DS)不仅与(?)有关,而且受气固流动状况以及颗粒物性、床层直径等因素的影响.通过对大量数据的分析,得到预测曳力系数的经验关联式(平均相对偏差小于5%)c_D/c_(DS)=1.685(?)~(0.253)(Re_r/Re_t)~(-1.213)(d_p/D)~(0.105)     

双流体模型中曳力及恢复系数对气固流动的影响

应用双流体模型CFD模拟的方法,从恢复系数和曳力两方面,研究了气固密相流化床中颗粒之间和气固相之间的相互作用对床内非均匀流动结构形成与变化的影响.计算结果表明颗粒间非弹性碰撞和气固间曳力的增大均使气固两相流动的非均匀性增大.通过比较二者对非均匀流动结构的影响,发现气固间曳力是形成非均匀流动结构的决定因素.从碰撞耗散、颗粒动能和颗粒势能的角度分析了二者的作用机理,发现恢复系数和曳力对流动结构的作用主要区别在于对颗粒团聚和床层膨胀的影响程度不同.     

65 t/h高低差速循环流化床流动特性模拟

对一台65 t/h高低差速循环流化床炉内流动特性进行二维数值模拟。采用基于颗粒动力学理论的欧拉双流体模型来描述气固流动,湍流模型、气固曳力模型和不同粒径颗粒间曳力模型分别采用RNG k-ε per phase模型、Gidaspow模型和Schiller-naumann模型,并应用商业计算流体力学软件Fluent进行数值计算,得到炉内颗粒速度分布、压力分布和颗粒浓度分布,并将压力分布与实测值进行对比。在欧拉双流体模型中分别采用单粒径固相模型和多粒径固相模型,并对模拟结果进行对比分析。结果表明,单粒径固相模型能够较好预测高低差速循环流化床炉内流动特性,为其优化设计、运行及大型化提供了理论依据。     

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.     

Hydrodynamic modelling of dense gas-fluidised beds: comparison and validation of 3D discrete particle and continuum models

A critical comparison of a hard-sphere discrete particle model, a two-fluid model with kinetic theory closure equations and experiments performed in a pseudo-two-dimensional gas-fluidised bed is made. Bubble patterns, time-averaged particle distributions and bed expansion dynamics measured with a nonintrusive digital image analysis technique are compared to simulation results obtained at three different fluidisation velocities. For both CFD models, the simulated flow fields and granular temperature profiles are compared. The effects of grid refinement, particle-wall interaction, long-term particle contacts, particle rotation and gas-particle drag are studied. The mechanical energy balance for the suspended particles is introduced, and the energy household for both CFD models is compared. The most critical comparison between experiments and model results is given by analysis of the bed expansion dynamics. Though both models predict the right fluidisation regime and trends in bubble sizes and bed expansion, the predicted bed expansion dynamics differ significantly from the experimental results. Alternative gas-particle drag models result in significantly different bed dynamics, but the gap between model and experimental results cannot be closed. In comparison with the experimental results, the discrete particle model gives superior resemblance. The main difference between both CFD models is caused by the neglect of particle rotation in the kinetic theory closure equations embedded in the two-fluid model. Energy balance analysis demonstrates that over 80% of the total energy is dissipated by sliding friction. Introduction of an effective restitution coefficient that incorporates the additional dissipation due to frictional interactions significantly improves the agreement between both models.     

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。     

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     

Modification of Ergun equation for application in trickle bed reactors randomly packed with trilobe particles using computational fluid dynamics technique

Based on a slit model, a pellet scale model has been developed for calculation of drag force imposed on trilobe catalyst particles in a packed bed reactor. The drag coefficient for single gas phase flow in a porous media has been calculated by CFD simulation and the results compared to the Ergun equation. The results show that the drag coefficient predicted by Ergun equation should be modified for various bed porosities, particle aspect ratio and gas densities. Therefore, a correction factor has been proposed to correct the Ergun equation constants in various conditions for trilobe particles. Comparison between the proposed corrected Ergun equation results and experimental data indicates considerable agreement.