流化床密相区颗粒扩散系数的CFD数值预测

应用离散颗粒模型直观获得颗粒运动情况,并从单个颗粒和气泡作用的角度分析颗粒运动和混合,证实气泡在床层中上升、在床层表面爆破以及气泡上升引起的乳化相下沉运动对颗粒混合起关键作用。应用基于颗粒动理学的双流体模型系统地对床宽分别为0.2、0.4、0.8 m的二维流化床在鼓泡区和湍动区的气固两相流动行为进行数值模拟。受离散颗粒模型启发,在双流体模型计算结果基础上,引入理想示踪粒子技术计算床内平均颗粒扩散系数。计算结果表明,颗粒横向扩散系数(D_x)总体上随流化风速增大而增大,但受床体尺寸影响较大;颗粒轴向扩散系数随流化风速增大而增大,受床体尺寸影响较弱。文献报道的密相区颗粒横向扩散系数分布在10^-4～10^-1 m²·s^-1数量级。本文提出的计算方法在数量级上与文献实验结果吻合,表明在大尺寸流化床且高流化风速下,颗粒横向扩散系数远大于小尺寸鼓泡流化床,为不同研究者实验结果的分歧提供了理论依据,也为预测大型流化床内颗粒扩散速率提供了放大策略。     

采用切圆射流及旋流布风板改进鼓泡床行为

用侧面切圆射流和旋流布风板造成了鼓泡流化床内气固流动的新流型。本文将改进鼓泡床与普通鼓泡床进行比较,在同样的空气量下,气泡尺寸减小,床面波动降低,加强了固体颗粒的横向混合,延长了颗粒床内停留时间。还研究了侧面切圆射流占总风量的份额、射流速度、喷入位置及角度对流化质量的影响。     

组合约束型提升管出口气固分布器的压降

在一套组合约束型提升管冷态实验装置上,通过实验研究了不同操作条件下提升管出口气固分布器的压降,并与常规气体分布器压降进行了对比。实验结果表明,在零床层及有床层的操作模式下,气固分布器压降均随提升管内表观气速和颗粒循环强度的增加而增大,在颗粒循环强度较低时,气固分布器压降曲线变化的斜率随着表观气速的增加而增大,在颗粒循环强度较高时,气固分布器压降曲线变化的斜率随着表观气速的增加而减小;随着开孔率及上部流化床层压降增加,气固分布器压降呈降低趋势,当流化床层压降达到一定程度后,分布器各孔方可实现有效布气,此后气固分布器压降趋于近似不变;在相同表观气速及开孔率下,气固分布器压降大于常规气体分布器压降。     

Convective solids transport in large diameter gas fluidized beds

It is demonstrated that the convective solids transport occurring in large diameter gas fluidized beds can be predicted quantitatively on the basis of measured properties of the bubble phase. Based on the fundamental findings of Rowe and co-workers [5], who have shown the solids mixing in gas fluidized beds for particle diameters greater than 100 μm to be caused solely by the action of rising bubbles, an equation has been derive from extensive measurements of the bubble development in a 1 m diam. fluidized bed of quartz sand which relates the convective solids mass flow due to solids transport in the bubble wakes to easily determinable parameters. The predictions of this relationship are found to bein good agreement with direct measurements of the convective solids transport carried out by Schmalfeld [21] on a pilot scale in a semicylindrical bed of 0.8 m diam.     

竖直管气固鼓泡流化床传热机理的CPFD模拟

针对细颗粒气固鼓泡流化床中床料与竖直传热管壁面间的传热行为,在前期实验的基础上,采用计算颗粒流体力学(CPFD)方法从颗粒在传热壁面更新的角度,深入分析了传热特性与壁面气固流动行为之间的关联性。结果表明,模拟得到的传热管壁面颗粒更新通量和基于颗粒团更新模型的颗粒团平均停留时间均能很好解释实验测得的传热系数变化规律,这证实颗粒团更新是影响传热过程的控制性因素。模拟还发现随加热管从床层中心向边壁的移动,加热管周向方向上颗粒更新通量和传热系数的不均匀性都呈增大趋势。随着表观气速的增大,气泡行为导致床层颗粒内循环流率增大,这是导致颗粒团在加热管壁面上的更新频率增大以及床层与壁面间传热系数增大的根源。     

静电效应对有无埋管气固鼓泡床内气泡特性的影响分析

采用基于双流体模型(TFM)耦合静电模型的方法,研究颗粒的静电对有无埋管气固鼓泡床内气固流动特性和气泡特性的影响。首先在无静电场存在的条件下,利用双流体模型对自由鼓泡床和埋管鼓泡床内的流动情况进行模拟并与实验结果进行对比;进一步耦合静电模型,考察静电对自由鼓泡床和埋管鼓泡床内床层的整体性质和气泡特性的影响。研究结果表明,在无静电场条件下采用双流体模型能较好地预测自由鼓泡床和埋管鼓泡床内的气固流动状况以及气泡的平均直径和气泡的上升速度。埋管的存在使鼓泡床内气固流动发生强烈扰动,并使气泡的平均直径和气泡的上升速度均呈振荡分布。静电的存在对自由鼓泡床和埋管鼓泡床内床层的平均固含率影响不大,但对气泡分布规律影响较大,使得自由鼓泡床内气泡数目减少,而埋管鼓泡床下部区域的气泡分布比较集中,上部有大气泡出现。     

A novel technique for particle tracking in cold 2-dimensional fluidized beds—simulating fuel dispersion

This paper presents a novel technique for particle tracking in 2-dimensional fluidized beds operated under ambient conditions. The method is applied to study the mixing mechanisms of fuel particles in fluidized beds and is based on tracking a phosphorescent tracer particle by means of video recording with subsequent digital image analysis. From this, concentration, velocity and dispersion fields of the tracer particle can be obtained with high accuracy. Although the method is restricted to 2-dimensional, it can be applied under flow conditions qualitatively resembling a fluidized-bed combustor. Thus, the experiments cover ranges of bed heights, gas velocities and fuel-to-bed material density and size ratios typical for fluidized-bed combustors. Also, several fluidization regimes (bubbling, turbulent, circulating and pneumatic) are included in the runs.A pattern found in all runs is that the mixing pattern of the tracer (fuel) solids is structured in horizontally aligned vortexes induced by the bubble flow. The main bubble paths always give a low concentration of tracer solids and with the tracer moving upwards, while the downflow of tracer particles in the dense bottom bed is found to take place in zones with low bubble density and at the sidewalls. The amount of bed material (bed height) has a strong influence on the bottom bed dynamics (development and coalescence of bubbles) and, consequently, on the solids mixing process. Local dispersion coefficients reach maximum values around the locations of bubble eruptions, while, in the presence of a dense bottom bed, an increase in fluidization velocity or amount of bed material enhances dispersion. Dispersion is found to be larger in the vertical than in the horizontal direction, confirming the critical character of lateral fuel dispersion in fluidized-bed combustors of large cross section.     

微型流化床内混合特性的数值模拟

微型流化床反应分析仪是中国科学院过程工程研究所研制的具有等温微分反应特性,且适合于气固反应分析的新仪器。细微样品与高温流化介质的瞬间混合是该仪器实现等温微分的必要条件。针对如何满足该要求,基于欧拉多流体模型对连接不同进样器的微型反应器本体进行了三维数值模拟,得到了不同喷口结构和位置下的流动图景及混合区浓度的相对标准偏差曲线,定量表征了各种进样器的混合质量。同时采用高速摄像手段获得了冷态实验中颗粒流动的快照,验证了模拟计算结果的可靠性。模拟结果对脉冲射流微量进样器结构的优化提出了如下建议：进样细管应避免采用弯角喷口,弯角结构会导致脉冲进样载流气喷出方向与流化气流相逆,使得细微颗粒试样堆积滞留,影响混合效果。     

埋管流化床颗粒流动行为的数值模拟

提出区域覆盖法,实现了正交结构化网格(DEM网格)与非规则网格(Fluent网格)的快速耦合,通过减少颗粒搜索量的方法,提高计算效率;采用向量法高效、精确定位颗粒在非规则网格内的位置,进而形成基于非规则网格的CFD-DEM数值模拟平台。在宽和高分别为250 mm和800 mm的二维多埋管流化床内对颗粒流动行为进行了数值模拟研究。结果表明:基于区域覆盖法建立的CFD-DEM模型能够模拟具有复杂几何结构流化床内的气固流动行为;多埋管流化床内的颗粒上升流磨损埋管的迎风面,在背风面形成颗粒浓度较低的区域;而颗粒下降流对埋管的影响与上升流的影响相反;多埋管流化床内的颗粒行为可划分为颗粒上升流、颗粒下降流、颗粒循环流和无颗粒区。     

Mixing dynamics in bubbling fluidized beds

Solids mixing affects thermal and concentration gradients in fluidized bed reactors and is, therefore, critical to their performance. Despite substantial effort over the past decades, understanding of solids mixing continues to be lacking because of technical limitations of diagnostics in large pilot and commercial‐scale reactors. This study is focused on investigating mixing dynamics and their dependence on operating conditions using computational fluid dynamics simulations. Toward this end, fine‐grid 3D simulations are conducted for the bubbling fluidization of three distinct Geldart B particles (1.15 mm LLDPE, 0.50 mm glass, and 0.29 mm alumina) at superficial gas velocities U/Umf = 2–4 in a pilot‐scale 50 cm diameter bed. The Two‐Fluid Model (TFM) is employed to describe the solids motion efficiently while bubbles are detected and tracked using MS3DATA. Detailed statistics of the flow‐field in and around bubbles are computed and used to describe bubble‐induced solids micromixing: solids upflow driven in the nose and wake regions while downflow along the bubble walls. Further, within these regions, the hydrodynamics are dependent only on particle and bubble characteristics, and relatively independent of the global operating conditions. Based on this finding, a predictive mechanistic, analytical model is developed which integrates bubble‐induced micromixing contributions over their size and spatial distributions to describe the gross solids circulation within the fluidized bed. Finally, it is shown that solids mixing is affected adversely in the presence of gas bypass, or throughflow, particularly in the fluidization of heavier particles. This is because of inefficient gas solids contacting as 30–50% of the superficial gas flow escapes with 2–3× shorter residence time through the bed. This is one of the first large‐scale studies where both the gas (bubble) and solids motion, and their interaction, are investigated in detail and the developed framework is useful for predicting solids mixing in large‐scale reactors as well as for analyzing mixing dynamics in complex reactive particulate systems. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4316–4328, 2017     

Modeling of Shallow Fluidized Bed Reactors: I. analytic solution of a simplified model

A fairly general dynamic model for shallow fluidized bed reactors is developed, and analytic solutions for the governing equations of the model are obtained after some simplification. The bubble size, the lateral dispersion coefficient of solids, the level of the excess fluid reactant and the structure of the bed are examined to determine their effects on the conversion of solids. It has been shown that the conversion of solids is influenced profoundly by the bubble size and that it is advantageous to employ a shallow fluidized bed reactor if a high conversion of solids is required.     

MAGNETICALLY STABILIZED FLUIDIZATION PART I GAS AND SOLIDS FLOW

A fluidized bed of magnetic particles, such as iron or magnetite, can be stabilized by applying an external magnetic field, as was shown earlier by Rosensweig and coworkers. The stabilization results in a suppression of bubble formation, little solids mixing and a much narrower residence time distribution of the gas flow; the gas flow rate in the dense phase is increased. In this experimental study the axial and radial mixing coefficients in the gas flow were determined as functions of several variables, such as gas flow rate and magnetic field strength. It appeared that the radial mixing coefficient is comparable to that in a fixed bed, and the axial mixing coefficient was greater than in a fixed bed but smaller than in a fluidized bed without stabilization. The axial mixing is the result of some channelling.

The mixing of the solids is very low, and if there is a continuous solids flow through the bed, deviations from plug flow can be reduced by increasing the magnetic field strength.

Apparently, the magnetically stabilized fluidized bed is well suitable for countercurrent gas-solid operations.     

BUBBLE DYNAMICS AND ELUTRIATION STUDIES IN GAS-FLUID1ZED BEDS

In order to adequately interpret the heat and mass transfer data taken in a gas-fluidized bed, it is essential to know the bubble dynamics and solids movement in the bed, and solids elutriation from the bed. To generate information on these aspects, an experimental facility has been designed, fabricated and successfully tested. This consists of a two-dimensional fluidized bed with its gas supply and cleanup system. The bubble dynamics and solids projection from the bed are recorded by a high-speed movie camera. The films are analyzed on a photo-optical data analyser and digitizer provided with an electronic graphics calculator connected to tape printer and a Teletype terminal interfaced with a computer. The analysis of recorded bed dynamics suggests that for large particles the bubbles grow to be non-spherical and these rise almost above the bed surface before bursting when the wake remains intact while the solids bulge at the bubble nose ruptures to release the bubble gas. It is concluded unambiguously that the solids projected in the freeboard originate from the bubble bulge, and not from the bubble wake as commonly believed. A series of experiments is proposed which will facilitate the development of a general quantitative theory for solids elutriation from industrial fluidized beds.

In addition, a fairly complete review of the work done on bubble dynamics, solids movement in the bed, and solids projection from the bed surface in two- and three-dimensional fluidized beds is presented. Thus, on the whole the present work reviews the state-of-the-art of these three different fluid-bed aspects, and reports new data.     

BUBBLE DYNAMICS AND ELUTRIATION STUDIES IN GAS-FLUID1ZED BEDS

In order to adequately interpret the heat and mass transfer data taken in a gas-fluidized bed, it is essential to know the bubble dynamics and solids movement in the bed, and solids elutriation from the bed. To generate information on these aspects, an experimental facility has been designed, fabricated and successfully tested. This consists of a two-dimensional fluidized bed with its gas supply and cleanup system. The bubble dynamics and solids projection from the bed are recorded by a high-speed movie camera. The films are analyzed on a photo-optical data analyser and digitizer provided with an electronic graphics calculator connected to tape printer and a Teletype terminal interfaced with a computer. The analysis of recorded bed dynamics suggests that for large particles the bubbles grow to be non-spherical and these rise almost above the bed surface before bursting when the wake remains intact while the solids bulge at the bubble nose ruptures to release the bubble gas. It is concluded unambiguously that the solids projected in the freeboard originate from the bubble bulge, and not from the bubble wake as commonly believed. A series of experiments is proposed which will facilitate the development of a general quantitative theory for solids elutriation from industrial fluidized beds.

In addition, a fairly complete review of the work done on bubble dynamics, solids movement in the bed, and solids projection from the bed surface in two- and three-dimensional fluidized beds is presented. Thus, on the whole the present work reviews the state-of-the-art of these three different fluid-bed aspects, and reports new data.     

Measuring and modelling lateral solid mixing in a three-dimensional batch gas—solid fluidized bed reactor

A 0.27 m diameter fluidized bed reactor has been designed to allow experimental measurement of the axial and radial mixing behaviour of the solids. A unique method has been developed which permits the continuous determination of solid tracer concentration with time at different radial and axial positions within the fluidized bed. Solids mixing has been described by a model in which vertical mixing is instantaneous and lateral mixing occurs by dispersion. The lateral solids dispersion coefficients have been evaluated at various operating conditions from the experimental results of tracer concentration versus time. Based on the results, a modification of an existing correlation is proposed.     

流动方向对循环流化床中颗粒混合行为的影响

对循环流化床提升管及下行床两种不同气固流动方式对颗粒混合行为的影响进行了较为深入的对比分析，发现在影响循环流化床颗粒混合的众多因素（如操作条件、床层直径、颗粒性质及床层内构件等）中，气固流动方向是影响颗粒轴向混合的最主要因素．当气固流动为顺重力场时（下行床），颗粒的轴向混合很小，流型接近平推流；当气固流动为逆重力场的提升管时，轴向颗粒混合将成倍增大，颗粒流动远离平推流流动．分析表明，下行床中颗粒混合仅为单一的弥散颗粒扩散，而提升管中则存在着两种颗粒混合机制：弥散颗粒扩散及颗粒团扩散．弥散颗粒的扩散基本以平推流的形式通过循环流化床，提升管中大量的颗粒轴向返混归因于颗粒团的严重返混并由此形成了提升管中颗粒停留时间的双峰分布．     

回料流入口高度对循环流化床提升管流动特性的影响

The axial pressure drop profile and the radial solids distribution were measured in a circulating fluidized bed for evaluating the effects of return gas-solids stream position on the riser flow properties.The saturation carrying capacity of gas for Geldart B typed particles and the flow mode of return gas-solids stream in the bed were discussed.It was found that arranging the inlet at a higher position of the riser would make the bottom bed leaner when U₀ was high and G_s was low.When G_s increased,the longer influenced region of return particles and a small air-staging through lifting the loosening air injection position made the bottom bed become denser significantly.The deceleration and residence of return particles caused a relatively denser but asymmetrical region in the vicinity of inlet.But much more symmetrical solids distribution profile was found in the upper and lower regions far away from the inlet.The effects of inlet height on the flow properties of the riser with air-staging also were analyzed.The secondary air injection below the solids inlet could not cut off the solids exchange in the bed.The bed solids concentration increased when the particles inlet moved to a higher position of the bed when air-staging was adopted.Using CO₂ as tracer,the dispersion of the loop-seal-fluidizing air for transmitting the return particles was investigated.It was found that the loop-seal fluidizing air dispersion rate was low but can be enhanced by the secondary air injection.     

Hydrodynamic characteristics of gas–solid fluidization at high temperature

Effect of temperature on the hydrodynamics of bubbling gas–solid fluidized beds was investigated in this work. Experiments were carried out at different temperatures ranged of 25–600°C and different superficial gas velocities in the range of 0.17–0.78 m/s with sand particles. The time‐position trajectory of particles was obtained by the radioactive particle tracking technique at elevated temperature. These data were used for determination of some hydrodynamic parameters (mean velocity of upward and downward‐moving particles, jump frequency, cycle frequency, and axial/radial diffusivities) which are representative to solids mixing through the bed. It was shown that solids mixing and diffusivity of particles increases by increasing temperature up to around 300°C. However, these parameters decrease by further increasing the temperature to higher than 300°C. This could be attributed to the properties of bubble and emulsion phases. Results of this study indicated that the bubbles grow up to a maximum diameter by increasing the temperature up to 300°C, after which the bubbles become smaller. The results showed that due to the wall effect, there is no significant change in the mean velocity of downward‐moving clusters. In order to explain these trends, surface tension of emulsion between the rising bubble and the emulsion phase was introduced and evaluated in the bubbling fluidized bed. The results showed that surface tension between bubble and emulsion is increased by increasing temperature up to 300°C, however, after that it acts in oppositely.     

气固搅拌流化床内压力脉动特性的数值模拟

在传统气固流化床中引入搅拌桨,可减轻聚合物颗粒的黏附并强化流态化过程。采用计算流体力学(CFD)方法对搅拌流化床内的压力脉动特性进行数值模拟,考察流态化过程中的气泡行为。模拟过程采用多重参考坐标系方法解决搅拌桨区域的运动问题,由欧拉双流体模型和颗粒动力学方法模拟气固两相流。床层压力脉动的统计分析和功率谱分析表明,随着搅拌桨转速的增加,流化床内的压力脉动标准偏差和功率谱幅值变小,床层内的平均气泡尺寸减小,床层可由鼓泡流态化向散式流态化转变。     

Numerical and experimental studies on the flow multiplicity phenomenon for gas-solids two-phase flows in CFB risers

The flow multiplicity phenomenon in circulating fluidized bed (CFB) risers, i.e. under the same superficial gas velocity and solids circulation rate, the CFB risers may sometimes exhibit multiple flow structures, was numerically and experimentally investigated in this study. To investigate the flow multiplicity phenomenon, the experiments of gas-solids two-phase flows in a 2-D CFB riser with different flow profiles at the inlet of the CFB riser were conducted. Specially designed gas inlet distributors with add-ons are used to generate different flow profiles at the inlet of the CFB rise. The CFD model using Eulerian-Eulerian approach with k-ε turbulence model for each phase was employed to numerically analyze the flow multiplicity phenomenon. It is experimentally and numerically proved that for gas-solids two-phase flows, the flow profiles in the fully-developed region are dominated by the flow profiles at the inlet. The solids concentration profile is closely coupled with the velocity profile, and the inlet solids concentration and velocity profiles can largely influence the fully-developed solids concentration and velocity profiles.