循环硫化床上升管中动态行为的拟流体模拟

The kinetic theory of granular flow (KTGF) is modified to fit the Einstein′s equation for effective viscosity of dilute flow. A pseudo-fluid approach based on this modified KTGF is used to simulate the dynamic formation and dissipation of clusters in a circulating fluidized bed riser. The agglomeration of particles reduces slip velocity within particle clusters, and hence results in two reverse trends: discrete particles are lifted by air while particle clusters fall down along the wall. The dynamic equilibrium of these two types of motion leads to the characteristic sigmoid profile of solid concentration along the longitudinal direction. The predicted solid velocity, lateral and longitudinal profiles of solid volume fraction and annulus thickness are in reasonable agreement with experimental results.     

Mean and fluctuating gas phase velocities inside a circulating fluidized bed

Gas phase velocities is an area in circulating fluidized beds (CFB) that has traditionally received little attention. The dynamics and motion of particles or clusters inside the bed has been the main focus of research. This is because particles dominate the fluid mechanics and heat transfer inside a CFB. However, gas phase motions also effect particle motion. Gas eddies or fluctuations can play an important role in transporting particles to and from the wall. They also help in providing a uniform temperature throughout the bed by promoting mixing. This paper deals with how particles effect the mean and fluctuating gas velocities throughout the cross-section of a riser.Gas velocities were measured inside a cold scale model CFB using a shielded hot wire anemometer. At the centerline, typical mean gas velocities were measured which were approximately twice the superficial gas velocity. These high velocities are likely caused by the negligible net gas upflow in the annulus region. The presence of many dense, downward flowing clusters in the annulus makes this a reasonable assumption.Previous work on gas phase turbulence in two phase flows has typically used either laser measurement techniques in very small diameter risers or in larger risers with very low particle concentration. The general results have shown that smaller particles, on the same order of magnitude as those typically used in CFB and FCC reactors, tend to damp out the gas phase fluctuations. This implies that gas phase motion behaves close to a laminar fashion. This present research measures gas phase fluctuations with typical particle concentrations inside a CFB (∼1-5%). The results indicate that at larger particle concentrations where clusters are formed, the gas phase fluctuations increase dramatically. This suggests that length scales based on cluster size, as opposed to particle size, should be used in estimating the increased levels of gas fluctuations caused by the solid phase. Hence, models which ignore the effect of clusters on the gas or which treat the gas phase as laminar like flow, yield a misleading picture of the flow dynamics inside a CFB riser.     

提升管与下行床颗粒团聚行为的离散颗粒模拟

从微观机理出发,采用计算流体力学和离散单元方法（CFD-DEM）结合的模型对二维提升管和下行床气固流动体系进行了数值模拟。模拟选用了粒径为520 μm、密度为2620 kg·m-3的球形颗粒和周期性边界条件,展示了气固并流逆重力场和顺重力场运动的颗粒聚团瞬态图像,定性或半定量地揭示了两个不同体系的颗粒微观聚集行为。提升管中颗粒聚团较为严重,且表现明显的颗粒返混现象;下行床中的颗粒聚团比较松散,且具有与宏观流动相同的流速方向,几乎无颗粒返混。通过统计分析获得宏观时均流体力学行为,包括两相的相分布和速度分布,并与文献报道的实验现象进行定性的比较。     

气固下行流化床反应器Ⅱ气固两相的流动规律

气固下行流化床反应器气固两相流动过程是比较复杂的，沿轴向气固两相运动可分为第一加速、第二加速和恒速３个运动段，沿径向局部气体速度、颗粒速度和颗粒浓度都具有不同程度的不均匀性。而这种不均匀性是由气固两相顺重力场湍动运动所决定的。和循环床提升管相比，下行管反应器气固两相沿径向分布的不均匀性得到有效地改善，气固可以实现超短接触操作，因而是一种新型高效气固超短接触反应器     

提升管和下行床在催化裂化过程中的比较

在综合考虑流动、反应、传质的基础上,建立了适用于模拟提升管和下行床反应器中催化裂化过程的二维返混模型,并利用正交配置法进行数值求解,得到了各产物在两种反应器内的不同浓度分布规律。这处结果源于两者流动结构和混合状况的差异。和提升管相比,由于下行床内的气固两相流动更接近平推流,气固速度和颗粒浓度径向分布均匀,气固轴向返混小,因而可得到更高的汽油收率。     

多段分级转化流化床提升管速度场的研究

针对流化床煤气化过程中需要长气固接触时间和高固体浓度,开发了耦合灰熔聚流化床和提升管的多段分级转化流化床。为了研究多段分级转化流化床提升管中局部颗粒速度的径向、轴向分布,在不同的操作条件下,采用PV-6型颗粒速度测量仪在冷态实验装置中系统测定提升管内局部颗粒速度。实验结果表明:提升管中任何径向、轴向位置的颗粒速度随着操作气速的增大而增大,随循环量的增加而减小。操作条件对中心区颗粒速度变化的影响明显高于边壁区。颗粒的加速首先发生在提升管中心区域,然后向边壁区域扩展。颗粒速度径向分布的不均匀性沿轴向逐渐增大,并且受操作气速影响比较大。     

Clustering behaviour in gas–liquid–solid circulating fluidized beds with low solid holdups of resin particles

The flow in a gas–liquid–solid circulating fluidized bed is self‐organised and manifests itself with clustering of particles and bubbles. The clustering behaviour in the fluidized bed at low solid holdups of resin particles was experimentally investigated with a high‐speed image measurement and treatment technique of complementary metal oxide semiconductor to enhance the fundamental understanding on such a flow. Several new physical quantities were suggested to characterise such ordered flow structures. The main findings are as follows. The clusters of solid particles largely exist as doublets and triplets, the mixed groups of particles and bubbles mostly exist as one bubble carrying two to four particles. Increasing superficial liquid velocity, particle diameter or density weakens the aggregation degrees of both particle and mixed clusters in the riser and downer, except that the increase of superficial liquid velocity enhances the mixed clustering behaviour in the riser. The climbing of the auxiliary liquid velocity or liquid phase viscosity intensifies the aggregation behaviour, except that the increase of liquid phase viscosity reduces the mixed clustering degree in the riser. The influences of superficial gas velocity and surface tension of liquid phase on the clustering behaviour seem to be a little complex and the trends are not simply increasing or decreasing. The life cycle of solid particle clusters in the GLS riser is not sensitive to the operation conditions, being around 0.07 s. The mixed clusters' life cycle is more sensitive to the conditions and physical properties of phases, changing from 0.02 to 0.07 s.     

Numerical simulation of hydrodynamics in downers using a CFD-DEM coupled approach

The gas-solid flows in a two-dimensional downer of 10 m in height and 0.10 m in width were simulated using a CFD-DEM method, where the motion of particles was modeled by discrete element method (DEM) and the gas flow was described by Navier-Stokes equations. The simulations revealed a rich variety of developing flow structures in the downer under different operating conditions. The two-phase flow development can be clearly characterized by the micro-scale particle distributions in the downer. Near the inlet, the particle distribution is dominated by the distributor design. Then, the particles disperse in the column, forming a homogeneous transit region. After that clusters start to form and modulate the gas-solid flow field till the fully-developed state. The particle-scale simulation disclosed that the clusters are composed of loosely collected particles, and these particles have the same flow direction as the bulk flow so that no particle backmixing can be observed. As the particles in the downer have the tendency to maintain the inertia, the capability of lateral transfer of particles is relatively weak, which was illustrated by tracking the movement of the single particles and clusters. The simulations of the inlet effect on the hydrodynamics in the downer showed that the gas-solid flow structure and the mixing behavior are sensitive to the inlet design. An inappropriate design or operation would probably cause the undesired flow phenomena such as the wide distribution of residence times. The time-averaged hydrodynamics based on the transient simulations showed good agreement with the experimental findings in the literature. The simulation based on the CFD-DEM coupled approach provides a theoretical way to comprehensively understand the physics at micro- to macro-scales in the co-currently downward gas-solid flows.     

Modeling of a multizone gas‐phase polyethylene reactor with a cluster‐based approach

A circulating fluidized reactor of polyethylene was modeled with the proper hydrodynamics for a riser and downer and combined with a kinetic model based on the moment equations. The hydrodynamic model was able to predict the profiles of the following parameters through the riser and downer: cluster velocity, bed porosity, concentration of potential active sites, active sites, gas‐phase components, molecular weights, and reactor temperature. It was shown that one could control the monomer consumption and molecular weight, which are crucial in the reactor behavior and production properties, respectively, by setting different operating hydrodynamic conditions, such as the gas velocity in the riser and the solid circulation rate. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

快速流化床颗粒团絮特征的MP-PIC数值模拟

为了研究快速流化床颗粒的团絮特征,建立了基于多相质点网格法的快速流化床气固多相流三维数理模型,气相场采用大涡湍流模型,通过求解牛顿运动方程得到颗粒相运动信息,气固间相互作用力采用Gidaspow曳力模型,固体间作用力通过计算颗粒应力梯度得到。基于该模型,对三维快速流化床上升管（H=3 m、d=0.1 m）气固流动开展了数值模拟,并与实验进行了校正,研究了在气速工况U_g=5.28 m·s^-1下的颗粒（ρ_p=2650 kg·m^-3、d_p=250 mm）团絮性质,实现了对上升管内颗粒团絮的基本类型（条形团絮、马鞍形团絮、U形团絮）的成功预测,并揭示了不同类型团絮在上升管内形成、发展、聚并直至破碎的演化规律。结果表明,上升管径向颗粒团絮的平均颗粒浓度分布呈现中间低两边高的环核结构,颗粒团絮速度的分布与其相反;随着轴向高度的增加,颗粒团絮的颗粒浓度逐渐降低而速度逐渐增加,但达到一定高度后变化减缓。     

Comparisons of particle cluster diameter and concentration in circulating fluidized bed riser and downer using computational fluid dynamics simulation

The information of particle cluster dynamics is necessary for improving the performance of a circulating fluidized bed system. The main objective of this study is to compare the particle cluster diameters and concentrations from computational fluid dynamics simulation results between circulating fluidized bed riser and downer. The calculation methodologies are based on the concept of kinetic theory of granular flow and statistics. The mathematical model was verified by using the experimental dataset from literature and used for computing the particle cluster dynamics. In the circulating fluidized bed riser and downer, a dense and dilute core-annulus flow structures were obtained, respectively. The particle cluster in the circulating fluidized bed riser possessed more heterogeneity movements than that in the circulating fluidized bed downer. This can be explained by the system flow direction. About the particle cluster dynamics, the particle cluster diameters and concentrations in the circulating fluidized bed riser were higher than the ones in the downer. The calculated values were comparable to the empirical correlations. This confirms the validity of the calculation methodologies. Particle cluster dynamics and its example application inside circulating fluidized bed riser and downer were also discussed.     

DEM MODELING AND SIMULATION OF A DOWN-FLOW CIRCULATING FLUIDIZED BED

A mathematical model based on the distinct element method (DEM) was developed to investigate the hydrodynamics in a gas-solid down-flow circulating fluidized bed reactor (downer). The models consist of the equations of particle motion and fluid motion. The contact force is calculated by using the analogy of a spring, dashpot, and friction slider. Simulation results show that the radial solids holdup and particle velocity profiles are uniform in the core region. Near the wall, the solids holdup is higher with lower particle velocity. An increase in the particle size decreases the solids holdup and increases the particle velocity. The solids holdup decreases with superficial gas velocity but increases with solids circulation rate. Particle velocity increases with gas velocity and solids circulation rate. The solids holdup and particle velocity are almost uniform along the height of the downer except near the distributor. The hydrodynamic behavior from this simulation showed trends similar to those of the experimental results. The results obtained from this model fit better with the experimental results than Kimm's and Bolkan's models do.     

循环流化床上升管内颗粒团聚行为的动态模拟

本文利用改进的颗粒流体动力学封闭的拟流体模型对循环流化床上升管内的颗粒团聚现象进行了模拟。模拟发现上升管内颗粒团聚物存在动态的产生、发展和消散的过程。颗粒团聚物的形成使其内部的相间滑移速度减小,从而使部分颗粒随气流上升,部分颗粒沿壁画下降。这两种运动形式的动态平衡造成了上升管内轴向浓度分布的Ｓ型曲线。     

提升管-下行床耦合反应器内颗粒混合行为

引　言循环流化床中气固两相的流动有两种不同的方式 :提升管中为气固并流上行的逆重力场运动 ,下行床中为气固并流下行的顺重力场运动 .其差异表现为提升管内颗粒浓度、速度以及气体速度在径向上严重的不均匀 ,颗粒浓度概率密度分布以及速度的瞬时信号都表明了颗粒团 -空穴两相结构的存在[1] ,一些研究[2 ,3] 还发现提升管中颗粒的停留时间分布 (ＲＴＤ)曲线存在较大的拖尾甚至出现双峰 ,研究者认为提升管内存在弥散颗粒和颗粒团两种不同的混合机理 ;下行床则比提升管大大改善 ,气固速度、颗粒浓度沿径向分布要均匀得多 ,颗粒的ＲＴＤ曲线…     

下行床反应器热试装置设计与开发

气固并行向下流动的下行床反应器比提升管具有气固接触时间短、气固速度及浓度径向分布更为均匀、气固轴向返混大大减少、易实现高固气比操作等特点。在下行床反应器小型热试装置开发过程中,为发挥下行床的技术优势,重点研究和解决了反应器出入口结构、原料雾化、高温高流通量下固体循环量测量与控制等几个关键性的工程问题,为下行床反应器的热试研究和应用开发提供了基础。     

Flow behaviors in the downer of a large-scale triple-bed combined circulating fluidized bed system with high solids mass fluxes

The flow behaviors in the downer of a large-scale triple-bed circulating fluidized bed (TBCFB) gasifier cold model, which is composed of a downer (Φ 0.1 m×6.5 m), a bubbling fluidized bed (BFB, 0.75×0.27×3.4 m³), a riser (Φ 0.1 m×16.6 m) and a gas-sealing bed (GSB, Φ 0.158 m×5 m), were investigated. Sand particles with a density of 2600 kg/m³ and an average particle size of 128 μm were used as bed materials. Solids mass fluxes were in the range 113–524 kg/m² s. Average solids holdup in the developed region of the downer increased with increasing solids mass flux. The gas seal between the riser and the downer had a large effect on the solids holdup distribution in the downer. Compared with the solids holdup in the riser, a relatively low solids holdup was formed in the downer even at high solids loadings. A pressure balance model was set up to predict the solids mass flux for this TBCFB system. It was found that the static bed height in the GSB had a great effect on the solids mass flux. The possibilities of achieving a high density solids holdup in a downer were discussed.     

大型循环流化床流动结构分析

采用双光路光纤密度探头和激光多普勒测速仪测量了内径418mm,高18m的大型循环流化床提升管和下行床中的瞬态颗粒浓度信号和颗粒速度信号.对瞬态颗粒浓度和颗粒速度的概率密度分布分析表明,下行床中存在着和提升管中不同的微观流动结构,在提升管内流动结构存在明显的两相：即颗粒团相和空穴相,两相的固含率分别为接近1-ε_mf和0.01～0.02.而在下行床中,虽然在边壁也存在着颗粒的团聚行为,但不能形成稳定的、固含接近于起始流化状态固含值的颗粒团相.这种流动结构的区别揭示了下行床中气固顺重力场运动和提升管逆重力场运动在流动机制上的差异.     

Transient dynamics of solid concentration in downer fluidized bed

This paper presents new results from experiments performed in a 15 cm diameter down-flow fast-fluidized bed. Tests were conducted at room temperature and near atmospheric pressure, with 125 μm glass beads. Superficial gas velocities range from 0 to 6 m/s. A needle capacitance probe was used to obtain measurement of instantaneous solids concentration (ε_s) in a small sampling volume of approximately 0.1 cm³. With the downer operating at steady-state, the capacitance probe provided a time-trace of the local solid fraction, indicating that ε_s was generally less than 0.01. However the measurements indicated that the local concentration did have excursions to significantly higher concentrations for short durations. This behavior is indicative of clusters in the down-flow fast-fluidized bed. While clusters are known to be a dominant feature of upward fast fluidization in risers, their presence has been questioned for solid/gas down flows. Sample data are presented to indicate the density, duration, and time fraction of clusters observed in this downer fluidized bed. Preliminary comparisons are made to the limited data available for comparable operating conditions in a riser.