DSMC-LES方法数值模拟鼓泡流化床内气泡和颗粒流动行为

基于稠密气体分子动力学和气固两相流体动力学,建立流化床稠密气固两相离散颗粒运动-碰撞解耦模型,采用直接模拟蒙特卡罗方法(DSMC)模拟颗粒间的碰撞,采用考虑颗粒脉动流动对气相湍流流动影响的大涡模拟(LES)研究气相湍流,单颗粒运动满足牛顿第二定律,颗粒相和气相相间作用的双向耦合由牛顿第三定律确定.数值模拟流化床中颗粒流动以及气泡的生成、长大和破碎过程,获得颗粒轴向和径向速度的概率密度分布,及颗粒浓度分布.计算结果表明床内气泡的形成造成床内颗粒的循环,使得流化床内颗粒具有不同的轴向和径向脉动速度,颗粒分速度分布近似服从高斯分布.颗粒温度随颗粒浓度增加,达到最大值后,随颗粒浓度增大而下降.流化床颗粒浓度脉动主要是低频部分,高频分量较低,表明在流化床内颗粒浓度脉动频率低,能量高,颗粒浓度脉动主频率为0.4～1.0Hz,其值与Pain et al.获得的颗粒浓度脉动主频率基本吻合.     

直接模拟蒙特卡罗方法研究循环流化床内颗粒聚团的流动

采用直接模拟蒙特卡罗方法(DSMC)模拟颗粒间的碰撞,用Lagrangian方法计算颗粒的运动.采用考虑颗粒脉动流动对气相湍流流动影响的大涡模拟(LES)研究气相湍流.数值模拟垂直管道内气固两相上升流动,对管内气相速度和颗粒相速度、浓度,以及聚团流动进行分析,得到气固两相上升流中颗粒流动的有关信息.     

循环流化床气体-颗粒团聚物两相流体动力特性的研究

基于气固两相流体动力学,建立循环流化床内气体-颗粒团聚物两相流动模型,模拟计算循环流化床内气固两相流动.理论预测得到循环流化床内颗粒团聚物的形成,得到颗粒"环-核"流动结构,模拟计算颗粒相浓度和速度以及气相速度分布与前人实验结果的趋势基本吻合.     

循环流化床中颗粒团聚物性质的PDPA测量

提出了一种利用相位多普勒粒子分析仪（PDPA）来测量气固循环流化床中颗粒团聚物性质的方法,并运用此方法初步考察了操作条件对循环流化床稀相区中颗粒团聚物性质的影响.在本实验操作条件下,颗粒团聚物的时间分率、频率、内部空隙率以及轴向速度等性质都存在轴径向的不均匀分布,具有较明显的环核特征;固体循环速率对颗粒团聚物性质径向分布影响不大,表观气速的变化可引起其轴向分布规律发生显著改变,但其径向的环核特性仍然存在.     

流化床颗粒团聚物水分分布测量新技术

在流化床中团聚物主要以宏观团聚物、微小团聚物、自由颗粒3种形式存在,通过TEB雾化喷嘴制造流化床颗粒团聚物,其中宏观团聚物沉积在流化床的底部位置,微小团聚物和自由颗粒均匀分布在整个流化床中。研究发现本文自主研制开发的多通道电导电路实验装置测量所得电导信号与流化床内水分含量存在线性比例的关系,利用流化床的持续流化效应造成团聚物破裂引发电导信号回升,实现了颗粒团聚物水分分布状态的测量。利用该装置测量了TEB雾化喷嘴气液比在0～4%范围内变化时流化床内颗粒及团聚物的水分分布状态,结果表明随着气液比的增加,微小团聚物、自由颗粒所包含的水分有所增加,宏观团聚物所包含的水分有所减小,即使在气液比增加到4%时,大量的水分仍然被宏观团聚物所吸收。     

基于DEM方法的旋转流化床纳米颗粒流动特性研究

基于DEM方法利用MFIX软件分别模拟了两种不同粒径纳米颗粒在旋转流化床内的流动。模拟结果与以往实验结果进行比较发现结果吻合较好,验证了旋转流化床模型的准确性和可靠性。对纳米颗粒在旋转流化床中的运动过程进行数值模拟,加入范德华力,得到了纳米颗粒分布、纳米颗粒速度矢量分布以及纳米颗粒床层压降在不同离心加速度下随气速的变化。结果表明,少部分纳米颗粒在旋转流化床上部游离,其余颗粒随旋转流化床运动,当纳米颗粒上升到60°旋转角附近时,颗粒出现回落,然后进行反复运动,由于纳米颗粒的运动受到颗粒堆积的影响,最终纳米颗粒在旋转流化床底部往复循环。纳米颗粒在旋转流化床内的压降均随离心加速度和气速的增加而增加,相同条件下TiO2颗粒的床层压降大于Al2O3颗粒的床层压降,而且与TiO2相比Al2O3颗粒先达到稳定状态。     

喷动流化床内宽筛分颗粒流动特性的研究

建立了喷动流化床气化炉,通过数值模拟方法分析了宽筛分颗粒时喷动流化床的流动特性.数值模拟中采用非稳态欧拉-欧拉多相流模型和双方程湍流模型,采用用户自定义函数(UDF)编程,编写出宽筛分颗粒模型.结果表明,不同粒径颗粒在密相区出现分层,喷动流化床内的流体动力学受颗粒尺寸分布以及颗粒间能量耗散和传递的影响.     

提升管内气固两相双组分颗粒流动的离散颗粒硬球模型的模拟

基于离散颗粒(DPM)硬球模型,数值模拟提升管内双组分颗粒气固两相湍流流动行为。应用Vreman的亚格子尺度（SGS）模型模拟气体湍流,建立考虑不同颗粒加速度效应的两颗粒碰撞最小时间计算模型。数值模拟预测了大颗粒和小颗粒的速度和浓度分布。研究结果表明小颗粒具有高的轴向速度和脉动速度,而大颗粒具有低的轴向速度和脉动速度。在床中心区域,小颗粒轴向速度分布出现3个峰值,对于大颗粒轴向速度仅出现两个峰值。在壁面区域大颗粒和小颗粒速度均出现两个峰值。沿床径向方向呈现床中心颗粒浓度低、壁面区域颗粒浓度高的环核流动结果。随着表观气速的增大,颗粒浓度沿径向和床高分布趋于均匀。在床中心区域模拟计算轴向颗粒速度、颗粒浓度和RMS速度与文献实验结果相吻合。在提升管内气体湍流对小颗粒流动具有一定的影响,颗粒间碰撞作用对颗粒相流动的影响大于气相湍流的影响。     

循环流化床生物质颗粒掺混流动特性CPFD模拟

针对循环流化床(CFB)燃煤锅炉掺混生物质颗粒对床内颗粒流动特性的影响,文中基于计算颗粒流体动力学(CPFD)方法对CFB内煤炭颗粒掺混生物质颗粒的混合流动特性进行模拟研究,考察床层压降,并分析生物质掺混比以及不同的掺混方式对颗粒流动特性的影响。结果表明：掺混质量分数15%生物质的混合颗粒最小流化速度为0.7 m/s;生物质掺混比例的增加使最小流化速度有所提升,但提升程度不大;此外,2种颗粒的堆积方式对颗粒流动特性存在影响,当生物质堆积在煤炭上方时,会阻碍床层内颗粒的正常流化,出现节涌现象。     

颗粒群绕流传质特性的格子Boltzmann模拟

为考察气固两相流中颗粒团聚对气固传质过程的影响,采用耦合传质的格子Boltzmann模型对在空间均匀分布的颗粒结构和非均匀分布的颗粒团聚物结构条件下的绕流过程进行了直接数值模拟,得到了气体绕颗粒流动的速度分布和伴随流动的浓度分布,发现颗粒团聚物对绕流过程的速度分布和浓度分布都具有明显的影响。通过对颗粒绕流的气固传质过程进行定量分析发现,在两种结构条件下计算得到的传质舍伍德数均随着雷诺数的增大而呈指数函数形式增大,并且在均匀结构条件下的传质舍伍德数一般为非均匀结构条件下的3~5倍,颗粒团聚物的存在将严重影响颗粒绕流过程的气固传质效率。通过格子Boltzmann方法建立的气固相间传质模型,可以为研究气固两相流介尺度结构的传递特性提供理论依据。     

Numerical study on the cluster flow behavior in the riser of circulating fluidized beds

In this paper, the cluster flow behavior in the riser of circulating fluidized beds (CFBs) was predicted by means of the Euler–Lagrange approach. Gas turbulence was modeled by means of Large Eddy Simulation (LES). Particle collision was modeled by means of the direct simulation Monte Carlo (DSMC) method. An extended cluster identification method was used to obtain the solid concentration and velocities of clusters. The flow behavior of falling clusters in the near wall region and the up-flowing and down-flowing clusters in the core of riser were analyzed respectively. At the same time, some cluster transient flow characteristics observed in the CFBs experimental studies were obtained in present simulation. Simulated results showed that there exists a low-pressure zone between the tails of a down-flowing cluster in the core of riser and a new cluster can be formed in the near wall region and transferred from the near wall region to the core of riser under the effect of upgoing gas. Simulated results have a reasonable agreement with the previous experimental findings.     

大涡模拟方法模拟鼓泡床内气固两相流动

采用大涡模拟（LES）方法模拟气相湍流,颗粒动理学方法考虑颗粒相碰撞产生的动量和能量传递和耗散,采用颗粒相大涡模拟方法（LESp）模拟颗粒脉动导致的能量耗散,同时考虑介观尺度对颗粒相压力的影响,建立了气体-颗粒LES-θ-LESp双流体模型,研究鼓泡流化床内气固两相流动的特性。数值模拟与文献实测颗粒速度和实测颗粒浓度结果具有相同的变化趋势。     

Multiresolution analysis on identification and dynamics of clusters in a circulating fluidized bed

A new wavelet‐threshold criterion was developed to distinguish the cluster and the void phases from the transient solids holdup/concentration fluctuation signals when measured in a 108 mm‐i.d. × 5.75 m‐high circulating fluidized bed with FCC particles (d_p = 78 μm, ρ_p = 1,880 kg/m³). An appropriate level of approximation subsignal was systematically specified as a threshold for cluster identification, based on multiresolution analysis (MRA) of wavelet transformation. By the established threshold, the dynamic properties of clusters including the appearance time fraction of clusters F_cl, average cluster duration time τ_cl , cluster frequency f_cl, and local average solids holdup in clusters ε_sc , at different radial and axial positions were determined under the turbulent, transition and fast fluidization flow regimes. The results also describe the dynamic properties of clusters and flow patterns in the splash zone along with the dense bottom region of the circulating fluidized beds. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Numerical simulation of bubble and particles motions in a bubbling fluidized bed using direct simulation Monte-Carlo method

Flow behavior of bubbles and particles in a bubbling fluidized bed were numerically computed using Euler-Lagrange approach. Particle collision was simulated by means of the direct simulation Monte-Carlo (DSMC) method and hard-sphere model. The computed velocities and fluctuations of particles were in agreement with experimental data of Yuu et al. [S. Yuu, H. Nishikawa, T. Umekage, Numerical simulation of air and particle motions in group-B particle turbulent fluidized bed, Powder Technol. 118 (2001) 32-44]. The distributions of velocity, concentration, granular temperature and collision frequency of particles in a bubbling fluidized bed were analyzed. The wavelet multi-resolution analysis was used to investigate flow behavior of bubbles and particles. The bubble frequency of random-like bubble fluctuation was determined from the wavelet multi-resolution analysis over a time-frequency plane.     

Flow behavior and regime transition in a high-density circulating fluidized bed riser

Flow behavior and flow regime transitions were determined in a circulating fluidized bed riser (0.203 m i.d. × 5.9 m high) of FCC particles (, ). A momentum probe was used to measure radial profiles of solids momentum flux at several heights and to distinguish between local net upward and downward flow. In the experimental range covered (; ), the fast fluidization flow regime was observed to coexist with dense suspension upflow (DSU). At a constant gas velocity, net downflow of solids near the wall disappeared towards the bottom of the riser with increasing solids mass flux, with dense suspension upflow achieved where there was no refluxing of solids near the riser wall on a time-average basis. The transition to DSU conditions could be distinguished by means of variations of net solids flow direction at the wall, annulus thickness approaching zero and flattening of the solids holdup versus G_s trend. A new flow regime map is proposed distinguishing the fast fluidization, DSU and dilute pneumatic transport flow regimes.     

Monte Carlo modeling of fluidized bed coating and layering processes

A stochastic modeling approach based on a Monte Carlo method for fluidized bed layering and coating is presented. In this method, the process is described by droplet deposition on the particle surface, droplet drying and the formation of a solid layer due to drying. The model is able to provide information about the coating coverage (fraction of the particle surface covered with coating), the particle‐size distribution, and the layer thickness distribution of single particles. Analytical solutions for simplified test cases are used to validate the model theoretically. The simulation results are compared with experimental data on particle‐size distributions and layer thickness distributions of single particles coated in a lab‐scale fluidized bed. Good agreement between the simulation results and the measured data is observed. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2670–2680, 2016     

Sequential modular simulation of circulating fluidized bed reactors

Bypassing the mathematical complexity of equation-oriented approaches in predicting the performance of chemical reactors has recently stimulated a significant amount of interest. Among chemical reactors, circulating fluidized bed reactors (CFBRs) have secured an important role in a broad range of applications in energy sectors due to their advantages, including high fluid-solid contact efficiency, uniform temperature, and enhanced heat and mass transfer rates. Accordingly, modelling and predicting the performance of these reactors is of great importance. In this study, a sequence-based model was developed to predict the behaviour of CFBRs. Complex phenomena in CFBRs were mimicked by two sub-models, namely the hydrodynamics module, which addressed the physical changes, and the reaction kinetics module, which described the chemical evolution of species. The performance of the proposed model was validated with a library of catalytic ozone decomposition experimental data in CFBRs. This work introduces a new infrastructure for modelling CFBRs, which may be combined with the current process simulation software, such as Aspen Plus©, for advanced process modelling applications.     

Flow regimes and critical velocity in a circulating fluidized bed

A laboratory scale circulating fluidized bed, 50 mm i.d. 4.97 m high, has been operated with different solid inventories in the downcomer. The operating conditions cover a wide range of superficial gas velocities and solid circulation rates. A critical gas velocity U_cr is defined as the gas velocity beyond which the interface between the dense bed and the dilute bed cannot be observed in the circulating fluidized bed at any solid circulation rate. Three different fluidization regimes exist at gas velocities lower than U_cr; they are: the dilute transport bed, the dense bed and the bed with an interface between the bottom dense bed and the upper dilute freeboard. An additional fluidization regime exists at gas velocities greater than U_cr where no interface can be found at any solid circulation rate. U_cr increases with increasing solid inventory. The height of the interface is significantly affected by the solid inventory, and is also greatly affected by the design of the pneumatic valve. The whole bed becomes a dense bed after the interface extends to the exit region of the bed.     

新型流化床内流体重构的流体力学特性变化及其数值模拟

针对前期研究开发的新型三重环流与多重环流流化床,构建气液两相流的二维数值模型,分析微观流场、液体运动速度和气相含率,剖析环流数对流化床流体力学特性的影响和对内循环过程的改善,寻找反应器运行过程节能的结构与优化的操作条件。通过数值模拟发现:基于单重环流,多重环流作用主要改变流体在上升区和下降区之间的相互混合和交汇,增大环流数可缩短流体运动的循环路程和时间,有利于加强相际之间的混合,但床内整体流态仍接近推流,微循环的存在所占比例较小;多重环流流化床的流体力学性能优于三重环流,随着环流次数的增多,液速在径向和轴向上分布更均匀,气泡逆流进入下降区,有利于反应器整体氧传质效率的提高。通过流化床结构改变实现的流体重构支持流体力学性能的改善,是高负荷有机废水好氧生物处理期待选择的开发方向。