基于两相两区模型的气固流化床DEM模拟

气固流化床DEM模拟中,通常采用面积加权平均法计算局部空隙率,为了考虑网格中显著非均匀结构对局部空隙率的影响,提出一个计算局部空隙率的两相两区模型。该模型将网格中的非均匀结构虚拟划分为稀相和密相,将实际网格区域划分为稀区和密区,并采用时空关联性原理识别稀区和密区;模型还采用自适应方法计算网格中颗粒分布的非均匀度;将非均匀度作为非均匀结构的影响权重计算密区颗粒的局部空隙率。模拟了鼓泡流化床,模拟结果表明:与传统的DEM相比,基于两相两区模型的DEM能更好地模拟气泡形态,且能捕捉气泡冒出床层和气泡破裂的复杂现象。     

60米高循环流化床内物料浓度分布的冷态试验

利用电厂循环流化床锅炉现有的结构和设备, 搭建提升管高度60m、内径400mm的超高循环流化床冷态实验台, 重点研究了流化风速和颗粒密度对提升管内轴向和径向空隙率分布的影响。实验结果表明:空隙率分布形式与流化风速和物料密度密切相关, 对于一定的床料高度, 在底部密相区一直有床料堆积的情况下, 随着流化风速的增加, 提升管底部密相区空隙率增大, 上部稀相区的空隙率减小并且其在径向的分布变得更加不均匀;在一定的流化风速下, 密度较小的物料将更多的被带入上部稀相区, 上部稀相区的空隙率减小, 其在径向分布将变得更加不均匀。     

非均匀气固两相系统中多尺度传质模型

建立了适用于气固循环流化床的多尺度传质模型 .从过程与尺度的角度出发 ,将非均匀气固两相流中的传质过程分解为静态与动态的过程 ,并将前者分解为稀相内、密相内以及稀密相间 3个尺度下的传质 ;在用多尺度能量最小 (EMMS)模型求解已知物系性质和操作条件的非均匀气固两相流体动力学参数的基础上 ,借助于前人的研究结果 ,利用相对滑移速度、空隙率等参数求解传质系数 ,求得轴向的浓度场分布 ,并讨论非均匀两相流动结构对传质效率的影响     

循环流化床中宏观非均匀结构的计算机模拟

运用颗粒运动分解轨道模型模拟了循环流化床中的宏观非均匀结构,模拟结果给出了与实验结果相吻合的空隙率分布、颗粒速度分布及气体速度分布．因此,颗粒运动分解轨道模型能够用于循环流化床中的宏观非均匀结构的模拟。     

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.     

高粘度流体流态化的特殊性

高粘度流体流态化具有与普通气固、液固流态化所不同的特殊规律.实验发现(1)操作条件改变时,高粘度流体流化床整体结构的变化是一个缓慢的过程;(2)高粘度流体流化床局部结构的变化也是一个缓慢的过程;(3)极高粘度下流化床会出现整体结构与局部结构的背离;(4)高粘度流体流化床在床层不同高度具有不同的非均匀性.这些特殊规律在设计流体粘度对流态化影响的实验中必须予以充分重视.     

振动流化床床层均匀流化行为

在φ148mm的振动圆柱型流化床中,采用光导纤维浓度测量仪测量了床层局部空隙率的波动．对空隙率波动偏差的分析表明,振动参数、颗粒物性和操作参数均对床层均匀流化行为有不同程度的影响,且振幅和振动频率越大越能明显提高以小米或玻璃珠为物料的床层的均匀稳定性,两颗粒物性和操作参数直接影响振动能量在床层中的传播．根据实验结果,提出并定义了一个振动均匀操作判别系数,这对选择实际振动流化床的操作范围具有一定的指导意义．     

鼓泡床床层高度的细网格DEM模拟

气固流化床DEM的细网格模拟中,采用传统方法计算网格空隙率和局部空隙率会对模拟结果造成较大偏差,给出一个精确面积分数模型和一个完全依赖颗粒环境的局部空隙率模型,从而更加合理地计算网格空隙率和局部空隙率。采用二倍颗粒直径的细网格模拟了小规模鼓泡流化床,模拟的气泡形状和尺寸与实验结果接近。模拟结果表明:采用给出的面积分数模型和局部空隙率模型能较好地模拟鼓泡流化床床层高度随时间变化的波形。     

鼓泡床床层高度的细网格DEM模拟

气固流化床DEM的细网格模拟中,采用传统方法计算网格空隙率和局部空隙率会对模拟结果造成较大偏差,给出一个精确面积分数模型和一个完全依赖颗粒环境的局部空隙率模型,从而更加合理地计算网格空隙率和局部空隙率。采用二倍颗粒直径的细网格模拟了小规模鼓泡流化床,模拟的气泡形状和尺寸与实验结果接近。模拟结果表明：采用给出的面积分数模型和局部空隙率模型能较好地模拟鼓泡流化床床层高度随时间变化的波形。     

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。     

Bubble phase voidage and dense phase voidage in thin two-dimensional fluidized beds

Bubble phase and dense phase voidages have been studied in a series of thin two-dimensional fluidized beds using a light transmission technique. It is proposed to divide the bubble phase voidage into a “visible” and an “invisible” portion with respect to light transmission. The variation of visible bubble phase voidage with bed thickness follows a logarithmic relationship. The analysis is especially relevant for the design of fluidized photoreactors. The effects of flow rate, particle size and bed thickness on bubble phase voidage have been examined. Average dense phase voidage, which varies with flow rate and particle size was found to exceed that at minimum fluidization in many cases. The present results show that variable dense phase porosity found in three-dimensional fluidized beds is also present in flat fluidized photoreactors.     

Investigation of nonuniformity in a liquid–solid fluidized bed with identical parallel channels

Previous work has demonstrated that multiphase flow through identical parallel channels and multiple cyclones can give rise to significant nonuniformity among the flow paths. This article presents results from a study where the distribution of voidage and flux through parallel channels in liquid–solid fluidized beds is investigated. Experiments and computational fluid dynamics simulations were performed with 1.2 mm glass beads fluidized by water where a cross baffle divided a 191 mm diameter column into four identical parallel channels. Voidages were measured by optical fiber probes. Simulations from a three‐dimensional unsteady‐state Eulerian–Eulerian model based on FLUENT software showed good agreement with the experimental results. Despite the symmetrical geometry of the system, the average voidage and particle velocities in one channel differed somewhat from those in the others. Increasing the superficial liquid velocity could increase voidage greatly and affect the degree of nonuniformity in the four channels. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

生物流化床的流化特性

本文对生物流化床中流化速度与空隙率的关系进行了研究。实验发现Richardson—Zaki式不能用于描述生物床。并依据实验数据,得出数学模型,更为完善地描绘生物床的流化特性。     

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.     

Axial liquid dispersion in a liquid–solid circulating fluidized bed

Although axial liquid dispersion has been studied extensively in particulate fluidized beds, no data has been reported previously in a liquid–solid circulating fluidized bed (LSCFb). In this work, the axial liquid dispersions at various radial positions were measured in an LSCFB of 76 mm in diameter and 3.0 m in height using a dual conductivity probe. The results reveal that the axial liquid dispersion is affected not only by the operating conditions but by the radial positions as well. A local axial dispersion model is proposed to describe the axial liquid dispersion at various radial positions. The local axial liquid dispersion coefficients determined by the proposed model are greater at the axis than near the wall region of the riser. This nonuniformity of axial liquid dispersion is believed to be caused by the radial nonuniform distribution of liquid velocity, and bed voidage in the LSCFB can significantly affect the axial liquid dispersion.     

Scale‐dependent nonequilibrium features in a bubbling fluidized bed

We investigate experimentally the nonequilibrium features in a pseudo 2‐D bubbling fluidized bed. Velocities of individual particles are measured by using a particle tracking velocimetry (PTV) method, and void fractions are obtained with the Voronoi tessellation. A bimodal shape of probability density function (PDF) for particle vertical velocity is found in not only time‐averaged but also time‐varying statistics, which is caused by the transition between the dense and dilute phases and breaks the local‐equilibrium assumption in continuum modeling of fluidized beds. The results of time‐varying radial distribution function and voidage distribution also confirm this finding. Moreover, the analysis of voidage, particle velocity, granular temperature and turbulent kinetic energy of particles shows that there is no scale‐independent plateau over the interface, and it seems hard to find a scale‐independent plateau to separate the micro‐ and meso‐scales of fluidized beds, which require sub‐grid meso‐scale modeling for continuum or coarse‐graining methods of gas‐fluidized systems. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2364–2378, 2018     

湍动流化床的流型与流型过渡

分别在直径为114mm 和244mm 的流化床中,采用压力探头和光导纤维探头考察了湍动流态化的上、下限及实验判别准则。发现床内局部点处的床空隙率随气速的变化符合于修正的 Richardson-Zaki 关系,并以此可以揭示出局部区域进入湍动流态化的流型过渡(上限)。压力脉动法的测量结果(U_c)还表明:此时,床内湍动流态化区域实际上已经占有相当的份额。另一方面,分析床内压力及床密度的脉动信号随气速的变化,可以判定脱离湍动流态化时的流型转变(下限)。本文还应用 Kolmogoroff 的湍流理论定性地分析了湍动流态化的湍流结构。     

MASS AND MOMENTUM TRANSFER WITH NON-NEWTONIAN FLUIDS IN FLUIDIZED BEDS

A new model for non-Newtonian fluid flows through fluidized beds is developed by extending the Richardson-Zaki concept. In the proposed model, the existing correlations for an isolated single sphere are simply extended to those for a fluidized bed by incorporating a voidage function. The model is found to be in a good agreement with other correlations for flows of a non-Newtonian fluid through multi-particle systems. The model is also applied to interpret mass transfer in fluidized beds.     

MASS AND MOMENTUM TRANSFER WITH NON-NEWTONIAN FLUIDS IN FLUIDIZED BEDS

A new model for non-Newtonian fluid flows through fluidized beds is developed by extending the Richardson-Zaki concept. In the proposed model, the existing correlations for an isolated single sphere are simply extended to those for a fluidized bed by incorporating a voidage function. The model is found to be in a good agreement with other correlations for flows of a non-Newtonian fluid through multi-particle systems. The model is also applied to interpret mass transfer in fluidized beds.