Comparative analysis on gas–solid drag models in MFIX-DEM simulations of bubbling fluidized bed

In this study,the open-source software MFIX-DEM simulations of a bubbling fluidized bed(BFB)are applied to assess nine drag models according to experimental and direct numerical simulation(DNS)results.The influence of superficial gas velocity on gas–solid flow is also examined.The results show that according to the distribution of time-averaged particle axial velocity in y direction,except for Wen–Yu and Tenneti–Garg–Subramaniam(TGS),other drag models are consistent with the experimental and DNS results.For the TGS drag model,the layer-by-layer movement of particles is observed,which indicates the particle velocity is not correctly predicted.The time domain and frequency domain analysis results of pressure drop of each drag model are similar.It is recommended to use the drag model derived from DNS or fine grid computational fluid dynamics–discrete element method(CFD-DEM)data first for CFD-DEM simulations.For the investigated BFB,the superficial gas velocity less than 0.9 m·s^(-1) should be adopted to obtain normal hydrodynamics.

Numerical simulation of dense particle-gas two-phase flow using the minimal potential energy principle

A simulation method of dense particle-gas two-phase flow has been developed. The binding force is introduced to present the impact of particle clustering and its expression is deduced according to the principle of minimal potential energy. The cluster collision, break-up and coalescence models are proposed based on the assumption that the particle cluster are treated as one discrete phase. These models are used to numerically study the two-phase flow field in a circulating fluidized bed （CFB）. Detailed results of the cluster structure, cluster size, particle volume fraction, gas velocity, and particle velocity are obtained. The correlation between the simulation results and experimental data justifies that these models and algorithm are reasonable, and can be used to efficiently study the dense particle-gas two-phase flow.

Comparative study of two fluid model and dense discrete phase model for simulations of gas-solid hydrodynamics in circulating fluidized beds

Computational fluid dynamics(CFD)has become a valuable tool to study the complex gas-solid hydrodynamics in the circulating fluidized bed(CFB).Based on the two fluid model(TFM)under the Eulerian-Eulerian framework and the dense discrete phase model(DDPM)under the Eulerian-Lagrangian framework,this work conducts the comparative study of the gas-solid hydrodynamics in a CFB riser by these two different models.Results show that DDPM could be used to predict gas-solid hydrodynamics in the circulating fluidized bed,and there are differences between TFM and DDPM,especially in the radial distribution profiles of solid phase.Sensitivity analysis results show that the gas-solid drag model exhibits significant effects on the results for both the two models.The specularity coefficient and the restitution coefficient in the TFM,as well as the reflection coefficient and the parcel number in the DDPM,exhibit less impact on the simulated results.

Experimental and numerical investigation of liquid-solid binary fluidized beds： Radioactive particle tracking technique and dense discrete phase model simulations

Liquid-solid binary fluidized beds are widely used in many industries. However, the flow behavior of such beds is not well understood due to the lack of accurate experimental and numerical data. In the current study, the behavior of monodisperse and binary liquid-solid fluidized beds of the same density but dif- ferent sizes is investigated using radioactive particle tracking （RPT） technique and a dense discrete phase model （DDPM）. Experiments and simulations are performed in monodisperse fluidized beds containing two different sizes of glass beads （0.6 and I mm） and a binary fluidized bed of the same particles for vari- ous bed compositions. The results show that both RPT and DDPM can predict the mixing and segregation pattern in liquid-solid binary fluidized beds. The mean velocity predictions of DDPM are in good agree- ment with the experimental findings for both monodisperse and binary fluidized beds. However, the axial root mean square velocity predictions are only reasonable for bigger particles. Particle-particle interac- tions are found to be critical for predicting the flow behavior of solids in liquid-solid binary fluidized beds.

Gas-solid flow in a high-density circulating fluidized bed riser with Geldart group B particles

We carried out experiments to explore and characterize the gas-solid flow dynamics of Geldart group B particles in a dense circulating fluidized bed riser. By reducing the pressure drop across the solid control valve and increasing the solid inventory in the storage tank, a high solid circulation rate and a solid holdup above 0.075 throughout the riser were simultaneously achieved. At a solid-to-gas mass flux ratio of approximately 105, flow transitioned from fast fluidization to a dense suspension upflow. In the axial direction of the riser, solid holdup had an exponential profile, increasing with increasing solid circulation rate and Jot decreasing superficial gas velocity. From the riser＇s center to its wall, the solid holdup increased markedly, exhibiting a steep parabolic profile. Increasing the solid circulation rate increased the radial non-uniformity of the solid concentration, while increasing the superficial gas velocity had the opposite effect, In our dense circulating fluidized bed riser, Geldart group B particles had similar slip characteristics to Geldart group A particles,

基于机器学习的Geldart A类加重质流化床的床层膨胀特性研究

为了预测Geldart A类加重质的膨胀特征,通过床层塌落试验,研究了床层流化过程中的气泡相和乳化相的组成与操作因素之间的关联,构建了操作气速、静止床高以及床层膨胀高度的数据集,进行了数据统计分布和相关性分析,利用具有最佳超参数的GBDT模型成功模拟了膨胀高度和影响变量之间的非线性关系,分析了特征变量的敏感性。结果表明,随着气速的逐渐增加,乳化相与气泡相的膨胀呈现先增加并略有减小的规律。对于不同的初始床高与床层膨胀高度,气泡相组成不受其变化的影响且组成比例相对稳定。操作气速的重要性得分是0.68,是膨胀高度最敏感的变量。而静止床高的重要得分仅为0.32,表明此变量对床层膨胀的影响较小。此外,由部分相关性分析可知,床层的膨胀高度对操作气速的依赖性具有一定的敏感性区间。

Multi-scale numerical simulation of fluidized beds: Model applicability assessment

In the past few decades,multi-scale numerical methods have been developed to model dense gas-solidflow in fluidized beds with different resolutions,accuracies,and efficiencies.However,ambiguity needsto be clarified in the multi-scale numerical simulation of fluidized beds:(i)the selection of the submodels,parameters,and numerical resolution;(ii)the multivariate coupling of operating conditions,bed configurations,polydispersity,and additional forces.Accordingly,a state-of-the-art review is performed to assess the applicability of multi-scale numerical methods in predicting dense gas-solid flow influidized beds at specific fluidization regimes(e.g.,bubbling fluidization region,fast fluidization regime),with a focus on the inter-particle collision models,inter-phase interaction models,collision parameters,and polydispersity effect.A mutual restriction exists between resolution and efficiency.Higherresolution methods need more computational resources and thus are suitable for smaller-scale simulations to provide a database for closure development.Lower-resolution methods require fewercomputational resources and thus underpin large-scale simulations to explore macro-scale phenomena.Model validations need to be further conducted under multiple flow conditions and comprehensivemetrics(e.g.,velocity profiles at different heights,bubbles,or cluster characteristics)for furtherimprovement of the applicability of each numerical method.

Particle Measurement Sensor for in situ determination of phase structure of fluidized bed

Based on three-dimensional （3D） acceleration sensing, an intelligent particle spy capable of detecting, transferring, and storing data, is proposed under the name of Particle Measurement Sensor （PMS）. A prototype 60-mm-dia PMS was tested to track its freefall in terms of velocity and displacement, and served as a particle spy in a fluidized bed delivering the in situ acceleration information it detects. With increasing superficial gas velocity in the fluidized bed, the acceleration felt by PMS was observed to increase. The variance of the signals, which reflect the fluctuation, increased at first, reaching a maximum at the gas velocity （Uc） which marks the transition from bubbling to turbulent fluidization. Through probability density distribution （PDD） analysis, the PDD peak can be divided into the emulsion phase peak and the bubble phase peak. The average acceleration of emulsion and bubble phase increased, while the variance of both phases reached a maximum at Uc, at the same time. However, the difference between the variances of two phases reached the maximum at Uc. Findings of this study indicate that PMS can record independent in situ information. Further, it can provide other in situ measurements when equipped with additional multi-functional sensors.

两相流动对流化床燃烧行为的影响

循环床锅炉沿床高的烟气浓度及燃烧份额分布测试结果证明 ,鼓泡流化床和循环流化床的重要差异表现为密相区燃烧行为的根本不同 ,由于床料平均粒径较低 ,循环床密相区的流动不同于鼓泡床 ,导致气固两相之间的传质阻力增加 ,从而影响燃烧反应 ,密相区的燃烧行为表现为欠氧。循环床锅炉沿床高乃至分离器都有燃烧反应发生 ,建立了考虑气固相间传质阻力的流化床密相区燃烧模型 ,并与实际循环流化床锅炉的测试数据比较 。

大型气－固湍流流化床径向空隙率分布的研究

在一套７１０／４０００×８７０／１１０００ｍｍ大型有机玻璃流化床中，测定了大比重的ＦＣＣ催化剂在湍流流化床密相密度沿径向的分布规律。试验结果表明，湍流流化床密相区密度沿径向有一个明显的分布。采用多项式模型对试验数据进行了关联。

FCC湍流流化床密相床的膨胀

The bed expansion in the dense region of FCC turbulent fluidized bed has been studied in a cold fluidized bed made of plexiglass with 710×4000/870×ll000mm and a RFCC regenerator(800,000T／a）.By using dimensional analysis,a calculation equation of dense phase density correlated by dimensionless groups is obtained.The equation may be use ful for process engineering design of commercial units.

煤在流化床中的热解Ⅰ.浓相温度对煤脱挥发分行为的影响

利用一个内径４８ｍｍ，具有锥形分布板的流化床对神木煤（４５～８０目）的热解行为进行了研究。流化床稀浓两段温度可以分别控制。浓相段温度４００～９００℃，稀相段温度控制在５８０℃，得到了浓相段温度与热解产物产率的关系。结果表明，提高浓相段温度有利于Ｈ２，ＣＯ，ＣＨ４及Ｃ２Ｈ４产率的提高，浓相段温度在６００～６５０℃左右液相产品产率达到最大值。通过与以往的研究者所得结果的比较，表明在稀相段温度较高的情况下，即使挥发分停留时间很短，也会有相当程度的二次反应发生。而本文的结果是在最大限度地减小了稀相段气相二次反应的情况下得到的，更准确地反映了流化床浓相温度对热解的影响。

外置溢流管型双层流化床冷模研究

在 10 0mm× 3 0 0 0mm的双层流化床中研究了溢流管对床层稳定控制的影响。划分了双层流化床操作的 4种可能的状态 ,其中包括上层密相高度低于或等于下层密相高度的两种稳定状态和高于下层密相高度的两种不稳定操作状态。当气速较小时 ,有无溢流管的操作状况相似。而当气速较高时 ,由于溢流管能够有效返回上层颗粒 ,从而操作稳定性增加。

下排气中温分离器型循环流化床锅炉的研究与设计

根据我国循环流化床锅炉燃煤宽筛分的特点，从循环倍率和密相区吸热量、燃烧分额、出口温度及分离效率等关系深入分析了密相区热量平衡，对中温下排气分离器内三维速度场进行分析研究，并以４１０ｔ／ｈ循环流化床锅炉方案设计为例，提出了宽筛分，中、低循环倍率，中温下排气分离器，п型布置的循环流化床的构思。

稠密气固两相流凸台绕流的声发射测试

近几年循环流化床锅炉水冷壁磨损较为严重,虽然采用防磨凸台可以有效减缓壁面磨损,然而基于凸台优化设计的文献还鲜见报道,因此文中使用声发射设备研究凸台绕流过程中稠密气固两相流颗粒与壁面接触程度问题,在一冷态实验台密相区,采用声发射对相同尺寸不同截面形状凸台下方颗粒与壁面接触程度进行测量,并用小波包分解处理信号。实验结果表明,0~10.5 k Hz低频段信号可以反应颗粒与壁面接触程度,重点分析该特征频段能量百分比,提出方形凸台为理论优化凸台,对进一步改进循环流化床防磨技术提供指导意见。

循环流化床中颗粒内循环与循环流化床锅炉的设计

本文从循环床锅炉密相区的热平衡计算出发，探讨了密相区内受热面面积及密相区高度与飞灰循环倍率、密相区燃烧份额的关系，并以４种典型煤种为例，分析了煤种变化对密相区高度的影响。设计计算和运行经验相结合，在密相区热平衡分析中，引入了床内粒子循环的概念，从而对密相区内热平衡和受热面面积的确定有更深入的理解。

鼓泡床、湍动床固含率的相似分布规律

在内径为４７４ｍｍ的流化床中，采用双光路光纤密度探头分别研究了流化床密相区、稀相区空隙率径向分布，发现无论是在密相区还是在稀相区，鼓泡床、湍动床中的固含率径向分布均比提升管均匀得多。鼓泡床、湍动床中的固含率径向分布符合相似分布规律，即局部固含率与截面平均固含率成正比，仅是径向位置的函数，与操作气速无直接关系。

不同曳力模型对提升管内气固流动特性的影响

在考虑稠密气固两相流中颗粒具有非均匀流动特性的情况下,提出了基于颗粒团聚效应的气固相间曳力模型,数值模拟循环流化床提升管内气固两相的流动特性。模拟计算时考虑气固相间的作用力,壁面处气相采用无滑移边界条件,固相采用考虑颗粒与壁面的碰撞和颗粒与壁面的简单摩擦作用的壁面边界条件。结果表明,考虑颗粒团聚效应的曳力模型能很好地反映提升管内气固两相流动特性,模拟结果比应用Gidaspow曳力模型的计算结果更接近文献[16]的试验结果。

流化床密相区内宽筛分颗粒气固流动研究

应用质量守恒、动量守恒等基本控制方程以及湍流、颗粒流动力学模型对流化床焙烧炉建立了宽筛分颗粒气固流动数学模型,并进行数值模拟.研究得出床层由静止启动到平稳运行过程大约需要6s;不同粒径颗粒在密相区出现分层,小于0.2mm的颗粒明显上升,易从溢流口排除;模拟分析了压强波动情况,大波动主要是颗粒团回落引起;同时模拟出了密相区颗粒流的环流和边界流.模拟结果与现场100t/d的工业流化床矿石焙烧炉上测量数据基本吻合.

油页岩循环流化床燃烧室密相区物料颗粒与燃烧特性

在 65t/h油页岩循环流化床锅炉上进行了燃烧室密相区物料特性与燃烧特性的工业试验。得到了极具片状结构特点的油页岩颗粒特性 ,经燃烧破碎磨损后的密相区床料和循环物料的颗粒特性与燃烧特性及相应的锅炉运行参数。试验结果对油页岩循环流化床锅炉的放大设计与运行调节具有一定的参考价值。同时为进一步建立油页岩循环流化床燃烧室密相区的流动、燃烧、传热、磨损、扬析与夹带模型奠定了基础。