Particle-scale simulation of fluidized bed with immersed tubes

In order to simulate gas-solids flows with complex geometry, the boundary element method was incorporated into the implementation of a combined model of computational fluid dynamics and discrete element method. The resulting method was employed to simulate hydrodynamics in a fluidized bed with immersed tubes. The transient simulation results showed particle and bubble dynamics. The bubble coalescence and break-up behavior when passing the immersed tubes was successfully predicted. The gassolid flow pattern in the fluidized bed is changed greatly because of the immersed tubes. As particles and gas are come in contact with the immersed tubes, the gas bubbles will be deformed. The collisions between particles and tubes will make the tubes surrounded by air pockets most of the time and this is unfavorable for the heat transfer between particles and tubes. __________ Translated from Chemical Engineering, 2007, 35(11): 21–24 [译自:化学工程]     

沉浸管式流化床的颗粒尺度模拟

为模拟具有复杂几何结构的气固流动系统,文中将计算流体力学和离散单元法与边界元方法结合起来,对沉浸管式流化床内颗粒及气泡的运动行为进行了数值模拟。模拟计算得到的瞬态流型图揭示了气泡绕流沉浸管束时出现的合并和破碎状态及颗粒群的详细运动行为,发现床内气固二相的流动受到沉浸管束存在的显著影响。当颗粒及气相的流动受到沉浸管的阻碍而绕管流动过程中气泡会发生变形,变得扭曲狭长且易被撕碎。同时颗粒与管道壁面碰撞会造成气固二相复杂的动态运动形式,床内的管道大部分时间会被气穴包围,将严重阻碍管道与颗粒之间的传热。     

Discrete element simulation of particle flow in arbitrarily complex geometries

Conventional simulations of dense particle flows in complex geometries usually involve the use of glued particles to approximate geometric surface. This study is concerned with the development of a robust and accurate algorithm for detecting the interaction between a spherical particle and an arbitrarily complex geometric surface in the framework of soft-sphere discrete element model (DEM) without introducing any assumptions. Numerical experiments specially designed to validate the algorithm shows that the new algorithm can accurately predict the contact state of a particle with a complex geometric surface. Based on the proposed algorithm, a new solver for simulation of dense particle flows is developed and implemented into an open source computational fluid dynamics (CFD) software package OpenFOAM. The solver is firstly employed to simulate hydrodynamics in a bubble fluidized bed. Numerical results show that a 3D simulation can predict the bubble size better than a 2D simulation. Subsequently, gas–solid hydrodynamics in an immersed tube fluidized bed is simulated. Results show that bubble coalescence and breakup behavior around the immersed tubes are well captured by the numerical model. In addition, seven different particle flow patterns around the immersed tubes are identified based on the numerical results obtained.     

Particle‐scale simulation of the flow and heat transfer behaviors in fluidized bed with immersed tube

A kind of new modified computational fluid dynamics‐discrete element method (CFD‐DEM) method was founded by combining CFD based on unstructured mesh and DEM. The turbulent dense gas–solid two phase flow and the heat transfer in the equipment with complex geometry can be simulated by the programs based on the new method when the k‐ε turbulence model and the multiway coupling heat transfer model among particles, walls and gas were employed. The new CFD‐DEM coupling method that combining k‐ε turbulence model and heat transfer model, was employed to simulate the flow and the heat transfer behaviors in the fluidized bed with an immersed tube. The microscale mechanism of heat transfer in the fluidized bed was explored by the simulation results and the critical factors that influence the heat transfer between the tube and the bed were discussed. The profiles of average solids fraction and heat transfer coefficient between gas‐tube and particle‐tube around the tube were obtained and the influences of fluidization parameters such as gas velocity and particle diameter on the transfer coefficient were explored by simulations. The computational results agree well with the experiment, which shows that the new CFD‐DEM method is feasible and accurate for the simulation of complex gas–solid flow with heat transfer. And this will improve the farther simulation study of the gas–solid two phase flow with chemical reactions in the fluidized bed. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Hydrodynamics of gas-solid flow around immersed tubes in bubbling fluidized beds

A numerical study was conducted based on the gas-solid two-fluid model using the body-fitted coordinate system to analyze the behavior of particles and bubbles flow in bubbling fluidized beds without and with immersed tubes. The kinetic theory of granular flow was implemented in the model. The images of simulated instantaneous particle concentration and velocity gave the process of the formation, coalescence and eruption of bubbles. The effects of the tube pitch and superficial gas velocity on the fluidization in a bubbling fluidized bed were investigated. Calculated bubble frequencies without and with immersed tubes were in agreement with previous experimental and simulation findings. The wavelet multi-resolution analysis was used to analyze the simulated data of instantaneous particle concentration. From the random-like particle concentration fluctuations, the fluctuating components due to particle flow and bubble motion can be extracted based on the wavelet multi-resolution analysis over a time-frequency plane.     

Numerical simulation of pulsed fluidized bed with immersed tubes using DEM-LES coupling method

The present work is a 2-D numerical simulation of pulsed fluidized bed with immersed tubes using DEM-LES coupling method. The pulsed inflow of gas phase is modeled as U₀(1+sin(2πft)), in which four pulsating frequencies of f=5, 10, 15 and 20 of velocity inflow are used. The discrete element method (DEM) simulation for particle motion coupled with the large eddy simulation (LES) for gas phase is used. The fluidized bed with five immersed tubes of staggered arrangement and six immersed tubes of in-line arrangement is simulated, respectively. It is found that the pressure drop, the mean drag force and the mean pressure gradient force experienced by particles are forced oscillated. The different effects of pulsed fluidization on the circumferential distribution of particle-tube collision on the outer surface of tubes at different pulse frequencies and modes of arrangement of immersed tubes are numerically analyzed. Finally, it is found that the pulsed motion of fluid with high frequency leads to suppression of particle fluctuating motion.     

带浸没管的惰性粒子振动流化床传热特性

采用带浸没加热管的惰性粒子振动流化床对膏状物料干燥进行了实验研究。考察了加料速率、进气温度、进气速度、加热管功率、振动强度等参数对床温和体积传热系数的影响,得出了计算体积传热系数的关联式。结果表明,在流化床中增设振动和浸没加热管装置,能大大强化传热传质,体积传热系数随加料量、振动强度、加热管功率、进风速度的增加而增大,随进气温度的增加而减小。其结果对惰性粒子流化床干燥器的设计和改进具有重要的指导意义。     

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。     

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.     

内置水平管式鼓泡流化床动力学的数值模拟

基于欧拉双流体模型,在水平管表面采用渐进式网格,对内置水平管式鼓泡流化床的动力学特性进行数值模拟,研究表观气速对时均空隙度与膨胀率的影响。模拟结果显示:气泡在水平管排区域存在合并、破裂等运动行为;在管排区域随着表观气速的增加,时均空隙度增大,并且相比于无管式流化床沿径向方向更加均匀;内置水平管式流化床的时均膨胀率高于无沉浸管式流化床的时均膨胀率,且随表观气速的增加有逐渐接近的趋势。     

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.     

Euler multiphase-CFD simulation on a bubble-driven gas–liquid–solid fluidized bed

An integrated flow model was developed to simulate the fluidization hydrodynamics in a new bubble-driven gas–liquid–solid fluidized bed using the computational fluid dynamic (CFD) method. The results showed that axial solids holdup is affected by grid size, bubble diameter, and the interphase drag models used in the simulation. Good agreements with experimental data could be obtained by adopting the following parameters: 5 mm grid, 1.2 mm bubble diameter, the Tomiyama gas–liquid model, the Schiller–Naumann liquid–solid model, and the Gidaspow gas–solid model. At full fluidization state, an internal circulation of particles flowing upward near the wall and downward in the centre is observed, which is in the opposite direction compared with the traditional core-annular flow structure in a gas–solid fluidized bed. The simulated results are very sensitive to bubble diameters. Using smaller bubble diameters would lead to excessive liquid bed expansions and more solid accumulated at the bottom due to a bigger gas–liquid drag force, while bigger bubble diameters would result in a higher solid bed height caused by a smaller gas–solid drag force. Considering the actual bubble distribution, population balance model (PBM) is employed to characterize the coalescence and break up of bubbles. The calculated bubble diameters grow up from 2–4 mm at the bottom to 5–10 mm at the upper section of the bed, which are comparable to those observed in experiments. The simulation results could provide valuable information for the design and optimization of this new type of fluidized system.     

Prediction on immersed tubes erosion using two-fluid model in a bubbling fluidized bed

A numerical study based on a gas-solid two-fluid model using body-fitted coordinates to predict the immersed tube erosion rate in a bubbling fluidized bed has been conducted. Computations have been performed at various s/D ratios (ratio of the spacing between two tube centers to the diameter of the tubes) for a bed into which one, two, three or four immersed tubes were inserted. A monolayer kinetic energy dissipation model is implemented to simulate the erosion on the surfaces of the immersed tubes at the body-fitted coordinates. The effect of the s/D ratios on the erosion rate in the bubbling fluidized bed is investigated. A granular temperature distribution provides an indication of the regions on the immersed tubes that could be susceptible to fluctuation intensity of the solid phase. Wake properties influenced the erosion rate remarkably. Calculated erosion rates on the surfaces of the immersed tubes were in good agreement with previous experimental and numerical findings.     

Particle‐resolved direct numerical simulation of gas–solid dynamics in experimental fluidized beds

Particle‐resolved direct numerical simulations (PR‐DNS) of a simplified experimental shallow fluidized bed and a laboratory bubbling fluidized bed are performed by using immersed boundary method coupled with a soft‐sphere model. Detailed information on gas flow and individual particles’ motion are obtained and analyzed to study the gas–solid dynamics. For the shallow bed, the successful predictions of particle coherent oscillation and bed expansion and contraction indicate all scales of motion in the flow are well captured by the PD‐DNS. For the bubbling bed, the PR‐DNS predicted time averaged particle velocities show a better agreement with experimental measurements than those of the computational fluid dynamics coupled with discrete element models (CFD‐DEM), which further validates the predictive capability of the developed PR‐DNS. Analysis of the PR‐DNS drag force shows that the prevailing CFD‐DEM drag correlations underestimate the particle drag force in fluidized beds. The particle mobility effect on drag correlation needs further investigation. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1917–1932, 2016     

梯度磁场下气固流化床中磁颗粒运动数值模拟

在外加竖直方向梯度磁场的气固鼓泡流化床中,考虑铁磁颗粒受到的梯度磁场力和颗粒间磁感应力,对气相采用流体力学方法(CFD),颗粒相采用离散元法(DEM),建立二维磁鼓泡流化床数学模型,模拟不同磁场强度下全磁颗粒圆形床料的气固流动过程,分析了不同磁场强度对磁流化床中气泡生长、颗粒运动、床层压降和磁颗粒受力的影响。研究结果表明在沿高度磁感应强度递减的梯度磁场中,磁颗粒在颗粒间磁感应力的作用下凝聚成链,破坏了大气泡的形成。随着磁场强度增加,颗粒扩散系数减小,颗粒间磁感应力和梯度磁场力增大;气体与颗粒相间作用力先减小、后增加;而颗粒接触力先增加、后减小。     

CFD–DEM simulation of tube erosion in a fluidized bed

The erosion of the immersed tubes in a bubbling‐fluidized bed is studied numerically using an Eulerian–Lagrangian approach coupling with a particle‐scale erosion model. In this approach, the motion of gas and particles is simulated by the CFD–DEM method, and an erosion model SIEM (shear impact energy model) is proposed to predict the erosion of the tubes. The model is validated by the good agreement of the simulation results and previous experimental data. By analyzing the simulation results, some characteristics of the tube erosion in the fluidized bed are obtained, such as the distribution of the erosion rate around the tube, the variation of the erosion rate with the position of the tube, the effect of the friction coefficient of particles on the erosion, the relationship between the maximum and the average erosion rate, etc. The microscale behavior of particles around the tubes is also revealed and the linear relationship between the erosion and the shear impact energy is confirmed by the simulation results and experiment. The agreement between simulation and experiment proves that the microscale approach proposed in this article has high accuracy for predicting erosion of the tubes in the fluidized bed, and has potential to be applied to modeling the process in other chemical equipment facing solid particle erosion. © 2016 American Institute of Chemical Engineers AIChE J, 63: 418–437, 2017     

Effect of local disturbance on the particle-tube collision in bubbling fluidized bed

A numerical investigation on the effect of local disturbance of gas turbulence on the probability distribution function (PDF, for shot) and the rate of particle-tube collision in bubbling fluidized bed is carried out. A two-dimensional (2D) fluidized bed with 6 and 18 immersed tubes (a tube bank) and a three-dimensional (3D) bed with two immersed tubes are simulated, respectively. The local disturbance of gas turbulence is introduced by a blowing gas flow injected from the underneath of the immersed tubes (2D) and from the walls of the bed (3D), respectively. In two-dimensional, the blowing gas flow is injected upstream, downstream or not injected; whereas in the three-dimensional case the gas flow is injected from the side walls of the bed. A DEM-LES coupling simulation method is applied. The results indicate a great increase in particle-tube collision rates and a change in the PDF profiles when the local disturbance is introduced, especially for the case of upstream disturbance. Two main mechanisms are interpreted, i.e. the turbulence augmentation and the formation of weaker velocity regions which both contribute to the changes in characteristics of particle-tube collision but act in independent ways.     

布风方式对流化床混合特性的影响

通过将离散单元法同计算流体力学相结合,对流化床内物料混合过程进行了研究。给出了水平布风板均匀布风、倾斜布风板非均匀布风2种情况下的示踪颗粒场历变过程。模拟结果表明:瞬时颗粒场组图能够较为直观表征床内混合现象;其中,在均匀布风情况下,床内气泡横向运动受到限制,颗粒整体横向运动能力较弱,混合方式以扩散混合为主;而对于非均匀布风流化床,床内存在较大的横向颗粒浓度梯度,对流混和起主要作用,且混合速度较为迅速。     

DEM-LES study of 3-D bubbling fluidized bed with immersed tubes

Based on Euler-Lagrange frame, a true three-dimensional numerical simulation of bubbling fluidized bed embedded with two immersed tubes is presented. The solid phase is composed of 178,200 particles of diameter and simulated by discrete element method (DEM, a soft-sphere approach). The gas phase is computed through solving the volume-averaged four-way coupling Navier-Stokes equations in which the Smagorinsky SGS tensor model is used in large eddy simulation (LES). Particle-tube collision is particularly treated as a transformation of DEM. The volume segmentation of a particle sphere for void fraction calculation is solved via a numerical sub-division approach. The numerical results are compared with the experimental results for validation. The results obtained with and without the LES model are also compared. The numerical results show a strong correlation between gas-particle interaction, particle-particle interaction, pressure drop, particle back mixing motion and bubble motion, and all of them follow a similar pattern of synchronous periodic variation though the periodicity may vary depending on different flow conditions. The effects of SGS tensor on evolution of fluidized bed are found in various aspects. Finally, the distribution of particle-tube impact frequency is given.     

Modeling gas‐particle two‐phase flows with complex and moving boundaries using DEM‐CFD with an immersed boundary method

An immersed boundary method (IBM) has been developed and incorporated into the coupled discrete element method and computational fluid dynamics (DEM‐CFD) approach to model particulate systems consisting of a compressible gas and solid particles with complex and/or moving boundaries. The IBM is used to deal with the interaction between gas and complex and moving boundaries by using simple rectangular grids to discretize the fluid field. The developed method has been applied to simulate some typical powder handling processes (e.g., gas fluidization with an immersed tube, segregation in a vertically vibrated bed, and pneumatic conveying). Good agreement is achieved between the present simulation results and the experimental ones reported in the literature. It has been demonstrated that the capacity of DEM‐CFD is enhanced with the incorporation of IBM, which can be used to simulate a wide range of problems that could not be handled with the conventional DEM‐CFD method. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1075–1087, 2013