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     

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     

Measurements and computation of low mass transfer coefficients for FCC particles with ozone decomposition reaction

The mass transfer coefficients and Sherwood numbers for catalyzed fluid cracking catalyst particles were measured and computed in a two‐dimensional (2‐D) bubbling fluidized bed, with ozone decomposition reaction. The measured and computed Sherwood numbers, using 3‐ and 2‐D kinetic theory based computational fluid dynamics simulations, were of the order of 10^?6–10^?2. The low Sherwood numbers were in reasonable agreement with the literature data for small particles, at low Reynolds numbers. The computational fluid dynamics simulations showed that it is possible to compute conversions in fluidized bed reactors without using the conventional model with empirical mass transfer coefficients. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

A CFD‐PBM‐PMLM integrated model for the gas–solid flow fields in fluidized bed polymerization reactors

Although the use of computational fluid dynamics (CFD) model coupled with population balance (CFD‐PBM) is becoming a common approach for simulating gas–solid flows in polydisperse fluidized bed polymerization reactors, a number of issues still remain. One major issue is the absence of modeling the growth of a single polymeric particle. In this work a polymeric multilayer model (PMLM) was applied to describe the growth of a single particle under the intraparticle transfer limitations. The PMLM was solved together with a PBM (i.e. PBM‐PMLM) to predict the dynamic evolution of particle size distribution (PSD). In addition, a CFD model based on the Eulerian‐Eulerian two‐fluid model, coupled with PBM‐PMLM (CFD‐PBM‐PMLM), has been implemented to describe the gas–solid flow field in fluidized bed polymerization reactors. The CFD‐PBM‐PMLM model has been validated by comparing simulation results with some classical experimental data. Five cases including fluid dynamics coupled purely continuous PSD, pure particle growth, pure particle aggregation, pure particle breakage, and flow dynamics coupled with all the above factors were carried out to examine the model. The results showed that the CFD‐PBM‐PMLM model describes well the behavior of the gas–solid flow fields in polydisperse fluidized bed polymerization reactors. The results also showed that the intraparticle mass transfer limitation is an important factor in affecting the reactor flow fields. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1717–1732, 2012     

DEM‐CFD modeling of particle systems with long‐range electrostatic interactions

To investigate dynamic behaviors of monocharged particle systems, a direct truncation (DT) method and a hybrid particle‐cell (HPC) method are implemented into the discrete element method coupled with computational fluid dynamics (DEM‐CFD) with defined cutoff distances. The DT method only considers electrostatic interactions between particles within the cutoff distance while the HPC method computes electrostatic interactions in the entire computational domain. The deposition process of monocharged particles in a container in air was simulated using the developed DEM‐CFD. It was found that using the DT method, the macrostructure, evolution of granular temperature, and radial distribution function of the particle system were sensitive to the specified cutoff distance. In contrast, using the HPC method, these results were independent of the specified cutoff distance, as expected. This implies that, although electrostatic interactions between particles with large separation distances are weak, they should be considered in DEM‐CFD for accurate modeling of charged particle systems. © 2015 The Authors AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers AIChE J, 61: 1792–1803, 2015     

磁场对湿颗粒流化床系统中介尺度结构影响机制研究

湿颗粒系统在自然界及工业过程非常普遍,例如喷雾造粒、反应器中矿物黏结、催化以及制药等,这其中含有大量典型介尺度结构如颗粒聚团、结块以及气泡等结构,这些结构的存在导致颗粒系统的流动及热质传递特性发生明显改变。针对鼓泡流化床湿颗粒系统中颗粒聚团以及气泡等介尺度结构,应用离散单元模型并引入外加磁场,研究磁场作用下湿颗粒系统中介尺度结构的演化机制,探究磁场力、液桥力、接触力对气泡演化过程的影响。研究发现,在不考虑磁场的条件下,颗粒易形成聚团并存在气泡边界不规则等现象,引入外加匀强磁场后,磁场力对鼓泡流化床内气泡结构存在破坏和抑制作用。     

Effect of particle size on flow and mixing in a bladed granular mixer

A number of studies have modeled flow and mixing of granular materials using the discrete element method (DEM). In an attempt to reduce computational costs, many of these DEM studies model particles larger than the actual particle size without investigating the implications of this assumption. Using DEM, the influence of the modeled particle size on flow and mixing in a bladed granular mixer is studied. The predicted flow microdynamics, including mixing rates, are strongly dependent on the particle diameter. The effect of particle size on macroscopic advective flow also is significant, particularly for dilute flow regions. These results suggest that the influence of particle size needs to be taken into consideration when using larger particles in DEM mixing simulations. To guide scale‐up efforts, particle‐size‐based scaling relationships for several key flow measurements are presented. © 2014 American Institute of Chemical Engineers AIChE J, 61: 46–57, 2015     

Experimental and numerical study on a bubbling fluidized bed with wet particles

As liquid bridge between particles acts an important role in the particle system, it is of considerable significance to analyze the flow hydrodynamics of wet particles in fluidized beds, which will improve the reactor design and process optimization. Thus, experimental and numerical investigations on wet particles in a bubbling fluidized bed are conducted in current work. On experimental side, particle image velocimetry (PIV) technology is employed with a designed bubbling fluidized bed. The silicone oil is used in this work because it is nonvolatile and transparent. On numerical side, a modified discrete element method (DEM) numerical method is developed by compositing an additional liquid‐bridge module into the traditional soft‐sphere interaction model. Most of the physical parameters are chosen to correspond to the experimental settings. Good agreements of particle velocity are found between the DEM simulation and PIV measurement. The performance of different liquid contents and superficial gas velocities are examined. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1970–1985, 2016     

A system‐size independent validation of CFD‐DEM for noncohesive particles

For the first time, CFD‐DEM simulations of small‐scale fluidized beds are quantitatively validated against large‐scale experiments. Such validation is possible via the identification of a measurement independent of system size, namely defluidization. CFD‐DEM inputs (particle properties and operating conditions) are measured directly. Sphericity is found to be critical, even for highly spherical particles. This size‐independent method of validation is valuable since it allows for validation of CFD‐DEM models without restrictions on system sizes or particle sizes. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4051–4058, 2015     

Effective particle diameters for simulating fluidization of non‐spherical particles: CFD‐DEM models vs. MRI measurements

Computational fluid dynamics—discrete element method (CFD‐DEM) simulations were conducted and compared with magnetic resonance imaging (MRI) measurements (Boyce, Rice, and Ozel et al., Phys Rev Fluids. 2016;1(7):074201) of gas and particle motion in a three‐dimensional cylindrical bubbling fluidized bed. Experimental particles had a kidney‐bean‐like shape, while particles were simulated as being spherical; to account for non‐sphericity, “effective” diameters were introduced to calculate drag and void fraction, such that the void fraction at minimum fluidization (ε_mf) and the minimum fluidization velocity (U_mf) in the simulations matched experimental values. With the use of effective diameters, similar bubbling patterns were seen in experiments and simulations, and the simulation predictions matched measurements of average gas and particle velocity in bubbling and emulsion regions low in the bed. Simulations which did not employ effective diameters were found to produce vastly different bubbling patterns when different drag laws were used. Both MRI results and CFD‐DEM simulations agreed with classic analytical theory for gas flow and bubble motion in bubbling fluidized beds. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2555–2568, 2017     

颗粒移动床内不稳定运动的计算颗粒动力学模拟

采用考虑滚动摩擦的三方程离散单元法（DEM）模型对侧开孔的移动床中的颗粒流动进行了数值研究。结果表明，计算颗粒动力学(CGD)方法可对复杂颗粒系统内颗粒的运动行为进行准确的预测，包括时均速度场和脉动速度场。讨论了模型中颗粒摩擦参数的重要影响，并对颗粒流动表现出的间歇现象进行了分析。颗粒流动与流体流动有相似之处，都存在随机的脉动，但颗粒流的随机脉动机理与流体中的湍流机理有很大不同，颗粒流动会表现出很强的不连续性。     

Flow of sphero-disc particles in rectangular hoppers—a DEM and experimental comparison in 3D

Flows of “sphero-disc” granular particles in a rectangular hopper are studied both experimentally using high-speed video recording and mathematically using the discrete element method (DEM). The flow behaviour of particles and their arching and discharging in the hopper are analysed and compared with the DEM results for three hopper openings. In general, good agreement is shown on particle static packing, the flow behaviour and hopper discharging rates and the arching effect when flow ceases due to an inadequate hopper outlet opening. Spherical particles with a similar volume to the disc-like particles are also tested and compared and a clear effect of particle shape on flow rates is shown. Although some minor discrepancies are shown, these are likely to be caused by the practical difficulties in matching the exact particle parameters between the simulations and the experiments. The DEM is shown to be a powerful tool to analyse the interactions between irregularly shaped particles and demonstrates a great potential in analysing detailed particle packing structure and flow patterns, which may lead to the elaboration of a novel method for hopper design. Further work will focus on developing DEM to model a wider range of particle shapes and hopper geometries, use of DEM for flow and structure analysis, and the development of more sophisticated measuring tools such as tomography to validate the DEM model.     

An extended DEM–CFD model for char combustion in a bubbling fluidized bed combustor of inert sand

This paper proposes a transient three-phase numerical model for the simulation of multiphase flow, heat and mass transfer and combustion in a bubbling fluidized bed of inert sand. The gas phase is treated as a continuum and solved using the computational fluid dynamics (CFD) approach; the solid particles are treated as two discrete phases with different reactivity characteristics and solved on the individual particle scale using an extended discrete element model (DEM). A new char combustion submodel considering sand inhibitory effects is also developed to describe char particle combustion behavior in the fluidized bed. Two conditions, i.e. a single larger graphite particle and a batch of smaller graphite particles, are used to test the prediction capability of the model. The model is validated by comparing the predicted results with the previous measured results and conclusions in the literature in terms of bed hydrodynamics, individual particle temperature, char residence time and concentrations of the products. The effects of bed temperature, oxygen concentration and superficial velocity on char combustion behavior are also examined through model simulation. The results indicate that the proposed model provides a proximal approach to elucidate multiphase flow and combustion mechanisms in fluidized bed combustors.     

Numerical study on microscopic mixing characteristics in fluidized beds via DEM

In this paper, discrete element method (DEM), combined with computational fluid dynamics (CFD), is used to investigate the micro-mixing process in fluidized beds (FBs) of uniform particles. With the aid of snapshots and adoption of Lacey and Ashton indexes, mixing evolvement for two cases, fluidized bed using horizontal distributor with even gas supply and fluidized bed using inclined distributor with uneven gas supply, is discussed in detail. Results indicate that the Ashton index appears to be more effective in assessing the mixing dynamics in this work. Further analyses illustrate that in the case of horizontal distributor incorporated with even gas supply, diffusive mixing pattern is predominant, since bubbles lateral motion is reduced in such a bed; whereas, there is a faster convective mixing process in a fluidized bed using inclined distributor with uneven gas feed, followed by shear mixing. Generally, localized air supply induces the density gradient of particle distribution in the bed, which is the basic agent of convective particle stream. The analyses are confirmed by the comparison of solid flux during the simulations of the two cases. In addition, the mixing mechanism and the mixing time scale agree well with published experimental results.     

循环流化床多组分颗粒气固两相流动模型和数值模拟

基于稠密气体分子运动论和颗粒动力学,考虑多组分颗粒中颗粒组分与颗粒组分、颗粒组分内颗粒之间的相互作用以及气体与颗粒之间的相互作用,提出多组分颗粒非等温颗粒气固两相流动模型.以颗粒压力、径向分布函数、黏度、颗粒碰撞耗散等耦合各颗粒组分间和颗粒间的相间作用.采用大涡模拟方法模拟气相湍流流动.提出了多组分颗粒的径向分布函数计算方法.对循环流化床上升管中双组分颗粒气固两相流动特性进行了数值模拟,模拟结果揭示了上升管中双组分颗粒气固两相流动的环-核流动结构,得到了平均颗粒粒径的轴向和径向分布规律,计算结果与文献中实验结果相吻合.     

基于DEM模拟气固循环流化床提升管内颗粒聚团特性

采用CFD-DEM的方法对气固循环流化床提升管内的气固流动特性进行模拟,建立了基于图像处理的分析颗粒聚团的方法,重点研究了颗粒聚团在床层内的整体分布以及颗粒聚团的特性,包括颗粒聚团的倾角、球形度以及长短轴比的概率密度分布以及它们在床层内的轴向和径向上的分布特性。研究结果表明,聚团在床层内的分布较宽,较小的聚团居多,边壁区域附近易形成较大的聚团。聚团的数目沿床层高度方向先增加后减少。聚团倾向于以偏离球形聚团、较大的倾角形式存在,其长短轴比值在2～4之间。