Numerical Investigation of Three‐Dimensional Bubble Column Flows: A Detached Eddy Simulation Approach

Numerical simulations were performed employing detached eddy simulation (DES) in a three‐dimensional transient Euler‐Euler framework for bubble columns, and all the computational fluid dynamics results were compared with a k‐? model and available experimental data. The numerical results are in good agreement with the experiments in predicting the time‐averaged axial velocity and turbulent kinetic energy profiles. The flow‐resolving capabilities of the DES model are highlighted, and it is shown that the DES turbulence model can be efficiently used for simulating flow field and turbulent quantities in the case of bubble columns.     

涡轮桨搅拌槽内单循环流动特性的大涡模拟

利用大涡模拟方法研究了涡轮桨搅拌槽内的单循环流动特性,采用Smagorinsky-Lilly动力亚格子模式,与文献实验及模拟数据进行了详细的比较. 结果表明,叶片后方的双尾涡偏向槽底运动,上尾涡在30o处已开始衰减. 800000个非均匀分布的计算网格和30个桨叶旋转周期的样本数据统计可获得准确的大涡模拟数据. 时均速度、均方根速度和湍流动能的大涡模拟值与实验数据一致,而k-e模型的模拟值与实验不符. 桨叶区呈现较强的各向异性,这是导致k-e模型预测不准确的主要原因. 对于搅拌槽内的复杂流动,大涡模拟方法是一个非常有效的工具.     

用大涡模拟结合改进的内外迭代法数值模拟搅拌槽中的液相流动

1 INTRODUCTION Stirred tank reactors are widely encountered in the chemical, pharmaceutical, and hydrometallurgical proc- esses. The fluid motion in stirred tanks is three-dimensional, complex, and covers a wide range of spatial and temporal scales. In the area surrounding the impeller, the flow is highly turbulent and swirling. The numerical simulation of such reactor systems is helpful in quantifying the effects of the impeller type, geometry, and the operational conditions in order to …     

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     

涡流桨搅拌槽内流动非稳定性的大涡模拟

The aim of this work is to investigate the flow instabilities in a baffled, stirred tank generated by a single Rushton turbine by means of large eddy simulation （LES）. The sliding mesh method was used for the coupling between the rotating and the stationary frame of references. The calculations were carried out on the ＂Shengcao-21C＂ supercomputer using a computational fluid dynamics （CFD） code CFX5. The flow fields predicted by the LES simulation and the simulation using standard κ-ε model were compared to the results from particle image velocimetry （PIV） measurements. It is shown that the CFD simulations using the LES approach and the standard κ-ε model agree well with the PIV measurements. Fluctuations of the radial and axial velocity are predicted at different frequencies by the LES simulation. Velocity fluctuations of high frequencies are seen in the impeller region, while low frequencies velocity fluctuations are observed in the bulk flow. A low frequency velocity fluctuation with a nondimensional frequency of 0.027Hz is predicted by the LES simulation, which agrees with experimental investigations in the literature. Flow circulation patterns predicted by the LES simulation are asymmetric, stochastic and complex, spanning a large portion of the tanks and varying with time, while circulation patterns calculated by the simulation using the standard κ-ε model are symmetric. The results of the present work give better understanding to the flow instabilities in the mechanically agitated tank. However, further analysis of the LES calculated velocity series by means of fast Fourier transform （FFT） and/or spectra analysis are recommended in future work in order to gain more knowledge of the complicated flow phenomena.     

Pilot‐scale investigation and CFD modeling of particle deposition in low‐dust monolithic SCR DeNOx catalysts

Deposition of particles in selective catalytic reduction DeNO_x monolithic catalysts was studied by low‐dust pilot‐scale experiments. The experiments showed a total deposition efficiency of about 30%, and the deposition pattern was similar to that observed in full‐scale low‐dust applications. On extended exposure to the dust‐laden flue gas, complete blocking of channels was observed, showing that also in low‐dust applications soot blowing is necessary to keep the catalyst clean. A particle deposition model was developed in computational fluid dynamics, and simulations were carried out assuming either laminar or turbulent flow. Assuming laminar flow, the accumulated mass was underpredicted with a factor of about 17, whereas assuming turbulent flow overpredicted the experimental result with a factor of about 2. The simulations showed that turbulent diffusion in the monolith channels and inertial impaction and gravitational settling on the top of the monolith were the dominating mechanisms for particle deposition on the catalyst. © 2013 American Institute of Chemical Engineers AIChE J, 59: 1919–1933, 2013     

LDA measurements and CFD simulations of an in‐line high shear mixer with ultrafine teeth

Hydrodynamics of a pilot‐scale in‐line high shear mixer (HSM) with double rows of ultrafine rotor and stator teeth, including the velocity profiles and power consumptions, were measured using laser Doppler anemometry and a torque transducer, respectively. Computational fluid dynamics simulations were conducted using the standard k‐ε turbulence model with first‐ and second‐order accuracy and large eddy simulation (LES) with the standard Smagorinsky–Lilly subgrid scale model. Predictive capabilities of the different turbulence models and discretization schemes were assessed based on the experimental data. It is found that the current LES can predict accurately the flow patterns for the strongly rotating and locally anisotropic turbulent flows in the complex in‐line HSM. The results obtained are fundamental to explore potential applications of the in‐line teethed HSMs to intensify chemical reaction processes. © 2013 American Institute of Chemical Engineers AIChE J, 60: 1143–1155, 2014     

Experimental and computational study of the “end of the vortex” phenomenon in reverse‐flow centrifugal separators

The “end of the vortex” (EoV) phenomenon, a flow instability that plays a crucial role in cyclone design and operation is studied in this article. In the experimental part of the study, tests were carried out to understand the origin and nature of the EoV and to study the effects of the flowrate through, and the length of, the cylindrical cyclone on the EoV. In the theoretical part, computational fluid dynamics (CFD) models, in agreement with the geometrical configurations and operating conditions used in the present and earlier experimental studies, were constructed and investigated. Three‐dimensional simulations were carried out using the large eddy turbulence model with the commercial CFD package Star‐CD. Bending of the vortex core to the wall of the vessel and its precessional motion, constituting the phenomenon of the EoV, was observed in the simulations in most geometries. The results are in a good agreement not only with the present experimental results but also with previous experimental observations. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Novel aeration of a large‐scale flat sheet MBR: A CFD and experimental investigation

Having previously established that the hydrodynamic effect introduced by slug bubbles is more effective and economic in fouling amelioration in flat sheet MBRs (FSMBR) than conventional bubbling, this work is focused on its implementation in a commercial FSMBR. The overall objective is to enhance the hydrodynamic effect on fouling control through the use of two‐stage large‐sized bubble development (coalescence and split). Computational Fluid Dynamics (CFD) was used to predict hydrodynamic features and substantial agreement was observed with experimental measurements. The critical height for bubble development space was determined to be circa 250 mm. Slug bubbles could be introduced into 14 channels, resulting in six‐fold stronger shear stress than that from single bubbles. Energy demand could be reduced by circa 50% compared with industry average usage and the shear stresses developed would, for most applications, be sufficient to ameliorate fouling. Furthermore, the specific air demand per permeate would be halved. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2721–2736, 2018     

A particle‐scale index in the quantification of mixing of particles

Discrete element method (DEM) is a useful tool for obtaining details of mixing processes at a particle scale. It has been shown to satisfactorily describe the flow structure developed in bladed mixers. Here, the advantage is taken of the microstructure gained from DEM to evaluate how best to quantify the microstructure created by mixing. A particle‐scale mixing index (PSMI) is defined based on coordination numbers to represent the structure of a particle mixture. The mixture quality is then analyzed qualitatively and quantitatively in three different ways: a macroscopic mixing index based on the conventional approach, coordination number, and PSMI. Their effectiveness is examined based on DEM data generated for different particle loading arrangements and binary mixtures of particles with various volume fractions, size ratios, and density ratios. Unlike the two other methods, PSMI reveals in a straightforward manner whether a binary mixture of different particles is mixing or segregating over time, while being able to detect particle‐scale structural changes accompanying the mixing or segregation processes in all the mixtures investigated. Moreover, PSMI is promising in that it is not influenced by the size and number of samples, which afflict conventional mixing indexes. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

LES–Lagrangian‐particles‐simulation of turbulent reactive flows at high Sc number using approximate deconvolution model

Large eddy simulation (LES) with the approximate deconvolution model is combined with Lagrangian particles simulation (LPS) for simulating turbulent reactive flows at high Schmidt numbers Sc. The LES is used to simulate velocity and nonreactive scalar while reactive scalars are simulated by the LPS using the mixing volume model for molecular diffusion. The LES–LPS is applied to turbulent scalar mixing layers with a second‐order isothermal irreversible reaction at Sc = 600. The mixing volume model is implemented with the IEM, Curl's, and modified Curl's mixing schemes. The mixing volume model provides a correct decay rate of nonreactive scalar variance at high Sc independently of the number of particles. The statistics in the LES–LPS with the IEM or modified Curl's mixing scheme agree well with the experiments for both moderately‐fast and rapid reactions. However, the LPS with the Curl's mixing scheme overpredicts the effects of the rapid reaction. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2912–2922, 2016     

Numerical simulations of a full‐scale cable tray fire using small‐scale test data

This paper presents a computational fluid dynamics (CFD)‐based modeling strategy for the prediction of cable tray fire development. The methodology is applied to a set of five horizontal trays (each 2.4‐m long and 0.45‐m wide) that are positioned with a 0.3‐m vertical spacing and set up against an insulated wall. Each tray contains 49 power PVC cables. Ignition is performed with an 80‐kW propane burner centrally positioned at 0.2 m below the lowest tray. A collection of four groups of cables per tray (made of one homogeneous material) is considered. These groups are separated by longitudinal slots of air to “mimic” their relatively “loose arrangement.” The thermal properties and surface ignition temperature are estimated from cone calorimetry (CC). When the ignition temperature is reached, the cables burn according to a prescribed heat release rate per unit area (HRRPUA) profile obtained from CC, as is or in a modified shape. A realistic flame pattern is predicted. Furthermore, using only data from CC, the peak HRR is underpredicted, and the time to reach the peak is overpredicted. The proposed “design” for the modified HRRPUA CC‐profile significantly improves the results.     

涡轮桨搅拌槽内流动特性的大涡模拟

利用大涡模拟方法研究了涡轮桨搅拌槽内的流动特性,采用了三种亚格子模式:标准Smagorinsky-Lilly模式(SLM)、Smagorinsky-Lilly动力模式(DSLM)和亚格子动能动力模式(DKEM),并将模拟结果与标准k-ε模型及文献实验数据进行了详细的比较.结果表明:大涡模拟方法可获得搅拌槽内的瞬态流场;对桨叶区时均速度及湍流动能的预测与实验数据相吻合,比标准k-ε模型计算结果有明显改进,三种亚格子模型中DSLM和DKEM模拟结果更好.同时分析了大涡模拟中桨叶端部附近湍流动能估计偏差的原因,发现主要是由于对轴向湍流均方根速度的预测偏差造成的.大涡模拟方法为搅拌槽内非稳态、周期性的湍流流动和湍流特性的研究提供了强有力的工具.     

埋管流化床内不同粒径颗粒传热行为的欧拉-拉格朗日模拟研究

将有限元方法、基于非结构化网格的计算流体力学方法与离散单元法结合,建立了CFD-DEM-FEM耦合方法,并在此基础上采用k-ε湍流模型及考虑颗粒间及气固间作用的多向耦合传热模型,对埋管流化床内的流动和传热行为进行了模拟和分析.通过计算结果从微观尺度探讨了埋管流化床内的传热机制,分析了影响床内传热的关键因素,得到了换热管道周围固含率和传热系数的分布规律,考察了颗粒直径对埋管周围传热系数的影响.数值模拟结果与实验数据基本一致,证实了CFD-DEM-FEM耦合方法模拟复杂气固流动和传热的可行性和准确性.     

机械镀锌过程中锌粉运动的模拟分析

基于离散单元法,采用EDEM离散元软件模拟机械镀锌过程,研究了冲击介质以不同速率冲击时锌粉的运动特性.结果表明,在机械镀锌过程中,锌粉先被纵向压缩,最后通过在纵、横方向进行重排和压缩变形得到镀锌层.上层锌粉主要发生变形和重排,中、下层锌粉以重排为主.当冲击速率为1.0 m/s时,所得机械镀锌层光滑平整,附着力为0级.模...     

Investigating dry reforming of methane with spatial reactor profiles and particle‐resolved CFD simulations

Dry reforming of methane (DRM) over nickel in a fixed‐bed reactor of spheres was studied experimentally and with CFD simulations. Temperature and mole fraction profiles were measured in a dedicated profile reactor as function of axial coordinate. Particle‐resolved CFD simulations took into account conjugate heat transfer, surface‐to‐surface radiation, and surface reactions described by microkinetics. Energy transport of CFD simulations were verified by studying heat transfer without chemical reactions. DRM experiments could not be reproduced with the original microkinetics formulation, even with the axial temperature profile applied. A detailed analysis of the microkinetics showed that thermodynamic inconsistencies are present, which are amplified by high surface coverage of CO*. After modifying the mechanism the experiments could be reproduced. This study shows how complex interactions between local transport phenomena and local kinetics can be quantified without relying on transport correlations. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4436–4452, 2016     

Micromechanic modeling and analysis of the flow regimes in horizontal pneumatic conveying

Pneumatic conveying is an important technology for industries to transport bulk materials from one location to another. Different flow regimes have been observed in such transportation processes, but the underlying fundamentals are not clear. This article presents a three‐dimensional (3‐D) numerical study of horizontal pneumatic conveying by a combined approach of discrete element model for particles and computational fluid dynamics for gas. This particle scale, micromechanic approach is verified by comparing the calculated and measured results in terms of particle flow pattern and gas pressure drop. It is shown that flow regimes usually encountered in horizontal pneumatic conveying, including slug flow, stratified flow, dispersed flow and transition flow between slug flow and stratified flow, and the corresponding phase diagram can be reproduced. The forces governing the behavior of particles, such as the particle–particle, particle‐fluid and particle‐wall forces, are then analyzed in detail. It is shown that the roles of these forces vary with flow regimes. A general phase diagram in terms of these forces is proposed to describe the flow regimes in horizontal pneumatic conveying. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

Density segregation of dry and wet granular mixtures in gas fluidized beds

The Discrete Element Method combined with Computational Fluid Dynamics was coupled to a capillary liquid bridge force model for computational studies of mixing and segregation behaviors in gas fluidized beds containing dry or wet mixtures of granular materials with different densities. The tendency for density segregation decreased with increasing fluidizing velocity, coefficient of restitution, and amount of liquid present. Due to the presence of strong capillary forces between wet particles, there was a high tendency for particles to form agglomerates during the fluidization process, resulting in lower segregation efficiency in comparison with fluidization of dry particles. Particle‐particle collision forces were on average stronger than both fluid drag forces and capillary forces. The magnitudes of drag forces and particle‐particle collision forces increased with increasing fluidizing velocity and this led to higher mixing or segregation efficiencies observed in dry particles as well as in wet particles at higher fluidizing velocities. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4069–4086, 2015