The effect of axial impeller geometry on the link between power and flow numbers

Stirred vessels are used to facilitate mixing processes across a wide range of industries. Their performance can often be predicted with certain characteristics of the agitator, like the dimensionless power and flow numbers. Since there exists a large number of agitator designs and geometries, it is desirable to be able to predict these characteristics using models rather than rely on previous experimental data. In this study, we use an angular momentum balance combined with computational fluid dynamics to correlate the power, flow, and mixer geometry across a wide range of down-pumping pitched blade turbine geometries. The models developed from the results allow us to predict the power from the flow (or vice versa) for the geometries investigated. We tested two methods for the measurement of the flow rate and found that the choice of measurement method can affect the relationship between the power, flow, and impeller geometry.     

旋流脱气性能影响因素的CFD模拟

液体脱气是工业生产环节不可缺少的部分。脱气式旋流器能够在线、快速地进行脱气工作, 但目前发展处于初期阶段, 对于其脱气性能影响因素的探究还不全面。本文采用计算流体动力学(CFD)的方法, 运用流体力学软件Fluent, 在H₂O-CO₂的两相体系、湍流模型采用Reynolds应力模型、多相流模型采用Mixture混合模型的条件下, 对不同柱径比的液体脱气式旋流器进行数值模拟, 利用旋流离心场的离心力和压力梯度对微小气泡进行分离, 获得了对5～50μm不同直径气泡的分离性能以及进口流速对气泡分离性能的影响关系。研究发现, 在一定条件下, 随着柱径比值的增大, 脱气效率总体上先逐渐增大, 超过一个极限值后迅速减小, 但柱径比越大, 越有利于小气泡颗粒的迁移分离;分离效率随着进口流速增加而增加, 到达极限之后效率降低。     

Numerical and experimental analysis of forced convection in rib-roughened channels with moisture-permeable walls

Ribbed features can promote mixing and improve convective heat and mass transfer in channels. This can be beneficial in certain exchangers, such as Energy Recovery Ventilators (ERVs), which employ moisture-permeable membranes to transfer both latent and sensible heat. In this work, a computational fluid dynamics model with experimental validation was used to study angled rib mixing features in compact channels. Results show that, relative to a smooth channel, the ribs increase the channel Sherwood and Nusselt numbers by a larger fraction than the corresponding increase in friction factor. For a typical commercial grade ERV, total effectiveness can be improved by over 10% for an equal pressure drop by adding ribs and slightly increasing the channel height. A custom test stand and rib-forming technique were developed to validate the simulation predictions. The experiments confirm the net benefit of ribbed channels and agree with the simulation results within experimental uncertainty.     

水力旋流器内非牛顿流体多相流场的数值模拟

利用一种非牛顿流体黏度修正模型描述水力旋流器内高浓度矿浆的非牛顿流动特性,并结合雷诺应力模型(RSM)、混合多相流模型(Mixture)以及拉格朗日颗粒追踪模型(LPT)建立了一种适用于模拟水力旋流器内非牛顿流体多相流场的数学模型。模拟结果与报道的实验值的相对误差均在10%以内,表明了该模型的可靠性。结果表明,非牛顿流体黏度的空间分布与矿浆密度的空间分布类似。沿零轴速包络面(LZVV)的轮廓存在一个高密度环,其原因为某粒径范围内的颗粒受到的径向合力为零,颗粒群沿LZVV做高速旋转运动。分散相的空间分布取决于不同粒径的颗粒受力。对于不同粒径的单位质量颗粒,向外离心力的数值大约为向内压力梯度力的两倍左右,使得大颗粒进入下行流并在底流口收集。随着颗粒粒径的减小,总体向内且具有波动性的流体曳力呈指数增长。向内的流体曳力将部分颗粒推向轴心,经上行流逃逸,同时也增强了颗粒运动的随机性。当颗粒粒径小于一定值后,流体曳力远远大于离心力和压力梯度力,颗粒运动的随机性非常强,宏观表现为均匀分布。     

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.     

考虑地面积尘时置换通风的数值模拟

引言 置换通风系统始于北欧,较多应用于工业通风.近几年来,由于其合理的温度分布、较高的通风效率和明显的节能特性,置换通风系统已经越来越多地应用到民用建筑中.在北欧新建的办公建筑中,约有50%～70%的部分采用了置换通风系统[1].     

混合澄清槽数值模拟与实验研究进展

稀土是不可再生的重要战略资源,在高新技术材料中具有不可替代的战略作用。混合澄清槽具有操作稳定性好、级效率高、结构简单、易放大等优点,是稀土分离工业中使用最为广泛的萃取设备。本文概述并分析了应用于混合澄清槽模拟的数值模型,结果表明Eulerian-Eulerian多相流模型、k-?湍流模型和多参考系模型因使用简便、精度可靠、对计算机性能要求不高而被当前的研究者们大量采用。此外,针对混合澄清槽抽吸性能、混合特性和澄清特性三大重要性能指标,分别总结了影响各性能的主要参数和调控方法,分析表明在抽吸性能和混合时间方面的研究较为成熟,在液滴聚并破碎数值研究、澄清方式和改进等方面的研究相对薄弱,构建高精度网格、采用更细致分析流场时空发展趋势的大涡模拟和引入种群平衡模型等方法将是下一步深入研究混合澄清槽的重要方向。     

基于颗粒直径大小对环形空间携屑影响的数值模拟

钻井是开发石油资源一个非常重要的环节和技术手段,井中岩屑易在井眼底部形成岩屑床,导致下钻遇阻、蹩泵甚至卡钻,因此岩屑运移机理的研究对钻井作业来说至关重要。通过对现场实际情况的调研,得到井下上返颗粒的直径范围,进行数值模拟。运用Solid Works对井下环形空间进行建模与装配,得到井下环形空间的三维模型。使用软件建立井下环形空间的简化模型,通过应用FLUENT中动网格部分的UDF编写成功地实现了钻杆的旋转。运用FLUENT数字模拟软件,通过对流体非定常流的数值模拟,对岩屑在环形空间的运移规律进行研究,模拟条件更加接近实际情况,结果更加精确,可以为钻井井眼清洁工作提供科学参考。     

Reconstruction of large-scale flow structures in a stirred tank from limited sensor data

We combine reduced order modeling and system identification to reconstruct the temporal evolution of large-scale vortical structures behind the blades of a Rushton impeller. We performed direct numerical simulations at Reynolds number 600 and employed proper orthogonal decomposition (POD) to extract the dominant modes and their temporal coefficients. We then applied the identification algorithm, N4SID, to construct an estimator that captures the relation between the velocity signals at sensor points (input) and the POD coefficients (output). We show that the first pair of modes can be very well reconstructed using the velocity time signal from even a single sensor point. A larger number of points improves accuracy and robustness and also leads to better reconstruction for the second pair of POD modes. Application of the estimator derived at Re = 600 to the flows at Re = 500 and 700 shows that it is robust with respect to changes in operating conditions.     

蒸汽喷射真空泵性能的CFD模拟研究

采用有限体积法离散控制方程,标准κ-ε湍流模型,近壁面处使用壁面函数修正的方法对蒸汽喷射真空泵的超音速混合过程进行数值模拟。计算并分析了第二喉管与工作蒸汽喷嘴喉管面积比、喷嘴出口截面与混合段入口截面间的距离及混合段的锥度等结构参数及工作蒸汽的压力和温度、引射流体及混合流体压力等热力参数对真空泵操作性能的影响。数值计算结果表明,几何参数的改变极大地影响着波系结构,在一定的设计工况下,总存在一个最佳的面积比及一个最优的相对位置对应于最大的喷射系数,其在物理上的表现形式为通过工作蒸汽喷嘴所产生的激波系刚好能够通过第二喉管。混合段的锥度在一定范围内对真空泵的性能无显著影响,等压混合理论较等面积混合理论具更优的操作性能。根据喷射泵内的物理现象及喷射系数的变化规律将蒸汽喷射真空泵的操作状态分为临界状态、亚临界状态和回流状态三类,同时指出临界点为最佳工作点。     

传热管外液体流动数值模拟研究

采用计算流体力学方法(CFD),对传热管外液体流动过程进行了模拟研究.建立了传热管三维网格模型,考察了不同布液密度下液体在传热管表面铺展形态、流型、液膜厚度以及绕管停留时间的变化规律.该研究结果对于加深理解降膜流动过程,指导蒸发器优化设计和工艺条件优化有一定的借鉴意义.     

Prediction of gas density effects on bubbly flow hydrodynamics: New insights through an approach combining population balance models and computational fluid dynamics

Detailed measurements and computational fluid dynamics (CFD) investigation of the hydrodynamics in a bubble column containing internal features causing flow disturbances are presented for both air and helium gases. An optical needle probe has been used to measure profiles of bubble size, bubble velocity, and gas holdup at different locations across the cross section of the column. An approach combining CFD with population balances is able to represent observed multiphase flow phenomena such as the effect of the pipes to remix and redistribute the gas as well as the tendency of the gas to channel through a slit in the pipes rather than go around the pipes. The comparison of CFD simulation to experimental measurements reveal that the overall decrease in gas holdup observed when switching from air to helium gas can be explained by swarm effects, whereas the steeper decrease in the gas holdup profile across the column is due to coalescence effects. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3764–3774, 2018     

Analysis of Mobility Effects in Particle-Gas Flows by Particle-Resolved LBM-DEM Simulations

Particle-resolved direct numerical simulations are performed to simulate the flow through particle assemblies that are either static or freely moving to demonstrate the influence of particle mobility. To obtain a comprehensive understanding for this influence essential parameters such as the Reynolds number, solids volume fraction, particle-fluid density ratio, collision parameters and particle shape are varied. The influence of particle mobility is assessed by evaluating the particle-fluid forces, the particle ensemble structure and particle velocities. It is found that the ability of existing correlations for static particle systems to predict drag and lift forces correctly in dynamic particle-gas flows is limited and that drift forces perpendicular to the drag force play an important role.     

A Critical Review of the Application of Drop-Population Balances for the Design of Solvent Extraction Columns: I. Concept of Solving Drop-Population Balances and Modelling Breakage and Coalescence

Population-balances are a powerful method to predict the population behavior of drops in chemical-engineering equipment such as solvent extraction columns. In such columns a complex interaction of different phenomena, namely drop sedimentation, mass transfer, drop breakage and coalescence as well as axial dispersion occurs. In this article the concept of drop-population balances is discussed in detail as well as possible solution methods. Also, a critical review of existing models accounting for breakage and coalescence taking place in extraction columns is presented. Future parts of this series will be devoted to modelling mass-transfer and sedimentation as well as on application of single-drop based modelling.     

Coupled simulation of the flue gas and process gas side of a steam cracker convection section

A coupled simulation of the flue gas and process gas side of the convection section of a steam cracker is performed, making use of the CFD software package Fluent. A detailed overview of the operating mode of the different heat exchangers suspended in the convection section is obtained. The asymmetric inlet flow field of the flue gas in the convection section, and the radiation from the convection section walls leads to large differences in outlet temperatures of the tubes located in the same row. The flow fields and temperature fields in the tubes of a single heat exchanger differ significantly with e.g., outlet temperatures of the hydrocarbon‐steam mixture ranging from 820 K to 852 K. Moreover, the simulations reveal the presence of hot spots on the lowest tube row, possibly causing fouling. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Sag in commercial thermoforming

Our previous analytical solution gives sag advancing implicitly as , or for , sag advances with the cube root of time for a thin wide rectangular Newtonian isothermal sheet. This previous analytical work applies to sheets that are pinned along just two edges, and not all the way around. Corresponding sagometer experimental results confirmed this cube root relation. This work compares the prediction with measured commercial thermoforming behavior on rectangular sheets that are, of course, pinned all the way around. Then sag parallel superposition is used to extend for a sheet pinned all the way around. We evaluate sag parallel superposition using a finite element method (FEM) employing ANSYS Polyflow. The equation assumes sagging sheet cylindricity, and from our FEM we find that this assumption is reliable when . We compare sag measured in commercial thermoforming, using high‐impact polystyrene (HIPS) sheets that are pinned all the way around, by extending with parallel superposition. It is found that the time evolution of the commercial sag follows nearly exactly the same shape as the isothermal prediction. We measure sag runaway, and although the isothermal analysis , predicts the sag runaway time accurately, our isothermal theory overpredicts the amount of sag in the nonisothermal commercial operation by as much as a factor of 14. It is also shown how to use sheet sag measurements from commercial thermoforming to deduce the Newtonian viscosity of a thermoforming resin at a temperature that is above its softening point. © 2013 American Institute of Chemical Engineers AIChE J, 60: 1529–1535, 2014     

非对称陶瓷膜管渗透性能的CFD模拟研究

陶瓷膜因其化学稳定性好、机械强度大等优点得到广泛应用。计算流体力学(CFD)的快速发展使得计算模拟成为研究和优化陶瓷膜管结构性能的有效手段。为了优化非对称结构陶瓷膜管的结构和操作参数,对其渗透性能进行了CFD计算模拟。针对非对称结构陶瓷膜管的膜层和过渡层的厚度在10 μm级的特点,采用Navier-Stokes方程和Darcy定律来分别描述膜管内和膜多孔介质内的纯水流动,利用多孔介质模型描述膜管的主体支撑层,用多孔跳跃边界简化膜管的膜层和过渡层,利用Konzey-Carmen方程对膜元件各层的渗透率进行估算。计算结果与实验值吻合较好,为优化陶瓷膜管的通道结构提供了便捷的工具。     

臭氧催化氧化反应器模拟与分析

臭氧催化氧化法是一种高效的污水处理技术,是目前污水高级处理的主要手段之一。传统的建模方法无法研究反应器内污水浓度的时空分布和操作条件对反应器的影响,本研究利用计算流体力学（CFD）,耦合多孔介质流动与传质和化学反应动力学多物理场模型,研究臭氧催化氧化过程中目标污染物浓度随时间和空间的分布情况,计算结果与实验结果有良好的一致性。进一步研究臭氧浓度和流量、循环水流量、催化剂层高度、催化剂颗粒大小等对臭氧催化氧化处理废水效率的影响,评估出最优的实验方案。结果表明,在不改变当前反应器主体结构的情况下,最优的操作条件是：臭氧浓度30～40mg/L,臭氧进口流量40～60mL/min,循环水量200～250mL/min,催化剂层填充高度600～800mm,催化剂颗粒半径大小为2mm。该研究有助于理解、设计和优化污水处理反应器。     

动量增强型喷射器性能实验与分析

基于增大混合区域,增强动量交换效率的优化思想,添加三维扰流元件的动量增强型喷射器能够提高设备性能。以此为研究对象,本文在空气介质下对其性能进行了实验和数值计算研究。相较传统喷射器,动量增强型喷射器能显著提高喷射器单体设备性能。在超音速情况下,驱动流量不受影响,实验测定动量增强元件在增压性能变化不大的情况下,提升临界区引射率E_R达5%,数值计算结果与模拟结果吻合。对数值计算所得流场分析发现：在喷射器驱动喷嘴尾部设置动量增强元件,一方面能够使流体混合边界出现花瓣形褶皱,增大流体混合面积,同时增厚混合区域达39.28%,整体拓展混合空间;另一方面,动量增强元件能够促使驱动流体轴向速度转化为周向、径向速度,诱发周向旋流,增强径向扰流,变喷射器传统二维动量交换为三维动量交换。动量增强元件亚声速情况下,引射性能进一步提升,但其临界压力点降低,增压能力损失。