微细颗粒物在过滤介质中过滤特性的CFD-DEM模拟

基于CFD-DEM(离散单元法)方法模拟了微细颗粒物在纤维过滤介质中的气-固两相流动特性,模拟时,充分考虑了颗粒群组成、粒径分布、颗粒间及颗粒与纤维间的反弹作用以及颗粒团聚等因素,分析了纤维过滤中颗粒群的运动特性和微细颗粒的沉积形式。结果表明:采用CFD-DEM模拟过滤介质的过滤过程以及微细颗粒在介质表面沉积过程和形式的方法是方便且可行的,模拟结果与前人的实验观测结果基本吻合;在过滤过程中,表面过滤的贡献较大,大部分的颗粒在介质表面即被捕集,进入到介质内部的部分粒径较小的颗粒经深层过滤作用而被捕集;大量的颗粒捕集是由颗粒-颗粒捕集机制来实现的;不同颗粒体系的颗粒群其过滤效果也有所差别,对于本文所研究的过滤介质模型,多颗粒体系的过滤效率比单一的颗粒体系的过滤效率高20%左右。     

Interface control for resolved CFD-DEM with capillary interactions

In this work, a resolved CFD–DEM coupling model for the simulation of gas-liquid-solid flows is developed: the interface capturing method based on the colour function is employed for fluids (i.e. a gas and liquid) whilst Discrete Element Method (DEM) is used for particles. The Volume Penalisation (VP) method is adopted to consider the hydrodynamic interactions between fluids and particles along with the Immersed Free Surface (IFS) method, which artificially extends the gas-liquid interface into the interior of the particle to account for the wettability. The unique point of the proposed model is that the thickness of the gas-liquid interface can be controlled by using both interface compression and diffuse interface techniques simultaneously. From the simulation results, it is presented that the accurate evaluation of the surface tension force as well as the capillary force can be achieved by appropriately controlling the interface thickness. Moreover, the major two methods in the literature to calculate the capillary force are compared in this work. The validity of the proposed model is presented for both static and dynamic cases. The behaviour of two colliding particles with a dynamic liquid bridge is then simulated to demonstrate the applicability of the proposed model to a complex three-phase system.     

缩径管内两相流砂粒碰撞和冲蚀特性研究

针对目前建模方法在描述两相流中砂粒碰撞行为方面存在的不足,运用计算流体力学和离散单元耦合方法建立了压裂工况下的缩径管两相流模型,实验验证了仿真模型的准确性。分析了缩径管内砂粒间碰撞及缩径斜面受力规律,研究发现,缩径斜面冲蚀深度随砂比增大而增大,砂比大于40%时,相同时间的冲蚀深度增量减小,并趋于最大值;缩径斜面冲蚀深度随黏度增大而减小,流体黏度对缩径斜面外缘比对内缘的冲蚀深度影响大。     

颗粒粒径对深海采矿矿浆泵性能的影响

为掌握矿浆泵内部颗粒的运动规律,考虑颗粒的体积效应和相内碰撞效应,应用CFD?DEM耦合算法,对扬矿模型泵内的粗颗粒?海水两相流实施了双向耦合非定常流动计算,分析了不同粒径的颗粒在矿浆泵内的空间分布和运动特征.研究结果表明:采用两相耦合的求解方式可以较好地模拟矿浆泵内的两相流场特性;流动域内颗粒浓度略高于管线平均值,叶...     

高湿黏性颗粒在聚四氟乙烯微孔膜滤料表面沉积特性的数值模拟

基于聚四氟乙烯(PTFE)微孔膜滤料扫描电镜(SEM)图像,建立PTFE微孔膜滤料微观结构模型,采用计算流体力学和离散单元法(CFD?DEM)耦合的方法对黏性颗粒在微孔膜滤料表面沉积特性进行模拟,引入液桥力模型,忽略范德华力的作用,统计计算域内颗粒的受力情况,分析了不同表面能条件下3~6 ?m粒径颗粒在微孔膜滤料表面的沉积特性,将模拟结果与黏附效率的经验公式进行对比。结果表明,黏附效率与经验值、颗粒受力与液桥力模型的相对误差均在6%以内,CFD?DEM耦合计算方法可用于模拟不同环境湿度条件下的颗粒沉积;过滤风速、粒径与黏性是影响沉积形态的重要因素,提高过滤风速及增大颗粒粒径与黏性,颗粒更易在滤料表面形成稳定的树突结构,黏附效率及含尘压降增加。环境相对湿度影响两物体间液桥体积,接触力影响颗粒沉积,当增加表面能与液桥体积时,接触力及液桥力均相应增加,根据受力平衡原理,环境相对湿度对颗粒沉积影响很大。     

基于CFD-DEM的水下清淤机器人吸泥管道流场分析

基于DDPM的CFD-DEM耦合数值分析方法,研究了自行式水下清淤机器人在不同工作俯角、泵流量条件下吸泥管内泥沙颗粒的流动状态。仿真结果表明,不同工作俯角下,低功率工况会引发吸泥管道堵塞趋势; 泥浆流经弯管处所产生的二次涡流会加剧管壁磨损,导致动能损失,从而影响输送效率。     

Bubble dynamics in bubbling fluidized beds of ellipsoidal particles

This article presents a CFD-DEM study on the effect of particle shape on bubble dynamics in bubbling fluidized beds. The particles used are ellipsoids, covering from disk-type to cylinder-type. The phenomena such as bubble coalescence and splitting are successfully generated, and the results are compared with literature, showing a good agreement. The results demonstrate that the bubble forming/rising regions and patterns are influenced significantly by particle shape. Ellipsoidal particles have asymmetrical bubble patterns with two or more circulation vortices while the bubbles for spherical particles form at the bed centerline and rise through the center of the bed. Hence, the vertical mass flux at the bed centerline for spheres is always positive, and ellipsoids have negative or positive vertical mass fluxes. The solid mixing estimated based on the dispersion coefficient revealed poor mixing for ellipsoids. Spherical particles have a larger bubble size and higher bubble rising velocity than ellipsoids.     

CFD-DEM study of the effects of solid properties and aeration conditions on heat transfer in fluidized bed

The solid properties are of significant influence on the thermal performance of the fluidized bed. In order to provide valuable information for the application of this equipment, a numerical study is carried to clarify the effects of solid properties on the heat transfer characteristics in a lab-scale fluidized bed by means of the CFD-DEM method. Specially, two aeration conditions, i.e. the same superficial velocity and the same fluidization number, are considered. The results show that the violent convective mechanism at bed bottom plays a significant role in the heating of the bed material. The entrainment of rising bubbles and hence solid mixing are the key factors to get better temperature uniformity of the bed during the heating process. With the decrease of particle density and size, the internal circulation of solid phase is strengthened under the same superficial velocity, while slightly weakened under the same fluidization number. Obvious resemblance can be captured between solid mixing and temperature uniformity, and the enhanced solid mixing usually leads to homogeneous temperature distribution of the bed. It can be found that the heating rate decreases with increasing solid density regardless of aeration setup. In addition, it is positively related to particle diameter under the same fluidization number, while keep unchanged under the same superficial velocity. Furthermore, enhanced solid mixing and better temperature uniformity can be captured with increasing solid heat capacity, which confirms that gas temperature shows considerable effect on gas-solid flow.     

Hybrid high-temperature fouling and erosion characteristics of gas-particle flow around heating tube via CFD-DEM

In this study, hybrid fouling and erosion of heating tube are presented via CFD-DEM. To analyze their characteristics, new high-temperature fouling and erosion sub-models, using a soft-sphere model considering the temperature-corrected material properties, are raised. The prediction of coupling morphologies is solved by clustered particles and dynamic mesh. The magnification factors of time are applied to the computation speedup for a long-time fouling and erosion. Results indicate that good agreements, of the critical adhesion velocity, high-temperature fouling and erosion on the heating tube, have been found between the experimental measurements and numerical predictions. As the inlet flow temperature goes up, the fouling amount increases, and oppositely the erosion amount decreases. As the surface temperature increases, both fouling and erosion amounts increase. As the particle size increase, the primary determinant of fouling varies from the impact efficiency to the impact efficiency and the critical adhesion velocity. Most innovative is to reveal the competition between the fouling and the erosion. In addition, reducing the surface temperature, as a way of temperature control, should be the priority to reduce fouling and erosion.