非流态化流体—颗粒流

本文应用多相流理论与颗粒介质力学研究非流态化流体—颗粒两相流动力学。建立了一系列动力学方程式,将流体压力、颗粒间接触压力、床层空隙率与颗粒和流体的性质、床的几何结构、操作条件相关联。这些方程的计算结果与大量实验数据相吻合。本文还归纳了垂直非流态化两相流的十三种操作状态,绘制了垂直移动床流体颗粒流动相图,定量地指明了各种操作状态存在的条件及其相互关系,并对理想料封状态进行了重点讨论。     

气固流态化中颗粒介尺度结构的动力学研究

气固流化床中的团聚等介尺度结构严重影响气固的整体流化特性、传热、传质和反应效率,因此介尺度结构的研究至关重要。简要回顾颗粒介尺度结构的定义与分类,然后从流体-颗粒、颗粒-颗粒之间的力和非弹性碰撞相互作用出发,总结分析了颗粒介尺度结构形成原因和动力学演化过程的研究现状和发展动态。重点关注细颗粒与超细纳米颗粒形成的介尺度结构,即结块,并针对结块形成原因,讨论了颗粒间范德华力计算以及结块的力平衡理论模型,提出了细颗粒介尺度结构进一步的研究方向,即研究带有颗粒-颗粒作用力的介尺度结构的动态演化规律,为精准调控介尺度结构提供理论依据。     

用颗粒流的动力学理论模拟提升管反应器流动特征

引　言大量实验结果[1～ 3]表明 ,循环流化床内气相和颗粒相的两相流体系中 ,颗粒浓度在提升管的横截面上存在显著的非均匀分布 .颗粒在某些条件下聚集于管壁 ,在某些条件下也可能聚集于管中央[4 ].颗粒浓度分布的非均匀性可以分为两种类型 ,一是颗粒层 (Ｐａｒｔｉｃｌｅ -Ｌａｙｅｒｉｎｇ) [5 ],其微观结构近似颗粒固定床 ,颗粒在沿平均速度方向上有排列成床的趋势 ,但仍保持流体特性 ;另为颗粒群(Ｐａｒｔｉｃｌｅ -Ｐａｃｋｅｔｓ) [6 ],颗粒群在流化床中的行为如同单相湍流中的流体涡团 ,作无规则运动 ,不断地形成、分解 .催化裂化提…     

撞击流中颗粒旋转特性

搭建了研究撞击流中颗粒旋转特性的气固两相撞击流实验台,使用高速摄像机拍摄一个截面为0.15 m×0.08 m的撞击区域内固体颗粒的运动。利用所搭建的实验台设置了单喷口和双喷口两种实验方式来研究颗粒旋转影响因素,得出撞击流内颗粒的旋转特性。结果表明:固体颗粒在气相中运动过程一直伴随着其自身的旋转;气相场对颗粒转速的影响较小,可忽略不计;相同实验条件下,颗粒直径越小其转速越大;颗粒以及气相速度越大,则固体颗粒在碰撞后的转速越大,当加速气相速度为25 m·s^-1,氧化铝陶瓷直径为0.003 m时,颗粒碰撞前后转速差平均值可达280 r·s^-1;颗粒间碰撞过程中,颗粒相对运动偏置角度对转速变化影响很小。     

反应传递多尺度耦合的拟颗粒模拟

反应与传递过程的耦合是众多化工过程的重要特征之一。随着过程精准调控要求的不断提高和过程强化与微化工等领域的迅速发展,传统上宏观与微观分离的反应与传递描述方式遇到了诸多挑战。拟颗粒模拟耦合软球和硬球两类分子模拟方法的优势,显著简化了分子间作用模型,极大提高了计算效率,为描述宏微观之间的介尺度上反应与传递紧密耦合的复杂现象提供了一种有效手段。本文简要回顾该方法提出的背景,阐述其基本思想,并展示和分析其在气固多相吸附和催化、气液微流动等问题中的应用前景。     

气—固流化床颗粒的内循环流动

从多尺度范围考察了气－固流化床内尾涡颗粒流和乳化相颗粒流的运动规律,将分散的尾涡颗粒流和乳化相颗粒流连续介质化,从流变学角度定义了颗粒洗粘度,用流体力学方法建立了内循环流动结构的多尺度,连续介质流模型,较好地揭示了颗粒循环循环流动的规律。实验观测支持模型预测结果。     

气-固流化床中颗粒的内循环流动

从多尺度范围考察了气－固流化床内尾涡颗粒流和乳化相颗粒流的运动规律,将分散的尾涡颗粒流和乳化相颗粒流连续介质化,从流变学角度定义了颗粒流粘度,用流体力学方法建立了内循环流动结构的多尺度、连续介质流模型,较好地揭示了颗粒循环流动的规律。实验观测支持模型预测结果。     

气固流化床内宽筛分硅粉颗粒流化特性的数值模拟

为了探索气固流化床内宽筛分硅粉颗粒的流化特性,作者利用计算流体力学CFD软件,采用Eulerian气固多相流模型及SIMPLE算法,模拟了二维气固流化床内不同粒级硅粉颗粒在不同操作件下的气固流化特性;分析了气泡生成、长大和破裂的过程,研究了床内气固两相的流动特性.结果表明:模拟计算值与实验值吻合较好,最大相对误差为10...     

从质量流向漏斗流转变过程中的动力学分析

球床模块式高温气冷堆的堆芯是全陶瓷型包覆铀燃料制成的球形颗粒,与石墨颗粒混合堆积而成,堆芯颗粒流的流态取决于颗粒尺度的平移、旋转等动力学量,以及力链、涡旋等介尺度物理量。为了分析颗粒的平移、旋转等动力学量对颗粒流流态的影响。基于筒仓颗粒流的物理模型,首先开展了筒仓颗粒流流变过程的实验测量,并使用基于 Hertz-Mindlin和 RVD （relative velocity dependent）滚动摩擦接触模型的离散单元法 （distinct element method, DEM）,研究了锥形筒仓颗粒流流变过程中球形颗粒的动力学量。进一步,基于DEM计算结果进行分析,发现筒仓自上而下呈现出质量流向漏斗流过渡的混合流状态。在筒仓混合流的不同流型区域中,平移速度和旋转速度之间的相关性是相反的;颗粒间的相对切向运动较大的区域集中在漏斗流区域与边壁区域。了解筒仓流变过程中颗粒的动力学特征,有助于优化筒仓颗粒流动,并减少颗粒表面的磨损。     

Quantitative comparison of hydrodynamic and elastoplastic approaches for modeling granular flow in silo

Hydrodynamic and elastoplastic theories are two commonly used continuum approaches for modeling granular materials. Here, the elastoplastic approach is extended to incorporate the rheology of dense granular flow, which therefore allows a quantitative comparison with the hydrodynamic approach under the same setting of rheological laws, material parameters, and numerical method. The flow patterns yielded by two approaches are apparently similar and the discharge rates are close. Yet the elastoplastic approach creates a narrow dome-like flow zone in contrast to the wide cone-like flow zone generated by the hydrodynamic approach. The shear localization is also less prominent in elastoplastic modeling owing to the existence of elastic deformation. The stresses predicted by two approaches match well in flow zones but show significant differences in stagnant zones. The proposed elastoplastic approach incorporating flow rheology can be used generally in both solid and fluid states of granular materials. © 2019 American Institute of Chemical Engineers AIChE J, 65: e16533, 2019     

Extension of the kinetic theory of granular flow to include dense quasi-static stresses

Fluidized bed technology has diverse industrial applications ranging from the gasification of coal in the power industry to chemical reactions for the plastic industry. Due to their complex chaotic non-linear behaviour understanding the hydrodynamic behaviour in fluidized beds is often limited to pressure drop measurements and a mass balance of the system. Computational fluid dynamics has the capability to model multiphase flows and can assist in understanding gas-solid fluidized beds by modeling their hydrodynamics. The multiphase Eulerian-Eulerian gas-solid model, extended and validated here improves on the kinetic theory of granular flow by including a closure term for the quasi-static stress associated with the long term particle contact at high solid concentrations. Similar quasi-static models have been widely applied to slow granular flow such as chute flow, flow down an incline plane and geophysical flow. However combining the kinetic theory of granular flow and the quasi-static stress model for the application of fluidized beds is limited. The objective of the present paper is to compare two quasi-static stress models to the experimental fluidized bed data of Bouillard et al. [4]. A quasi-static granular flow model (QSGF) initially developed by Gray and Stiles [18] is compared to the commonly used Srivastava and Sundaresan [37]. Both models show good agreement with the experimental bubble diameter and averaged porosity profiles. However only the QSGF model shows a distinct asymmetry in the bubble shape which was documented by Bouillard et al. [4].     

Quantitative estimation of volume changes of granular materials during silo flow using X-ray tomography

The aim of the paper is to show the potential of the X-ray tomography to quantitatively measure volume changes in granular materials during silo flow in a rectangular model bin. The experiments were carried out with initially dense sand. The bin walls were smooth or very rough. In the first step, continuous X-ray radiographs were conducted. The results of volume changes were presented in form of 1D cross-sectional plots and 2D images. They were directly compared with corresponding experimental results obtained with a Particle Image Velocimetry (PIV) method allowing for measuring displacements on the solid surface. Measurement errors in both methods were discussed.     

Application of particle image velocimetry (PIV) for deformation measurement during granular silo flow

The paper presents results of deformation measurements in dry cohesionless sand during free flow in small rectangular model silos using a non-invasive, indirect method called particle image velocimetry (PIV). It is an optical technique for measuring surface deformations from successive digital images. Laboratory model tests were performed with a mass and funnel flow silo to investigate the kinematics of the flowing sand. The measurements were carried out for granular flow in model silos without inserts and in a funnel flow silo equipped with three different types of inserts: cone-in-cone, inverted cone and double cone. The effect of the initial sand density and roughness of silo walls on the volumetric and deviatoric strain in sand was investigated. The accuracy of measurements was discussed. Advantages and disadvantages of PIV were outlined. The results were qualitatively compared with sand displacements obtained with colored sand layers and with the aid of X-rays (initially dense sand).     

Determination of bulk solid concentration changes during granular flow in a model silo with ECT sensors

Experiments in a cylindrical model silo were carried out with different initial densities of sand and silo wall roughness. Solid concentration changes during granular flow in the model silo were measured with electrical capacitance tomography (ECT) sensors. During silo flow, strong dynamic effects connected with booming sound occurred. Local one-dimensional (1D) and cross-sectional 2D evolutions of solid concentrations in dry sand during silo discharge were observed. The 1D phenomena were estimated from the raw data and the 2D phenomena were obtained from the reconstructed data by solving an inverse problem with a linear back projection algorithm.     

Application of ECT to solid concentration measurements during granular flow in a rectangular model silo

The paper presents results of concentration changes in cohesionless sand during dynamic mass flow in a rectangular model silo composed of a bin and hopper. Electrical capacitance tomography (ECT) was used. Sensors were located outside the silo along both the periphery and height. Local horizontal one-dimensional and cross-sectional two-dimensional evolutions of solid concentrations in dry sand during silo discharge were determined. The first ones were estimated from the raw data and the latter were obtained with the aid of the reconstructed data using a Linear Back Projection algorithm (LBP) to solve an inverse problem. Experiments in a model silo were carried out with two different initial sand densities and wall roughness grades. The measured results with ECT were compared with corresponding ones obtained with a Particle Image Velocimetry (PIV) method.     

模拟颗粒流动的离散元方法及其应用

介绍了离散元 (DEM)方法的基本原理、颗粒运动控制方程和颗粒相互作用力的数学模型。综述了DEM在流化床和固定床反应器 ,以及一些单元操作如料仓卸料过程、混合过程等中的最新应用和研究结果 ,表明DEM能够反映过程的本质机理 ,可以利用基本的数据模拟复杂的颗粒流动系统。最后指出了DEM发展中亟待解决的问题     

Kinetic theory of aggregation in granular flow

This article presents a mathematical formulation of the aggregation kinetics in granular flow. The traditional kinetic theory and its generalized application to granular flow does not allow for particle size to change with time thus cannot be used to describe particle flow with aggregation taking place. In this article, a collision success factor, quantifying the completely inelastic collision of particles, is introduced into the evaluation of collision rate. The kinetic transport equations are then transformed to include source terms that account for the effects of particle size and aggregation. The analytical solution of the collision success factor is obtained by integrating the relative velocity distribution function over its velocity domain from 0 to a critical value, which corresponds a balance between the repulsion and attraction forces in a collision. The factor has been found to depend on the mixture granular energy and the critical relative collision energy. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

Flow behaviors of a large spout-fluid bed at high pressure and temperature by 3D simulation with kinetic theory of granular flow

Flow behaviors of a large spout-fluid bed (I.D. 1.0 m) at high pressure and temperature were investigated by Eulerian simulation. The gas phase was modeled with k − ε turbulent model and the particle phase was modeled with kinetic theory of granular flow. The development of an internal jet, gas-solid flow patterns, particle concentrations, particle velocities and jet penetration depths at high pressure and temperature at different operating conditions were simulated. The results show that the bed operated at an initial bed height larger than the maximum spoutable bed height resembles the flow patterns of jetting fluidized beds. The radial profiles of particle velocities and concentrations at high temperature and pressure have the similar characteristic shapes to those at ambient pressure and temperature. The particle concentrations and velocities appear to depend on the bed heights when increasing pressure while keeping the gas velocities and temperature constant. The particle velocities in the lower region of the bed increase with increasing pressure, while they tend to decrease in the middle and upper regions of the bed. The particle concentrations have an opposite dependency with increasing pressure. They decrease in the lower region of the bed but increase in the middle and upper regions of the bed. Besides, the jet penetration depths are found to increase with increasing pressure.