粗糙颗粒动理学及稠密气固两相流动的数值模拟

考虑颗粒碰撞过程中摩擦作用,给出了粗糙颗粒碰撞动力学.引入颗粒相拟总温来表征颗粒平动和转动脉动能量的特征.基于气体分子运动论,建立颗粒碰撞中平动和旋转共同作用的粗糙颗粒动理学,给出了颗粒相压力和黏度等输运参数计算模型.运用基于颗粒动理学的欧拉-欧拉气固两相流模型,数值模拟了流化床内气体颗粒两相流动特性,分析了颗粒旋转流动对颗粒碰撞能量交换和耗散的影响.模拟得到的流化床内径向颗粒浓度和提升管内颗粒轴向速度与他人实验结果相吻合.模拟结果表明随着颗粒浓度的增加,颗粒相压力和能量耗散逐渐增加,而颗粒拟总温先增加后下降.随着颗粒粗糙度系数的增加,床内平均颗粒相拟总温和能量耗散增加,表明颗粒旋转产生的摩擦将导致颗粒旋转脉动能量的改变,影响床内气体-颗粒两相宏观流动特性.      

参-强激励联合作用下输流管的分岔和混沌行为研究

研究输送脉动流的两端固定输流管道在其基础简谐运动激励下的分岔和混沌行为,考虑管道变形的几何非线性和管道材料的非线性因素,推导了系统的非线性运动方程,并应用Galerkin方法对其进行了离散化处理。通过采用数值模拟方法,对系统的运动响应进行仿真,重点探讨了流体平均流速、流速脉动振幅以及基础简谐运动激励振幅对系统动态特性的影响。结果表明,系统在不同的参数下会发生围绕不同平衡点的周期和混沌等运动,并在系统中发现了两条通向混沌运动的途径:倍周期分岔和阵发混沌运动。     

含颗粒污染物的油液均质流的数值分析

含颗粒污染物的油液是浓度很稀、粒径极小的伪均质流,为了掌握颗粒污染物在输送过程中的浓度分布,利用一维扩散方程构建了污染油液的数学模型;通过特征线法数值求解,获得了污染油液中各相的动态特征。结果表明,油液压力和速度沿管长呈脉动规律运动,且随着时间的延长逐渐衰减;颗粒污染物对油液速度具有极好的跟随特性;颗粒污染物的浓度分布也随着油液流速的变化而呈现规律性的变化;在不同运行时间内油液压力沿管长的衰减趋势不同,油液速度沿管长的变化趋势与压力的趋势相反;颗粒污染物速度和浓度分布沿管长与油液速度具有紧密的联系。     

稠密气固两相流各向异性颗粒相矩方法

基于气体分子动力学和颗粒动理学方法,考虑颗粒速度脉动各向异性,建立颗粒相二阶矩模型.应用初等输运理论,对三阶关联项进行模化和封闭.考虑颗粒与壁面之间的能量传递和交换,建立颗粒相边界条件模型.采用Koch等计算方法模拟气固脉动速度关联矩.考虑气体-颗粒间相互作用,建立稠密气体-颗粒流动模型.数值模拟提升管内气固两相流动特性,模拟结果表明提升管内颗粒相湍流脉动具有明显的各向异性.预测颗粒速度、浓度和颗粒脉动速度二阶矩与Tartan等实测结果相吻合.模拟结果表明轴向颗粒速度脉动强度约为平均颗粒相脉动强度的1.5倍,轴向颗粒脉动能大约是径向颗粒脉动能3.0倍.     

气固两相流动的湍流脉动扩散数学模型及其应用

本文提出了气固两相流动的湍流扩展数学模型，本模型用k-ε双方程模型求解气相湍流场，并根据气流脉动的频谱、能谱曲线提出了随机富工级数来模拟气相脉动速度，用拉氏方法描述颗粒的运动，故称为脉动频谱随机颗粒轨道模型。本文还给出了本模型在气固多相射流和流化床内应用的实例。     

系统压力对含悬浮颗粒油液的影响研究

油液在运行过程中不可避免地会产生颗粒物,影响油液的正常使用,甚至出现设备故障,因而分析含悬浮颗粒油液的动态特征,掌握在不同压力变化条件下油液及颗粒物的变化规律具有重要意义。利用两相流体理论建立了含悬浮颗粒油液的悬浮流动力学模型,通过特征线法进行了数值求解,将数值结果与实验数据比较,具有较好的一致性;根据所建模型,分析了不同系统压力条件下悬浮流中各相的脉动规律。结果表明,流场中各相参数的脉动幅值随着系统压力的增加而增大;管路始端和终端各相参数的脉动时刻分别位于1/4脉动周期(T)的奇数倍和偶数倍处,管路中段各相参数的脉动时刻则位于T/8的奇数倍处;悬浮颗粒速度会受到油液速度拖曳力作用,其变化趋势与油液速度基本一致,颗粒浓度分布与油液压力的变化趋势完全相反。     

PHCA实验中犬心脏的非线性动力学研究

按照非线性理论,实施了犬的深低温停循环（Profound Hypothermia and Circulatory Arrest,简称PHCA）实验,并应用混沌理论对实验中采集的心电信号进行了研究,得出以下结论：（1）混沌特征参数可反映心脏的总体动态特征,并可作为心血管疾病早期诊断的依据。（2）在正常的生理状态下心脏的运动是混沌的,而在病理状态下则趋于有序。     

受限颗粒体对制动系统非线性振动的影响

制动系统在工作时,往往受到沙粒、尘土以及磨屑等受限颗粒体的影响,这些受限颗粒体在摩擦副中的高度分布具有较强的随机性,一定程度诱发了制动系统的非线性振动. 本文中基于制动片切向振动模型,引入了新的受限颗粒体摩擦模型,提出了用波动系数来描述受限颗粒体高度分布随机性的强弱. 发现在特定参数下,当此系数为0时,制动片切向振动为周期运动;但是当此系数不为0时,制动片切向振动呈现拟周期或混沌运动,此时的切向振动分岔特性图的稳定轨道也会出现数量或分布的变化,甚至表现出混沌特性. 同一时变信号内,受限颗粒体引发制动片切向非线性振动包括发散、收敛以及拟周期运动等多种形式.      

稠密气固两相湍流流动的实验和数值模拟

基于气固两相流动模型计算循环流化床内稠密气固两相流湍流动,颗粒动理学方法模拟颗粒相湍动能,ＳＧＳ模型模拟气相湍流,采用γ－射线密度计和非等速取样管测量局部颗粒浓度和流率,利用ＦＦＴ方法计算颗粒浓度功率谱密度。模拟计算得到上升管内气相和固相速度和浓度分布等。同时数值模拟与Ｔｓｕｊｉ等和Ｋｎｏｗｌｔｏｎ等试验结果进行了比较,结果表明数值模拟计算与实验结果相吻合。     

装药量及水深对水下爆炸气泡动态特性的影响

以气泡体积加速度模型为基础研究水下爆炸气泡运动的初始条件,采用MSC.DYTRAN 非线性 有限元软件,结合开发的定义流场初始条件与边界条件的子程序,研究水下爆炸气泡运动特性,包括气泡的脉 动、坍塌以及射流等运动特性,并将气泡脉动体积计算结果与实验及边界积分方法计算结果进行对比,验证了 有限元模型的正确性与有效性。以此为基础,得到初始水深、装药量与气泡的脉动体积、最大半径、周期以及 射流速度之间的关系,计算结果与经验公式具有较好的一致性。得到一些有规律性的曲线,可为相关水下爆 炸气泡动态特性研究提供参考。     

Statistical and frequency analysis of the pressure fluctuation in a fluidized bed of non-spherical particles

In this paper, the pressure fluctuation in a fluidized bed was measured and processed via standard devia- tion and power spectrum analysis to investigate the dynamic behavior of the transition from the bubbling to turbulent regime. Two types （Geldart B and D） of non-spherical particles, screened from real bed materials, and their mixture were used as the bed materials. The experiments were conducted in a semi- industrial testing apparatus. The experimental results indicated that the fluidization characteristics of the non-spherical Geldart D particles differed from that of the spherical particles at gas velocities beyond the transition velocity Uo The standard deviation of the pressure fluctuation measured in the bed increased with the gas velocity, while that measured in the plenum remained constant. Compared to the coarse particles, the fine particles exerted a stronger influence on the dynamic behavior of the fluidized bed and promoted the fluidization regime transition from bubbling toward turbulent. The power spectrum of the pressure fluctuation was calculated using the auto-regressive （AR） model; the hydrodynamics of the flu- idized bed were characterized by the major frequency of the power spectrum of the pressure fluctuation. By combining the standard deviation analysis, a new method was proposed to determine the transition velocity Uk via the analysis of the change in the major frequency. The first major frequency was observed to vary within the range of 1.5 to 3 Hz.     

Statistical and frequency analysis of the pressure fluctuation in a fluidized bed of non-spherical particles

In this paper, the pressure fluctuation in a fluidized bed was measured and processed via standard deviation and power spectrum analysis to investigate the dynamic behavior of the transition from the bubbling to turbulent regime. Two types (Geldart B and D) of non-spherical particles, screened from real bed materials, and their mixture were used as the bed materials. The experiments were conducted in a semi-industrial testing apparatus. The experimental results indicated that the fluidization characteristics of the non-spherical Geldart D particles differed from that of the spherical particles at gas velocities beyond the transition velocity U_c. The standard deviation of the pressure fluctuation measured in the bed increased with the gas velocity, while that measured in the plenum remained constant. Compared to the coarse particles, the fine particles exerted a stronger influence on the dynamic behavior of the fluidized bed and promoted the fluidization regime transition from bubbling toward turbulent. The power spectrum of the pressure fluctuation was calculated using the auto-regressive (AR) model; the hydrodynamics of the fluidized bed were characterized by the major frequency of the power spectrum of the pressure fluctuation. By combining the standard deviation analysis, a new method was proposed to determine the transition velocity U_k via the analysis of the change in the major frequency. The first major frequency was observed to vary within the range of 1.5 to 3 Hz.     

Apparent viscosity of mixed particle system in pulsed gas–solid separation fluidized bed

The apparent viscosity reflects the resistance of the fluidized medium in the bed to the beneficiation particles, which directly affects the separation time and mismatch content. So, the falling-ball method was used to measure the apparent viscosity of a binary medium in a pulsed fluidized bed by varying the gas velocity, pulsation frequency, and fine particle content. The results show that with increasing gas velocity and fine particle content, the apparent viscosity of the bed gradually decreased, whereas it first decreased and then increased with pulsation frequency increasing and achieved a minimum value in the range of 4–6 Hz. Within limits, the adjustment of gas velocity and fine content can effectively reduce the apparent viscosity and improve the separation process. A model for predicting the apparent viscosity in a pulsation separation fluidized bed was established with good accuracy.     

Hydrodynamics of gas-solid fluidization of a homogeneous ternary mixture in a conical bed： Prediction of bed expansion and bed fluctuation ratios

Hydrodynamic characteristics of fluidization in a conical or tapered bed differ from those in a columnar bed because the superficial velocity in the bed varies in the axial direction. Fixed and fluidized regions could coexist and sharp variations in pressure drop could occur, thereby giving rise to a noticeable pressure drop-flow rate hysteresis loop under incipient fluidization conditions. To explore these unique properties, several experiments were carried out using homogeneous, well-mixed, ternary mixtures with three dif- ferent particle sizes at varying composition in gas-solid conical fluidized beds with varying cone angles. The hydrodynamic characteristics determined include the minimum fluidization velocity, bed fluctuation, and bed expansion ratios. The dependence of these quantities on average particle diameter, mass fraction of the fines in the mixture, initial static bed height, and cone angle is discussed. Based on dimensional analysis and factorial design, correlations are developed using the system parameters, i.e. geometry of the bed （cone angle）, particle diameter, initial static bed height, density of the solid, and superficial velocity of the fluidizing medium. Experimental values of minimum fluidization velocity, bed fluctuation, and bed expansion ratios were found to agree well with the developed correlations.     

Flow structure and bubble dynamics in supercritical water fluidized bed and gas fluidized bed: A comparative study

Supercritical water (SCW) fluidized bed is a new reactor concept for hydrogen production from biomass or coal gasification. In this paper, a comparative study on flow structure and bubble dynamics in a supercritical water fluidized bed and a gas fluidized bed was carried out using the discrete element method (DEM). The results show that supercritical water condition reduces the incipient fluidization velocity, changes regime transitions, i.e. a homogeneous fluidization was observed when the superficial velocity is in the range of the minimum fluidization velocity and minimum bubbling velocity even the solids behave as Geldart B powders in the gas fluidized bed. Bubbling fluidization in the supercritical water fluidized bed was formed after superficial velocity exceeds the minimum bubbling velocity, as in the gas fluidized bed. Bubble is one of the most important features in fluidized bed, which is also the emphasis in this paper. Bubble growth was effectively suppressed in the supercritical water fluidized bed, which resulted in a more uniform flow structure. By analyzing a large number of bubbles, bubble dynamic characteristics such as diameter distribution, frequency, rising path and so on, were obtained. It is found that bubble dynamic characteristics in the supercritical water fluidized bed differ a lot from that in the gas fluidized bed, and there is a better fluidization quality induced by the bubble dynamics in the supercritical water fluidized bed.     

Influence of cycle time distribution on coating uniformity of particles in a spray fluidized bed by using CFD-DEM simulations

Cycle Time Distribution (CTD) plays a critical role for determining uniformity of particle coating in spray fluidized beds. However, the CTD is influenced by both geometrical structure and operating conditions of fluidized bed. In this study, a spray fluidized bed of coating process is simulated by a comprehensive Computational Fluid Dynamics-Discrete Element Model (CFD-DEM). To achieve different behaviors of CTD, some modifications are designed on a pseudo-2D internally circulating fluidized bed, which traditionally composes of a high-velocity upward bed and low-velocity downward bed. These modifications include making the air distributor slope and/or laying a baffle in the downward bed. First, the CTD and evolution of particle size distribution under different bed structures are compared. The CTD directly influences the coating uniformity. By making the particles flowing along a parallel direction in the downward bed through the geometrical modifications, the CTD becomes narrower and the coating uniformity is significantly improved. Second, under the optimized bed structure, the influence of operating conditions on the coating uniformity is studied. Properly increasing the fluidization gas velocity and the fluidization gas temperature and reducing the liquid spray rate can improve the coating uniformity.     

An analysis of the chaotic motion of particles of different sizes in a gas fluidized bed

The dynamic behavior of individual particles during the mixing/segregation process of particle mixtures in a gas fluidized bed is analyzed. The analysis is based on the results generated from discrete particle simulation, with the focus on the trajectory of and forces acting on individual particles.Typical particles are selected representing three kinds of particle motion:a flotsam particle which is initially at the bottom part of the bed and finally fluidized at the top part of the bed; a jetsam particle which is initially at the top part of the bed and finally stays in the bottom de-fluidized layer of the bed; and a jetsam particle which is intermittently joining the top fluidized and bottom de-fluidized layers. The results show that the motion of a particle is chaotic at macroscopic or global scale, but can be well explained at a microscopic scale in terms of its interaction forces and contact conditions with other particles, particle-fluid interaction force, and local flow structure. They also highlight the need for establishing a suitable method to link the information generated and modeled at different time and length scales.     

Effect of magnetic field orientation on fluidized beds of magnetic particles: Theory and experiment

Geldart-A fluidized beds of fine particles experience a jamming transition between a fluid-like state and a solid-like state at a certain superficial gas velocity, that depends on the relative strength of interparticle attractive forces with respect to particle weight. Interparticle forces provide the bed with a certain tensile strength in the jammed state. In the work presented here we analyze the behavior of a fluidized bed of magnetic particles subjected to an externally applied magnetic field, which contributes to enhance interparticle forces. The importance of the magnetic contribution to interparticle forces is measured by the changes in the tensile strength and the superficial gas velocity at the jamming transition. The link of the field orientation with the microstructure of the bed is discussed.     

Effect of magnetic field orientation on fluidized beds of magnetic particles： Theory and experiment

Geldart-A fluidized beds of fine particles experience a jamming transition between a fluid-like state and a solid-like state at a certain superficial gas velocity, that depends on the relative strength of interparticle attractive forces with respect to particle weight, lnterparticle forces provide the bed with a certain tensile strength in the jammed state. In the work presented here we analyze the behavior of a fluidized bed of magnetic particles subjected to an externally applied magnetic field, which contributes to enhance interparticle forces. The importance of the magnetic contribution to interparticle forces is measured by the changes in the tensile strength and the superficial gas velocity at the jamming transition. The link of the field orientation with the microstructure of the bed is discussed,