三螺杆分散混合数值模拟及实验分析

为了研究平行同向啮合三螺杆挤出机的分散混合效果,采用离散元法对三螺杆挤出机中聚丙烯(PP)/低密度聚乙烯(PE-LD)两种塑料颗粒在加料段的混合情况进行数值模拟.首先建立了三螺杆挤出机的三维模型,在EDEM软件中仿真出不同螺杆转速下挤出机的混合效果,通过对收集盒分层处理计算得出混合过程中混合均匀度的变化趋势.分析结果表...     

褐煤颗粒堆积角数值模拟及试验研究

通过离散元数值模拟并结合试验装置,对比研究了褐煤颗粒的堆积行为。研究结果表明:离散元法可用于描述褐煤的运动轨迹和状态;本文建模方法和经验证的褐煤物性参数,可用于后续工业装置中局部模型和整体模型的数值模拟,对工作开展起到了一定的推动作用。     

水力压裂理论模型及数值计算方法综述

通过用两套方程描述水力压裂过程中涉及到的岩石变形、裂缝扩展以及流体流动与滤失等力学问题,介绍了有限元法、扩展有限元法、边界元法、离散元法这四种常规数值计算方法的基本原理、研究现状,分析了各个方法的优缺点和适用范围。     

基于微流动混合的微纳米粒子合成进展

综述了近三年来人们采用普通层流与聚焦层流、分散相与连续相形成技术、液滴技术等微流控技术角度来合成微纳米粒子的研究进展。这些进展反映出,微纳米粒子合成的微流控技术正在从连续微流动层流向离散态的分段流与液滴流技术的方向发展。文中对围绕基本微混合方式形成的特殊微流控结构被用于粒子合成的研究也给予了关注。最后对微流控微混合方式合成粒子技术过程中存在的一些基本问题作了讨论。     

离散元法及其在单螺杆挤出机计算机模拟中的应用

离散元法(Discrete Element Method,简称DEM)为模拟每个粒子运动及其相互接触行为的数值方法,已成功运用于工业过程的分析和产品设计领域。介绍了离散元法的基本原理,及其在单螺杆挤出机计算机模拟中的应用,并对其发展趋势进行了探讨。     

新型单螺杆CRD分散混合器的理论研究与应用

介绍了一种以增强拉伸流动机理提高单螺杆挤出机混合效果的新观点，以及新分散混合元件——CRD混合器，介绍该元件的结构特点、用边界元法（BEM）进行理论分析与模拟仿真的结果，以及国外采用新型混合器的实用螺杆情况。该混合器使物料在高剪切区产生拉伸流动，并使其多次通过高剪切区，可获得良好分散混合效果。     

分形结构团聚体在剪切流场中分散行为的三维数值模拟

为深入了解颗粒团聚体的分散机理及其动态细节,在经典离散元理论模型中引入范德华力和流体拖曳力,开发了三维颗粒离散元程序DEMix3D,对具有分形结构的颗粒团聚体在纯剪切流场中的分散过程进行数值模拟。结果表明:团聚体在流场中先发生变形然后再分散,分散过程中存在剥蚀和破碎两种分散方式;相同剪切流场下,初始团聚体的分形维数越小其分散效率越高。     

针-环电极配置下电场对气泡分散特性的影响

在多相反应系统中,外加电场因具有促进离散相在连续相中的分散并强化相间传质的优势而受到广泛关注。基于此,本研究设计并构建了静电液气分散实验测量系统,通过在针-环电极间施加高压直流电产生非均匀电场,以乙醇为连续相、空气为离散相,在考虑气体流量和施加电压影响因素的基础上,采用高速摄像技术对电场作用下的气泡分散过程进行了可视化测量。结果表明,气泡在乙醇中的分散主要呈现滴状、混合和喷雾3种模式。以量纲为1气泡直径（ζ）表征的气泡尺寸随着电邦德数（Bo_E）的增大显著减小,随着气体韦伯数（We_g）的增加而增大,其中在混合模式和喷雾模式下的微气泡尺寸基本在20μm以下。增大Bo_E和We_g均能促进气泡的产生,但Bo_E的影响效果更加明显。同时,发现气泡分散模式的转变主要取决于Bo_E,增大We_g使气泡分散模式的转变趋势延缓。此外,基于本研究实验数据建立了ζ与Bo_E和We_g相关的气泡尺寸预测模型,针对ζ ≤ 5的气泡,该模型结果与实验结果高度吻合。通过低能量消耗实现小气泡的生产一直以来是技术难题,采用电场强化气泡分散为发展相应技术实现强化多相系统中相间混合和传质提供了新思路。     

新型微发泡气体注射器注气过程可视化研究

气体注射器是微发泡注塑过程中一个重要装置,本文研究了新型气体注射器的注气过程,通过高速摄像和计算机数据采集系统采集的压力曲线分析注气过程,发现气体从气体注射器进入可视化装置时呈分散状态,有利于在微发泡过程中气体与熔体的分散混合。通过压力数据计算注气流量并配合高速摄像图像分析发现,注气压差和初始水压对注气流量影响很大。通过研究注气过程和注气量,发现新型微发泡气体注射器注气过程稳定可控,可用于微发泡注塑实验。     

固体推进剂固相组分的混合研究

研究了含固体组分的两相体系固体推进剂混合过程的特点。用固体组分的浓度、粒径及其分布等参量研究分析了改性双基推进剂代料的混合特点。根据实验结果，认为改性双基推进剂的混合过程属分散混合。固体推进剂的性能将取决于分散混合后固体颗粒的尺寸。     

Mixing of particles in rotary drums: A comparison of discrete element simulations with experimental results and penetration models for thermal processes

The transverse mixing of free flowing particles in horizontal rotating drums without inlets has been simulated by means of the Discrete Element Method (DEM) in two dimensions. In the simulations the drum diameter has been varied from 0.2 to 0.57 m, and the rotational frequency of the drum from 9.1 to 19.1 rpm, for drum loadings of 20% or 30%, and average particle diameters of 2.5 and 3.4 mm. The choice of operating parameters allows for comparison with experimental data from literature. Though simple models for inter-particle interactions have been implemented, the overall agreement is good. The results are presented and discussed in terms of mixing times and mixing numbers that means numbers of revolutions necessary for uniform mixing of the solids. In this way, comparison with penetration models, as typically applied to modelling of thermal processes, is possible. The limitations of such continuum models are pointed out, along with the potential of DEM to replace them, in the long term.     

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     

A study of the mixing and segregation mechanisms in the Bohle Tote blender via DEM simulations

The flow of spherical particles in a tumbling blender is investigated using discrete element methods (DEM). Simulations are performed on a collection of particles that are mono-disperse and bi-disperse in size. The mono-disperse system is primarily used to assess the quality of mixing as a function of fill level and time. Results reveal that radial convection is faster than axial dispersion transport. This slow dispersive process hinders mixing performance in this geometry. We also find that both axial dispersion and radial convection worsen as the blender fill level is increased. This trend is corroborated by recent laboratory experiments performed in an identical geometry. Particle velocity profiles indicate that the flow is composed of two regions: i) a high velocity layer cascading atop ii) a nearly ‘solid body’ rotation region. Segregating mechanisms are investigated using bi-disperse systems, which show that small particles segregate in pockets at both extremes of the axis of rotation.     

Investigating mixing in a multi-dimensional rotary mixer: Experiments and simulations

Mixing is an important but poorly understood aspect in petrochemical, food, ceramics, fertilizer and pharmaceutical processing and manufacturing. Segregation and mixing phenomenon occur in most systems of powdered or granular solids and have a significant influence on their behavior. Deliberate mixing of granular solids is an essential operation in the production of industrial powder products usually constituted from different ingredients. The knowledge of particle flow and mixing in a blender is critical to optimize the design and operation. Since performance of the product depends on blend homogeneity, the consequence of variability can be detrimental. A common approach to powder mixing is to use a tumbling blender, which is essentially a hollow vessel horizontally attached to a rotating shaft. This single axis rotary blender is one of the most common batch mixers among in industry, and finds use in myriad of application as dryers, kilns, coaters, mills and granulators. In most of the rotary mixers, the radial convection is faster than axial dispersion transport. This slow dispersive process hinders mixing performance in many blending, drying and coating applications. A double cone mixer is designed and fabricated which rotates around two axes, causing axial mixing competitive to its radial counterpart. Discrete Element Method (DEM) based numerical model is developed to simulate the granular flow within the mixer. Digitally recorded mixing states from experiments are used to fine-tune the numerical model. Discrete pocket samplers are also used in the experiments to quantify the characteristics of mixing. A parametric study of the effect of initial loading, particle size, fill ratio, vessel speeds, on the granular mixing is investigated by experiments and numerical simulation. Incorporation of dual axis rotation enhances axial mixing by 60 to 90% in comparison to single axis rotation. Mixing is achieved faster with front-back initial loading than with side-side loading. Particle size and fill level are found to have no significant effect on mixing characteristics.     

Numerical investigation of granular mixing in an intensive mixer:Effect of process and structural parameters on mixing performance and power consumption

Discrete element method (DEM) simulations of particle mixing process in an intensive mixer were con-ducted to study the influence of structural and process parameters on the mixing performance and power consumption. The DEM model was verified by comparing the impeller torque obtained from simulation with that from experiment. Impeller and vessel torque, coordination number (CN) and mixing index (Relative standard deviation) were adopted to qualify the particle dynamics and mixing performance with different parameters. A method based on cubic polynomial fitting was proposed to determine the critical mixing time and critical specific input work during the mixing process. It is found that the mixing performance and energy efficiency increases with the decrease of impeller offset. The mixing perfor-mance is improved slightly with the increase of blade number and the impeller with 3 blades has the highest energy efficiency due to its low input torque. Results indicate that the energy efficiency and the mixing performance increase with the decrease of filling level when the height of granular bed is higher than that of blade.     

Continuous versus discrete modelling of heat transfer to agitated beds

Heating of free-flowing particles by contact with the wall of a rotary drum without inserts has been simulated in two dimensions by means of the thermal version of the Discrete Element Method (DEM). The results are in qualitative agreement with existing experimental data and with the classical penetration model (PM) for the following limiting cases: heat transfer controlled by a contact resistance at the wall of the drum; heat transfer to agitated beds with significant bed-side resistance; heat transfer to the stagnant bed. The latter can be used to establish an equivalence (calibration) between the discrete (DEM) and the continuous (PM) modelling approach. Thermal mixing times can be derived from asymptotic overall heat transfer coefficients obtained via thermal DEM for agitated beds. They are found to be significantly smaller than purely mechanical mixing times. For the investigated conditions, they are also much smaller than previous recommendations based on the PM. The ability of thermal DEM to provide information not accessible to the penetration model, like temperature distributions, is discussed. It is pointed out that a decrease of the high computational cost of the method is necessary in order to enhance its applicability.     

清洁燃料二甲醚

通过介绍液化石油气中二甲醚掺混问题产生的行业背景及相关标准的制定情况,说明了二甲醚作为替代液化石油气的清洁燃料,应纯烧、而非掺混的重要性,并提出了建立二甲醚标准化体系、完善二甲醚检测方法及扩大二甲醚应用领域等建议。     

Analytical and Experimental Evaluation of Dispersive Mixing Performance of Special Rotor Segments in a Corotating Twin-Screw Extruder

An intermeshing corotating twin-screw extruder is often used for producing polymer composites. In this study, we discussed mixing performance of special rotor segments in molten zone. These rotor segments have several tip clearances for achievement of self-cleaning and effective mixing, and these are named various clearance mixing technology. We studied about dispersive mixing performance of configuration, which consists of various clearance mixing technologys (various clearance mixing technology configuration) using numerical analyses and experiments. For the analytical evaluation of mixing performance, we have adopted a particle tracking method. As evaluation indices of dispersive mixing performance, we focused on following three values (1) maximum value in a history of first principal stress of each particle, (2) time-integrated shear rate of each particle, and (3) time-integrated first principal stress of each particle. And we made the probability distributions of the indices. We adopted weighted probability as approximation of volume probability in each region. The results were compared with those of kneading disk segments configuration (kneading disk configuration). Furthermore, we validated the accuracy of the analytical evaluation by performing experiments under same conditions as those of numerical analyses. For the experimental evaluation of mixing performance, we used twin-screw extruder. Polypropylene was selected as suspending media. And CaCO₃ was selected as filler. To compare just the mixing performance of various clearance mixing technology configuration with that of kneading disk configuration, side feeding and screw configuration which had third mixing segments zone were adopted. From the experimental result, it is found that various clearance mixing technology configuration dispersed better than kneading disk configuration. And from the comparison between the experimental evaluation and the analytical evaluation, it is found that the first principal stress is more appropriate for evaluation index of dispersive mixing. Finally, it can be mentioned that various clearance mixing technology configuration has better mixing performance than kneading disk configuration, and large stress is important for dispersive mixing.     

数值模拟研究螺纹元件的混合性能

使用FEM方法数值模拟了同向双螺杆螺纹混合元件和普通螺纹元件流道中硬聚氯乙烯的流动过程,计算了2种螺纹元件流道内熔体的非等温流场,使用粒子示踪法统计分析了2种螺纹元件流道内熔体的混合性能.研究结果表明,螺纹混合元件具有较均匀的剪切速率分布、较平缓的温度分布和较强的分散与分布混合能力,但是其输送能力较小.