轻质环氧基复合材料隔热性能的实验与数值研究

通过实验和数值模拟方法研究了空心玻璃微球/环氧树脂复合材料隔热性能。首先使用ANSYS/APDL建立代表性体积单元,探讨了模拟过程中模型尺寸、填料体积分数、填料平均粒径及基体与填料导热系数之比(λ_m/λ_p,λ_m保持不变)对复合材料导热系数的影响。数值研究结果表明:当代表性体积单元尺寸大于500μm时,模型尺寸对复合材料导热系数的计算结果影响较小;复合材料导热系数随填料体积分数和λ_m/λ_p的增大而减小,且基本不受填料粒径影响。其次,将复合材料导热系数实验值和计算结果进行对比,比较发现导热系数计算结果和实验值及Agari模型理论值吻合较好,证明了计算方法的可靠性。同时,空心玻璃微球的掺入有效地降低了树脂的密度。轻质隔热的空心玻璃微球/环氧树脂复合材料有潜力成为一种具有广泛应用前景的节能环保材料。     

液固流化床的混合离析模型

本文从颗粒的微观运动出发，采用随机过程模型描述了定常态下液固流化床内二元颗粒的混合和离析现象。在φ１０４×１５００ｍｍ的流化床内，以水为流化介质，对两种不同大小的同种颗粒二元体系在不同流速、不同组成、不同粒径比情形下进行了测定，确定了模型参数关联式，获得了颗粒沿床层的浓度分布关系。将模型计算值与实验值及文献值作了比较，证实了该模型的可行性     

气固两相流中颗粒运动强化器壁对流传热的机理

提出了颗粒碰撞壁面过程中颗粒打破边界层和颗粒与壁面导热的强化传热模型,以分析气固两相流中颗粒强化对流传热的机理,通过模型计算研究了风速、颗粒浓度和粒径等因素对颗粒在壁面的停留特性、颗粒在边界层的扰动及对导热和边界层破坏两种强化机制传热比重的影响．以气固两相流横掠圆管的传热过程为例进行计算,计算与实验结果一致,并获得了具有实用价值的关联计算式．     

EFFECTIVE THERMAL CONDUCTIVITY OF BINARY MIXTURES AT HIGH SOLID TO GAS CONDUCTIVITY RATIOS

The effective thermal conductivity of single size and binary mixtures of packed particle beds with stagnant gas at high solid/gas conductivity ratios is determined by a deterministic, unit cell approach. The model results are shown to be in good agreement with experimental data for various gas pressures and solid to gas thermal conductivity ratios up to 1300. A set of correlations for effective conductivity of binary mixtures as a function of gas pressure and particle size is derived. The effect of particle swelling on the effective conductivity of binary mixtures is studied by performing a parametric study of the contact area between the particles     

PLGA nano- and microparticles for the controlled release of florfenicol: Experimental and theoretical study

In this study, PLGA particle systems were studied for the controlled release of florfenicol, a broad spectrum antibiotic used in veterinary treatments. The emulsion-solvent evaporation technique was used for particle preparation. To evaluate the particle size, entrapment efficiency, and drug release behavior, factors such as solvent type, emulsification time and methods, and drug to polymer ratio were investigated. The results showed that the use of ethyl acetate and 2.5 min of ultrasonication or 30 min of homogenization can lead to sub-micron and micron-sized particles, respectively. Sizes between 200–300 nm and 2–3 μm were obtained for ultrasonication and homogenization procedures, respectively. Entrapment efficiencies were around 20% for all systems and release profiles were size dependent. In addition, a mathematical model was implemented to simulate the florfenicol transport. The model predicts the florfenicol release and takes into account the particle size, polymer molecular weight, and autocatalytic polymer degradation. Simulation results are in good agreement with experimental results. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47248.     

Simulation of a composite cathode in solid oxide fuel cells

Incorporating the mechanistic model for oxygen reduction at YSZ/LSM interface, a complete micro-model for YSZ/LSM composite cathode considering all forms of polarization was developed which established the interrelationship among the transport phenomena, electrochemical processes and the microstructure of the composite cathode. The exchange current densities of the rate-limiting steps used in the simulation were obtained by fitting the proposed mechanistic model to the DC polarization curves. Simulation was conducted to predict the optimal design parameters, e.g. cathode thickness, particle size, particle size ratio and YSZ volume fraction, for a LSM/YSZ composite cathode. Except for the YSZ volume fraction beyond the percolation thresholds, the predicted results seem to be in good agreement with the experimental and literature data. Incorporating with reliable experimental data, the model can be used as a tool to guide the design of high performance cathodes.     

FACTORS GOVERNING SURFACE MORPHOLOGY OF SPRAY-DRIED AMORPHOUS SUBSTANCES

Changes in particle morphology (size, shape, and appearance) have been monitored during drying of drops of foods and food-related materials. The apparatus produces a single stream of drops of uniform size, using a vibrating-orifice device for drop production. The drop size and the time-temperature history of the drops as they fall can be varied and controlled.

Qualitative observations are reported for drying of aqueous solutions of lactose, maltodextrin, skim milk, and coffee extract, with different feed concentrations. Particular emphasis is placed upon the tendency for development of folds upon the particle surface. A mechanistic model is developed, relating the tendency for folding to the extent of viscous flow of surface material in response to a surface-energy driving force. This model gives semi-quantitative agreement with observations for solutes of different molecular weight (and hence different viscosity) and for different feed concentrations.     

气液逆喷洗涤器除尘性能的实验研究

在冷模实验装置上考察了气液逆喷洗涤器的除尘效率,研究了液气体积比、洗涤液喷头入口压力及进口粉尘质量浓度对除尘效率的影响。结果表明,除尘效率随液气体积比、洗涤液入口压力和进口粉尘质量浓度的增加而增大;在液气体积比大于6 L/m3时,气液逆喷洗涤器的除尘效率便可达99%以上;对于1—5μm的可吸入颗粒,其粒级效率大于97.5%;对5μm以上粒子,粒级效率则为100%。对洗涤器粒级除尘效率的理论计算显示,计算值与实验测量值吻合较好。文中的计算可用于气液逆喷洗涤器的性能预估。     

Predicting the effective thermal conductivity of polydispersed beds of softwood bark and softwood char

In this paper, the effective thermal conductivity (ETC) of softwood bark and softwood char particle beds which are highly polydispersed has been studied theoretically and experimentally. Use of the linear packing theory and unit cell model of heat conduction enabled to express ETC of polydisperded beds as a function of particle size distribution. Each of the softwood bark and softwood char samples were sieved into seven fractions. The initial porosity and binary packing size ratio of the particles have been characterized as a function of mean sieve size. ETC of polydispersed beds of bark and char has been predicted as a function of particle size distribution. Model predictions were in good agreement with the experimental measurements. The proposed approach can be used to predict the ETC of any size distribution of softwood bark and softwood char beds without measuring the in situ bed porosity.     

PARTIUE FLOW AND CONTACf HEAT TRANSFER CHARACTERISTICS OF STIRRED GRANULAR BEDS

This paper presents a new contact heat transfer model for estimation of wallto- bed heat transfer rates based exclusively on information on particle flow and mixing within a stirred granular bed. The effects of solids flow ability, bed height, blade rotational speed, size of the annular wall-to-blade clearance, vessel diameter and aeration of the bed on the overall solids mixing patterns, particle renewal rates and contact heat transfer in vessels agitated with flat paddles are presented and discussed. The model is shown to yield satisfactory agreement With expenrnental data.     

管道流动体系中水合物颗粒粒径分布特性数值模拟

研究管道内水合物颗粒的粒径分布特性对深水流动安全保障具有十分重要的意义。为模拟管道流动体系中水合物颗粒的粒径分布特性,本文首先建立了基于水合物颗粒聚集动力学的群体平衡模型。该模型重点考虑水合物颗粒在流动过程中的碰撞频率、聚并效率、破碎频率及破碎后子颗粒的粒径分布函数,可较好地刻画管内水合物颗粒的流动行为。随后,根据文献中的实验装置建立三维几何模型,利用FLUENT 14.5软件对上述群体平衡模型和相关固液两相流模型进行联合求解,借此模拟水合物颗粒初始粒径分布、水合物体积分数、管内流速和水合物颗粒初始粒径大小对管内水合物颗粒粒径分布类型及分布规律的影响。本文模拟结果与文献中相关实验数据吻合良好,可为水合物防治技术的发展提供技术支持。     

植物纤维浓硫酸水解动力学研究

研究了黄Ma杆浓硫酸水解的动力学，考察了酸浓度、固液比、粒度、温度对水解速率和糖产率的影响，建立了浓硫酸水解的动力学模型，该模型能较好地关联 实验数据。     

Analysis of iron particle growth in aerosol reactor by a discrete-sectional model

The growth of iron particles by thermal decomposition of Fe(CO)₅ in a tubular reactor was analyzed by using a one dimensional discrete-sectional model with the coalescence by sintering of neighboring particles incorporated in. A thermal decomposition of Fe(CO)₅ vapor to produce iron particles was carried out at reactor temperatures varying from 300 to 1,000°C, and the effect of reactor temperature on particle size was compared with model prediction. The prediction exhibited good agreement with experimental observation that the primary particle size of iron was the largest at an intermediate temperature of 800°C. Model prediction was also compared with Giesen et al.’s [1] experimental data on iron particle production from Fe(CO)₅. Good agreement was shown in primary particle size, but a considerable deviation was observed in primary particle size distribution. The deviation may be due to an inadequate understanding of the sintering mechanism for the particles within an agglomerate and to the assumption of an ideal plug flow in model reactor in contrast to the non-ideal dispersive flow in actual reactor.     

Effect of temperature on particle size for vapor-phase synthesis of ultrafine iron particles

For the vapor-phase synthesis of iron particles from FeCl₂ at temperatures ranging from 800 to 950‡C the reason is sought why the model based on the classical nucleation theory brought an increase of particle size with temperature increase, in reverse to experimental observation. The nucleation rate according to the classical theory should decrease with a temperature increase, due to the decrease of super-saturation ratio resulting from the increase of vapor pressure. The decrease of nucleation rate ultimately leads to an increase of particle size. Yang and Qiu’s nucleation theory was applied in place of the classical theory. However, the same result as with the classical theory was obtained : the nucleation rate decreased with the temperature increase. Finally, an Arrhenius-type nucleation rate equation was introduced. The preexponential factor and the activation energy for nucleation were determined to be 1348.2 sec_-1 and 159.1 KJ/mol, respectively. With these values put into Park et al.’s model, good agreement was obtained in temperature dependence of particle size between model prediction and experimental data.     

Effects of particle shape and size on devolatilization of biomass particle

Experimental and theoretical investigations indicate particle shape and size influence biomass particle dynamics, including drying, heating rate, and reaction rate. Experimental samples include disc/flake-like, cylindrical/cylinder-like, and equant (nearly spherical) shapes of wood particles with similar particle masses and volumes but different surface areas. Small samples (320 μm) passed through a laboratory entrained-flow reactor in a nitrogen atmosphere and a maximum reactor wall temperature of 1600 K. Large samples were suspended in the center of a single-particle reactor. Experimental data indicate that equant particles react more slowly than the other shapes, with the difference becoming more significant as particle mass or aspect ratio increases and reaching a factor of two or more for particles with sizes over 10 mm. A one-dimensional, time-dependent particle model simulates the rapid pyrolysis process of particles with different shapes. The model characterizes particles in three basic shapes (sphere, cylinder, and flat plate). With the particle geometric information (particle aspect ratio, volume, and surface area) included, this model simulates the devolatilization process of biomass particles of any shape. Model simulations of the three shapes show satisfactory agreement with the experimental data. Model predictions show that both particle shape and size affect the product yield distribution. Near-spherical particles exhibit lower volatile and higher tar yields relative to aspherical particles with the same mass under similar conditions. Volatile yields decrease with increasing particle size for particles of all shapes. Assuming spherical or isothermal conditions for biomass particles leads to large errors at most biomass particle sizes of practical interest.     

Simulation of nanotube separation in field-flow fractionation (FFF)

A Brownian dynamics simulation based on a prolate spheroid particle model has been developed to model the separation of nanotubes in cross flow driven, field-flow fractionation (FFF). The particle motions are governed by stochastic forms of a linear momentum balance with orientation dependent drag and diffusion coefficients, and the Jeffrey equation with rotational diffusion. The simulation shows that nanotube scale particles would be expected to elute by a normal mode mechanism up to aspect ratios of about 1000, based on a particle diameter of 1 nm. Separation of nanotubes of different length is governed by the value of the retention variable for each component in agreement with theory. Elution profiles and average velocity through the device as a function of particle size, and the flow rates in the throughput and cross-flow directions are examined. The simulation shows that clean separations between components of different size is achieved when the ratio of the retention values is greater than 2.     

Packing Density of Composite Powder Mixtures

A model of particle packing in binary composite systems is developed. The effects of both inclusion surfaces and touching inclusions on the packing density are taken into account. To implement the model, a statistical approach is used to determine the number of inclusion contacts as a function of inclusion content. The statistical approach indicates that the average number of inclusion contacts is a linear function of the inclusion volume fraction, a result which agrees very well with independent computer simulations. The model suggests that the packing efficiency, defined by the ratio of the packing density to the ideal packing density (as originally derived by Furnas), is governed by the inclusion volume fraction ( f_i ) and the particle to inclusion size ratio ( r ). Good agreement is obtained between the theory and experimental literature data.     

FACTORS GOVERNING SURFACE MORPHOLOGY OF SPRAY-DRIED AMORPHOUS SUBSTANCES

ABSTRACT

Changes in particle morphology (size, shape, and appearance) have been monitored during drying of drops of foods and food-related materials. The apparatus produces a single stream of drops of uniform size, using a vibrating-orifice device for drop production. The drop size and the time-temperature history of the drops as they fall can be varied and controlled.

Qualitative observations are reported for drying of aqueous solutions of lactose, maltodextrin, skim milk, and coffee extract, with different feed concentrations. Particular emphasis is placed upon the tendency for development of folds upon the particle surface. A mechanistic model is developed, relating the tendency for folding to the extent of viscous flow of surface material in response to a surface-energy driving force. This model gives semi-quantitative agreement with observations for solutes of different molecular weight (and hence different viscosity) and for different feed concentrations.     

反应沉淀法制备亚微米粒子的粒度分布模拟与实验验证

采用基于粒数衡算方程及物料衡算方程的数值模拟方法模拟了包括反应、成核、生长及凝并的反应沉淀制备亚微米粒子的过程,其中颗粒间的凝并过程采用分级模型来模拟,产物颗粒的粒度分布（PSD）由离散化的粒数衡算方程求得.以BaSO₄的反应沉淀体系作为研究体系,将实验测量数据与数值模拟结果进行对比,验证了所构建的数学模型的正确性与适用性.应用此模型进行分析,发现沉淀时间、产物过饱和度及凝并对粒度及粒度分布有着显著的影响.在此基础上,提出了反应沉淀法制备亚微米粒子过程中颗粒粒度及粒度分布的控制方法.     

Quantitative characterization of dispersed particle size,size distribution,and matrix ligament thickness in polypropylene blended with metallocene ethylene–octene copolymers

The average rubber particle size, size distribution, and matrix ligament thickness between particles in polypropylene blends containing metallocene catalyzed ethylene–octene copolymers have been quantitatively analyzed, as functions of blend composition and phase viscosity ratio. Comparison has been made between experimental data and those predicted from a number of theoretical models. All blends showed two‐phase morphology, with interestingly a bimodal distribution of the rubber particle size. The ranges and averages of rubber particle size were mainly determined by blend composition and viscosity ratio between the phases, irrespective of comonomer content along the rubber chains. The logarithmic relationship between the matrix ligament thickness and rubber concentration was observed. The values of ligament thickness obtained from the experiments and theoretical models were not in agreement. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2140–2149, 2001