Combined Homogeneous and Heterogeneous Crystal Nucleation in Glasses

The temperature dependence of combined homogeneous and heterogeneous crystal nucleation is examined within the framework of the classical theory. It is demonstrated that nucleation can have a local maximum rate at one or two temperatures. The physical parameters which cause large changes in the structure of the nucleation curve are elucidated. Also, the influence of saturation effects on the structure of nucleation curves is illustrated. The utilization of these results for the interpretation of experiments and for the production of controlled crystal nucleation in glass is considered.     

Simulation of Homogeneous Particle Nucleation in a Free Turbulent Jet

The homogeneous particle nucleation in a free turbulent jet is simulated at Reynolds numbers of 3400 and 34,000 by using large eddy simulation (LES) coupled with our newly developed equivalent mean nucleation method (EMNM) for the present study. The simulated velocity field, passive scalar concentration, and nucleated particle concentration are compared with various available measurements in the literature. Both the simulated mean velocity and mean scalar concentration show self-similarity properties and agree well with the experimental measurements quantitatively in the literature. Using the EMNM, it is found that the nucleation primarily occurs in a very narrow region around the jet centerline. It is also found that the position of maximum nucleation is a function of the vapor temperature and concentration at the jet exit and its surrounding background (i.e., T ₀, T _∞, c ₀, and c _∞), and the function varies monotonically with respect to these four parameters. Present numerical simulation and analysis also show that the group, (where is the mean particle number concentration, u ₀ is the jet velocity, and d is the jet exit diameter), is constant along the axial direction of the jet, which was first observed in the experiment of Lesniewski, T. K., and Friedlander, S. K. (1998). Particle Nucleation and Growth in a Free Turbulent Jet. Proc. R. Soc. Lond. A, 454:2477–2504. However, a different physical explanation of the fact is provided. In this study, it is found that the maximum nucleation occurs around 20d away from the jet exit, other than in the shear layer (less than 5d), which was supposed by Lesniewski, T. K., and Friedlander, S. K. (1998). On the basis of the additional simulated results, more comprehensive and reasonable explanations of their measurement data are provided. In this study, an efficient and rigorous numerical method is presented to simulate the homogeneous particle nucleation in a free turbulent jet, and this has provided a deeper and thorough understanding of the process.     

鼓泡流态化向湍动流态化过渡的判别

通过对流化床床层压力脉动信号的计算机在线分析,对气-固密相流化床中鼓泡流态化向湍动流态化的转变过程进行了研究.在较大范围内考察了颗粒重度、颗粒尺寸和床层结构条件对这一流型转变的影响.给出了鼓泡流化向湍动流化转变速度U_c的计算式:U_c/(gd_ρ)~(1/2)=[k(D_f/d_ρ)·((ρ_ρ-ρ_f)/ρ_f)]~n并察明Geldart的颗粒分类方法亦可反映床层流型转变的特征,从而赋予了Geldart颗粒分类方法以新的内涵.     

Homogeneous Nucleation by Continuous Mixing of High Temperature Vapor with Room Temperature Gas

The formation of aerosol particles by homogeneous nucleation in a supersaturated vapor has been studied experimentally and theoretically. In the laboratory, a particle-free gas at room temperature is continuously mixed with a high-temperature gas containing dibutylphthalate vapor in a new device for the study of aerosol nucleation called a particle size magnifier. A highly supersaturated vapor is rapidly formed in the mixing zone of the particle size magnifier, and the resulting number concentrations of aerosol particles are measured under various temperatures, mixing ratios, and mixing methods. Measured number concentrations are compared with those predicted by the classical and Lothe-Pound nucleation theories. The measured concentrations lie between the predictions of the two theories, and the trends with temperature and saturation ratio are consistent with either nucleation theory, provided vapor depletion is considered.     

一种圆管紊流混合长度理论的修正

对普朗特混合长度理论在圆管紊流流动中的应用进行了修正。基于尼古拉兹的圆管紊流速度分布实验结果:在除管中心及其附近位置外的紊流核心区内精确符合对数分布,结合力平衡方程,得出混合长度表达式;考虑圆管紊流实际情况,对混合长度表达式进行合理假设并引入阻尼函数进行改进,形成适用于圆管不同半径位置处的混合长度计算公式,修正后的混合长度表达式在整个圆管半径范围内光滑连续。基于修正后的混合长度表达式,推导出圆管剪切速率分布函数,并得出速度分布沿半径是一条存在二阶导数的光滑曲线;对剪切速率分布函数进行数值积分,计算某一流动状态下的管道速度分布,与实验结果精确吻合,且在圆管中心位置和层流子层区边界不再存在速度突变,更符合流体实际流动情况。     

A Unified Model for the Mixing of Non‐Newtonian Fluids in the Laminar,Transition, and Turbulent Region

Summary: Non‐Newtonian fluid behavior has significant influence on quantities in chemical engineering like power input, mixing time, heat transfer etc. In the laminar flow region, the concept of effective viscosity by Metzner and Otto is well established. In the transition region between laminar and turbulent flow, the existing concepts use three and even more empirical parameters to determine the specific power input. Here, a unified and general but simple approach is introduced to calculate the power input for shear thinning fluids over the whole flow region using just one empirical parameter. The Metzner‐Otto relation is obtained as a limiting case for the laminar region. The empirical parameter of the new approach is related to the Metzner‐Otto constant. The concept is validated for eight different stirrer systems. Mixing time and maximum shear rate and heat transfer can also be calculated using this approach. The new concept presented should also be applicable for other apparatuses, e.g., static mixers.

Comparison of experimental data and a curve calculated according to the new method (solid line).     

Heterogeneous Nucleation in the Intergranular Phase of a SiAlON

A SiAlON ceramic with yttria as a sintering aid has been fabricated and subjected to a 1-h heat treatment at 1150°C. Microstructural characterization by analytical electron microscopy and diffraction analysis indicates that the residual intergranular phase crystallizes into a metastable hexagonal phase (Y₂SiAlO₅N) and mullite. The mullite appears as a characteristically shaped precipitate forming with a definite orientation relationship on the (10 1 0) facets of SiAlON grains. The absence of a residual amorphous film between the precipitate and the SiAlON indicates that the precipitation occurred in a heterogeneous manner on the surface of the SiAlON.     

利用面激光诱导荧光技术研究喷射器内液-液湍流混合特性

面激光诱导荧光(planar laser induced fluorescence,PLIF)是流场研究中的先进测试技术.今建立了液-液喷射混合器流场研究实验装置,利用PLIF技术对喷射混合器的湍流混合特性进行了研究,得到了喷射器内湍流混合的二维浓度场.对瞬时、时均浓度场的图像进行了分析,并利用离析度(intensity of segregation,IOS)的概念对时均混合效果进行了评价,得到了不同操作条件下IOS值沿射流方向的变化趋势图,结果表明流速比和绝对流速是影响混合的两个主要因素,液液喷射器中流速比越大.达到完全混合所需的距离就越短,在相同流速比的情况下,流速越大混合进行得就越快.     

T型混合器内湍流混合的多尺度模拟

建立了T型混合器内物料的宏观混合模型和微观混合模型,基于湍流k-ε模型,利用fluent6.2软件对T型混合器的宏观混合性能和微观混合性能进行了模拟研究.结果表明,T型混合器的侧管人口角度对混合性能具有重要的影响.对于宏观混合,在30°时混合效果最佳;而对于微观混合,60°时混合效果最佳.侧管人口速度与主管速度比越大,混合效果越好,达到完全混合所需要的长度越短.速度比相同时,混合效果不随绝对速度的变化而变化,达到完全混合所需要的混合长度相同.     

Analytical Approximation Schemes for Mean Nucleation Rate in Turbulent Flows

Nucleation rate is a very sensitive function of the temperature and vapor mole fraction. Analytical approximation schemes for the mean nucleation rate in turbulent flows are derived using Laplace’s approximation method. The schemes only require the derivative of the nucleation rate function and the probability density function (pdf) of the vapor mole fraction and/or temperature at the point of maximum nucleation rate. Based on the relation between the mole fraction and temperature, i.e., linearly correlated or not, different approximation schemes are developed. Numerical examples are constructed to investigate the accuracy of these approximation. In the examples, the pdfs of mole fraction and/or temperature in various turbulent flows are assumed to come from the beta distribution with five distinct forms. The mean nucleation rate of dibutyl phthalate (DBP) aerosol in these turbulent flows are calculated from the approximation schemes, and compared with exact numerical integration. The relative errors are less than 1% for cases when nucleation rate diminishes at the bounds of temperature fluctuations, and no more than 50% for all studied examples. Furthermore, the approximation schemes are not sensitive to the precise form of the pdfs. Hence, these developed approximation schemes can be used to estimate the mean nucleation rate in a broad range of turbulent flows conveniently.

Copyright 2014 American Association for Aerosol Research     

均相成核—水热法制备纳米氧化锌

制备纳米氧化锌的关键是成核与生长控制。采用尿素为均相沉淀剂 ,结合水热处理 ,温度 130℃ ,反应时间为 3～ 5h制备了粒径小 ,分布窄的纳米氧化锌。Zn2 +的均相成核与其浓度、OH- 的浓度有关 ,体系中Zn(OH) 2 的过饱和度越大 ,则成核数量越多 ,所制备的氧化锌颗粒越小 ;与以氢氧化锌胶体为前驱体的共沉淀技术相比 ,均相沉淀技术有利于氧化锌纳米晶粒均匀生长 ;此外 ,添加有机物作分散剂可以减弱微小晶粒间的叠合生长 ,有利于制备均一性较好的纳米氧化锌。     

Diffusion of a Submicronic Spray in an Homogeneous Turbulent Flow

The prediction of the diffusion of a spray in a gaseous environment is still a difficult problem to solve, even when the droplets are small enough to be considered a passive contaminant. The present work proposes to use a simulation of the random trajectories of the individual particles based on the method proposed by Ormancey (1983), and Ormancey and Martinon (1983). It is applied to the case of a nozzle of finite dimensions discharging passive particles in homogeneous turbulent airstream at a low enough concentration to avoid neighboring effects. The results of this simulation compare favorably with those of an experiment designed to test the method. They are also in agreement with the Lagrangian diffusion theory of Taylor (1936).     

方形气固流化床从鼓泡到湍动流态化转变速度预测模型

在368 mm×368 mm的方形气固流化床中对FCC颗粒进行了流态化实验,基于对床内总体压力脉动信号分析了从鼓泡流态化到湍动流态化的转变速度Uc与静床高度H0及床层面位置H的关系.结果表明,床层截面位置H较低或静床高度H0增加都使Uc增加,即鼓泡流态化到湍动流态化的流型转变是由床层上部逐渐向下扩展的递进行为.基于这一...     

Particle Mixing and Diffusion in the Turbulent Wake of a Sphere

The mixing and diffusion of monodisperse aerosols from a turbulent free stream into the wake created by a solid sphere were studied experimentally and analytically. Experimental measurements with regard to wake velocity profiles and momentum transport were taken in a tubular flow system by hot-wire anemometry. Particle concentration profiles, measured in the wake by means of a screen collection technique, were used to calculate mixing and diffusion parameters for the aerosols used. Experimental results are presented for the mean velocity and particle concentration profiles, wake centerline velocity and particle concentration, wake velocity and concentration half widths, and momentum and particle diffusivities. It is shown that the experimental results may be reasonably described in terms of two phenomenological theories, i.e., Prandtl's mixing length and constant diffusivity in the radial cross section of the wake. Certain departures from predicted concentration behavior are empirically analyzed resulting in the ability to predict overall particle mixing and diffusion characteristics in a turbulent wake.     

Attempts to Fill the Gap Between Enzymatic, Homogeneous, and Heterogeneous Catalysis

Bio-inspired and single site metal complex catalysts have been discussed to direct towards a rational design of solid heterogeneous catalysts. When concepts derived from catalytic antibodies, molecular imprinting and molecular recognition, and site isolation and modification by appropriate ligands are combined, with new techniques to prepare, tailor made solid materials, catalysts can be prepared that improve reaction rate and selectivity by increasing the concentration and activation of reactants in the vicinity of the active sites, and by stabilizing transition states or intermediate products. It is also shown that enzymatic, homogeneous and hetergeneous catalysts can be combined to perform “one-pot” cascade reactions.     

Homogeneous Nucleation of 2, 4-Dichlorophenol in Acid Aqueous Solutions

The interfacial tension of 2, 4-dichlorophenol/acidic solution, 23 ergs/cm², was calculated from Nielsen's equations which combine homogeneous nucleation theory and diffusion controlled growth. The parameters measured were induction periods in the millisecond range and the technique employed was photographing the appearance of turbidity on a film moving at 50cm/sec velocity. The calculated value was compared with direct measurements of 2, 4-dichlorophenol's surface tension in air and in solution.     

Effect of Mixing on the Nucleation and Growth of Titania Particles

Aerosol processes that produce titania particles by reacting gaseous precursors (such as titanium tetrachloride) initially must mix the precursor into the oxidizer at elevated temperatures to initiate the formation of product. Oftentimes the rate of reaction is sufficiently large as to be mixing limited. Thus the rate of mixing of the reacting species will control the chemistry and morphological properties of the particles that are produced. The interplay between mixing, nucleation, and growth in these systems is difficult to observe experimentally due to the small time scales that are involved and the spatial limitations of most diagnostics. An alternative approach is direct numerical simulation (DNS). DNS refers to a class of numerical solutions of the three-dimensional time-dependent governing equations for a particular system in which no turbulence modeling assumptions are made. To within the precision of the numerical algorithm, DNS can be thought of as a numerical experiment. Here we apply DNS to the mixing, reaction, nucleation, and growth of titania particles formed from the reaction of titanium tetrachloride with oxygen. The simulation solves for the velocity, species concentration, and eight moments of the particle size distribution using a combination of a pseudospectral method (for the velocity) and a compact finite difference scheme (for all of the scalars). The results show that increasing the rate of mixing increases the rate of particle formation while decreasing the variance in the particle size distribution. However, for a given extent of reaction, poorer mixing leads to larger mean particle sizes and larger standard deviations. The results are most easily interpreted in terms of the reaction volume between the unmixed reactants, where most of the reaction occurs. Based on this analysis, we present rules of thumb for controlling the particle size distribution in aerosol reactors.     

Kinetics of Heterogeneous Nucleation in Supersaturated Vapor: Fundamental Limits to Neutral Particle Detection Revisited

We examine the nucleated (with barrier) activation of perfectly wetting (zero contact angle) particles ranging from essentially bulk size down to approximately 1-nm mass diameter. While similar studies trace back to the pioneering work of Fletcher, we present here a novel approach to the analysis based on general area constructions that enable key thermodynamic properties, including surface and bulk contributions to nucleation work, to be interpreted geometrically with reference to the Kelvin curve. The kinetics of activation are described in more detail in terms of the mean first passage time (MFPT) for barrier crossing. MFPT theory and benchmark calculations are used to develop and test a new approximate-but-simpler-to-use analytic expression for the barrier crossing rate. The present study is motivated by recent condensation particle counter (CPC) studies that appear to finally establish the long-predicted detection of “sub-Kelvin” particles in the nano-size regime. Corresponding states thermodynamic and kinetic scaling approaches are used to facilitate the correlation and selection of optimal CPC working fluids and operating conditions based on a new metric for heterogeneous nucleation, the signal-to-noise ratio, and physical and chemical properties.

Copyright 2012 American Association for Aerosol Research