Asymptotic Behavior of Aerosol Growth in the Free Molecule Regime

Although the asymptotic behavior of aerosol growth by condensation/evaporation and by coalescence has been well established independently, no complete theory is available for such simultaneous processes. In this paper, through the use of a supercomputer, it is shown that asymptotic limit-size distributions exist for mass conserved systems when condensation/evaporation and coalescence occur simultaneously in the free molecule regime. Unlike the independent growth process either by condensation/evaporation or by coalescence, “crossover” or change from one dominating growth process to another growth process takes place continuously. It is also demonstrated that substantial savings in central processing unit time on a supercomputer can be achieved by restructuring the numerical algorithm and by vectorizing the inner DO loops.     

喷射状态下大孔径复合筛板塔中的液滴分布

The droplet size distribution with large-holed compound sieve tray operating in the spray regime is measured by using a double electrical probes technique in a cold model column of 400 mm diameter. The results indicate that the hole F-factor Fo and surface tension are the main factors which influence the liquid dispersion expressed by the Sauter mean diameter D32. A correlation of D32 on surface tension, viscosity, F-factor, weir height and liquid flow rate is proposed.     

Condensational Growth and Evaporation in the Transition Regime

The condensational growth and evaporation of liquid droplets in the transition regime and in the continuum regime are considered. An analytical solution that includes both transition regime corrections for mass and heat transfer and the Kelvin effect is derived. The solution represents an improvement over earlier analytical solutions, which are valid only in the continuum regime. The growth (evaporation) times predicted by the analytical solution are compared with the times obtained by a numerical solution. In the transition regime the new analytical expression estimates the growth time within 20%. The closer the saturation ratio is to unity, and the smaller the latent heat of vaporization of the substance in question, the better is the accuracy of the new expression     

不同分子量聚丁烯-1的非等温结晶动力学研究

采用差示扫描量热法研究了不同降温速率下,不同分子量聚丁烯-1(PB、PB0.5％、PB1.5％、PB3.0％)的非等温结晶动力学,并用Avrami和Jeziorny法进行处理分析,获得PB相关非等温动力学参数,并利用Kissinger方程计算其结晶活化能.结果表明:当PB分子量降低时,结晶速率变快.不同分子量PB的结晶...     

Effect of Molecular Transfer on Nonisothermal Reactive Mixing

A closed set of Langevin-type equations is derived, which involves the scalar dissipation rate as an independent variable and enables one to study the effect of the molecular transfer coefficients on the turbulent micromixing rate.     

Evaporation and Condensational Growth of Liquid Droplets in Nonisothermal Gas Mixtures

A theoretical study and coupled analytical model of the growth rates of a droplet in a nonisothermal gas mixture are presented. The derived model explicitly accounts for the influence of the properties of the gaseous media on the transport processes in an aerodisperse system. The Stefan flux as well as temperature and composition dependencies of the transport coefficients are accounted for simultaneously, still resulting in a convenient analytical expression. Such an expression is especially useful to use in a numerical GDE solution routine, in which much time cannot be spent in the individual physical processes. The results of previous studies are compared to our model predictions. Numerical calculations are performed for a wide range of saturation ratios for the water droplet-humid air system and compared with experimental data. The presented new model is an improvement on the existing ones, especially in cases in which the mass transfer rates are large, resulting in a nonuniform temperature field.     

Monte Carlo Simulation of Droplet Nucleation in RAFT Free Radical Miniemulsion Polymerization

Abstract

The early stages of the reversible addition/fragmentation transfer (RAFT) miniemulsion polymerization were simulated, focusing on the effect of the RAFT agent on droplet nucleation. For highly reactive RAFT agents, a large number of free radicals (N_c ) needed to be captured by a droplet in order to initiate polymerization in the droplet, which was totally different from the behavior of regular miniemulsion polymerization. More interestingly, it was found that droplet size had a significant influence on N_c value. It was shown that the RAFT agent has a significant influence on miniemulsion polymerization, leading to long induction periods and retardation of polymerization. In addition, miniemulsion droplets with different sizes are nucleated at different times, which could lead to very low nucleation efficiency. The results would be very helpful in understanding and designing a RAFT miniemulsion polymerization system.     

刮膜式分子蒸馏中液体流动形态的研究

分子蒸馏是一种高效的液液分离技术,但由于缺乏对其流体力学性质的认识,使得该项技术还多停留在实验室阶段.本实验通过高速摄像法,以水和丙三醇溶液作为测试液体,在冷模常压下,对在不同操作工况下蒸发壁面上的液膜形态进行图像记录,来研究刮膜式分子蒸馏设备内的流体力学性质.实验发现在整个操作过程中壁面主要出现4种不同的液膜形态:点状分布、线状分布、部分成膜和整体成膜.根据实验数据作出的壁面液体流型图证实了液膜形态与过程的转速,进料速度及物料性质有关.除此之外利用量纲分析法对影响成膜的各因素进行分析,得到与成膜过程相关的三个无量纲数∏1、∏2和∏3.使用非线性最小二乘法对实验数据进行拟合,得到整体成膜状态下方程∏3=f(∏1,∏2)的具体形式,再利用丙三醇作为测试液体验证了方程∏3=f(∏1,∏2)的准确性.结果表明成膜临界速度u*cr作为判断壁面流型的标准是可行的,其大小与过程的雷诺数、韦伯数及设备的高径比有关.成膜临界速度u*cr经验关联式的提出为分子蒸馏设备的理论研究和工业应用提供了参考.     

悬垂水滴表面天然气水合物的生长特性

针对天然气水合物的喷雾法制备,设计并搭建了悬垂水滴显微实验装置,研究了不同温度和压力条件下天然气水合物在悬垂水滴表面的生长形态,建立了水滴表面水合物膜的生长模型,并通过实验研究了驱动力、过冷度、水合反应速率、气体扩散速率及十二烷基硫酸钠(SDS)浓度对水合物膜生长的影响. 结果表明,水合物膜的生长速率随过冷度的增加而增加,实验条件下测得水滴表面水合物膜的生长率为0.24~0.39 mm/s. 水滴表面水合物膜的生长为内扩散控制,水合物的活化能为13.01 kJ/mol. 当SDS浓度为550 mg/L时,界面反应速率常数有最大值0.0027 m/s.     

Growth Interruption Effect on the Fabrication of GaAs Concentric Multiple Rings by Droplet Epitaxy

WE PRESENT THE MOLECULAR BEAM EPITAXY FABRICATION AND OPTICAL PROPERTIES OF COMPLEX GAAS NANOSTRUCTURES BY DROPLET EPITAXY: concentric triple quantum rings. A significant difference was found between the volumes of the original droplets and the final GaAs structures. By means of atomic force microscopy and photoluminescence spectroscopy, we found that a thin GaAs quantum well-like layer is developed all over the substrate during the growth interruption times, caused by the migration of Ga in a low As background.     

Water Vapor Depletion in the DMT Continuous-Flow CCN Chamber: Effects on Supersaturation and Droplet Growth

The continuous-flow streamwise thermal-gradient cloud condensation nuclei counter (CFSTGC) is a commercially available instrument that is widely used for laboratory and field measurements of cloud condensation nuclei (CCN). All studies to date assume that the supersaturation profile generated in its growth chamber is not influenced by the condensation of water vapor upon the growing CCN. The validity of this assumption, however, has never been systematically explored. This work examines when water vapor depletion from CCN can have an important impact on supersaturation, measured CCN concentration, and droplet growth. A fully coupled numerical flow model of the instrument is used to simulate the water vapor supersaturation, temperature, velocity profiles, and CCN growth in the CFSTGC for a wide range of operation and CCN concentrations. Laboratory CCN activation experiments of polydisperse calibration aerosol (with a DMT CFSTGC operated in constant flow mode) are used to evaluate the simulations. The simulations and laboratory experiments are then generalized using a scaling analysis of the conditions that lead to supersaturation depletion. We find that CCN concentrations below 5000 cm^?3 (regardless of their activation kinetics or instrument operating conditions) do not decrease supersaturation and outlet droplet diameter by more than 10%. For larger CCN concentrations, a simple correction can be applied that addresses both the depression in supersaturation and droplet size.     

Study on the Characteristics of Gas Molecular Mean Free Path in Nanopores by Molecular Dynamics Simulations

This paper presents studies on the characteristics of gas molecular mean free path in nanopores by molecular dynamics simulation. Our study results indicate that the mean free path of all molecules in nanopores depend on both the radius of the nanopore and the gas-solid interaction strength. Besides mean free path of all molecules in the nanopore, this paper highlights the gas molecular mean free path at different positions of the nanopore and the anisotropy of the gas molecular mean free path at nanopores. The molecular mean free path varies with the molecule’s distance from the center of the nanopore. The least value of the mean free path occurs at the wall surface of the nanopore. The present paper found that the gas molecular mean free path is anisotropic when gas is confined in nanopores. The radial gas molecular mean free path is much smaller than the mean free path including all molecular collisions occuring in three directions. Our study results also indicate that when gas is confined in nanopores the gas molecule number density does not affect the gas molecular mean free path in the same way as it does for the gas in unbounded space. These study results may bring new insights into understanding the gas flow’s characteristic at nanoscale.     

Nonisothermal Degradation Kinetics of Ultra-High Molecular Weight Polyethene Composites Filled with Carbon or Aramid Fibers

The thermal degradation in static air of three types of composite materials based on ultra-high molecular weight polyethene with unmodified and modified carbon fibers or aramid fibers have been investigated under nonisothermal conditions at a heating rate of 10 K min^?1. The Coats–Redfern method was used to determine the kinetic parameters. The analysis of the result obtained by the Coats–Redfern method shows that the thermal degradation process of these composites corresponds to a diffusion-controlled reaction (D₅ mechanism, three-dimensional diffusion described by Zhuravlev–Lesokhin–Tempelman equation). It was found that the composites with unmodified and modified carbon fibers or aramid fibers possess the highest thermal stability at 3 mass % fiber content. The activation energy, frequency factor and the changes of entropy, enthalpy and Gibbs energy for the active complex of the composites were also measured.     

Nonisothermal bubble growth in polymeric foams

The dynamics of a large number of bubbles separated by distances of the order of their radii in highly viscous fluids with specific application to foams is investigated. The growth of bubbles is due to diffusion of gas from the fluid and the momentum transfer between the fluid and the bubbles. Equations governing the growth of a single bubble in a shell of fluid containing limited dissolved agas are coupled with the transport equations for the fluid under non-isothermal conditions. The resulting set of equations are solved numerically for a system of bubbles growing along the axial direction in a mold. The results predict a bubble size distribution along the axial direction with large bubbles close to the melt front and smaller bubbles close to the gate, which results in a density distribution in the molded article. Experimental studies on structural foam under nonisothermal conditions are performed. The transient bulk foam density is measured by monitoring the melt front as the foam expands. The predicted values of the foam density are compared with the experimental results and the sources of error are discussed.     

非等温催化剂颗粒上活性组分的最佳分布

本文讨论了一维多孔性催化剂颗粒在非等温情况下活性组份的最佳分布.活性组份集中在离颗粒中心某一距离(?)处的无限薄层上可得到最大的有效因子ηmax.该最佳位置决定于非等温效应、传质阻力以及反应速率的相对大小.文中给出了确定该位置和最大有效因子的分析表达式.得到的ηmax皆恒大于或等于一.活性分布偏离最佳分布或活性层厚度增大皆使有效因子减小;且活性层偏向最佳位置的内侧有效因子减小较少、而偏向最佳位置外侧有效因子急剧减小.     

Note on Nonisothermal Flow in Field-Flow Fractionation

Abstract

Myers et al. (1) derived an expression for the velocity profile of a fluid flowing under an applied pressure gradient between parallel flat plates separated by a small distance h. The coefficient of viscosity (μ) was allowed to vary between the plates, say, as the result of a (linear) temperature gradient established by controlling the temperature on each plate to a different value. In this note an error in their hydrodynamic analysis is corrected, and sample calculations are given to demonstrate the possible quantitative effect of this error on the fluid velocity profile.