计算机模拟技术在絮体分形成长研究中的应用

颗粒聚集形成絮体是水处理混凝过程中的中心现象,絮体的结构、行为和性能与混凝效果密切相关。计算机模拟为研究颗粒聚集过程提供了一条新的途径,而分形理论则提供了新的理论观念。本文对絮体的分形结构模型、絮体成长过程的计算机模拟方法和模拟结果的分析方法进行了系统的介绍,以期推动此项技术的应用研究。     

基于分形学的膜面滤饼特性研究进展

膜污染问题限制了膜分离技术在实际工程中的广泛应用。从分形解析的角度深入探讨混凝-膜过滤过程中滤饼层的微观结构对控制膜污染情况至关重要。总结和回顾了运用分形理论表征滤饼层重要性质参数孔隙率、比阻、渗透率研究的前沿动态,对絮体特性和滤饼层特性两者内在关系进行分析评述,以期为混凝-膜过滤工艺优化提供参考。     

腐殖酸絮凝体的形态学特征和混凝化学条件

以腐殖酸为混凝去除对象,通过图像法观察腐殖酸絮凝体的形态,研究了在混凝过程中以及不同化学条件下絮凝体分形维数的变化规律。实     

不同水处理工艺的混凝效果比较

该文以宁夏宁东地区黄河水为研究对象,考察了不同混凝条件（常规混凝沉淀池、微涡旋混凝沉淀池、反冲洗水回流沉淀池）下三种水处理工艺对混凝效能的影响。结果表明微涡旋改造有助于提高絮凝池的混凝效果,絮体的沉降性得到改善,“跑矾”现象得到缓解,絮体颗粒数目较折板絮凝池减少,沉淀池出水浊度明显降低;滤池反冲洗水回流技术可以有效地提高浊度的去除率以及改善絮体的沉降性能,且絮体颗粒数目较微涡旋絮凝池有明显减少,说明增加水体中颗粒数目可以有效地提高混凝效果;由分形维数的数据可以看出,微涡旋改造和反冲洗水回流可以明显提高絮体的分形维数,改善了絮体的沉降性。     

混凝技术及其发展

混凝是工业用水和废水处理中最基本，也是极为重要的处理过程。认识、研究、掌握混凝理论及其应用技术，无疑对提高水处理技术水平具有重要的意义。作者介绍了混凝技术实践应用的理论要点、混凝技术及其新发展，以便为混凝技术在工程设计和运行中的应用提供有益的参考。     

分形理论及在高分子科学中的应用和发展

介绍了分形理论的主要内容，包括分形定义、特性和测定分形维数的方法，概括和评述了近年来分形理论在高分子链结构、结晶过程、溶液中、高分子材料断裂、磨损以及其他方面的研究和应用，并展望了分形理论今后的发展趋势。     

分形理论在活性炭脱硫中的应用

分析了应用分形理论来指导活性炭法脱硫的可能性。简介了分形理论，同时对目前分形理论在活性炭领域的研究进展进行了综述。指出了在研究活性炭的精细结构、吸附等温式、活性炭表面活性位分布与反应概率分布之间的关系、脱硫机理这四个主要方向上，分形理论可能为研究人员提供一种新的研究工具和研究思路。     

强化混凝技术研究与发展方向

强化混凝是指水处理常规混凝处理过程中,在保证浊度去除效果的前提下,通过提高混凝剂的投加量和控制pH值来实现提高有机物去除率的工艺过程。依据国内外进行过的试验研究及应用,综述了强化混凝技术的研究进展及结果,在此基础上探讨了强化混凝技术的研究发展方向。     

强化混凝技术在水处理方面的研究进展

混凝工艺是水处理过程中的一个重要阶段,决定了后续流程的最终出水。强化混凝技术在原有常规混凝工艺作用的基础上改进了其对于城市污水、工业废水以及微污染原水水处理效果不佳的问题。本文着重围绕强化混凝技术近年来的研究情况,结合相关研究成果综述强化混凝技术的最新研究进展,为后续的相关实验和工程技术作参考。     

PIV技术在絮体特征和流场测量中的研究进展

介绍了粒子图像测速(PIV)技术的设备组成及工作原理,并从絮体检测和流场测量的角度叙述其在混凝研究中的研究进展。该技术应用于絮体检测克服了流场扰动、絮体失真和时滞性等问题,可获取粒度、分形维数等表征絮体特性的特征参数。针对复杂流场具有三维立体、时变性、非定向性等特性,PIV可获取丰富的流场信息,通过流场测量结果反映水力条件对混凝效果的影响。     

分形维数在聚合磷硫酸铁絮凝研究中的应用

通过测定絮凝体的分形维数判断聚合磷硫酸铁的絮凝效果,并结合分形理论,利用正交设计原理,对絮凝体形成的影响因素OH-/Fe,PO43-/Fe、硫酸铁投加量进行研究。结果表明,聚合磷硫酸铁絮凝最佳条件为:OH-∶Fe=2∶5,PO43-∶Fe=1∶20,1 mol/L硫酸铁溶液的投加量5 mL/L。     

Coagulation Coefficient of Agglomerates with Different Fractal Dimensions

A coagulation coefficient of agglomerates with different fractal dimensions has not been considered in the past, even though there is a possibility of variations in the fractal dimension of agglomerates at any instant. In this study, a Brownian dynamics simulation was performed with simultaneous collision and sintering, and variations in the fractal dimension of agglomerates were observed. A coagulation coefficient expression for agglomerates with two different fractal dimensions was proposed. The coagulation coefficient based on the different fractal dimensions was at most 140% higher than that based on the average fractal dimension. To determine an accurate coagulation coefficient of agglomerates, the fractal dimension of each agglomerate has to be considered.     

Bivariate population dynamics simulation of fractal aerosol aggregate coagulation and restructuring

In the present work a previously developed model of fractal aggregates evolution from an initial morphology (as described by their fractal dimension) towards to that defined by the prevailing coagulation mechanism is extended in two directions. Firstly a new constitutive law for the fractal dimension of the aggregate resulting from a coagulation event is generalized and secondly a restructuring mechanism is added to the population balance model. Several techniques from detailed Monte Carlo simulations to simple monodisperse (in both volume and fractal dimension) approximations are employed for the solution of the corresponding bivariate coagulation equation. The parametric evolution of the fractal dimension of aggregates for the case of Brownian coagulation in the continuum regime is studied and the results indicate that the existence of restructuring makes the evolution dynamics of the fractal dimension distribution of the aggregate population much richer than in the case of simple coagulation examined previously. As an application of the present approach, the morphological data of Xiong and Friedlander [(2001) Morphological properties of atmospheric aerosol aggregates. Proceedings of the National Academy of Sciences of USA, 98, 11851–11856] on atmospheric aggregates are examined and are shown to be consistent with a combined coagulation–restructuring process.     

COSIMA—a computer program simulating the dynamics of fractal aerosols

The sectional aerosol behavior code COSIMA simulates the time evolution of the structural, dynamical, and optical properties of airborne agglomerate particles as well as their heterogeneous chemical interactions with reactive trace gases utilizing a formalism based on fractal scaling laws. The modeled processes include diffusion to the walls and sedimentational deposition, Brownian and gravitational coagulation, molecular transport from the gas phase to the accessible particle surface, surface adsorption and reactions, gas phase reactions, and dilution effects due to sampling (e.g. during aerosol chamber experiments). The effect of hydrodynamic interactions and shielding on particle mobility is considered within the framework of the Kirkwood–Riseman theory. Rayleigh–Debye–Gans theory is used to deal with light absorption and scattering. The code is validated against new experimental data on the dynamics of Diesel and graphite spark soot as well as recent theoretical and simulation results. Applying the Kirkwood–Riseman formalism to compute the mobility of fractal like agglomerates significantly enhances coagulation rates as well as wall and depositional loss but does not affect the form of the self preserving size distributions attained in the long time regime if Brownian coagulation dominates the aerosol dynamics.     

Evolution of aggregate size and fractal dimension during Brownian coagulation

Fractal aggregate coagulation is described within a general framework of multivariate population dynamics. The effect of aggregate morphology on the coagulation rate, is taken into account explicitly, introducing in addition to aggregate particle size, the aggregate fractal dimension, as a second independent variable. A simple constitutive law is derived for determining the fractal dimension of an aggregate, resulting from a coagulation event between aggregates with different fractal dimensions. An efficient Monte Carlo method was implemented to solve the resulting bivariate Brownian coagulation equation, in the limits of continuum and free molecular flow regimes. The results indicate that as the population mean fractal dimension goes from its initial value towards its asymptotic value, the distribution of fractal dimension remains narrow for both flow regimes. The evolution of the mean aggregate size in the continuum regime is found to be nearly independent of aggregate morphology. In the free molecular regime however, the effects of aggregate morphology, as embodied in its fractal dimension, become more important. In this case the evolution of the aggregate size distribution cannot be described by the traditional approach, that employs a constant fractal dimension.     

Effect of Volume Fraction on Transient Structural Behavior of Aerosol Particles Using Off-Lattice Kinetic Monte Carlo Simulation

Fractal-like aggregates exhibit interesting properties that determine their physicochemical advantages, and thus, the control and prediction of aggregation is critical for many applications. An off-lattice kinetic Monte Carlo (KMC) simulation was performed to investigate the aggregate evolution from primary particles to three-dimensional fractal aggregates, at three different volume fractions. We have found that at low volume fraction, aggregation kinetics is slow, and aggregate morphology is widely open and stringy, with fractal dimension of (D_f) 1.8, in which the system is constantly preserved in the dilute regime. In denser volume fractions, however, the aggregation kinetics appears to be accelerated and aggregate morphology is more compact and less stringy due to the transition from dilute to dense regimes. Moreover, the volume fractions determine what kind of coagulation mechanism may occur to produce aggregates with different morphologies. At low volume fraction, coagulation is predominated by coagulation between aggregates in which the maximum probability of interpenetration event is only 18%. This suggests that aggregates at low volume fraction can maintain their self-similarity behavior. While at high volume fraction, coagulation is predominated by two subsequent coagulation mechanisms, namely, primary particle–aggregate and aggregate–aggregate interaction. The probability of interpenetration event increases up to 40%. In addition, the interpenetration process as well as the primary particle–aggregate coagulation, particularly in the dense regime, could produce superaggregates with a hybrid structure with a high fractal dimension at large size scales and a low fractal dimension at small scales. A detail mechanism for the formation of superaggregates was discussed.

Copyright © 2015 American Association for Aerosol Research     

Characterization of floc size, strength and structure under various coagulation mechanisms

Flocs generated by various coagulation mechanisms exhibit different size, strength and structure. The properties and fractal dimensions of flocs formed under three common coagulation mechanisms, i.e. charge neutralization, sweep and bridging, were investigated at various hydraulic conditions. The results showed that the floc size decreased with the increasing average velocity gradient G and the stable floc size exponent γ was of the following hierarchy: charge neutralization (0.6107) > sweep (0.5618) > bridging (0.3674). Furthermore, fractal dimensions of flocs were the highest when formed by sweep and the lowest when generated by bridging flocculation. The mass fractal dimensions measured by light scattering were between 2.0 and 3.0 and the floc strength was between 0.01 and 0.58 N m^− 2. An intrinsic unity of the relationship among floc size, fractal dimensions, floc strength under the three coagulation mechanisms was demonstrated.     

高分子絮凝剂对泥沙絮体分形结构的影响

黄河泥沙架桥絮凝沉降实验与电镜实验研究表明,絮体结构具有分形特性。文章讨论了高分子絮凝剂的架桥絮凝机理以及高分子所带电荷种类、高分子的分子量、投加剂量与浓度对絮体分形结构的影响,并用表征絮体结构分形特性的参数“分维D”定量分析了这种影响规律。同时,对比了相同含沙量下无机混凝剂絮体与高分子架桥絮体结构达最佳时的分形特征及所需的混凝控制指标Gt值。     

分形理论在高聚物科学中的应用

介绍了分形理论的特点,并概述混沌理论在高分子领域的应用。列举出分形理论在高聚物分子链结构、溶液及凝胶化反应、高分子材料断裂、结晶过程等方面的应用。