【作者】 贺维鹏；

【导师】 南军；

【作者基本信息】 哈尔滨工业大学 ， 市政工程， 2012， 博士

【摘要】 絮凝是水处理的核心操作单元之一。该过程中形成絮体的粒度、结构和强度等对絮凝效果及后续的固液分离行为都具有重要的影响。因此,能否形成性能良好的絮体,是絮凝工艺的关键,这主要取决于絮凝机理及絮体成长过程。然而,由于絮体形成与它所处的物化条件和水动力学环境之间关系密切,整体上呈现纷繁复杂性和具有混沌性,再加之试验和检测手段的制约,致使人们对絮体成长与多项影响因子在动态絮凝过程中的相互作用仍缺乏足够的认识。本文以絮凝试验为基础研究手段,并通过对反应器内水动力学环境(三维流场)和絮体分形成长过程的计算机模拟,尝试将试验现象与结果同数值计算有机结合起来,进而对动态絮凝过程中絮体分形成长与流场协同作用机制进行了深入探讨。试验研究中,以聚合氯化铝(PAC)为絮凝剂、高岭土和腐植酸悬浊液为试验水样,借助水中絮体形态原位识别技术对方形反应器内不断运动的絮体进行实时监测和原位分析,以期更为准确地对动态絮凝过程中絮体粒度和结构的演变特征进行定量描述。通过对具有不同几何结构特征的方形反应器内三维流场进行数值模拟,并将计算结果与相应工况下的絮凝试验现象相联系,初步考察了反应器内水流结构与絮体成长的相互作用关系。研究结果表明,只有在适宜的高宽比(如H=D)、挡板条件(如B=0.10D)和桨叶高度(如C=0.33H)时,才能确保反应器内桨叶区及其附近的紊动耗散率较大和水流循环时间较短,使尽可能多的颗粒参与到流体的大循环中,同时还可最大限度地消除“死水区”,实现颗粒在桨叶区及其附近充分碰撞凝聚,加快絮凝反应,获得理想的絮凝效果。研究还发现,较大的絮凝强度可减弱上述条件对絮凝后期絮体形态演变的影响;较长的沉淀时间也可削弱各工况时剩余浊度的差异。为了深入探索絮体内部构造,对传统有限扩散凝聚(DLA)模型进行了适当改进,在模型中考虑了不同粒子来源时总凝聚粒子数以及粒子可运动区域、粘结方式和粘附几率等的影响,并借助新模型实现了单一或多个凝聚核时颗粒聚集过程的可视化。通过系统分析单凝聚核时虚拟絮体的成长及形态统计特性,发现絮体在粒度增加的同时,其致密性开始下降,结构变得疏松多孔,强度也随之降低,致使絮体抵抗水流剪切破坏的能力变弱。此外,在全圆周粒子源条件下,可实现“絮凝核心”在各个方向上充分发育,为随机粒子提供更多的粘附位置,最终使所形成虚拟絮体的粒度比半圆周粒子源的要大,其结构也比后者的要致密。多凝聚核时的模拟结果表明,在多个凝聚核的颗粒聚集过程中,各凝聚集团的分形成长之间存在着竞争作用,且凝聚核的间距越小,竞争作用越激烈,这对深化认识絮体的成长机制有重要意义。研究还发现,回转半径内网格占有率的增加是絮体密度增大和结构变得致密的主要原因,也是导致其边界分形维数减小和空隙率下降的主要因素。在絮体分形结构虚拟模拟的基础上,分别对絮体破碎及再形成过程絮体形态的动态变化进行了数值模拟和试验分析,以考察颗粒破碎行为对其形态演变的影响及絮体的分形成长特征。研究发现:(a)在虚拟絮体成长过程中,存在一个使其由各向同性向各向异性过渡的临界时刻,之后各枝杈对彼此生长的抑制作用增强;(b)絮体抵抗剪切破坏能力主要取决于其质心附近颗粒的空间分布,而受远离质心的区域内颗粒重组的影响较弱;(c)破碎后形成的絮体碎片,为再形成阶段悬浮颗粒(或微小团簇)的重组提供了更多的附着位点,它们的粒度及结构决定着再形成过程絮凝核心的性能。研究认为,在具体操作时需要合理控制絮凝体系的物化条件,既不能使絮体过度破碎,也不能让破碎后的絮体过度生长,这均不利于改善最终形成絮体的性能。基于前文及相关文献的研究,本文建立了一种新的絮体动态生长模型,并借助该概念模型,通过对低剪切条件下以及多级搅拌时絮体形态演变特征的分析,深入探讨了动态絮凝过程中絮体分形成长与流场的协同作用机制。研究发现,由絮体破裂及随之发生的再形成行为引起的重组过程中形成凝聚体的粒度比由絮体破损及随后的再形成过程形成的要大,结构也更为致密,这有助于絮凝反应池的优化设计,也为形成特定絮体结构奠定了理论基础。此外,由于分形凝聚体的自相似性,对于任一絮凝强度,絮体边界分形维数进入稳定状态的耗时比其平均粒径的要少,并且随着絮凝强度的增大,二者达稳耗时的差值呈先减小而后增大的变化趋势。更多还原

【Abstract】 Flocculation is one of the most important operational units in water treatment. The size, structure and strength of flocs formed during this process significantly affect flocculation efficiencies and subsequent solid/liquid separation behaviors. Therefore, forming flocs of good quality is considered fundamental to the process of flocculation, and is highly dependent upon flocculation mechanisms and floc growth processes. However, as floc formation is greatly related to the physicochemical conditions and the hydrodynamic environments, making it rather complicated and chaotic characterized, it is not yet well known how flocs have formed during flocculation and how multinomial factors interact to reach a dynamic balance.The objective of this study is to better understand the synergetic mechanisms of floc growth and flow field in water during dynamic process of flocculation by integrated combination of flocculation phenomena and numerical results. Firstly, the three-dimensional flow field and the fractal growth process of flocs were respectively simulated by computer. Secondly, a series of flocculation tests were performed and an in-situ recognition technique of floc morphology in water was used to monitor/capture the moving flocs in rectangular reactors. The captured images of flocs were then analyzed to accurately characterize the evolution of floc size and structure during flocculation. For each test, kaolin and humic acid suspension was used as testing water sample and polyaluminum chloride (PAC) was selected as the flocculant.In order to investigate the interaction between flow distribution and floc growth in flocculating reactors, three-dimensional flow fields were simulated in rectangular reactors with different geometrical structural characteristics, and then the numerical results were used to analyze flocculation testing phenomena in corresponding conditions. As expected, a flocculating reactor, with an appropriate height-to-width ratio (e.g., H=D), rational baffle sizes (e.g., B=0.10D) and an optimal impeller height (e.g., C=0.33H), could effectively accelerate flocculation process and produce an ideal efficiency of flocculation. This was because the geometrical structures mentioned above might not only ensure that average turbulent dissipation rate was larger and flow circulation time was shorter in the vicinity of the impeller, making as many particles as possible involved in the whole circulation of flow, but also maximize the elimination of“dead zones”existed in rectangular reactors, increasing the opportunity of particle collisions. Moreover, it was found that a higher intensity of flocculation could weaken the effect of above conditions on floc morphological evolution in late stage of flocculation. Also, a longer settling time might narrow differences of residual turbidity produced in each working condition.The traditional model of limited-diffusion aggregation (DLA) was modified properly by considering effects of particle sources, aggregated particle number, particle movement region, particle adhesive types and probability, etc. With the help of the modified model, particle aggregation process was visualized for a single aggregating core and multiple aggregating cores, in order to explore the internal structure of flocs. By analyzing virtual-floc growth and its statistical morphological properties, it was found that when the size of flocs increased, their structure became loose and porous, resulting in a relatively low strength, making them susceptible to breakage by fluid shear. Furthermore, virtual flocs formed by whole-circle particle source had a larger size and a more compact structure than those formed by half-circle particle source, because for the former particle source,“flocculating core”could fully grow in all directions due to more adhesive points provided for random particles. The numerical results for multiple aggregating cores showed that during the process of particle aggregation for multiple aggregating cores, fractal-growth competition existed among all aggregates, and the smaller the distance between aggregating cores, the more intense the competitive effect, which has important implications for increasing understanding of floc growth mechnisms. It was also found that the increase of grid occupancy rate in radius of gyration resulted in the increase of floc density and the formation of flocs with more compact structures, which were the main factors for the decrease in floc boundary fractal dimension and void ratio.Based on the simulation of fractal structure of virtual flocs, numerical and experimental studies on the processes of floc breakage and subsequent re-formation were carried out to investigate the effect of particle breakage behaviors on morphological evolution and fractal-growth characteristics of flocs. The results showed that: (a) during virtual-floc growth, there was a critical moment for a transition from isotropic to anisotropic, and after this moment the inhibition of inter-branch growth became intense; (b) the ability to resist fluid shear was highly dependent upon the spatial distribution of particles in the vicinity of aggregate mass center, but weakly related to restructure of particles far away from the mass center; (c) the size and structure of floc pieces after breakage determined the nature of flocculating core during re-formation, as a result of more adhensive points provided for suspended particles (or micro-aggregates) in the process of re-formation. It suggested that in an operational sense, physicochemical conditions for flocculation system needed a reasonable control, because excessive breakage and excessive re-growth after breakage were both harmful for improving the quality of flocs fromed in the late stage of flocculation.According to the study above and some correlative literature, a fractal growth model of floc was proposed and used to analyze evolutional characteristics of floc morphology under low-shear conditions and in variable-level stirring, with the goal of deep exploration about the synergetic mechanisms of floc growth and flow field in water during dynamic process of flocculation. The results showed that fragmentation followed by re-formation seemed to be more effective in forming larger and more compact aggregates than the restructuring process due to erosion and re-formation. This finding may provide useful insights for the design of flocculating reactors and establish a theoretical foundation for the formation of flocs with specific structures. Additionally, flocculation time that was required to reach steady state for floc average size and boundary fractal dimension firstly decreased and then increased with increasing intensity of flocculation, but steady state was attained faster for floc structure than for size at the same shear during whatever flocculation, possibly due to the self-similarity of fractal aggregates.更多还原