Study On Synergetic Mechanism And Numerical Simulation Of Floc Fractal Growth And Flow Field

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.