絮凝体的分形特征和致密型絮凝体形成

传统混凝条件下形成的随机型絮凝体具有分形特征．其分形维数Df与其密度函数ρ3∝dp^-Kp的指数Kp之间具有Df=3-K，的关系．通过建立分步成长絮凝体模型．讨论了在絮凝过程中逐次导入颗粒间的空隙率对絮凝体密度和构造的影响．模型参数分析的结果进一步证明了分步成长的絮凝体是一个典型的分形，其分形维数取决于空隙率ε和颗粒结合个数m．降低ε或提高m均有利于提高Df，使絮凝体由松散型向致密型过渡．脱水收缩和逐一附着模式是达到这一目的的两种操作模式．前者可以通过延长机械搅拌时间来实现，而后者通过造粒流化床实现．实验结果表明两种方式均能提高形成的球状颗粒的密度．但是前者所形成的团粒依然具有颗粒密度随粒径增大而降低的特点，其分形维数为2．40～2．47；而逐一附着模式所形成的团粒密度基本上与粒径无关．其分形维数接近于3．通过讨论造粒流化床操作条件．并将试验得到的致密型絮凝体密度和常规絮凝体密度进行比较．说明该方法实现逐一附着型絮凝体操作是促使絮凝体致密化的有效途径．

Fractal nature of flocs and compact floc formation

Random flocs formed under conventional operation condition are of fractal nature.There is a simple relation of D f = 3 - K p between D f,the fractal dimension and K p,the floc density index in the floc density function ρe∞dKp p.Through a stepwise agglomeration model,the process of floc growth was discussed.It was shown that at each agglomeration step,additional void water was entrapped in the floc and thus affected floc density and structure.By analyzing the parameters of the agglomeration model,it was further proved that a floc formed in this manner is of fractal nature with its fractal dimension depending on the void ratio ε and agglomeration number m.A decrease in ε or increase in m may result in an increase in D f,which implies a transition of floc from loose structure to compact.Mechanical syneresis and one-by-one attachment are two pathways for compact floc formation-the former can be realized by prolonged mechanical agitation and the later by the fluidized pellet bed operation.Experimental results show that spherical pellets of high density can be obtained by both methods.However,the pellets formed by mechanical syneresis still show a tendency of decrease in their density with increase in particle size,and the fractal dimension of such kind of pellets is about 2.40-2.47.In contrast,the pellets formed through one-by-one attachment show almost identical density regardless of the particle size,and the fractal dimension can be considered to be near 3.The optimum condition for fluidized pellet bed operation was also discussed and comparison was made on the density of compact flocs with that of random flocs formed by conventional operation.The fluidized pellet bed method is proved to be the most effective way to achieve floc compaction.