Zhu Zhongfan, Peng Dingzhi, Dou Jie
College of Water Sciences, Beijing Normal University, Beijing 100875, China.
Center for Spatial Information Science, The University of Tokyo, Kashiwa 277-8568, Japan E-mail:
Water Sci Technol. 2018 Feb;77(3-4):861-870. doi: 10.2166/wst.2017.603.
In this study, Couette flow experiments were performed to estimate the temporal evolution of the 2D and perimeter-based fractal dimension values of kaolinite flocs during flocculation. The fractal dimensions were calculated based on the projected surface area, perimeter length and length of the longest axis of the flocs as determined by sampling observation and an image-processing system. The 2D fractal dimension, which relates the longest axis length and projected surface area of flocs, was found to decrease with the flocculation time, corresponding to the production of some porous flocs from the flow shear. This fractal dimension finally reached a steady state, which resulted from a dynamic equilibrium among the floc growth, floc breakage and floc restructuring. The perimeter-based fractal dimension, which characterizes the relationship between the projected surface area and the perimeter of flocs, increases with flocculation time because the flow shear increases the collisions among the primary particles, and some irregular flocs are formed. The perimeter-based fractal dimension reaches a steady level because of the balance among floc aggregation, breakage and restructuring. In addition, a stronger turbulent flow shear makes the steady state of fractal dimensions occur early during flocculation.
在本研究中，进行了库埃特流实验，以估计絮凝过程中高岭土絮体基于二维和周长的分形维数值的时间演变。分形维数是根据通过采样观测和图像处理系统确定的絮体的投影表面积、周长和最长轴长度来计算的。与絮体最长轴长度和投影表面积相关的二维分形维数随絮凝时间而降低，这对应于流动剪切产生了一些多孔絮体。该分形维数最终达到稳定状态，这是由絮体生长、絮体破碎和絮体重组之间的动态平衡导致的。基于周长的分形维数表征了絮体投影表面积与周长之间的关系，随着絮凝时间的增加而增加，因为流动剪切增加了初级颗粒之间的碰撞，并形成了一些不规则絮体。由于絮体聚集、破碎和重组之间的平衡，基于周长的分形维数达到稳定水平。此外，更强的湍流剪切使得分形维数的稳定状态在絮凝早期出现。
