三峡库区黏性泥沙在紊动剪切作用下的絮凝试验研究

An experimental study on the effect of turbulent shear on flocculation of cohesive sediment in Three Gorges Reservoir

采集三峡库区忠县黄花城河段黏性泥沙进行紊动剪切作用下的絮凝试验研究。试验装置主要包括沉降柱、絮体分离室和絮体图像采集系统,试验中采用振动格栅产生近似均匀紊流场,并通过ADV(Acoustic Doppler Velocimetry)测量流速来校核流体剪切率和格栅振动频率的关系。试验结果表明:(1)该黏性泥沙絮凝效果明显,絮体最大粒径Df,95为73~126μm、平均粒径Df,50为18~33μm,絮体粒径分布受紊动剪切率和泥沙浓度的影响较大;(2)随着紊动剪切率的增大(从3.84 s-1至30.07 s-1),Df,95呈先增大后减小的趋势;当剪切率为19.94 s-1时,Df,95达到最大值,大絮体(粒径大于96μm)的数量百分数也达到最大值;(3)相同紊动剪切率条件下,不同浓度(0.3~1.0 g/L)泥沙絮凝后的絮体粒径分布,基本符合"泥沙浓度越高,Df,95越大"的趋势。

Flocculation experiments on the effect of turbulent shear was conducted on the cohesive sediment collected in Huanghuacheng section of Zhongxian County along Three Gorges Reservoir. The experimental instruments included cylindrical settling column,flocculation separation chamber,and image collecting system. An approximately uniform turbulence flow was generated using oscillating grids in the cylindrical settling column,and the relation between the shear rate and the frequency of oscillating grids was calibrated using an ADV（Acoustic Doppler Velocimetry）. The results showed that：（1） The apparent flocculation of cohesive sediment was observed,with the largest floc size Df,95 ranging 73—126 μm and the mean size Df,50 ranging 18—33 μm,which were both strongly affected by the turbulent shear rate and the sediment concentration.（2） Df,95 demonstrated a trend of increase followed by decrease with the turbulent shear rate（from 3. 84 s-1 to 30. 07 s^-1）,with its peak occurring at the shear rate 19. 94 s^-1,when the percentage of large floc（larger than 96 μm in diameter） also reached the vertex.（3） Floc size distribution corresponding to different concentrations（0. 3-1. 0 g/L） at the same shear rate basically followed a tendency of Df,95 increases with the sediment concentration.