Based on the investigation of the operation conditions for different wastewater treatment plants around the world,it is found that the distribution of the air bubbles in the aeration tank is not uniform in temporal and spatial scale,which in turn results in the difference of oxygen transfer.The effluent quality and the operation cost would be affected accordingly.The experiments were conducted in a plug-flow aeration tank model in which the oxygen transfer in clean water was measured.The high speed camera and digital image processing technique was employed to investigate the shape and variation characteristics of air bubble cloud.The air flow ratios of a single diffuser disc are 0.25 m3/h,0.5 m3/h,1.0 m3/h and 2.0 m3/h, respectively.The results showed that the ratio of axis length to water depth is around 1.03 to 1.30,with the identical shape of flow velocity distribution.The cloud axis length and bubble residence time increased first and then decreased as the air flow rate increased.The air bubble cloud shape became irregular and the shape variation frequency was high as the increasing of air flow ratios.For multiple aeration disc design,the bubble plume upstream had an effect on the shape of bubble plume downstream,and the width of bubble plume was almost the same in both air flow direction and lateral direction.The average width of bubble plume was greater at the position of 0.8h0 and 0.5h0 than that at 0.2h0.The width of bubble plume at different water depths varied with time,and the trend was more significant at 0.8h0.For air discharge rate 0.25 m3/h,1.0 m3/h and 2.0 m3/h,the average width of bubble plume was 1.0~1.5,1.6~2.5 and 1.75~3.5 times of the diameter of the aeration disc,respectively.There are mainly three regions of bubble plume in plug-flow aeration tank,namely,the adjustment region at the bottom area,the free surface region and the turbulent diffusion region at the middle,with different bubble mechanics and kinematics characteristics.With the increase of air flow discharge,the residence time of air bubble increased and then decreased with the diffusion width.These characteristics of bubble plume may provide theoretical support on the numerical simulation of bubble mechanics and kinematics in plug-flow aeration tank.%目前各种曝气池运行时曝气产生的气泡云团形态差别很大,气泡群时空分布不均,导致氧转移效率在曝气池中的分布存在差异,能耗高.为了弄清楚盘式曝气气泡云团的形态特征及其时空分布,利用高速摄像技术和摄影测量技术,对室内清水曝气实验模型中的气泡云团轴线长度、径向尺度和形态演化进行了观测.结果表明:实验范围内气泡云团轴线形状与水流流速分布曲线形状基本类似,气泡群轴线长度与水深之比介于1.03~1.30之间.曝气量由0.25 m3/h增大到2.0 m3/h时,气泡云团轴线长度和气泡的驻留时间先增大后减小.随着曝气量增大,气泡云团形状变得不规则,且随时间变化越频繁.多个曝气盘曝气时,上游的气泡云团会影响下游气泡云团的形态.在流向和展向上,气泡云团宽度基本相同;在水深方向上,由曝气盘向水面沿程增大,0.5h0、0.80处气泡云团平均宽度明显大于0.20处的宽度.不同水深处,气泡云团的宽度随时间随机变化;0.80处气泡云团宽度随时间的变化最剧烈.气泡群的平均宽度随曝气量增大而增加,曝气量为0.25、1.0、2.0 m3/h时,气泡云团的平均宽度分别为曝气盘直径的1.00~1.50倍、1.60~2.50倍和1.75~3.50倍.盘式曝气形成的气泡云团在水中可被划分为3个区域:底部的调整区、近自由面区和中部的紊动扩散区,不同区域内的气泡的受力和运动特性不同.气泡云团分区为气泡受力和运动的建模计算提供了依据.
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