Abstract We present an experimental study to investigate the impact of bubble size on behavior of bubble plumes from the integral point of view. Two distinct types of bubble plume were generated under the same volumetric gas flow rate: many small bubbles using an air-stone diffuser and a few large bubbles using a single-orifice diffuser. Using Particle Image Velocimetry, we measured the velocity field in the plumes, and derived the integral characteristics for comparison, including the evolution of plume width, centerline velocity, volume flux, momentum flux, momentum amplification factor, and the entrainment coefficient. The measurement data show that the evolution of all the parameters in the plumes are influenced strongly by the different bubble sizes and population. The plumes with many small bubbles behave closer to the single-phase coherent plumes, showing a linear growth of plume width and a decreasing centerline velocity. These characteristics are modified in the plumes with fewer but larger bubbles. With the same total initial buoyancy flux from the source, fewer but larger bubbles give rise to weaker mean flow characteristics, i.e., smaller plume velocity and weaker fluxes of volume and momentum. This modification is attributed to the different mechanisms in fundamental transport of energy and momentum between mean and fluctuating components. In the single-orifice cases, the plumes are influenced by stronger but less frequent bubble wakes, compared to those more coherent bubble plumes in the air-stone cases. Higher momentum amplification factors were found in bubble plumes with fewer but larger bubbles, indicating a stronger ratio of turbulence to mean in these plumes. Despite the modification, all bubble plumes support a typical shear entrainment process within 40 cm height-of-rise. Hence, the integral model and the universal scaling using the plume length scale D and the bubble slip velocity Ws were found to describe the integral behavior of bubble plumes generally well in the range of our experimental parameters, regardless of the different bubble sizes and population.

中文翻译：

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气泡尺寸对静止和非分层水中气泡羽流整体特性的影响

摘要 我们提出了一项实验研究，从积分的角度研究气泡尺寸对气泡羽流行为的影响。在相同体积气体流速下产生两种不同类型的气泡羽流：使用气石扩散器产生的许多小气泡和使用单孔扩散器的一些大气泡。我们使用粒子图像测速仪测量了羽流中的速度场，并得出了用于比较的积分特征，包括羽流宽度的演变、中心线速度、体积通量、动量通量、动量放大因子和夹带系数。测量数据表明，羽流中所有参数的演变受到不同气泡大小和数量的强烈影响。带有许多小气泡的羽流表现得更接近于单相相干羽流，显示出羽流宽度的线性增长和降低的中心线速度。这些特性在羽流中被修改，气泡更少但更大。在来自源的相同总初始浮力通量下，较少但较大的气泡产生较弱的平均流动特性，即较小的羽流速度和较弱的体积和动量通量。这种修改归因于平均分量和波动分量之间能量和动量基本传输的不同机制。在单孔口情况下，与气石情况下那些更连贯的气泡羽流相比，羽流受更强但频率更低的气泡尾流的影响。在气泡较少但较大的气泡羽流中发现了更高的动量放大因子，表明这些羽流中湍流与平均值的比率更强。尽管进行了修改，但所有气泡羽流都支持 40 厘米上升高度内的典型剪切夹带过程。因此，无论气泡大小和数量如何，积分模型和使用羽流长度尺度 D 和气泡滑移速度 Ws 的通用缩放都可以很好地描述气泡羽流在我们的实验参数范围内的整体行为。