3.1.2 Influence of diameter and shape of the release pipe on the morphological characteristics of microbubbles
The effect of the diameter of the release pipe on the particle size distribution of microbubbles was further explored. It can be seen from Fig. 3 that when the diameter of the release pipe (25 cm) is the same as the pipe, the particle size of microbubbles is concentrated between 20 to 60 μm, and the peak value of the curve appears at 40 μm. When the diameter increased to 32 cm, the particle size distribution curve of microbubbles shifted slightly to the right (20~70 μm), and the peak value appeared at 45 μm. When the diameter of the release pipe decreased to 20 cm, the particle size distribution curve moved to the right, and the peak value appeared at 60 μm, and the range of the particle size became larger (20~100 μm). According to the cumulative frequency distribution of bubble size (Fig. 3 (b)), the distribution proportion of microbubbles with particle size less than 50 μm is 35% (diameter=20 cm), 71% (diameter=25 cm) and 67% (diameter=32 cm), respectively.
In conclusion, when the diameter of the release pipe is the same as that of the pipe, the proportion of microbubbles with particle size within 50 μm is the largest, and the particle size distribution is concentrated. This is because when the dissolved air water is released at the pressure reducing valve, the diameter of the release pipe increases, which weakens the turbulence intensity, the spatial density of microbubbles is relatively reduced, the collision rate between microbubbles is reduced, and the microbubbles are relatively stable. However, when the diameter of the release pipe decreases, the resistance of the microbubbles along the path increases, and the pressure loss of the microbubbles is great. At the same time, the spatial density of the microbubbles increases relatively, and the collision probability between the microbubbles increases, leading to the merger or rupture of some microbubbles. Both lead to the larger particle size of the microbubbles.