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.