Cavitation usually refers to the generation of cavities and the subsequent dynamic behaviors when a liquid suffers from a sufficient pressure drop. According to the mode of production, cavitation can usually be divided into acoustic cavitation, hydrodynamic cavitation, light cavitation and particle cavitation. It has been widely used in sonochemistry, biomedical, environmental science and many other fields.
Acoustic cavitation is usually high intensive but limited to the throughput, while hydrodynamic cavitation as an alternative method is easy to be scaled up but limited to the intensity.
The method of combining the ultrasonic cavitation and hydrodynamic cavitation taking place simultaneously in the same space, named hydrodynamic-acoustic-cavitation (HAC), can make full use of the advantages of the two kinds of cavitation to maximize the extent of hydroxyl radial generation.
Recently, researcher WU Pengfei and his colleagues from the Institute of Acoustics (IOA) of the Chinese Academy of Sciences explored the mechanism and dynamics of hydrodynamic-acoustic cavitation systematically, and provided a direct observation and a preliminary physical model on hydrodynamic-acoustic-cavitation.
From a physical viewpoint, hydrodynamic-acoustic-cavitation refers to cavitation in a liquid where acoustic wave and flow coexist.
The researchers established an experimental setup (Figure 1) consisting of a vertical multistage pump with an inverter to control the flow rate, a nozzle with a circular hole placed in a rectangular channel with two plexiglass windows mounted on front and back for optical observation, and a self-made ultrasonic transducer. Tap water was used in the experiment.
Figure 1. Experimental setup. (Image by WU Pengfei)
Via a high-speed camera equipped with a long-distance microscope, the researchers investigated bubble cloud dynamics in HAC and proposed a method for cavitation characterization (Figure 2) including a relationship between the light intensity distribution on lens sensor of the camera and the cavitation state variable.
Figure 2. Schematic illustration of the characterization method for cavitation. (Image by WU Pengfei)
Since cavitation is easily affected by the existence of the hydrophone in the measurement, like cavitation bubbles tend to gather around the hydrophone in this experiment, the optical detection method is rather noninvasive.
The researchers found that, compared with hydrodynamic cavitation or acoustic cavitation individually, HAC has significantly widened the range and enhanced the strength of cavitation (Figure 3), which may overcome the shortcomings of scale limitation of ultrasonic cavitation and low intensity of hydrodynamic cavitation.
Figure 3. Cavitation intensity distribution: (A) Hydrodynamic cavitation. (B) Hydrodynamic-Acoustic-Cavitation. (C) Acoustic cavitation. (Image by WU Pengfei)
Simulated results showed that the HAC bubble is far more violently expanded and collapsed compared with the hydrodynamic cavitation bubble and the acoustic cavitation bubble, which may be a mechanism why HAC has significantly enhanced the strength of cavitation.
What should be noted is that the present model is quite preliminary, the influence of bubble-bubble interaction, the instability of flow on bubble dynamics should be further considerate in future works.
The study entitled "Mechanism and Dynamics of Hydrodynamic-Acoustic Cavitation (HAC)" was published online in Ultrasonics Sonochemistry. It was supported by the National Natural Science Foundation of China.
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