When the partial pressure in the liquid decreases, the process of the formation, development and collapse of vapor or gas cavities (cavitations) inside the liquid or at the liquid-solid interface is called cavitation.
Cavitation is a phenomenon peculiar to liquids, which occurs in the low pressure area inside the liquid, especially at the interface between the liquid and the solid. Cavitation in the traditional sense is the phase change process between liquid and vapor, including three stages of occurrence, development and disappearance. The occurrence stage is called "cavitation incipient", accompanied by a large increase in noise radiation; the disappearing stage is called "cavitation collapse", which releases huge pressure and heat energy, which may cause damage to materials and may also be used To serve a variety of industrial projects; between birth and collapse is the stage of cavitation development, called "cavitation development", which mainly interferes with the movement of cavitation objects and changes the force, such as the drop in thrust, Vibration of components, etc., in some cases can also reduce drag and reduce the drag coefficient even by an order of magnitude.
Ultrasonic cavitation
All liquid-related ultrasonic processing technologies use ultrasonic cavitation as a driving force. Ultrasonic cavitation refers to the rapid and repetitive growth-closing-destroying motion of tiny bubbles in the liquid substance under the action of ultrasound, and a series of physical effects caused by it. Because the effect of ultrasonic cavitation is very complicated, the research content involves many subjects such as acoustics, chemistry, optics, and fluid mechanics, and it is impossible to separate the cavitation effect from the mechanical effect and the photoelectric effect in practice. The physical, chemical, and biological effects caused by cavitation have quite specific properties, and these properties have important theoretical value and huge application potential. For example, the collapse of cavitation bubbles will generate micro-jets in a very small space around the cavitation bubbles and radiate cavitation noise to the outside. The high temperature and high pressure generated when the cavitation bubble collapses can be used to clean, cut, and break objects, improve the surface properties of materials, and strengthen the process. Ultrasonic cavitation has become a hot spot in current research.
The production of cavitation
When a liquid is treated with high-intensity ultrasonic waves, the sound waves propagating into the liquid medium will produce alternating high-pressure (compression) and low-pressure (sparse) cycles, the rate of which depends on the frequency. During the low-pressure cycle, high-intensity ultrasound will create small vacuum bubbles or voids in the liquid. When the bubbles reach a volume where they can no longer absorb energy, they collapse violently during the high-pressure cycle. This phenomenon is called cavitation. During the explosion, the local area will reach very high temperature (about 5,000K) and pressure (about 2,000atm). The collapse of the cavitation bubbles also causes the liquid jet to have a velocity of up to 280 m/s, and the resulting shear force mechanically destroys the cell membrane and improves material transfer. Depending on the ultrasound parameters used, ultrasound has a destructive or constructive effect on cells, depending on the ultrasound parameters used.
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