Cavitation Effect
Ultrasonic cavitation is formed during the positive pressure of ultrasonic molecular structure empty, emission intensity ultrasound in dissolved gas or liquid in the liquid vapor bubble growth and burst, eliminate the phenomenon. To be specific, the ultrasonic cavitation is strong in ultrasonic liquid caused by a kind of peculiar physical phenomena, is a typical nonlinear acoustic problems. In theory, the pure liquid molecular binding force is very strong, so has a very high tensile strength. But usually the actual liquid mixed with some tiny bubbles due to various reasons, constitutes the weak link of the liquid, when alternating pressure formation of the negative pressure phase is strong enough, the liquid will be opened first at the weak link, thus forming the cavity and raised; Then in the next coming under positive pressure cavity will be compressed, and then quickly closed. The tiny air bubbles shrink quickly with ultrasonic vibration, closure, rupture process of ultrasonic cavitation. Tiny bubbles existing in the liquid is called cavitation nuclei, the minimum pressure required to produce cavitation or liquid cavitation threshold, the sound intensity values of radial said cavitation threshold suggests that only to the difficulty of the generated by ultrasonic cavitation, it with all sorts of effects of cavitation are two different things. The influencing factors of ultrasonic cavitation threshold, the domestic and foreign scholars have conducted a lot of research found that the different nature of the liquid, and its surface tension coefficient, density, saturation vapor pressure, viscosity; For the same kind of liquid and its temperature, static pressure, gas content, etc. There is a close relationship with impurity; In addition, in the liquid state are the same situation, cavitation threshold with the frequency of the radiation sound wave, wave type, the waveform parameter changes.
According to the different dynamic behavior of bubbles can be empty tumble for steady-state cavitation and transient cavitation. The former refers to the fluid acoustic field exists in the appropriate amount of bubbles, they are under the effect of alternating pressure may enter human body resonance state, when the sound wave frequency is close to the dynamical process of bubbles when the body characteristic of resonance frequency, the process is often accompanied by a series of second-order phenomenon occurs, including radiation effects and sound with micro bubble pulsation of flow; While the latter refers to the high sound field in the dynamic process of bubbles is more complex and intense, in the sound field, negative pressure phase present in the liquid cavitation nucleus expands rapidly, then suddenly contracted and collapse under positive pressure phase, when the gas volume: volume contraction to hours, and its temperature can be as high as thousands of degrees, the water vapor in the air bubbles is decomposed into H and OH radicals, free radicals and quickly react with other ingredients, in addition in the cavitation bubble collapse sound and luminescence, impact the phenomenon such as high-speed jet accompanied by. Both, happens to be medium is the presence of bubbles.
Anyhow, bubbles in the closing stage, due to the static pressure in the liquid, is a combination of pressure and liquid surface tension, bubble wall closure speed will be faster and faster, at the instant of the closed completely, at this time due to the sharp compression inside the bubble and bubble liquid should also be considered a is compressed, so at the instant of the quick make bubbles produced a series of discharge, light, high temperature and high pressure, high energy density shock wave, micro flow, micro fusion, strange extreme physical conditions such as high precision clock. This is also produce biological effect of physical basis.
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