Ultrasonic cavitation is a phenomenon in which the reduction of pressure to or below the liquid’s vapour pressure leads to the formation of small vapour-filled cavities in the liquid. When subjected to higher stress, these cavities, called “bubbles” or “voids”, collapse and generate shock waves. A shock wave is strong, very close to the imploded bubble, but rapidly weakens as it propagates away from the implosion.
Table of Contents
- 1 The three stages of cavitation
- 2 Application of ultrasonic cavitation
- 3 The generation of ultrasonic cavitation
- 4 Influencing factors of ultrasonic cavitation
- 5 What is the cavitation threshold
The three stages of 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 a phase change process between liquid and vapour, including three stages of occurrence, development and disappearance.
- Cavitation in the traditional sense is a phase change process between liquid and vapour, 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 enormous pressure and heat energy, which may cause damage to materials. It can be used in many industrial projects.
- Between birth and collapse is the cavitation development stage, called “cavitation development, ” which mainly interferes with cavitation objects’ movement. Changes in force, such as thrust drop, component vibration, etc., can also reduce drag and reduce the drag coefficient by order of magnitude in some cases.
Application of ultrasonic cavitation
Ultrasonic cavitation powers all liquid-related ultrasonic treatment technologies. Ultrasonic cavitation refers to the movement of tiny bubbles in a liquid substance under the action of ultrasound, which periodically grows, close, and collapse with the frequency of ultrasound, and a series of physical effects produced thereby. Because the development of ultrasonic cavitation is very complicated, the research content involves many subjects such as acoustics, chemistry, optics, and fluid mechanics. It is impossible to separate the cavitation effect from the photoelectric effect’s mechanical development in practice.
The physical, chemical and biological effects caused by cavitation have quite specific properties, and these properties have essential theoretical value and colossal application potential. For example, the collapse of cavitation bubbles will generate micro-jets in a tiny space around the bubbles and radiate cavitation noise to the outside. The high temperature and high pressure developed when the bubble collapses can clean, cut, break objects, improve the surface properties of materials, and strengthen the process. Ultrasonic cavitation has become a current research focus.
The generation of ultrasonic cavitation
High-intensity ultrasonic waves are added to the liquid for processing， The sound waves transmitted to the liquid medium will alternately generate high-pressure (compression) and low-pressure (reflection) cycles. The cycle rate depends on the ultrasonic frequency. High-intensity ultrasonic waves create small vacuum bubbles or voids in the liquid during the low-pressure process. When the bubbles attain a volume at which they can no longer absorb energy, they collapse violently during a high-pressure cycle. This phenomenon is termed cavitation. During the implosion, very high temperatures (approx. 5,000K) and pressures (approx. 2,000atm) are reached locally. The cavitation bubble’s implosion also results in liquid jets of up to 280m/s velocity.
Influencing factors of ultrasonic cavitation
The strength of ultrasonic cavitation is related to the acoustic parameters and the liquid’s physical and chemical properties.
Ultrasonic intensity refers to the ultrasonic power per unit area, and the generation of cavitation is related to the ultrasonic intensity. When the ultrasonic intensity of general liquid increases, the cavitation intensity increases. But after reaching an absolute value, cavitation tends to be saturated. Increasing the ultrasonic power will generate many useless bubbles, thereby increasing the scattering attenuation and reducing the cavitation intensity.
The lower the ultrasonic frequency, the easier it is to produce cavitation in the liquid. In other words, to cause cavitation, the higher the frequency, the greater the sound intensity required. For example: To produce cavitation in water, the power needed for the ultrasonic frequency at 400kHz is 10 times greater than that at 10kHz; that is, cavitation decreases as the frequency increases. The frequency range generally used is 20-40kHz.
Surface tension and viscosity coefficient of liquid
The greater the liquid’s surface tension, the higher the cavitation intensity, and the less likely to produce cavitation. A liquid with a large viscosity coefficient is challenging to create cavitation bubbles, and the loss during propagation is also large, so cavitation is also challenging to make.
The higher the liquid temperature, the more beneficial the generation of cavitation, but when the weather is too high, the vapour pressure in the bubble increases, so the buffering effect is enhanced when the bubble is closed, and the cavitation is weakened.
What is the cavitation threshold
The cavitation threshold is the lowest sound intensity or sound pressure amplitude that makes the liquid medium produce cavitation. Only when the alternating sound pressure amplitude is greater than the static pressure can it appear negatively. And only when the negative pressure exceeds the liquid medium’s viscosity, cavitation occurs.
The cavitation threshold varies with different liquid media. Different temperatures, pressures, cavitation nuclei radius and gas content have different cavitation thresholds for the same liquid medium.
Influencing factors of cavitation threshold
- The cavitation width is related to the bubble radius in the medium. The smaller the radius, the higher the cavitation threshold.
- The cavitation threshold is related to the duration of the sound wave. The longer the sound wave radiation time, the lower the cavitation threshold.
- The cavitation threshold is related to the static pressure of the environment. The greater the static pressure, the higher the cavitation threshold.
- The cavitation threshold is related to the medium’s viscosity; the viscosity is large, the surface tension is enormous, and the cavitation threshold is high.
- The chemical threshold is related to the gas content of the liquid. The less the gas content, the higher the cavitation threshold.
- The cavitation threshold is related to the liquid temperature, and the increase in liquid temperature is beneficial to cavitation.
When the temperature is too high, the bubble’s vapour pressure increases, which enhances the buffering effect during the bubble closure period and weakens the cavitation.