Ultrasonic heating effect
When ultrasonic waves propagate in the medium, due to the internal friction of the propagation medium, part of the sound wave energy will be absorbed by the medium and converted into heat energy to increase the temperature of the medium. Compared with other heating methods, this method of heating achieves the same effect. This effect of increasing the temperature of the medium is called the thermal effect of ultrasound.
When ultrasonic waves propagate in the medium, large-amplitude sound waves will form periodic shock waves with a sawtooth wave surface, causing a large pressure gradient at the wave surface. Vibration energy is continuously absorbed and converted into heat by the medium to increase the temperature of the medium. The absorbed energy can increase the overall temperature of the medium and the local temperature outside the boundary. At the same time, due to the ultrasonic vibration, the medium produces strong high-frequency oscillation, and the medium rubs against each other to generate heat, which can increase the temperature of the solid and fluid medium. When ultrasonic waves penetrate the interface of two different media, the temperature rise is greater. This is because the characteristic impedance on the interface is different, which will cause reflection, forming a standing wave, causing friction between molecules and heating.
The thermal action of ultrasound can produce two forms of thermal effects. One is the thermal effect produced by continuous waves, and the other is the instantaneous thermal effect. The thermal effect of continuous wave is due to the absorption of the medium and internal friction loss. The continuous action of ultrasound within a certain period of time causes a temperature rise in the sound field area of the medium. The instantaneous thermal effect mainly refers to the instantaneous high temperature generated by the closing of the cavitation bubble.
The thermal mechanism of ultrasound has the following aspects:
Transition occurs when ultrasonic vibration passes through the medium;
The medium point is contracted periodically, so that the temperature increase center occurs in the compression phase of the ultrasonic wave;
It is formed at the boundary of different tissues. Due to the stratification of the tissues and the different dielectric impedances, reflections will be generated and standing waves will be formed, causing relative movement between molecules to generate friction and forming heat. At that time, there will be a local temperature increase at the corresponding position of the standing wave antinode. .
Among these factors, the absorption of the medium is the main factor for heat formation.
Because there is no substance that can convert all sound energy into mechanical energy and chemical energy, it will generate heat more or less, so the thermal effect is a unique characteristic of ultrasound. The thermal effect of ultrasound can be manifested as the overall heating caused by ultrasound, the local heating at the boundary, the local heating of the wave front when the shock wave is formed, and so on.
