Ultrasonic Cleaning From The Physics Point Of View

The material presented in the article is intended primarily for the staff of organisations that use ultrasonic cavitation treatment plants for liquid media in ultrasonic equipment. The information is kindly provided by Hilsonic, a far-famed British company, now specialising in the production of high-tech ultrasonic cleaning equipment for 7 years.

The principle of operation of the ultrasonic reactor & the effect of static pressure

The functioning of ultrasonic reactor is based on the ultrasonic cavitation in the thin layer of pumped liquid to be treated through the reactor. The ultrasonic oscillator generates the ultrasonic frequency voltage received by ultrasonic transducer, which converts the high frequency voltage in an ultrasonic frequency and mechanical vibrations. These vibrations are transmitted to the emitter, which comprises a hub, whereby high frequency fluctuations are amplified and on the output they may generate up to 100 microns or more. Converter with a radiator attached to the body of the reactor through the support mechanism.

In case of fluctuations of the radiator at an ultrasonic frequency to be treated in the reactor liquid medium having alternation of compression and expansion, which create additional pressure change in a relatively constant static pressure of this environment. These pressure fluctuations in the liquid medium are defined by sound pressure, emitted by transmitter.

As a result, the liquid medium is observed closely associated with the sound pressure effect called ultrasonic cavitation, which refers to the formation of steam and gas in a liquid medium (cavitation) cavities in the negative phase of the sound pressure of the acoustic oscillations of ultrasonic frequency, followed by slamming them into the positive phase sound pressure with the formation of shock waves.

The emergence of cavitation bubble and the effects associated with it slamming depend on a number of parameters: acoustic characteristics (sound pressure and frequency), the thermodynamic parameters (pressure and external temperature) and parameters of the liquid (density, viscosity, surface tension, vapor pressure and solubility of liquid gas therein).

Cavitation stages

The process of developing a single cavitation bubble passes through three stages.

During the first step the cavity is expanded from the initial cavitation nucleus combined cycle (that always presents in the liquid in large amounts) due to pressure reduction (elongation phase) in the liquid phase under the influence of the negative acoustic pressure. This process is determined by the difference between the values ​​of the variable sound pressure and constant static pressure.

During the second stage, the process of cavitation bubble collapse takes place under the influence of the positive phase of the sound pressure (compression phase). This process is determined by the sum of the values ​​of the variable sound pressure and constant static pressure. As a result, the process of collapsing cavitation bubble is very fast – the speed of the cavity wall is about 250 m/s; wherein the steam-gas mixture, always located within the cavity, is compressed under normal pressure to 3000 atm., and the temperature inside the cavitation bubble reaches 6,000 Kelvin degrees mark.

The third stage is conditioned by the process of secondary cavitation bubble expansion due to the fact that the vapor-gas mixture, compressed to several thousand atmospheres causes cavitation cavity rapidly expanding with a velocity of 250 m/sec. This step can be identified with a point explosion. At this stage, the effect of the alternating sound pressure and constant static pressure can be ignored, as these pressures have virtually no effect on the process of expanding secondary cavitation bubble.