IX Международная студенческая научная конференция Студенческий научный форум - 2017

Grinding of loose materials in order to increase the contact surface, improving rheological activity for the intensification of a number of chemical, physical and physical-chemical processes - the most common process steps in industrial production of various materials. It is the grinding of materials of different origin in our time is spent more than 20% of electricity generated in the world, which will undoubtedly once again emphasizes the importance of the technological process of grinding granular materials used in various fields of human industrial activity. Bulk solids, as a raw material for the manufacture of products for different purposes, are very diverse both in its composition and on basic physical and chemical properties. A variety of basic characteristics comminuted materials dictates the need to select the most efficient and economically viable method accordingly this type of grinding a hard material, and consequently the type of grinding mechanisms. As is known, for coarse and medium crushing hard materials are used methods of crushing and splitting activities, and for fine grinding - machines abrasive and impact. The increasingly growing need for large quantities of fine granular materials makes us look for new ways of effective mechanical grinding. Mechanical size reduction takes place in mills of various designs, looks very attractive for dispersed systems. However, there is a limit of mechanical grinding in mills of solids, interfering in some cases, the achievement of sustainable grinding down to the nano; furthermore, high power load on the feed material cause intensive interaction with the nanoparticles formed dispersion medium. Mechanical dispersion is associated with exposure to the material pressure and temperature. The only physical process that combines high pressure and temperature at the lowest cost for their education - the process of cavitation.

Under cavitation in liquids understand the formation of steam and gas-filled cavities or bubbles when the local pressure drops in the liquid to vapor pressure. The ratio of gas and vapor content within the cavity may be different (theoretically from zero to one). Depending on the concentration of vapor or gas in the space referred to as steam or gas. It should be noted that a decrease in fluid pressure to the saturated vapor pressure may also be at boiling liquid or vacuuming. But these processes are distributed throughout the volume of fluid in contrast to cavitation which has a limited range. There are hydrodynamic cavitation that occurs due to a local pressure reduction in the flow of the fluid in the flow around a solid body and acoustic cavitation occurring when passing through the liquid acoustic vibrations.

Hydrodynamic cavitation occurs in those parts of the flow where the pressure is reduced to a certain critical value. Those present in the liquid gas or vapor bubbles, moving with the flow of fluid and falling to less than the critical pressure, the ability to acquire unlimited growth. After the transition to the high pressure zone growth stops, and bubbles begin to decrease. If bubbles contain a lot of gas when they reach the minimum radius, they are restored and perform multiple cycles of damped oscillations, and if small, the bubble collapses completely during the first cycle.

Thus, near the streamlined body create "cavitation zone" filled with moving bubbles. Reducing cavitation bubble occurs at high speed, and is accompanied by a sound pulse, the stronger, the smaller the gas bubble contains. If the degree of cavitation is such that there is a lot of bubbles and slams, the phenomenon is accompanied by a loud noise with a continuous spectrum from a few hundred hertz to several hundred kHz. The range is expanded to lower frequencies with increasing the maximum radius of the bubble.

Acoustic Cavitation - is the formation of cavities and slamming into the fluid under the influence of sound. The cavities are formed by the fracturing fluid during the half periods stretching cavitation nuclei and slam during half cycles of compression.

At the time of the collapse, the gas pressure and temperature reach significant values ​​(according to some sources up to 100 MPa and 10000 C). After the collapse of the cavity in the surrounding liquid is distributed spherical shock wave, fast fading in space.

Method of ultrasonic cavitation based on the effects of ultrasonic radiation on the liquid with the development of such an effect, as the acoustic cavitation occurring in the environment of the propagation of ultrasound. Acoustic cavitation is an effective means of sound wave energy concentration of low density in high energy density associated with the pulses and the collapse of cavitation bubbles. The overall picture of the formation of cavitation bubble is represented as follows. In the phase of the acoustic wave in the liquid vacuum vapor bubbles are formed. Steam bubbles entering into the liquid at a temperature lower than the saturation temperature of the process under the action slam vapor condensation and surface tension forces. In the phase of compression under the action of the acoustic wave of high pressure intensity collapse of bubbles increases.

Destruction (collapse) condensing the vapor bubble in contact with the surface of the object entails the formation of cumulative streams. The cumulative streams and destroying the surface layers of the solid surface by the kinetic energy of the fluid. It can be assumed that small rigid body particles whose dimensions are comparable with the cross section of cumulative jets, addicted to them and give an additional contribution to the process of destruction of the surface layers themselves and solid particles in the liquid.

In the ultrasonic range are most common piezoelectric and magnetostrictive oscillators cavitation. In these electroacoustic transducers used direct magnetostrictive and piezoelectric effect in alternating magnetic and electric fields. Range transducers excitation frequencies is very wide (8 ... 44 kHz and above).

The advantage of this method:

• grinding down to the nano structures

• obtaining a high pressure and temperature with minimal formation of

• the ability to influence the physical and chemical properties of the treated structures

• possibility to combine different processing methods

Disadvantages of this method:

• the complexity of the equipment performance

• high cost;

• the need for protection against electricity

bibliography:

1. Pernik A.D. The problems of cavitation. - L .: Shipbuilding 1966.

2. Peirsol I. Cavitation. - M .: Mir, 1975.

3. Collection of scientific works Sevastopol National University of Nuclear Energy and Industry: METHODS OF CAVITATION DISPERSION V.A. Gerliga, I.A. Prityka, A.S. Peasant

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