Page 38 - Төлентай Наслизат

1.1. What is the process of bubble formation ?

Cavitation (from lat. cavitas-emptiness) - the process of formation and subsequent
collapse of bubbles in the liquid flow, accompanied by noise and hydraulic shocks, the

formation of cavities in the liquid (cavitation bubbles, or voids), which may contain
rarefied steam. Cavitation occurs as a result of a local decrease in pressure in a liquid,
which can occur either by increasing its velocity (hydrodynamic cavitation), or by passing
an acoustic wave of high intensity during the half-life of the vacuum (acoustic cavitation),
there are other causes of the effect. Moving with the flow to an area with higher pressure

or during the half-life of compression, the cavitation bubble collapses, emitting a shock
wave. At its core, cavitation has the same mechanism of action as a shock wave in the air,
which occurs at the moment of overcoming the sound barrier by a solid body.

The phenomenon of cavitation is local and occurs only where there are conditions. It
cannot move in the environment. Cavitation destroys the surface of propellers, hydraulic
turbines, acoustic radiators, shock absorber parts, hydraulic couplings, etc. Cavitation

also benefits — it is used in industry, medicine, military equipment and other related
fields.According to Christopher Brennen's definition: "When a liquid is subjected to a
pressure below the threshold (tensile stress), then the integrity of its flow is violated, and
vaporous cavities are formed. This phenomenon is called cavitation. When the local
pressure of the liquid at some point falls below the value corresponding to the saturation
pressure at a given ambient temperature, then the liquid passes into another state, forming

mainly phase voids, which are called cavitation bubbles. Another formation of cavitation
bubbles by local energy supply is also possible. This can be achieved by focusing an
intense laser pulse (optical cavitation) or by sparking an electrical discharge."

In many sources, the physics of this phenomenon is explained as follows. The physical
process of cavitation is similar to the process of boiling a liquid. The main difference
between them lies in the fact that when the boiling of the phase change liquid is at an

average volume of liquid pressure equal to vapor pressure, then cavitation secondary to
fluid pressure above the steam pressure, and the pressure has a local character.However,
more recent studies have shown that the leading role in the formation of bubbles during
cavitation is played by gases released into the formed bubbles. These gases are always
contained in the liquid, and with a local decrease in pressure, they begin to be intensively

released into these bubbles.Since under the influence of variable local pressure of the
liquid bubbles can dramatically shrink and expand, the temperature of the gas inside the
bubbles varies widely, and can reach several hundred degrees Celsius. It is estimated that
the temperature inside the bubbles can reach 1500 °C[1]. It should also be borne in mind
that the gases dissolved in the liquid contain more oxygen as a percentage than in the air,
and therefore the gases in the bubbles during cavitation are chemically more aggressive
than atmospheric air-causing oxidation (reaction) of many usually inert materials.