The Phenomenon of Cavitation

The pressure wave caused by ultrasound transmitted in a liquid medium will, in turn, cause an oscillation of the molecules around their mean position. When a large negative pressure (Pc) is applied to the liquid, where Pc (rarefaction pressure) = Pa — Ph, the distance between molecules can overcome a critical distance R, under which the liquid breaks down so that cavitation bubbles form. The R distance for water is around 10"8 cm and the pressure involved is 10.1 x 10s kPa, where Pc = 2a/R, a is the surface tension. The cavitation process can be observed at much lower negative pressure (e.g. 10.1 x 104 kPa), as a result of the presence of gas nuclei as dissolved gas, suspended gas bubbles, or gas bubbles caused by heat fluctuations within the liquid. The cavitation threshold decreases with degassed liquids or as consequence of the increase in hydrostatic pressure.

Cavitation can be divided into two classes: transient and stable. Stable cavities oscillate around some equilibrium size (R0) over several rarefaction-compression cycles. In contrast, transient cavities usually exist over one acoustic cycle, increasing their size during the cycle and collapsing into smaller bubbles. The time required for a bubble to collapse is usually shorter than the period of the acoustic wave, and therefore Pm (i.e. pressure in the liquid at the moment of transient collapse, Pm = Ph + Pa) can be considered as constant during collapse. This time can be expressed as:

where Rm is the radius of the cavity at the moment of collapse.

Temperatures and pressures reached inside a cavi-tation bubble containing nitrogen in water at ambient temperature and pressure before collapsing are nearly 4200 K and 975 bar, respectively. The high temperature existing inside cavitation bubbles accounts for radical formation, whereas the shock wave caused by bubble implosion may be responsible for the increased chemical reactivity.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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