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Introduction to Cavitation

 

 

 

 

 

 

 

 

Cavitation science is a part of liquid physics which studies the motion of liquids near or beyond the limit of vaporization. It has been known in science and technology for more than hundred years. British fluid dynamics engineer and scientist, Osborne Reynolds was the first to discover this phenomenon in 1894 on English torpedo boats. Cavitation can occur in two ways. Hydrodynamic Cavitation is the kind of cavitation which occurs in flowing liquids while acoustic Cavitation is induced by an oscillating pressure field in a liquid almost at rest but the principles which govern the two are the same.

In general, vaporization happens if velocities of liquid are high, causing pressure to drop below a critical value at which the liquid continuum is broken at one or a few points. Cavitation is the occurrence of vapor cavities inside a liquid. It is well known that in static conditions a liquid changes to vapor if its pressure is lowered below the so-called vapor pressure. In liquid flows, this phase change is generally due to local high velocities which induce low pressures. The liquid medium is then “broken” at one or several points and “voids” appear, whose shape depends strongly on the structure of the flow.

Effects of Cavitation
Vapor cavities are produced and various unexpected effects such as noise, lower performance and vibrations would occur. If the cavitated region is in direct contact with the walls of an impeller blade or water-tunnel surface, noise isn’t the only result. The extremely high extinction pressure caused by the collapse of cavities will be exerted against the metal itself and cause actual erosion of the metal. Cavitation is generated not only in water, but also in any kind of fluid, such as liquid hydrogen. The generation of cavitation can cause severe damage in hydraulic machinery. Therefore, the prevention of cavitation is an important concern for designers of hydraulic machinery.

Applications of Cavitation 
Cavitation traditionally results in:
1. Sterilization of a liquid processed
2. Emulsification of oil and water
3. Disruption of long polymer chains in mineral oils: transition into a new structural condition
4. Size reduction (dispersion) to micron level of solid particles in a liquid
5. Intensification of chemical reactions into tens and even thousand times

For a long time, it was thought that cavitation phenomena should be avoided entirely because of the detrimental effects. Over the years it appeared that imposing such a constraint could be costly, though not necessarily justified on scientific grounds. Under meticulous control, a limited development of cavitation can be allowed. This promotes the development of high speed hydrodynamics and hydraulics. There is also great potential to utilize cavitation in various important applications, such as environmental protection. In industries, cavitation is often employed to homogenize, mix and break down suspended particles in colloidal liquid compounds, like paint mixtures. Many industrial mixing machines are designed based on this principle. Water purification devices have also been designed based upon cavitation in which pollutants and organic molecules can be broken down into micro level. Therefore, by carefully designing the systems and defining the parameters in which cavitation occurs, the science of cavitation can be harnessed to develop useful industrial applications.

References:

  • Fundamentals of Cavitation (Springer; 1 edition, August 3, 2004)
    JEAN-PIERRE FRANC Research Director (CNRS), Turbomachinery and Cavitation Research Group, Laboratory of Geophysical and Industrial Fluid Flows (LEGI) of the Grenoble University (Institut National Polytechnique de Grenoble (INPG) & Université Joseph Fourier (UJF), FranceJEAN-MARIE MICHEL Presently retired, was Research Director (CNRS) and Head of the Cavitation Research Group in the Laboratory of Geophysical and Industrial Fluid Flows (LEGO) of the Grenoble University (Institut National Polytechnique de Grenoble (INPG) & Université Joseph Fourier (UJF), France
  • Cavitation And Bubble Dynamics (Oxford University Press 1995)
    CHRISTOPHER EARLS BRENNEN PhD, Califiornia Institute of Technology