A form of localized boiling in a liquid, caused by sudden dramatic reductions in pressure, giving rise to pockets of gas. Cavitation can occur at the trailing edge of ship propellers, and in liquids subject to powerful sound waves.
Cavitation is a general term used to describe the behaviour of voids or bubbles in a liquid. Cavitation is usually divided into two classes of behaviour: inertial (or transient) cavitation and non-inertial cavitation. Inertial cavitation is the process where a void or bubble in a liquid rapidly collapses, producing a shock wave. Such cavitation often occurs in pumps, propellers, impellers, and in the vascular tissues of plants. Non-inertial cavitation is the process where a bubble in a fluid is forced to oscillate in size or shape due to some form of energy input, such as an acoustic field. Such cavitation is often employed in ultrasonic cleaning baths, and will also be observed in pumps, propellers etc.
Inertial Cavitation
Inertial cavitation was first studied by Lord Rayleigh in the late 19th century when he considered the collapse of a spherical void within a liquid. This phenomenon is termed cavitation inception and may occur behind the blade of a rapidly rotating propeller or on any surface vibrating underwater with sufficient amplitude and acceleration. Other ways of generating cavitation voids involve the local deposition of energy such as an intense focussed laser pulse (optic cavitation) or with an electrical discharge through a spark. Such a low pressure cavitation bubble in a liquid will begin to collapse due to the higher pressure of the surrounding medium.
Inertial cavitation can also occur in the presence of an acoustic field. Hence, inertial cavitation can occur even if the rarefraction in the liquid is insufficient for a Rayleigh-like void to occur. Ultrasonic cleaning baths usually utilise the inertial cavitation of microscopic gas bubbles for erosion of dirt from materials.
The physical process of cavitation inception is similar to boiling. Cavitation inception occurs when the local pressure falls sufficiently far below the saturated vapour pressure, a value given by the tensile strength of the liquid.
In order for cavitation inception to occur, the cavitation "bubbles" generally need a surface on which they nucleate.
Non-inertial cavitation
Non-inertial cavitation is the process where small bubbles in a liquid are forced to oscillate in the presence of an acoustic field, when the intensity of the acoustic field is insufficient to cause total bubble collapse. This form of cavitation causes significantly less erosion than inertial cavitation, and is often used for the cleaning of delicate materials, such as silicon wafers.
Problems
Cavitation is, in many cases, an undesirable occurrence. In devices such as propellers and pumps, cavitation causes a great deal of noise, damage to components, vibrations, and a loss of efficiency.
When the cavitation bubbles collapse, they focus liquid energy to very small volumes. The noise created by cavitation is a particular problem in submarines, as the noise destroys its stealth.
Beneficial uses
Although cavitation is undesirable in many circumstances, this is not always the case. For example, supercavitating torpedoes in use by the military envelop the torpedo in a large bubble of cavitation.
Cavitation can also be a boon in ultrasonic cleaning devices. These devices effect cavitation using sound waves and use the collapse of the cavitation bubbles to clean surfaces.
In industry, cavitation is often used to homogenize, or mix and break down suspended particles in a colloidal liquid compound, such as paint mixtures, or milk. It is usually achieved through impeller design, or by forcing the mixture through an annular opening that has a narrow entrance orifice with a much larger exit orifice: the drastic decrease in pressure as the liquid accelerates into the larger volume causes cavitation to take place. The outer surface of this type of mixing valve, upon which the cavitation bubbles are driven against to cause their implosion, undergoes tremendous stress, and is often constructed of super-hard or tough materials such as stainless steel, Stellite, or even polycrystalline diamond (PCD).
Cavitating water purification devices have also been designed, in which the extreme conditions of cavitation can break down pollutants and organic molecules. The light emitted from cavitation bubbles is termed sonoluminesence.
Hydrophobic chemicals are attracted underwater by cavitation as the pressure difference between the bubbles and the liquid water forces them to join together.
Biomedical application
Cavitation plays an important role for the destruction of kidney stones in shock wave lithotripsy (lithotriptor). Currently it is tested if cavitation can be used to transfer large molecules into biological cells (sonoporation).
Pumps and propellers
Major places where cavitation occurs are in pumps, on propellers, or at restrictions in a flowing liquid.
As an impeller's (in a pump), or propeller's (as in the case of a ship or submarine) blades move through a fluid, low pressure areas are formed as the fluid accelerates around and moves past the blades.
Cavitation in pumps may occur in two different forms:
Suction cavitation
Suction cavitation occurs when the pump suction is under a low pressure/high vacuum condition where the liquid turns into a vapor at the eye of the pump impeller. An impeller that has been operating under a suction cavitation condition has large chunks of material removed from its face causing premature failure of the pump.
Discharge cavitation
Discharge cavitation occurs when the pump discharge pressure is extremely high, normally occurring in a pump that is running at less than 10% of its best efficiency point.
Discharge cavitation is believed to be the cause of the cracking of joints.
Cavitation in engines
Some bigger diesel engines suffer from cavitation due to high compression and undersized cylinder walls. This layer will be exposed to the same cavitation, but rebuilds itself.
Vascular plants
Cavitation occurs in the xylem of vascular plants when the water potential becomes so great that dissolved air within the water expands to fill the plant cell - either vessel elements or tracheids. Plants are generally able to repair cavitated xylem, for example with root pressure, but for others such as vines, cavitation often leads to mortality.
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