Cavitation

Cavitation is the phenomenon where small and largely empty cavities are generated in a fluid, which expand to large size and then rapidly collapse, producing a sharp sound. Cavitation occurs in pumps, propellers, impellers, and in the vascular tissues of plants.

Overview

Following the definition of Christopher E. Brennen: A liquid when it is subjected to a low pressure (tensile stress) above a threshold it ruptures and forms vaporous cavities. This phenomenon is termed cavitation. When the local ambient pressure at a point in the liquid falls below the liquid's vapor pressure at the local ambient temperature, the liquid can undergo a phase change, creating largely empty voids termed cavitation bubbles. Other possibilities to generate cavitation bubbles involve the local deposition of energy. This can be achieved by focusing an intense laser pulse (optic cavitation) or with an electrical discharge through a spark.

The physical process of cavitation is almost exactly the same as that which occurs during boiling. The major difference between the two is how the phase change is effected. Boiling is when the local vapor pressure of the liquid rises above its local ambient pressure and sufficient energy is present to cause the phase change to a gas.

In order for cavitation to occur, the cavitation "bubbles" generally need a surface on which they nucleate. This surface can be provided by the sides of a container and by impurities in the liquid. It is generally accepted that hydrophobic surfaces stabilize small bubbles. These pre-existing bubbles start to grow unbounded when they are exposed a pressure below the a threshold pressure termed Blake's threshold.

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. Thereby, they create spots of high temperature and emit shock waves which are the source of noise. The noise created by cavitation is a particular problem in submarines, as the noise destroys its stealth.

The collapse of cavities involves very high energies, and can cause major damage. Cavitation can damage almost any substance. The pitting caused by the collapse of cavities produces great wear on components and can dramatically shorten a propeller or pump's lifetime.

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. By greatly reducing or eliminating contact with water, these torpedoes can travel significantly faster than conventional torpedoes.

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. Used in this manner, the need for sometimes environmentally harmful chemicals can be reduced in many industrial and commercial processes that require cleaning as a step. Still the details on how bubbles clean are not understood.

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. Many industrial mixing machines are based upon this design principle. 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. This method can be controlled with hydraulic devices that control the size of the inlet orifice, and this allows for adjustment to the process "on the fly", or for different substances. 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. Spectral analysis of light emitted in sonochemical reactions reveal chemical and plasma based mechanisms of energy transfer. The light emitted from cavitation bubbles is termed sonoluminesence.

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. The faster the blades move, the lower the pressure around it can become. As it reaches vapor pressure, the fluid vaporizes and forms small bubbles of gas. This is cavitation. When the bubbles collapse later, they typically cause very strong local shockwaves in the fluid, which may be audible and may even damage 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. This vapor is carried over to the discharge side of the pump where it no longer sees vacuum and is compressed back into a liquid by the discharge pressure. This imploding action occurs violently and attacks the face of the 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. It normally occurs in a pump that is running at less than 10% of its best efficiency point. The high discharge pressure causes the majority of the fluid to circulate inside the pump instead of being allowed to flow out the discharge. As the liquid flows around the impeller it must pass through the small clearance between the impeller and the pump cutwater at extremely high velocity. This velocity causes a vacuum to develop at the cutwater (similar to what occurs in a venturi) which turns the liquid into a vapor. A pump that has been operating under these conditions shows premature wear of the impeller vane tips and the pump cutwater. In addition due to the high pressure condition premature failure of the pump mechanical seal and bearings can be expected. Under extreme conditions this can break the impeller shaft.

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. Vibrations of the cylinder wall induce alternating low and high pressure in the coolant against the cylinder wall. The result is pitting of the cylinder wall that will eventually let cooling fluid leak into the cylinder and combustion gasses to leak into the coolant.

It is possible to prevent this from happening with chemical additives in the cooling fluid that form a protecting layer on the cylinder wall. This layer will be exposed from 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. In some trees, the sound of the cavitation is clearly audible. In the autumn the dropping temperature increases the formation of air bubbles in the tracheids of some plant species causing them to drop their leaves.

External links

References

For cavitation in plants, see Plant Physiology, by Taiz and Zeiger.

Fluid dynamics | Plants | Pumps

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