Cavitation

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Cavitation generating propellers

Cavitation ( Latin: cavitare "to hollow out") is the formation and dissolution of steam-filled cavities (steam bubbles ) in liquids . A distinction is made between two borderline cases, between which there are many transitional forms:

  • In steam cavitation or hard ( transient ) cavitation , the cavities mainly contain steam from the surrounding liquid. Such cavities collapse under the action of external pressure via bubble implosion (microscopic steam hammer ).
  • With soft gas cavitation , gases dissolved in the liquid enter the cavities and dampen their collapse, with stable gas cavitation they prevent it.

The cavitation number describes the tendency of a liquid to cavitate.

causes

The most common causes of cavitation are fast moving objects in a fluid , e.g. B. the impellers of centrifugal pumps , water turbines or propellers . According to Bernoulli's law , the higher the speed , the lower the static pressure of a liquid . If the static pressure falls below the evaporation pressure of the liquid, vapor bubbles form. These are then usually carried away with the flowing liquid into areas of higher pressure. When the static pressure rises again above the steam pressure, the steam suddenly condenses in the cavities and the steam bubbles collapse. Extreme pressure and temperature peaks occur.

The cause of cavitation, especially in centrifugal pumps, is the local pressure drop in the blade channel inlet of the impeller. They are inevitably connected with the flow around the blade leading edges and the transfer of energy from the impeller blades to the fluid to be pumped. Cavitation can also occur at other points on the pump where the pressure drops locally, for example at the leading edges of stator blades, housing tongues, split rings, etc.

Other causes are either the rise in the temperature of the pumped liquid, the drop in pressure on the inlet side of the pump, a geodetic suction height that is too high or the decrease in the inlet height.

Strong pressure fluctuations that trigger cavitation can also be generated by ultrasound . Cavitation occurs in the pressure minima of the oscillation.

The phenomenon of vapor bubbles, which arise when liquids boil and sometimes collapse again, is not cavitation, because it is not triggered by pressure fluctuations but by temperature changes.

Formation and implosion of cavitation bubbles

At a pressure of 1013.25  hPa , which corresponds to normal air pressure , water evaporates at 100 ° C. At a higher pressure the evaporation temperature is higher, at a lower pressure it is lower. For example, water evaporates at a system pressure of less than 23.37 hPa at a temperature of 20 ° C.

When the water evaporates, bubbles form because the water vapor at 20 ° C requires a space that is many times larger than that of liquid water. If the water pressure rises again, the evaporation process stops again, the water vapor created in the cavitation bubble condenses on the outer wall of the steam bubble, and the steam bubbles that have already formed collapse suddenly. The previously required space suddenly becomes smaller, the water has to fill this space again and flows back like an implosion , which results in very strong - albeit brief - pressure surges in the water , which can be several 100  MPa (1 MPa = 10  bar  = 1 N / mm²). This process creates pressure waves with high pressure peaks.

If the vapor bubbles are near or directly on a solid wall, e.g. B. the impeller blades, the implosion creates a jet of liquid (microjet) that hits the wall or impeller blade at high speed. These are highly stressed by the sudden pressure load; therefore the crater-shaped material erosion stirs when cavitation occurs.

Cavitation in a gear pump

Effects

Mechanical damage

If cavitation occurs on the surface of solid bodies (such as a ship's propeller ), so-called cavitation corrosion may occur. The surface material is deformed in microscopic parts due to the high mechanical loads. After a while, larger particles break out of the surface. The mechanism of this damage is briefly described in the previous section "Formation and implosion of cavitation bubbles".

Cavitation is usually undesirable in hydraulics . On the one hand, it reduces the efficiency , and on the other hand, it can lead to damage, because when the cavities are imploded, extremely high accelerations , temperatures and pressures occur briefly , which can damage the material. To prevent cavitation in pumps , make sure that the temperature of the liquid in the pump is not too high and that the suction pressure of the pump is not too low. High temperatures arise when the pump is running without any liquid being drawn off. In this case, the liquid should be pumped in a circuit (e.g. in a backwash line) or the pump should be switched off. Cavitation damage occurs, for example, with centrifugal fire pumps if the pump is only switched on to maintain the pressure in the line and no extinguishing water is drawn off. In the case of liquid ring vacuum pumps , the ratio of pressure and temperature in the pump chamber must not fall below a design-dependent value in order to avoid damage.

In rocket technology, cavitation leads to irregular combustion and damage to centrifugal pumps driven by gas turbines . In rockets, these pumps have to deliver large amounts of fuel at high pressure and are therefore particularly prone to cavitation. Cavitation is reduced by putting the entire fuel tank under (very little) overpressure and maintaining this overpressure by adding pressurized gas while the tank is emptying.

Noise development

The noise that occurs when imploding (bang, rattling) used to undo the camouflage of submarines - the boats could be located with passive sonar . Since low-cavitation propellers were developed, cavitation no longer plays a role in submarine location up to a certain speed depending on the depth of the dive. You can recognize low-cavitation propellers by the strongly curved blades with which they glide more gently and much more quietly through the water. At high speeds in shallow depths, however, a low-cavitation propeller also loses its cavitation-reducing ability. To avoid cavitation, the submarine must either go deeper to increase the water pressure or reduce the speed.

In military surface ships, low cavitation screws and systems for noise reduction are also used, e.g. B. the prairie masker system on US ships.

Effects in medical diagnostics

With sonography in medical diagnostics, there is a risk that strong energies (especially the negative peak pressures of the ultrasound) will cause cavitation within the body. This could have harmful thermal or mechanical effects. For example, the generation of free radicals or shock waves could destroy tissue parts. Some possible effects have already been observed in animal experiments in vivo . It is also believed that contrast agents increase the likelihood of cavitation occurring.

Effects in nature

Research by the Israel Institute of Technology shows that the maximum speed at which dolphins swim is limited by the cavitation effect . The pain in the caudal fins above a certain speed is the limiting factor. Some fish, such as tuna , do not have pain receptors in their caudal fin. They therefore advance into speed ranges in which cavitation effects arise.

Popcocks are able to create a cavitation bubble by closing their claws extremely quickly. The bang that occurs when the bladder collapses is, on the one hand, eponymous for these animals; on the other hand, the energy released can be sufficient to stun or kill prey.

Applications

technology

  • When cleaning objects in so-called ultrasonic baths , in which surface dirt is removed through cavitation. The cavitation is generated by ultrasound . The mechanical cleaning effect of the ultrasound can be further enhanced by using specially matched cleaning fluids and increasing the temperature of the bath.
  • In the military field, torpedoes are used, which move at very high speed under water in an artificially created cavity. This technology was first developed to be ready for use by the Russian Navy and has become known under the term supercavitation .
  • Sonochemistry : The high energies that occur when the bubbles collapse enable sonochemical reactions.
  • In the food industry, cavitation is used in high pressure homogenizers to break up fat droplets.

zoology

  • Crabs are able to generate cavitation bubbles and use them in the fight against conspecifics or to catch prey.
  • If the catch of a mantis shrimp misses its prey, it may be hit and stunned by the cavitation bubble created during the strike and can then be captured.

See also

literature

Web links

Wiktionary: Cavitation  - explanations of meanings, word origins, synonyms, translations
Commons : Cavitation  - collection of images, videos and audio files

Individual evidence

  1. ↑ J. Jenne: Cavitation in biological tissue. In: Ultrasound in Medicine. 22, pp. 200-207, doi: 10.1055 / s-2001-17913 .
  2. ↑ Dolphins swim so fast it hurts. In: newscientist.com. March 28, 2008, accessed January 5, 2015 .
  3. ↑ M. Versluis, B. Schmitz, A. von der Heydt, D. Lohse: How snapping shrimp snap: through cavitating bubbles . In: Science . tape 289 , no. 5487 . New York, NY September 22, 2000, pp. 2114-2117 , PMID 11000111 .
  4. ↑ B. Freudig, S. Tesch, H. Schubert: Production of emulsions in high pressure homogenizers - Part 2: Significance of cavitation for the droplet size reduction . In: Chemical Engineer Technology . 74, No. 6, June 1, 2002, ISSN  1522-2640 , pp. 880-884. doi : 10.1002 / 1522-2640 (200206) 74: 6 <880 :: AID-CITE880> 3.0.CO; 2-O .
  5. ^ SN Patek, WL Korff, RL Caldwell: Deadly strike mechanism of a mantis shrimp . In: Nature . 428, 2004, pp. 819-820. doi : 10.1038 / 428819a . "No"
  6. ↑ SN Patek, RL Caldwell: Extreme impact and cavitation forces of a biological hammer: strike forces of the peacock mantis shrimp . In: Journal of Experimental Biology . 208, 2005, pp. 3655-3664. doi : 10.1242 / jeb.01831 . PMID 16169943 . "No"