Cavitation

( " Erode " Latin cavitare ) Cavitation is the formation and dissolution of vapor-filled cavities ( steam bubbles) in liquids. There are two limiting cases between which there are many transitional forms. In Dampfkavitation or hard ( transient ) cavitation cavities contain mainly steam of the surrounding liquid. Such voids fall under the influence of external pressure by bubble implosion together ( microscopic chuff ). In the soft gas cavitation occur in the liquid dissolved gases in the cavities and dampen their collapse in the stable they prevent it.

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

• 2.1 Mechanical damage
• 2.2 noise
• 2.3 Effects in medical diagnostics
• 2.4 Impact in nature

• 3.1 technology
• 3.2 Zoology

Causes

The most common cause of cavitation are fast moving objects in the water such as impellers of centrifugal pumps, water turbines or propellers. According to the law of Bernoulli, the static pressure of a liquid is lower, the higher the speed. If the static pressure drops below the vaporization pressure of the liquid, vapor bubbles form. These are then usually entrained with the flowing liquid in areas of higher pressure. With the renewed increase in the static pressure above the vapor pressure of the vapor in the cavities condenses abruptly and collapse the vapor bubbles. This occurred in extreme pressure and temperature peaks.

The cause of cavitation is the local pressure drops in the vane passage of the impeller inlet, which are inevitably associated with the flow around the blade leading edge and the transfer of energy from the impeller blades to the conveying liquid, in particular for centrifugal pumps. Cavitation can also at other points of the pump where the pressure drops locally, such as at the leading edge of stator blades, housing tabs, split rings, etc., can occur.

Other causes are either the increase of the temperature of the liquid, the drop in pressure on the inlet side of the pump, too much Geodetic suction head or the reduction of the inlet height.

Strong pressure fluctuations cause the cavitation may be produced by ultrasound. It comes in the pressure minima of the vibration cavitation.

Cavitation occurs also during rapid immersion of a body in the water. When water jumping occur, caused for example by cavitation secondary splashes.

The phenomenon of vapor bubbles formed during boiling of liquids and coincide partly back in on itself, is no cavitation, because it is not triggered by pressure fluctuations but by changes in temperature.

Formation and implosion of cavitation bubbles

At a pressure of 1013.25 hPa, which corresponds to the normal air pressure, water evaporates at 100 ° C. At a higher pressure, the evaporation temperature is higher, lower, at a lower pressure. As water is evaporated at a pressure of only 23.37 hPa already at a temperature of 20 ° C.

On evaporation occur in the water bubbles, as the water vapor often a larger space than the liquid water at 20 ° C. If the water pressure rises again, the evaporation process again responds, the water vapor formed in the cavitation bubble condenses on the outer wall of the vapor bubble and the vapor bubbles already formed falling abruptly collapses. The space previously needed is suddenly smaller, the water has to fill in this space again and flows implosionsartig back, so very strong in the water - albeit brief - water hammers, can accept the orders of magnitude of several 100 MPa ( = 1000 bar ). The process will produce pressure waves with high pressure peaks. Are the vapor bubbles near or directly on a solid wall, for example, the impeller blades will arise when the implosion of a liquid jet, known as a microjet impinging on the wall or impeller blade at high speed and this highly stressed by the sudden pressure. Therefore, the crater-like material erosion stir in the occurrence of cavitation.

Effects of cavitation

Mechanical damage

Cavitation occurs at the surface of solid bodies ( such as a ship's propeller ) which leads to so-called Kavitationsfraß circumstances. The surface material is deformed by the high mechanical stresses in microscopically small parts. After some time breaking out of the surface larger particles. The mechanism of this damage is not yet fully understood.

Cavitation is usually undesirable in hydraulics. Firstly, it reduces the efficiency, on the other hand it can lead to damage, because the implosion of the cavities occur momentarily extremely high accelerations, temperatures and pressures, which can damage the material. To prevent cavitation in pumps, it must be ensured that the temperature of the liquid in the pump is not too high or the suction pressure of the pump is too low. High temperatures occur when the pump is running without fluid is removed. In this case, the liquid to be pumped in a circuit is switched off ( for example in a back-flushing line ) or the pump. Cavitation occur, for example in fire -fighting centrifugal pump, when the pump is only switched on in order to maintain the pressure in the line and no fire-fighting water is removed. In liquid ring vacuum pumps, the ratio of pressure and temperature in the pump chamber must to avoid damage does not fall below a type- dependent value.

In the missile art cavitation leads to an irregular combustion, and damage to the gas turbine -driven centrifugal pumps. These pumps have to pump large amounts of fuel at high pressure with missiles and are therefore particularly susceptible to cavitation. It reduces the cavitation by placing the entire fuel tank under ( very low ) pressure and keeps this pressure through deserts of compressed gas, while emptied the tank.

Cavitation in a centrifugal pump impeller

Cavitation damage to a boat propeller

Noise

The occurring during the implosion sound ( bang, rattle ) raised earlier times the camouflage of submarines on - the boats could be tracked with passive sonar. Since kavitationsarme propellers were developed, plays the cavitation for submarine detection up to a certain speed depending on the depth of immersion is no longer relevant. It can be seen kavitationsarme propeller to the highly curved ends to which they glide smoother and much quieter through the water. At high speeds at low depths but also a kavitationsarmer propeller loses its ability kavationsmindernde. To avoid cavitation, the submarine must then either dive deeper to increase the water pressure, or decrease the speed.

In military surface ships also kavitationsarme screws and systems are used to reduce noise, such as Prairie Masker system for U.S. vessels.

Effects in medical diagnostics

At sonography in medical diagnosis there is the risk that (in particular, the negative peak pressures of the ultrasound ) is formed by strong energy within the body of cavitation. This could have adverse thermal or mechanical effects. For example, pieces of tissue could be destroyed by the generation of free radicals or shock waves. Some possible effects have been observed in animal experiments in vivo. It is believed, moreover, that contrast agents increase the probability for the occurrence of cavitation.

Effects in nature

Studies of the Israel Institute of Technology show that the maximum speed, swim with the dolphins, is limited by Kavitationeffekte. The resulting above a certain speed pain in the tail fin is the limiting factor. Some fish, such as tuna, have no pain receptors in the caudal fin. They penetrate therefore exist in velocity areas where cavitation occurs.

Applications

Technology

• When cleaning objects in so-called ultrasonic baths in which cavitation surface dirt is removed. The cavitation is generated here by ultrasound. Through the use of specially adapted cleaning fluids and a higher temperature of the bath, the mechanical cleaning effect of ultrasound can be enhanced.
• In the military field torpedoes are used that move at very high speed under water in an artificially created cavity. This technology was first developed by the Russian Navy to the application stage and has become known as super-cavitation.
• Sonochemistry: By occurring during the collapse of the bubbles high energies sonochemical reactions are possible.

Zoology

• Bang crabs are able to generate cavitation bubbles and to use in the fight against conspecifics or to catch prey.
• Missed the fishing beat of a mantis cancer its prey, so this is hit and stunned under circumstances of the cavitation bubble produced during impact and can be captured thereafter.