Cavitation is, in most cases, an undesirable event. In devices such as propellers "Propeller (device)") and pumps, cavitation can cause a lot of noise, component damage, and a loss of performance "Performance (physics)"). To predict the occurrence of cavitation in a propeller, the cavitation number is used, while to predict it in a pump, the Thoma number is used.

This phenomenon is widely studied in naval engineering during the design of all types of ships because it shortens the useful life of some parts such as the propellers "Propeller (device)") and the rudders "Rudder (device)").

In the case of submarines, this effect is even more studied, avoided and unwanted, since it makes it impossible for these warships to maintain their operational characteristics of silence and undetectability due to the vibrations and noises that cavitation causes in the hull "Hull (nautical)") and the propellers "Propeller (device)").

The collapse of the cavities involves a large amount of energy that can cause enormous damage. Cavitation can affect almost any material. The pitting caused by the collapse of the cavities produces enormous wear on the different components and can greatly shorten the life of the pumps or propellers "Propeller (device)").

In addition to all of the above, the creation and subsequent collapse of bubbles creates friction and turbulence in the liquid. This contributes to an additional loss of performance "Performance (physics)") in devices subjected to cavitation.

Cavitation also occurs at the bottom of rivers where it is generated from irregularities in the bed, dissociating water and air. Both are subjected to pressure, giving rise, the latter, to bubbles that, with the force of the water, decompose into microscopic sizes, shooting out at high speed. This causes a strong impact on the bed that can be up to 60 t/m². Its importance lies in the constancy and repetition of the phenomenon, which favors its performance. Cavitation is a common erosion process in bridge piers.

Although cavitation is an undesirable phenomenon in most circumstances, this is not always the case. For example, supercavitation has military applications such as supercavitation torpedoes in which a bubble surrounds the torpedo, thus eliminating all friction with the water. These torpedoes can travel at high speeds underwater, even up to supersonic speeds. Cavitation can also be a positive phenomenon in ultrasonic cleaning devices. These devices make use of ultrasonic sound waves and take advantage of the collapse of bubbles during cavitation to clean surfaces.

Another application of cavitation is during the homogenization process of a product (e.g. biotechnological process) which is carried out using a sonicator characterized by the García-Atance number. This device has a tip that is immersed in the solution to be homogenized and vibrates at ultrasonic frequencies. These high frequency vibrations cause cavitation, which as already mentioned is the formation of small bubbles in the liquid medium. These bubbles collapse, releasing their mechanical energy in the form of shock waves equivalent to several thousand atmospheres of pressure, breaking cells present in the suspension. The duration of ultrasound needed depends on the type of cells, sample size and cell concentration. For bacterial cells such as E. coli, 30 to 60 seconds may be sufficient for small samples. Yeast cells, may need 2 to 10 minutes.

It is believed that cavitation may be the cause of 'joint cavitation'.[1]​.

Contenido

Los álabes de un rodete "Rodete (rotor)") de una bomba o de la hélice "Hélice (dispositivo)") de un barco se mueven dentro de un fluido. Cuando el fluido se acelera a través de los álabes se forman regiones de bajas presiones. Cuanto más rápido se mueven los álabes menor es la presión alrededor de los mismos. Cuando se alcanza la presión de vapor, el fluido se vaporiza y forma pequeñas burbujas de vapor "Vapor (estado)") que al colapsarse causan ondas de presión audibles y desgaste en los álabes.

La cavitación en bombas puede producirse de dos formas diferentes:.

Suction cavitation

Suction cavitation occurs when the pump suction is under low pressure/high vacuum conditions which causes the liquid to transform into vapor "Vapor (state)") at the impeller inlet "Impeller (rotor)"). This vapor "Vapor (state)") is transported to the discharge zone of the pump where the vacuum disappears and the vapor "Vapor (state)") of the liquid is again compressed due to the discharge pressure. At that moment a violent implosion occurs on the surface of the impeller "Impeller (rotor)"). An impeller "Impeller (rotor)") that has worked under suction cavitation conditions presents large cavities produced by the pieces of material removed by the phenomenon. This causes premature failure of the pump.

Discharge cavitation

Discharge cavitation occurs when the pump discharge is very high. This typically occurs in a pump that is operating at less than 10% of its optimal efficiency point. The high discharge pressure causes most of the fluid to circulate inside the pump instead of exiting through the discharge area. This phenomenon is known as slippage. As the liquid flows around the impeller, it must pass at a very high speed through a small opening between the impeller and the pump cutwater. This speed causes a vacuum in the cutwater (a phenomenon similar to that which occurs in a venturi) which causes the liquid to transform into vapor "Steam (state)"). A pump operating under these conditions shows premature wear of the impeller, cutwater and blades. Furthermore, due to the high operating pressure, premature failure of the pump's seals and bearings is to be expected. Under extreme conditions, the shaft "Axle (mechanical)") of the impeller "Impeller (rotor)") may break.

Cavitation can appear in the xylem of plants when the water potential becomes so great that the air dissolved within the water expands to fill the plant cell. Plants are generally able to repair damage caused by cavitation, for example with pumping pressure from the roots "Root (botany)"). In other types of plants such as vines, cavitation can lead to death. In some trees, cavitation is clearly audible.

El proceso físico de la cavitación es casi exactamente igual que el que ocurre durante la ebullición. La mayor diferencia entre ambos se debe a presión y temperatura dentro del diagrama de fases. La ebullición eleva la presión de vapor del líquido por encima de la presión ambiente local para producir el cambio a fase "Fase (termodinámica)") gaseosa, mientras que la cavitación es causada por una caída de la presión local por debajo de la presión de vapor que causa una succión.

For cavitation to occur, the "bubbles" need a surface to nucleate. This surface can be the wall of a container or tank, impurities in the liquid or any other irregularity, but normally the temperature of the fluid must be taken into account, which will largely be the possible cause of cavitation.

The determining factor in cavitation is the temperature of the liquid. By varying the temperature of the liquid, the vapor pressure also varies significantly, making it easier or more difficult for a given local ambient pressure for the vapor pressure to fall to a value that causes cavitation.

The position that the hydraulic machines must have, with respect to the water level, in the suction tank or in the return channel to avoid cavitation, is also studied. The approach is done taking a specific point on the pump or turbine as a reference level. Different parameters and concepts associated with cavitation are analyzed, such as: the net positive charge in the suction and the limitations that these concepts and parameters have in the sense of extrapolating results following the laws of similarity are presented. The need to further investigate the limitations of the cavitation coefficient in turbines is highlighted and a new relationship is proposed that allows estimating the static head as a pump of a reversible machine, from the values ​​of the cavitation coefficient of the machine operating as a turbine.

Cavitation is, in most cases, an undesirable event. In devices such as propellers "Propeller (device)") and pumps, cavitation can cause a lot of noise, component damage, and a loss of performance "Performance (physics)"). To predict the occurrence of cavitation in a propeller, the cavitation number is used, while to predict it in a pump, the Thoma number is used.

This phenomenon is widely studied in naval engineering during the design of all types of ships because it shortens the useful life of some parts such as the propellers "Propeller (device)") and the rudders "Rudder (device)").

In the case of submarines, this effect is even more studied, avoided and unwanted, since it makes it impossible for these warships to maintain their operational characteristics of silence and undetectability due to the vibrations and noises that cavitation causes in the hull "Hull (nautical)") and the propellers "Propeller (device)").

The collapse of the cavities involves a large amount of energy that can cause enormous damage. Cavitation can affect almost any material. The pitting caused by the collapse of the cavities produces enormous wear on the different components and can greatly shorten the life of the pumps or propellers "Propeller (device)").

In addition to all of the above, the creation and subsequent collapse of bubbles creates friction and turbulence in the liquid. This contributes to an additional loss of performance "Performance (physics)") in devices subjected to cavitation.

Cavitation also occurs at the bottom of rivers where it is generated from irregularities in the bed, dissociating water and air. Both are subjected to pressure, giving rise, the latter, to bubbles that, with the force of the water, decompose into microscopic sizes, shooting out at high speed. This causes a strong impact on the bed that can be up to 60 t/m². Its importance lies in the constancy and repetition of the phenomenon, which favors its performance. Cavitation is a common erosion process in bridge piers.

Although cavitation is an undesirable phenomenon in most circumstances, this is not always the case. For example, supercavitation has military applications such as supercavitation torpedoes in which a bubble surrounds the torpedo, thus eliminating all friction with the water. These torpedoes can travel at high speeds underwater, even up to supersonic speeds. Cavitation can also be a positive phenomenon in ultrasonic cleaning devices. These devices make use of ultrasonic sound waves and take advantage of the collapse of bubbles during cavitation to clean surfaces.

Another application of cavitation is during the homogenization process of a product (e.g. biotechnological process) which is carried out using a sonicator characterized by the García-Atance number. This device has a tip that is immersed in the solution to be homogenized and vibrates at ultrasonic frequencies. These high frequency vibrations cause cavitation, which as already mentioned is the formation of small bubbles in the liquid medium. These bubbles collapse, releasing their mechanical energy in the form of shock waves equivalent to several thousand atmospheres of pressure, breaking cells present in the suspension. The duration of ultrasound needed depends on the type of cells, sample size and cell concentration. For bacterial cells such as E. coli, 30 to 60 seconds may be sufficient for small samples. Yeast cells, may need 2 to 10 minutes.

It is believed that cavitation may be the cause of 'joint cavitation'.[1]​.

Contenido

Los álabes de un rodete "Rodete (rotor)") de una bomba o de la hélice "Hélice (dispositivo)") de un barco se mueven dentro de un fluido. Cuando el fluido se acelera a través de los álabes se forman regiones de bajas presiones. Cuanto más rápido se mueven los álabes menor es la presión alrededor de los mismos. Cuando se alcanza la presión de vapor, el fluido se vaporiza y forma pequeñas burbujas de vapor "Vapor (estado)") que al colapsarse causan ondas de presión audibles y desgaste en los álabes.

La cavitación en bombas puede producirse de dos formas diferentes:.

Suction cavitation

Suction cavitation occurs when the pump suction is under low pressure/high vacuum conditions which causes the liquid to transform into vapor "Vapor (state)") at the impeller inlet "Impeller (rotor)"). This vapor "Vapor (state)") is transported to the discharge zone of the pump where the vacuum disappears and the vapor "Vapor (state)") of the liquid is again compressed due to the discharge pressure. At that moment a violent implosion occurs on the surface of the impeller "Impeller (rotor)"). An impeller "Impeller (rotor)") that has worked under suction cavitation conditions presents large cavities produced by the pieces of material removed by the phenomenon. This causes premature failure of the pump.

Discharge cavitation

Discharge cavitation occurs when the pump discharge is very high. This typically occurs in a pump that is operating at less than 10% of its optimal efficiency point. The high discharge pressure causes most of the fluid to circulate inside the pump instead of exiting through the discharge area. This phenomenon is known as slippage. As the liquid flows around the impeller, it must pass at a very high speed through a small opening between the impeller and the pump cutwater. This speed causes a vacuum in the cutwater (a phenomenon similar to that which occurs in a venturi) which causes the liquid to transform into vapor "Steam (state)"). A pump operating under these conditions shows premature wear of the impeller, cutwater and blades. Furthermore, due to the high operating pressure, premature failure of the pump's seals and bearings is to be expected. Under extreme conditions, the shaft "Axle (mechanical)") of the impeller "Impeller (rotor)") may break.

Cavitation can appear in the xylem of plants when the water potential becomes so great that the air dissolved within the water expands to fill the plant cell. Plants are generally able to repair damage caused by cavitation, for example with pumping pressure from the roots "Root (botany)"). In other types of plants such as vines, cavitation can lead to death. In some trees, cavitation is clearly audible.