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The formation of vapor- or gas-filled cavities in liquids. If understood in this broad sense, cavitation includes the familiar phenomenon of bubble formation when water is brought to a boil under constant pressure and the effervescence of champagne wines and carbonated soft drinks due to the diffusion of dissolved gases. In engineering terminology, the term cavitation is used in a narrower sense, namely, to describe the formation of vapor-filled cavities in the interior or on the solid boundaries created by a localized pressure reduction produced by the dynamic action of a liquid system without change in ambient temperature. Cavitation in the engineering sense is characterized by an explosive growth and occurs at suitable combinations of low pressure and high speed in pipelines; in hydraulic machines such as turbines, pumps, and propellers; on submerged hydrofoils; behind blunt submerged bodies; and in the cores of vortical structures. This type of cavitation has great practical significance because it restricts the speed at which hydraulic machines may be operated and, when severe, lowers efficiency, produces noise and vibrations, and causes rapid erosion of the boundary surfaces, even though these surfaces consist of concrete, cast iron, bronze, or other hard and normally durable material.
Acoustic cavitation occurs whenever a liquid is subjected to sufficiently intense sound or ultrasound (that is, sound with frequencies of roughly 20 kHz to 10 MHz). When sound passes through a liquid, it consists of expansion (negative-pressure) waves and compression (positive-pressure) waves. If the intensity of the sound field is high enough, it can cause the formation, growth, and rapid recompression of vapor bubbles in the liquid. The implosive bubble collapse generates localized heating, a pressure pulse, and associated high-energy chemistry. See Sound, Ultrasonics
Both experiments and calculations show that with ordinary flowing water cavitation commences as the pressure approaches or reaches the vapor pressure, because of impurities in the water. These impurities, called cavitation nuclei, cause weak spots in the liquid and thus prevent it from supporting higher tensions. The exact mechanism of bubble growth is generally described by mathematical relationships which depend upon the cavitation nuclei. Cavitation commences when these nuclei enter a low-pressure region where the equilibrium between the various forces acting on the nuclei surface cannot be established. As a result, bubbles appear at discrete spots in low-pressure regions, grow quickly to relatively large size, and suddenly collapse as they are swept into regions of higher pressure.
the formation in a liquid of cavities (cavitation bubbles, or caverns), filled with gas, vapor, or a mixture of them. Cavitation bubbles are formed in locations where the pressure in the liquid becomes lower than a certain critical value pcr (in a real liquid, pcr is approximately equal to the saturated vapor pressure of the liquid at the given temperature). If the pressure drop occurs because of high local velocities in the flow of a liquid, then the cavitation is called hydrodynamic, whereas if it is the result of the passage of acoustic waves, it is called acoustic.
Hydrodynamic cavitation. Since tiny gas or vapor bubbles are always present in a real liquid, they become unstable and acquire the capacity for unlimited growth as they move with the flow and enter the pressure region where p< pcr. After a bubble passes into the zone of higher pressure and the kinetic energy of the expanding liquid is exhausted, it stops growing and begins to contract. If the bubble contains a sufficient quantity of gas, then upon attaining a minimum radius it is regenerated and undergoes several cycles of damped oscillation; however, if it contains insufficient gas, then it collapses completely during its initial period of existence. Thus, a fairly sharply outlined “cavitation zone” forms in the vicinity of a streamlined body (for example, a pipe with a local constriction).
Collapse of a cavitational bubble occurs with great speed and is accompanied by a sonic pulse (a type of hydraulic shock); the smaller the quantity of gas in the bubble, the stronger the pulse. If the degree of development of cavitation is such that many bubbles are generated and collapse at random times, the phenomenon is accompanied by loud noise, with a continuous spectrum from several hundred hertz to several hundred or thousand kilohertz. If the cavitational cavern collapses near the body, the multiple impacts lead to the destruction (cavitational erosion) of the surface of the body (the blades of hydraulic turbines, ship’s screws, and other hydraulic-engineering devices).
If the liquid were ideally uniform, and if the surface of the solid body with which it forms an interface were ideally wettable, then the rupture would occur at a pressure considerably lower than the saturated vapor pressure of the liquid. The tensile strength of water, with thermal fluctuations taken into account, is 150 meganewtons per sq m (MN/m2), or 1, 500 kilograms-force per sq cm (kgf/cm2). Real liquids are less strong. The maximum tensile strength of thoroughly purified water at 10°C is 28 MN/m2 (280 kgf/cm2). However, rupture usually occurs at pressures that are only slightly lower than the saturated vapor pressure. The low strength of real liquids is associatd with the presence in them of cavitation nuclei, which are poorly wettable regions of solids, solid particles with gas-filled cracks, microscopic gas bubbles preserved from dissolution by monomolecular organic layers, and ionic formations generated by cosmic rays.
For a particular shape of the streamlined body, cavitation arises at a certain definite value of the dimensionless parameter
K =2(p - ps)/pv2∞
that is completely defined for a particular point in the flow, where p is the hydrostatic pressure of the flow against the body, ps is the saturated vapor pressure, p is the density of the liquid, and v∞ is the velocity of the liquid at a sufficient distance from the body. This parameter is called the cavitation number, which is a similarity criterion in the simulation of hydrodynamic flows. An increase in the flow velocity after the start of cavitation leads to a rapid increase in the number of cavitational bubbles, which is followed by their combination into a single cavitation cavern, whereupon the flow becomes a jet flow. In this case the flow retains its non stationary nature only in the region of cavern collapse. Jet flow is formed particularly rapidly in the case of bluff bodies.
If atmospheric air or another gas is introduced into the cavern through a body in the vicinity of which cavitation is generated, the dimensions of the cavern increase. In this case a flow that corresponds to the cavitation number, found from the gas pressure pc within the cavern—that is, K = 2(p∞ — pc)/pv2∞ —rather than from the saturation pressure ps, will be established. The emergence of such a cavitational cavern will be described by the Froude number Fr = v2∞/gd, where g is gravitational acceleration and d is a certain characteristic linear dimension. Since pc may be much larger than ps, it is possible under such conditions, but at low velocities of the incident flow, to obtain fluxes corresponding to very low values of K—that is, to high degrees of development of cavitation. Thus, motion of a body through water with velocity of 6– m/sec may lead to flows around the body corresponding to velocities of up to 100 m/sec. Cavitation flows resulting from the supply of gas into the interior of the cavern are called artifical cavitation.
Hydrodynamic cavitation may be accompanied by a number of physicochemical effects, such as sparking and luminescence. The influence of electric currents and magnetic fields on cavitation that arises during flow around a cylinder in a water tunnel has been observed in a number of studies.
The study and prevention of cavitation are of great importance, since it has an adverse effect on the performance of hydraulic turbines, liquid pumps, ships’ screws, underwater sonic emitters, hydraulic systems of high-altitude aircraft, and other devices, decreases their efficiency, and produces damage. Cavitation may be reduced by increasing the hydrostatic pressure—for example, by placement of the device at a sufficient depth below the free surface of the liquid—as well as by appropriate choice of the corresponding design elements to decrease the harmful effects of cavitation. To reduce erosion of turbine blades, they are made from stainless steels and polished.
Experimental studies of cavitation are performed in cavitation tubes, which are ordinary water tunnels equipped with devices for static pressure control.
REFERENCESKornfel’d, M. Uprugos’ i prochnos’ zhidkostei. Moscow-Leningrad, 1951.
Birkhoff, G., and E. Sarantonello. Strui, sledy i kaverny. Moscow, 1964. (Translated from English.)
Pernik, A. D. Problemy kavitatsii, 2nd ed. Leningrad, 1966.
Osherovskii, S. Kh. “Kavitatsiia v generatorakh,” Energetika i elektrifikatsiia, 1970, no.1.
A. D. PERNIK
Acoustic cavitation. Irradiation of a liquid with sound having a sonic pressure amplitude in excess of a certain threshold value leads during the half-periods of rarefaction to the formation of cavitation bubbles on the cavitation nuclei, which consist most frequently of gaseous inclusions in the liquid, and on the oscillating surface of the acoustic emitter. For this reason, the cavitational threshold increases with decreasing gas content of the liquid, with increasing hydrostatic pressure, upon compression of the liquid by high hydrostatic pressure (of the order of 103kgf/cm2≅102 MN/m2), with cooling of the liquid, and with increasing frequency of the sound and decreasing duration of exposure. The threshold is higher for a traveling wave than for a standing wave. The bubbles collapse during the half-periods of compression, thus generating short-lived pulses (of the order of 10−6 sec) of pressure (up to 103 MN/m2 ≅ 104 kgf/cm2 and higher), which are capable of destroying very strong materials. Such destruction is observed at the surfaces of powerful acoustic emitters that are immersed in liquids. The pressure during the collapse of cavitational bubbles increases with decreasing sound frequency and increasing hydrostatic pressure; it is higher in liquids with low saturated vapor pressures. The collapse of bubbles is accompanied by adiabatic heating of the gas in the bubbles to temperatures of the order of 104 °C, which apparently leads to the luminescence of bubbles during cavitation (sonic luminescence)
Cavitation is accompanied by the ionization of gas in the bubbles. Cavitational bubbles group together to form cavitational regions of complex and variable shapes. The intensity of cavitation may be easily estimated from the destruction of thin aluminum foil in which holes are punched by the cavitating bubbles. The quantity and position of the holes formed during a fixed time interval make possible estimation of the intensity of cavitation and the configuration of the cavitation zone.
If the liquid is saturated with a gas, then the gas diffuses into the bubbles and prevents their complete collapse. The bubbles rise to the surface and thus reduce the gas content of the liquid. Intense oscillations of the gas-filled bubbles in the free liquid, as well as in the vicinity of solids, lead to micro flows of the liquid.
The occurrence of cavitation limits the possibility of further increase in the intensity of sound radiated into the liquid as a result of a decrease in its wave drag and the corresponding decrease in the load on the emitter. Acoustic cavitation and the physical phenomena connected with it generate a number of effects. Some of them, such as the breakdown and dispersion of solids, emulsification of liquids, and cleaning of surfaces and parts, are due to the impacts accompanying the collapse of bubbles and the micro flows in the vicinity of them. Other effects— for example, the initiation and acceleration of chemical reactions —are related to the ionization of gas in the bubbles. Because of these effects, cavitation is being used more widely in the development of new industrial processes and the advancement of existing processes. Many practical applications of ultrasound are based on the cavitation effect.
Acoustic cavitation is of great importance in biology and medicine. The pressure pulses that arise in cavitational bubbles lead to instantaneous disintegration of microorganisms and protozoa in aqueous mediums subjected to the action of ultrasound. Cavitation is used for the extraction of enzymes, hormones, and other biologically active substances from animal and plant cells.
REFERENCESBergmann, L. UVtrazvuk i ego primenenie v nauke i tekhnike. Moscow, 1956. (Translated from German.)
Roi, N. A. “Vozniknovenie i protekanie uPtrazvukovoi kavitatsii.” Akus-ticheskii zhurnal, 1957, vol. 3, issue 1, p. 3.
Sirotiuk, M. G. “Eksperimental’nye issledovaniia uPtrazvukovoi kavitatsii.” In Fizika i tekhnika moshchnogo uVtrazvuka, vol. 2. Moscow, 1968.
UVtrazvuk v gidrometallurgii. Moscow, 1969.
N. A. ROI