Cavitation

Cavitation damage (sometimes referred to as cavitation corrosion or cavitation erosion) is a form of localized corrosion combined with mechanical damage that occurs in turbulent or rapidly moving liquids and takes the form of areas or patches of pitted or roughened surface. Cavitation has been defined as the deterioration of a surface caused by the sudden formation and collapse of bubbles in a liquid. It has been similarly defined as the localized attack that results from the collapse of voids or cavities due to turbulence in a liquid at a metal surface. Cavitation also occurs in areas of high vibration such as on engine pistons and piston liners (Fig. 6.43). 

In some instances, cathodic protection has been successful in reducing or preventing cavitation damage, but because cavitation damage usually involves physical as well as electrochemical processes, it cannot always be prevented by this means. In some cases, inhibitors have been used successfully to limit cavitation corrosion, as in the water side of diesel engine cylinder liners. Cavitation is a problem with ship propellers, hydraulic pumps and turbines, valves, orifice 

Cavitation damage of a diesei engine piston iiner on the return stroke. (Courtesy of Defence R D Canada-Atiantic) 

Strictly speaking, the Bernoulli equation applies to flow along a streamline, however, in turbulent flow the bulk flow velocity can be used with little error. Thus the increase in the velocity as the liquid is accelerated through an orifice or over an impeller can result in a drop of the local static pressure. As the liquid slows down, after it passes a vena contracta or approaches the volute in a pump, the pressure rises again, leading to the collapse of the cavities formed by the previous drop in pressure (Fig. 6.39). Five different types of cavitation can be observed depending on the flow conditions and geometry [35,36]. 

The cavitation number ( r) is a dimensionless number that provides an estimate of cavitation tendency in a flowing stream as described in Eq. (6.9)  

Cavitation is the formation of cavities. This phenomenon has been found to take place in ceramics containing a glass phase. The final creep fracture in this case results from the accumulation of cavities. The factors controlling this kind of creep are the microstructure, volume of glass phase, temperature, and applied stress. These factors give rise to bulk and localized damage. 

Cavity nucleation in viscous grain boundary film under a tensile stress. 

Comparing equation (7.12) with equation (7.7), it is clear that at cavitation 

The value of Kc at fully open for a rotary valve, Kc, is considerably lower, typically 0.25. Now, however, the location of the effective throat and the flow field downstream of it are both dependent on valve travel. This explains the dependence of Kc on valve travel, X, for rotary valves, where Kc has been found to obey approximately the following equation set  

Good control will normally require the rotary valve to operate somewhere in mid-range. Substituting x = 0.5 in equation set (7.14) gives KdKc = 1.8. For a rotary valve with Kc = 0.26 (say), this implies Kc =0.46, substantially lower than for a globe valve. 

It may be seen from the foregoing that the friction coefficient, Cf, is less than unity at the point of liquid flashing for both globe and rotary values. It follows from equation (7.10) that the throat pressure will be lower than the outlet pressure at the onset of cavitation. In fact, inequality (7.2) contained in Section 7.3 will normally hold throughout the non-cavitating range also. 

In certain conditions, a thin layer of liquid, nearly static at the metal-liquid interface, can prevent impingement of the surface by the turbulent flow of the liquid. However, 

It is important to darify that the pump does not eavitate, although people in the industry tend to say that the pump is eavitating. It is more correct to say that the pump is in eavitation or the pump is suffering eavitation. In realit - it is the system that cavitates the pump, beeause the system controls the pump. 

Inadequate NPSHa establishes favorable conditions for cavitation in the pump. If the pressure in the eye of the impeller falls below the vapor pressure of the fluid, then cavitation can begin. 

The vapor pressure of a liquid is the absolute pressure at which the liquid vaporizes or converts into a gas at a specific temperature. Normally, the units are expressed in pounds per square inch absolute (psia). The vapor pressure of a liquid increases with its temperature. For this reason the temperature should be specified for a declared vapor pressure. 

At sea level, water normally boils at 212°F. If the pressure should increase above 14.7 psia, as in a boiler or pressure vessel, then the boiling point of the water also inereases. If the pressure decreases, then the water s boiling point also decreases. For example in the Andes Mountains at 15,000 ft (4,600 meters) above sea level, normal atmospheric pressure is about 8.3 psia instead of 14.7 psia water would boil at 184°F. 

Inside the pump, the pressure deereases in the eye of the impeller beeause the fluid veloeity inereases. For this reason the liquid ean boil at a lower pressure. For example, if the absolute pressure at the impeller eye should fall to 1.0 psia, then water eould boil or vaporize at about 100°F (see the Tables in Chapter 2 Properties of Water I and II). 

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Classic nucleation theory must be modified for nucleation near a critical point. Observed supercooling and superheating far exceeds that predicted by conventional theory and McGraw and Reiss [36] pointed out that if a usually neglected excluded volume term is retained the free energy of the critical nucleus increases considerably. As noted by Derjaguin [37], a similar problem occurs in the theory of cavitation. In binary systems the composition of the nuclei will differ from that of the bulk... 

Birkin P R, O Connor R, Rapple C and SilvaMartinez S 1998 Electrochemical measurement of erosion from individual cavitation events generated from continuous ultrasound J. Chem. See., Faraday Trans. 94 3365... 

Cavitation damage is a fonn of deterioration associated with materials in rapidly moving liquid environments, due to collapse of cavities (or vapour bubbles) in the liquid at a solid-liquid interface, in the high-pressure regions of high flow. If the liquid in movement is corrosive towards the metal, the damage of the metal may be greatly increased (cavitation corrosion). 

Brennen C E 1995 Cavitation and Bubbie Dynamics (New York Oxford University Press)... 

Hammitt F G 1980 Cavitation and Muitiphase Fiow Phenomena (New York McGraw-Hill)... 

There are several effects present in the region where the molecule meets the solvent shell. The first is referred to as a cavitation energy, which is the energy required to push aside the solvent molecules, thus making a cavity in... 

Calix-cavitates Calixciowns Calixin Calixpodands Calixspherands C. alkanolyticum... 

Sonochemistry can be roughly divided into categories based on the nature of the cavitation event homogeneous sonochemistry of hquids, heterogeneous sonochemistry of hquid—hquid or hquid—sohd systems, and sonocatalysis (which overlaps the first two) (12—15). In some cases, ultrasonic irradiation can increase reactivity by nearly a million-fold (16). Because cavitation can only occur in hquids, chemical reactions are not generaUy seen in the ultrasonic irradiation of sohds or sohd-gas systems. 

Fig. 1. Transient acoustic cavitation the origin of sonochemistry and sonoluminescence.

Microjet Formation during Cavitation at Liquid—Solid Interfaces... 

Fig. 3. Liquid jet produced during collapse of a cavitation bubble near a solid surface. The width of the bubble is about 1 mm.

For both aqueous and nonaqueous liquids, MBSL is caused by chemical reactions of high energy species formed duriag cavitation by bubble coUapse, and its principal source is most probably not blackbody radiation or electrical discharge. MBSL is predominandy a form of chemiluminescence. 

Spectroscopic Probes of Cavitation Conditions. Determination of the temperatures reached ia a cavitating bubble has remained a difficult experimental problem. As a spectroscopic probe of the cavitation event, MBSL provides a solution. High resolution MBSL spectra from sUicone oU under Ar have been reported and analy2ed (7). The observed emission comes from excited state has been modeled with synthetic spectra as a... 

Increases in the appHed static pressure increase the acoustic intensity necessary for cavitation, but if equal number of cavitation events occur, the coUapse should be more intense. In contrast, as the ambient pressure is reduced, eventuaUy the gas-fiUed crevices of particulate matter which serve as nucleation sites for the formation of cavitation in even "pure" Hquids, wiU be deactivated, and therefore the observed sonochemistry wiU be diminished. 

The choice of the solvent also has a profound influence on the observed sonochemistry. The effect of vapor pressure has already been mentioned. Other Hquid properties, such as surface tension and viscosity, wiU alter the threshold of cavitation, but this is generaUy a minor concern. The chemical reactivity of the solvent is often much more important. No solvent is inert under the high temperature conditions of cavitation (50). One may minimize this problem, however, by using robust solvents that have low vapor pressures so as to minimize their concentration in the vapor phase of the cavitation event. Alternatively, one may wish to take advantage of such secondary reactions, for example, by using halocarbons for sonochemical halogenations. With ultrasonic irradiations in water, the observed aqueous sonochemistry is dominated by secondary reactions of OH- and H- formed from the sonolysis of water vapor in the cavitation zone (51—53). 

Control of sonochemical reactions is subject to the same limitation that any thermal process has the Boltzmann energy distribution means that the energy per individual molecule wiU vary widely. One does have easy control, however, over the energetics of cavitation through the parameters of acoustic intensity, temperature, ambient gas, and solvent choice. The thermal conductivity of the ambient gas (eg, a variable He/Ar atmosphere) and the overaU solvent vapor pressure provide easy methods for the experimental control of the peak temperatures generated during the cavitational coUapse. 

The phenomenon of acoustic cavitation results in an enormous concentration of energy. If one considers the energy density in an acoustic field that produces cavitation and that in the coUapsed cavitation bubble, there is an amplification factor of over eleven orders of magnitude. The enormous local temperatures and pressures so created result in phenomena such as sonochemistry and sonoluminescence and provide a unique means for fundamental studies of chemistry and physics under extreme conditions. A diverse set of apphcations of ultrasound to enhancing chemical reactivity has been explored, with important apphcations in mixed-phase synthesis, materials chemistry, and biomedical uses. 

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