Cavitation properties

Theory Cavitational properties of ultrasound accelerate those organic reactions which involving free radical mechanism, hydrolysis, protonation, etc. However the sonochemical effects are negative for those reactions in which one of the reactants is volatile. 

In these studies, we used unfocused ultrasound with a frequency of 0.88 MHz and an intensity of 2 W/cm. Cavitation properties of gels containing impurities and without them were comparatively estimated using an IS-3 MS cavitation indicator developed at the Belarusian State University of Informatics and Radioelectronics. The device s operating principle is based on the measurement of the level of scattered noise in a frequency band from 2 to 10 MHz, received by a broadband hydrophone. An emitter 2 cm in diameter was in direct contact with the face of the cylindrical gel sample (2 cm high and 2.5 cm in diameter), that is, measurements were performed in the transducer s near field. The hydrophone with a sensitive element 2 mm in diameter was placed in the immediate vicinity from the side surface of the sample. 

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). 

The abrasion resistance of cobalt-base alloys generally depends on the hardness of the carbide phases and/or the metal matrix. For the complex mechanisms of soHd-particle and slurry erosion, however, generalizations cannot be made, although for the soHd-particle erosion, ductihty may be a factor. For hquid-droplet or cavitation erosion the performance of a material is largely dependent on abiUty to absorb the shock (stress) waves without microscopic fracture occurring. In cobalt-base wear alloys, it has been found that carbide volume fraction, hence, bulk hardness, has Httie effect on resistance to Hquid-droplet and cavitation erosion (32). Much more important are the properties of the matrix. 

The actual shape of the curve illustrated in Fig. 12.4 can vary according to factors such as metal properties and cavitation intensity. (Cavitation intensity relates to the number of bubbles created in a unit volume of fluid and the amount of energy transferred during the col-... 

The essential condition governing cavitation is an unstable fluid flow. An unstable fluid flow is, in turn, affected by the properties of the fluid... 

Normal reheat treatment can partially restore blade properties however, it does not appear to be capable of full property recovery, although the miscrostructures are comparable to new blades. This shortcoming implies that cavitation may be present and was not removed by conventional reheat treatment. Hot isostatic press (HIP) processing is an alternative that ensures void removal. It has demonstrated its ability to remove even gross internal shrinkage porosity in investment castings. The results of HIP treatment... 

Probably the most important single property of hydraulic oil is its viscosity. The most suitable viscosity for a hydraulic system is determined by the needs of the pump and the circuit too low a viscosity induces back-leakage and lowers the pumping efficiency while too high a viscosity can cause overheating, pump starvation and possibly cavitation. 

Applications Plasma spraying is used to apply coatings for protection against wear and corrosion, to prevent erosion or cavitations, and to provide electrical insulation or conductivity. It can also be employed to produce bearing surfaces, abrasive properties or resistance to wetting by molten metals. The coatings can also be applied to facilitate the joining of different materials. 

We can divide the surface contact properties of polymers into two categories those in which the surface remains intact when it comes into contact with another surface and the polymer surface s resistance to damage. Friction and cling fall into the first category. Surface damage can be caused by erosion, abrasion, or cavitation. 

The important liquid phase physicochemical properties which affect the cavitation phenomena and hence the extent of cavitational effects for the given application include vapor pressure, viscosity and surface tension. 

Both chemical and physical properties of the reaction medium will dictate the required level of cavitation power. High viscosity media with low vapor pressure will require higher energy to generate cavitation. The presence of entrained or evolved gases will facilitate cavitation, as will the presence or generation of solid particles. 

In the sonochemical reactors, selection of suitable operating parameters such as the intensity and the frequency of ultrasound and the vapor pressure of the cavitating media is an essential factor as the bubble behavior and hence the yields of sonochemical transformation are significantly altered due to these parameters. It is necessary that both the frequency and intensity of irradiation should not be increased beyond an optimum value, which is also a function of the type of the application and the equipment under consideration. The liquid phase physicochemical properties should be adjusted in such a way that generation of cavitation events is eased and also large number of smaller size cavities are formed in the system. 

Table 3.1 Optimum operating conditions for the hydrodynamic cavitation reactors No. Property Favorable conditions...

The uses of inorganic metal compounds and rare gases to probe the conditions of cavitation collapse have become some of the most important methods available in fundamental ultrasonics. Quantitative determination of collapse temperatures and pressures, and qualitative determination of fundamental aspects of the nature of the cavitation field have been achieved, largely through SL spectroscopic methods. The presence of salts has a marked influence on properties on the acoustic systems, such as the extent of coalescence and bubble size, and the sonochemical activity and SL intensity. 

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