Cavitation phenomena

Fig. 17. Cavitation phenomenon in pumps showing cavitation bubble distribution and rate of weight loss as a function of cavitation coefficient at constant...

Ultrasonic irradiation of a liquid leads to the generation of cavitation phenomenon which comprised of unique reaction fields in addition to physical and mechanical effects the formation of micro-meter sized bubbles, formation of bubbles with high temperature and high pressure conditions, formation of shock waves, and strong micro-stirring effects are produced. Table 5.1 shows representative ultrasound techniques to synthesize inorganic and metal nanoparticles and nanostructured materials. 

Early in the study of cavitation phenomenon, Minnaert observed that scientists... 

As result of the research at BurMines, a mechanism for initiation under "card-gap" conditions was suggested and the alternate explanation for the off-the-charge-end "plasma" phenomenon reported by Cook et al (Ref 34b, pp 1881-92) was provided. The "off-end" phenomenon is briefly described in Ref 38f. A theory of initiation of liquid expls formulated at the BurMines is described in this section under Cavitational Phenomenon... 

The cavitation phenomenon offers at present considerable technical interest, first of all, in connection with the destruction of materials and energy losses that arise in hydraulic machines, propellers, etc., when the velocity is increased. 

Besides small gas bubbles, other nucleation sites (e.g., at minute dust particles) may give rise to the cavitation phenomenon. Normally, the surface tension of water is too high to allow the formation of water vapor bubbles at the relatively small negative pressures created by the sonic field. However, at the surface of the dust particles the surface tension of water may be sufficiently low to create a water vapor bubble in the sonic field and thus start the cavitation process. 

Finally, it is worth mentioning that the cavitation phenomenon observed in low viscosity liquids is also caused by (explosive) boiling induced by sudden reduction of pressure, such as that occurring in regions behind moving surfaces, such as impellers, or as the result of flow acceleration (Bernoulli effect) (23). 

The concentration of volatile compounds in the cavitation bubbles increases with temperature thus, faster degradation rates are observed at higher temperatures for those compounds [23]. Conversely, in the case of nonvolatile substrates (that react through radicals reactions in solution), the effect of temperature is somehow opposed to the chemical common sense. In these cases, an increase in the ambient reaction temperature results in an overall decrease in the sonochemical reaction rates [24]. The major effect of temperature on the cavitation phenomenon is achieved through the vapor pressure of the solvent. The presence of water vapor inside the cavity, although essential to the sonochemical phenomenon, reduces the amount of energy... 

This section examines variabies influencing the cavitation phenomenon in a manner that can be adjusted to fuifii specific purposes. 

Tests involving new, more sophisticated measurement tools have provided new interpretations and equations for the cavitation phenomenon [14,15]. The thermal and non-thermal effects of non-inertial cavitation, and the chemical and mechanical effects of Inertial cavitation in relation to their impact on ultrasound safety have recently been Investigated [16]. 

The nature of the transmitting iiquid must aiways be taken into account with uitrasonic baths, and aiso with probes when they are not to be dipped in the sampie vessei. The nature of the iiquid infiuences the cavitation phenomenon viz. the estimation of the zones that wiii receive the maximum irradiation ampiitude). 

The cavitation phenomenon inoreases the polarity of the leachant, analytes and matrix. This inoreases the leaohing efficiency, which is frequently similar to or greater than that provided by oonventional Soxhiet leaching. 

After a brief description of the cavitation phenomenon and commercially available devices for the production of ultrasound, this chapter discusses its principal applications... 

Sir John Thornycroft and Sydney W. Bamaby, in 1894, were the first to describe the cavitation phenomenon. During trials of a new high-speed British Navy ship, they ascribed the strong vibrations found and the erosion observed in the propeller to strong turbulence leading to the formation of cavitational bubbles. Although the source of cavitation was turbulence, high-intensity ultrasound has similar effects. 

The outside zone is used to store the reserve liquid precursor in a mass reservoir, which surrounds the preform in a fully immersed manner. Outside of the preform the liquid forms a turbulent boiling fluid with many bubbles due to cavitation and the outgoing gases. The cavitation phenomenon is a two-phase process in which bubbles or voids in the heated liquid precursor are formed. 

Solvent vapor pressure also has a significant effect on the cavitation phenomenon because the intensity of cavitation decreases as the vapor pressure of the solvent increases. This is because more vapor is enclosed in the microbubble, which cushions the collapse, leading to lower collapse temperatures and pressures. On the other hand, solvents with low vapor pressure tend not to diffuse into the growing microbubble thereby reducing the size of the bubble, which lessens the intensity of bubble collapse. Thus, a delicate balance of solvent properties must be achieved to attain the desired sonication conditions. 

In presence of a base, a-bromo acetals can be converted into ketene acetals, products difficult to obtain by the classical processes. The reaction was studied in the presence of KOH in solvent-free conditions. The effects of a phase transfer agent as well as that of ultrasound, a non-classical method of activation especially efficient in heterogeneous solid-liquid systems due to cavitation phenomenon, were studied (Table 6) [87]. 

Desinent cavitation is defined as the threshold cavitation number when the flow returns back to single-phase flows from cavitating conditions. The desinent cavitation number was considerably higher than the incipient cavitation number. Although the cavitation hysteresis (difference between incipient and desinent cavitation) phenomenon has been observed in macroscale [9, 10], its effects seem to be ameliorated with decrease in size of the system [8]. 

If the matrix is a biological tissue for which it is desired to disrupt the cells and homogenize the resulting lysate, ultrasonic disruption is widely used, exploiting the cavitation phenomenon discussed in Section 4.3.2c. That earlier discussion was mainly concerned with extracting analytes from slurries of small particles and by implication referred mainly to thermally stable analytes. When... 

High power ultrasound applied to an alkoxide/water mixture makes it possible to obtain nanostructured materials. The process is driven by the acoustic cavitation phenomenon. The cavities act as nanoreactors, where the hydrolysis reaction starts. The products (alcohol, water, and silanol) help continue the dissolution of that immiscible mixture. 

Creates micro jets in the air, the distribution of which is correlated with the propagation of the cavitation phenomenon. 

The first observations concerning the role of the gas bubbles existent into a liquid supposed to the ultrasounds action belong to Boyle and Lemann (1923) and V.C. Sorensen (1936). The first ones underlined that the gaseous bubbles can penetrate into the spaces formed by cavitation and discovered the tendency of sound to remove the gases dissolved in the liquid medium. The cavitation phenomenon depends on the external pressure that can increase up to a certain limit from which gas removal starts [1145]. According to V.C. Sorensen the removing of 1 cm of gas from saturated water requires 51.2 kW at 197 kHz, 72.6 and 87.4 kW at 380 kW and 530 kHz, respectively. Therefore, the gas removing essentially depends by the ultrasound intensity [1146]. 

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