Cavitation process

Figure 12.1 is a simplified representation of the cavitation process. Figure 12. L4 represents a vessel containing a liquid. The vessel is closed by an air-tight plunger. When the plunger is withdrawn (B), a partial vacuum is created above the liquid, causing vapor bubbles to form and grow within the liquid. In essence, the liquid boils without a temperature increase. If the plunger is then driven toward the surface of the liquid (C), the pressure in the liquid increases and the bubbles...

The cavitation damage in this spacer was due to vibrations from operation of the engine. The localized nature of the damage in this case is an illustration of a common feature of cavitation. Pits formed by initial cavitation damage become preferred sites for the development of subsequent cavitation bubble formation due to the jagged, irregular contours of the pit. This tends to localize and intensify the cavitation process, especially in later stages of pit development. 

Ultrasound can thus be used to enhance kinetics, flow, and mass and heat transfer. The overall results are that organic synthetic reactions show increased rate (sometimes even from hours to minutes, up to 25 times faster), and/or increased yield (tens of percentages, sometimes even starting from 0% yield in nonsonicated conditions). In multiphase systems, gas-liquid and solid-liquid mass transfer has been observed to increase by 5- and 20-fold, respectively [35]. Membrane fluxes have been enhanced by up to a factor of 8 [56]. Despite these results, use of acoustics, and ultrasound in particular, in chemical industry is mainly limited to the fields of cleaning and decontamination [55]. One of the main barriers to industrial application of sonochemical processes is control and scale-up of ultrasound concepts into operable processes. Therefore, a better understanding is required of the relation between a cavitation coUapse and chemical reactivity, as weU as a better understanding and reproducibility of the influence of various design and operational parameters on the cavitation process. Also, rehable mathematical models and scale-up procedures need to be developed [35, 54, 55]. 

Balasundaram B, Harrison STL (2006) Disruption of Brewers yeast by hydrodynamic cavitation Process variables and their influence on selective release. Biotech Bioeng 94 303-311... 

The effect of the bulk solution temperature lies primarily in its influence on the bubble content before collapse. With increasing temperature, in general, sonochemical reaction rates are slower. This reflects the dramatic influence which solvent vapor pressure has on the cavitation event the greater the solvent vapor pressure found within a bubble prior to collapse, the less effective the collapse. In fact, one can quantitate this relationship rather well (89). From simple hydrodynamic models of the cavitation process, Neppiras, for example, derives (26) the peak temperature generated during collapse of a gas-filled cavity as... 

Kruus also conducted experiments in the presence of the radical scavenger diphe-nylpicryhydracyl (DPPH) and observed induction periods which were roughly proportional to concentration of DPPH employed. This clearly demonstrates the free radical nature of the polymerisation. By assuming that each of the monomer radicals produced by the cavitation process (Eq. 5.30) reacted with one DPPH molecule, he was able to deduce the following kinetic relationship ... 

In addition to the cavitation process related to the presence of a dispersed phase, the formation of voids in the plastic zone has been observed to occur also in the matrix phase. Kinloch and Huang stated that the plastic void growth succeeding cavitation also contributes to energy absorption and might become as important as the shear banding, especially at fairly elevated temperatures [161]. 

Temperature dependency of the rate of destruction is determined by the effect of temperature upon the cavitation process there is an optimum temperature at which the intensity of cavitation is maximum similarly, the rate and depth of destruction depend upon the frequency of acoustic vibrations. 

A rapid movement of fluids caused by a variation of sonic pressure subjects the solvent to compression and rarefaction. This movement can be described as a motion that alternatively compresses and stretches the molecular structure within the cavitation process. This rapid movement of fluids is... 

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. 

The oscillating gas bubbles are a continuous source of free-radicals as long as they remain in the antinode area of the sonic field, but one has also to envisage a catastrophic collapse that generates in addition to the free radicals a number of smaller bubbles. These serve as further nucleation sites for subsequent cavitation processes. 

It was observed that, under equal conditions, the yields of copper complexes are always higher in comparison with those of nickel. An increase in donor force of the solvent applied leads to more rapid formation of complexes an increase in viscosity leads to its delay. According to the physical-chemical study, the formed products are the same as those prepared by conventional methods from corresponding metal salts and ligands. It was established that a multimolecular layer of crystalline product is formed in the border metal-solution. Diffusion of metal atoms takes place through this layer due to cavitation processes [738], Another application of ultrasonic treatment for optimization of traditional synthetic methods is presented in the Experimental Procedures at the end of this section. 

According to studies reported in Ref. 738, a multimolecular layer of the product is formed on the metal surface. Since for its formation the presence of metal atoms or ions on the border between liquid and solid phases is needed, a diffusion of metal atoms through the compound layer is a necessary condition for such layer formation. The cavitation processes on the surface contribute to this. Since an energetic barrier should be mastered in the reaction route, a cavitation ultrasonic action has the same importance as triboplasma formed by metal friction [756]. 

Turning to Eq. 8.6-2, we note that the superheat appears in the denominator of the exponent term. As pointed out earlier, under ordinary conditions the maximum superheat of a given system at a fixed temperature can be obtained by reducing Po to a minimum value close to zero. However, further decrease in the local value of Pq can be obtained by a cavitation process due to the tensile stresses generated in the moving boundaries of the macrobubble. Street (42) showed that the bubble surface is stretched (i.e., the t, , and stress components are negative), and that for a viscoelastic liquid they are given by... 

Fig. 12. Strain introduced into PZT crystallites of PbZro.5Tio.5O3 by cavitational processing in the double-orifice processor (CaviPro 300) as a function of different sets of orifices, shown on the left side of the figure. Similar data using the single orifice processor (CaviMax) are shown on the right side of the figure.

For removal of surface contamination, rust scale, mill scale, and other bound moieties by wet or dry abrasion (including surface layers of metal itself) by wet or dry abrasion, erosion, and/or cavitation processes... 

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