Intensity cavitation

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-... 

Resilient materials such as rubber and some plastics may be useful in certain applications, especially under conditions of low cavitation intensities. However, such materials are subject to disbondment at the metal and elastomer interface at high cavitation intensities, even if the exposure is brief. 

If the cavitation intensity is low and corrosion is a significant accelerating factor, appropriate inhibitors can be useful. Notable successes have been achieved with diesel engine cylinder liners. 

Cavitation is more or less continuous, but the cavitation intensity is so low that only the coating of corrosion products is dislodged. Resistance of the corrosion products to cavitation relative to the resistance of the metal to cavitation is low, making the corrosion products more susceptible to damage. 

Note that localized corrosion having the appearance illustrated in Figs. 12.18 through 12.20 could be associated with brief exposure to a strong acid. In this case, however, all available information indicated that the tubes had never been exposed to an acid of any type. Cavitation was caused by high-frequency vibration of the tubes. The vibration apparently induced a threshold cavitation intensity such that rough or irregular surfaces produced cavitation bubbles, and smooth internal surfaces did not. 

The pump has experienced graphitic corrosion. Figures 17.10, 17.12, and 17.14 illustrate typical appearances of graphitically corroded cast iron. In addition, cavitation damage (see Chap. 12) has produced severe metal loss in specific areas (see Fig. 17.13). The soft, friable corrosion products produced by graphitic corrosion are susceptible to cavitation damage at relatively low levels of cavitation intensity. 

Graphitic corrosion of the cast iron produced a soft, mechanically weak corrosion product that is highly susceptible to cavitation damage, even at relatively low cavitation intensities. The black coating on the impeller surface is visual evidence of graphitic corrosion. The spongelike surface contours are typical of cavitation damage (see Chap. 12). 

Thermal conductivity of Ar(0.01772 W/mK) is higher than Xe (0.00565 W/mK) and hence the loss of heat from bubble containing Ar is more rapid as compared to Xe. So the bubble temperature with Xe is always higher than bubble with Ar gas, resulting in higher cavitational intensity. 

Intensification can be achieved using this approach of combination of cavitation and advanced oxidation process such as use of hydrogen peroxide, ozone and photocatalytic oxidation, only for chemical synthesis applications where free radical attack is the governing mechanism. For reactions governed by pyrolysis type mechanism, use of process intensifying parameters which result in overall increase in the cavitational intensity such as solid particles, sparging of gases etc. is recommended. 

Design of sonochemical reactors is a very important parameter in deciding the net cavitational effects. Use of multiple transducers and multiple frequencies with possibility of variable power dissipation is recommended. Theoretical analysis for predicting the cavitational activity distribution is recommended for optimization of the geometry of the reactor including the transducer locations in the case of multiple transducer reactors. Use of process intensifying parameters at zones with minimum cavitational intensity should help in enhancing the net cavitational effects. 

Chivate MM, Pandit AB (1995) Quantification of cavitation intensity in fluid bulk. Ultrason Sonochem 2 S19-S25... 

Moholkar VS, Sable SP, Pandit AB (2000) Mapping the cavitation intensity in an ultrasonic bath using the acoustic emission. AIChE J 46 684—694... 

A dimensionless number known as the cavitation number (Cv) has generally been used to relate the flow conditions with the cavitation intensity [8] ... 

Moholkar et al. [11] studied the effect of operating parameters, viz. recovery pressure and time of recovery in the case of hydrodynamic cavitation reactors and the frequency and intensity of irradiation in the case of acoustic cavitation reactors, on the cavity behavior. From their study, it can be seen that the increase in the frequency of irradiation and reduction in the time of the pressure recovery result in an increment in the lifetime of the cavity, whereas amplitude of cavity oscillations increases with an increase in the intensity of ultrasonic irradiation and the recovery pressure and the rate of pressure recovery. Thus, it can be said that the intensity of ultrasound in the case of acoustic cavitation and the recovery pressure in the case of hydrodynamic cavitation are analogous to each other. Similarly, the frequency of the ultrasound and the time or rate of pressure recovery, are analogous to each other. Thus, it is clear that hydrodynamic cavitation can also be used for carrying out so called sonochemical transformations and the desired/sufficient cavitation intensities can be obtained using proper geometric and operating conditions. 

High pressure homogenizers are especially suitable for the emulsification processes in the food, pharmaceutical and bioprocess industries. A general disadvantage of these type of reactors is that there is no precise control over the cavitationally active volume and the magnitude of the pressure pulses that will be generated at the end of the cavitation events (cavitational intensity), unless the valve seat designs are substantially modified. 

In the case of a venturi flow, the most economical technique for increasing cavitation intensity would be to reduce the length of venturi, but for higher volumetric flow rates there could be a limitation due to the possibility of flow instability and super-cavitation. A similar argument can be given for the enhancement in the cavitation intensity by reducing the venturi throat to pipe diameter ratio. 

As the frequency increases the pulsation and collapse of the bubble occurs more rapidly and more radicals escape from the bubble. However as the frequency increases the cavitation intensity decreases and this reduces the yield of radicals and consequently the number which reach the interface and bulk solution. 

This section briefly discusses the factors that significantly affect cavity formation and cavitational intensity (i.e., the temperature and pressure generated during collapse). 

The presence of dissolved gas is essential for cavitation to occur in a liquid. The dissolved gas molecules disrupt intermolecular bonding between solvent molecules and hence, serve as nucleation sites for cavitation. There are three properties of dissolved gases that have significant influence on the degree of nucleation and cavitational intensity solubility of gas in the liquid, ratio of specific heats (y or Cp/Cy), and thermal conductivity (2). More soluble gases reduce the cavitational effects because the bubbles formed redissolve... 

Only the first two types of cavitation are of suitable intensity for chemical or physical processing. In the case of cavitation reactors, two aspects of cavity dynamics are ofmain importance, the maximum size reached by the cavity before its violent collapse and the life of the cavity. The maximum size reached by the cavity determines the magnitude of the pressure pulse produced on the collapse and hence the cavitation intensity that can be obtained in the system. The life of the cavity determines the distance traveled by the cavity from the point where it is generated before the collapse and hence it is a measure of the active volume of the reactor in which the actual cavitational effects are observed. 

In another (somewhat different) approach, a probability density function (PDF) has been proposed (Moholkar and Pandit, 1997). This is used to map the cavity dynamics in the reaction medium covering all three phases of a cavity s lifetime growth, oscillation, and collapse. An ultrasonic reactor is considered highly efficient if the PDF shows peaks in the collapse regime at all of the locations in the cavitation field. This is an indication that pressure pulses exist throughout the medium and are not restricted to just a few locations. In other words, the cavitational intensity is uniformly distributed. If peaks occur in the growth and collapse regimes, it is desirable to place the reactor inside the sonicated medium at a location where the maximum probability of collapse is indicated. 

A rise in the discharge pressure and hence in the final recovered pressure results in an increase in the active cavitational volume downstream from the orifice, with increased cavitational intensity. 

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