Distribution of cavitational activity

There exists a wide distribution of cavitational activity in the sonoreactor with the maximum intensity observed just above the center of the transducer (for the standard arrangements such as ultrasonic horn/bath). The intensity varies both axially as well as in the radial direction with decreasing trends as we go away from the transducer... 

Prediction of cavitational activity distribution based on theoretical analysis of the bubble dynamics equations can be used to identify the regions with maximum pressure fields in a large scale reactor and then may be small reactors can... 

Fig. 2.10 Classification of different types of techniques for understanding cavitational activity distribution...

Cavitation medium gets disturbed due to the presence of external instrument such as thermocouple, hydrophone, aluminum foil, test tube etc. and hence we may not get a realistic picture of the cavitational activity distribution... 

The reactor design in terms of ratio of the diameter of the immersion transducer to reactor diameter, liquid height, position of the transducers and characteristics of the cell plays a important role in deciding the cavitational activity distribution and hence the efficacy of sonochemical reactors for the specific application. Based on a critical analysis of the existing literature, following important design related information can be recommended ... 

The position of the transducers in reactors based on the multiple frequency arrangement should be done in such a way that maximum and uniform cavita-tional activity is obtained. Theoretical analysis of the cavitational activity distribution as discussed earlier aids in arriving at an optimum location of the transducers. Similar argument holds true for the geometry of the reactor. 

At times the net rates of chemical/physical processing achieved using ultrasonic irradiations are not sufficient so as to prompt towards industrial scale operation of sonochemical reactors. This is even more important due to the possibility of uneven distribution of the cavitational activity in the large scale reactors as discussed... 

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. 

Geometry of the reactor Number of cavitational events and cavitational activity distribution Higher number of transducers of optimum shape so as to achieve uniform cavitational activity in the reactor... 

Cavitational activity was found to be maximum at the center axis of the transducer and diminishing away from the centerline in the same plane and also as one moves away from the transducers in the direction perpendicular to the tank bottom. For the case of a homogenous distribution of the bubbles, the influence on the pressure fields was negligible for a bubble fraction < 10 whereas above this value a remarkable reduction in the pressure amplitude was observed. For reactors of type 1 and 3, intensity at the opposite end nearly vanishes whereas considerable attenuation was observed for the reactor 2. For very high volume fractions of gas/vapor (0.2), the intensity vanishes even near the transducer surface. 

An attempt has also been made to evaluate the efficacy in terms of mapping of the cavitational activity at different radial and axial locations in the reactor, as uniform distribution of the cavitational activity is a must for an efficient large-scale operation. The local pressure amplitudes have been measured using a hydrophone (Bruel Kjaer Ltd., Type 8103, Denmark). The details of the measurement techniques can be obtained in the earlier study of Kanthale et al. (2003). 

It could be seen in that there is a direct correlation between diameter of the horn tip and size/size distribution of the microspheres. Smaller the horn tip diameter narrower the size distribution and smaller the average size of microspheres. In order to understand the mechanism for such a correlation, the active cavitation zone was analysed for these systems using sonochemiluminescence images (Fig. 2.15). 

A threshold of interfacial adhesion between both phases is needed to (a) promote the cavitation mechanism and (b) activate the crack-bridging mechanism. For rubbery particles, the former contributes much more than the latter to the total fracture energy. Adhesion is achieved by the use of functionalized rubbers that become covalently bonded to the matrix. Higher toughness values have been reported by the use of functionalized rubbers (Kinloch, 1989 Huang et al., 1993b). However, these experimental results also reflect the effect of other changes (particle size distribution,... 

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