Immersion transducer

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 ... 

With an increase in the diameter of immersion transducer relative to the reactor diameter, the cavitational activity increases till an optimum ratio, usually dependent on the application [51,52], The ratio mainly affects the level of turbulent... 

An immersible transducer is a radiating device accommodated in a housing which can be submerged in a liquid bath to energize the liquid and produce cavitation. An immersible transducer placed in a still tank therefore turns that tank into an ultrasonic cleaner. The immersible transducer is, in effect, a standard tank (turned inside out) with the radiating surface on the outside and the transducers on the inside. 

One inch immersion transducers (Panametrics) are used with resonance frequencies of respectively 0.48 MHz., 0.70 MHz., 0.90 MHz. and 4.0 MHz. These transducers can be used for measurements between -50°C and 80°C. The two transducers shown in Figure 4.9 are special high temperature transducers (resonance frequency 1.0 MHz.) which can be used up to 200°C. 

Large-scale batch sonochemical reactors can be designed on the basis of the performance of conventional laboratory sonicators if it is assumed that there are no serious scale-up factors. These are the cleaning bath reactors (indirect sonication) and reactors with immersible transducers or sonic probes (direct sonication). Continuous reactors use either wall-mounted transducers (indirect sonication) or sonic probes (direct sonication). 

Several designs are shown in Figure 22.12 (a) common batch cleaning bath reactor with wall mounted transducers (b) batch reactor with immersible transducers (c) batch reactor with sonic probe (d) continuous flow tubular reactor with wall-mounted transducers (e) the Harwell sonochemical reactor and (f) a shell-and-tube reactor. A number of other designs are discussed by Thompson and Doraiswamy (1999b). 

Reactors with immersible transducers (Figure 22.12b) give higher intensities and can be retrofitted easily. On the other hand, a mechanical stirrer cannot easily be used with certain types of transducers. As a result, the acoustic (and... 

Computations show (Goodwin, 1990) that this cost is much less than that for the cleaning bath reactor or for the reactor with immersible transducers. A practical problem with this reactor is that the tips are exposed to a very small fraction of the liquid, hence a very efficient method of circulation is necessary. 

The properties of the piezocomposite material mentioned above offer special benefits when the transducer is coupled to a material of low acoustic impedance. This especially applies to probes having plastic delay lines or wedges and to immersion and medical probes. These probes with piezocomposite elements can be designed to have not only a high sensitivity but also at the same time an excellent resolution and, in addition, the effort required for the probe s mechanical damping can be reduced. 

For immersion probes we also get similar improvements using piezocomposite transducers as demonstrated by the third example. In Fig. 8 we compare pulse form and frequency spectrum for a 2 MHz probe Z2K with 10 mm transducer diameter. The echo of the composite probe has 11 dB more amplitude and is clearly shorter than for the old design, also indicated by the increase in bandwidth from 45 to 76 %. 

Fig. 8 Pulse shape (top) and spectrum (bottom) for a 2 MHz immersion probe with PZT (left) and composite transducer (right)...

The Champ-Sons model is a most effieient tool allowing quantitative predictions of the field radiated by arbitrary transducers and possibly complex interfaces. It allows one to easily define the complete set of transducer characteristics (shape of the piezoelectric element, planar or focused lens, contact or immersion, single or multi-element), the excitation pulse (possibly an experimentally measured signal), to define the characteristics of the testing configuration (geometry of the piece, transducer position relatively to the piece, characteristics of both the coupling medium and the piece), and finally to define the calculation to run (field-points position, acoustical quantity considered). 

The method is now applied to an object "o" consisting of an aluminum rod immersed in a tank and fixed at the supposed center of the bench the section of the rod being smaller than the central wavelength the size of the square section of the rod is 4 mm and the wavelength is 6 mm (the central frequency of the transducer is 250 kHz). Figure la gives the time-sinogram of the object without correction ... 

Figure 1 Examples of anay transducers, a/ Dual anay probe for plate inspection b/ Immersion 45° SW focused anay probe for inner tube inspection, c/ Immersion encircling probe for tube inspection...

Two major sources of ultrasound are employed, namely ultrasonic baths and ultrasonic immersion hom probes [79, 71]- The fonuer consists of fixed-frequency transducers beneath the exterior of the bath unit filled with water in which the electrochemical cell is then fixed. Alternatively, the metal bath is coated and directly employed as electrochemical cell, but m both cases the results strongly depend on the position and design of the set-up. The ultrasonic horn transducer, on the other hand, is a transducer provided with an electrically conducting tip (often Ti6A14V), which is inuuersed in a three-electrode thenuostatted cell to a depth of 1-2 cm directly facing the electrode surface. 

The transducers are typically mounted on an outside wall of the cleaning tank, but may also be mounted on the inside of the tank below the solution level in a sealed container. Alkaline cleaning solutions are typically at the same concentration and temperature as for a normal immersion cleaner, but the time required to clean may be less because of the ultrasonic effect. Like electrocleaning, ultrasonic cleaning produces an extremely clean surface. The main drawback is the relatively high cost. 

Probe systems, also called as the ultrasonic horn are being most frequently used for the sonochemical research at laboratory scale of operation. A typical schematic representation of the setup of probe systems has been given in Fig. 2.5. These are typically immersion type of transducers and the most important advantage of using... 

Immersion types of transducers are poorest, when scale-up possibilities are considered, though very high intensities (pressures of the order of few thousands atmosphere) are observed very near to the horn. The intensity decreases exponentially as one moves away from hom and vanishes at a distance of 1-3 cm both in axial as well as radial directions depending on the maximum power input to the equipment and also on the operating frequency [11,32]. 

The extent of immersion of the transducer in an ultrasonic horn or the extent of liquid height, which affects the extent of reflection of the incident sound waves from the liquid surface as well as the reactor bottom, also shows an optimum value [53]. 

Recently, a very practical bubble pressure tensiometer was developed using elegant pressure transducer mechanics which only needs one capillary made from a high-tech polymer [51, 52]. The tensiometer is able to measure at different immersion depths but needs calibration in order to make the resulting data comparable to surface tension values from other sources. It was shown in a series of measure-... 

Laboratory rotors have been used to evaluate the drag resistance of immersed surfaces as well. The rotational frequency of the innermost cylinder is adjusted to the relevant speed (monitored by a tachometer) and after about 30 minutes of flow stabilisation a 50 Nm strain gauge transducer picks up the torque, M,. All measurements are based on the average of three replicate tests. A sketch of the set-up is shown in Figure 13 details on the set-up and calculations are described in Weinell et al. (2003). 

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