Cavitation Ultrasonic horn

Horn design is a very important aspect of ultrasonic engineering. The vibrational amplitude of the piezoelectric crystal itself is normally so small that the intensity of sonication attainable by direct coupling of the transducer to the chemical system is not large enough to cause cavitation. The horn acts as an amplifier for the vibration of the transducer and the precise shape of the horn will determine the gain or mechanical amplification of the vibration. It is for this reason that it is sometimes referred to as a... 

An ultrasonic horn transducer consists of a transducer unit attached to a horn (rod) usually made from titanium alloy and which has a length a multiple of half-wavelengths of the sound wave. For the commonly encountered 20-kHz horn this corresponds to 12.5 cm. The horn is then partially inserted into the fluid medium of interest and intense ultrasound is generated at its tip so that, for adequately large intensities, a cloud of cavitation bubbles is visible. This arrangement permits significantly higher ultrasonic intensities (10-1000 W cm ) to be applied than are achievable with a bath. 

The astute observer wiU see that the fluidized bed crystaUization system shown in Fig. 11-21 has an unusual feature. Flow sonication units are in position to operate on pumped slurry from the seed beds. In other systems, the sonicators are located intemaUy in the bottom of the column. The sonifiers are power ultrasonic horns, which create sufficient cavitation and impact energy to break the crystals along cleavage planes. 

Figure 48.1(b) shows the population density of crystals filmed under increasing ultrasonic power output levels. A single 0.1-sec pulse of ultrasound was applied at — 3.8°C in a 15wt% sucrose solution. The growing crystals are shovm 0.40 sec later. These results highlight that a commercial ultrasonic horn can be used to control the nucleation and the size distribution of ice crystals produced within the supercooled liquid. Further results by the first author have shown that as the power output level is increased, the level of transient cavitation increases (Chow-McGarva, 2004). This increase in transient cavitation could be correlated to an increase in the primary nucleation events and thus explain the increase in the observed number of crystals. 

A detailed study of the nucleation of ice by power ultrasoimd has been performed using a variety of high-speed photography systems with a particular focus on the influence of cavitation. The nucleation of ice has been shown to occur predominantly within the bubble cloud produced by a commercial ultrasonic horn. An investigation of a single oscillating bubble has confirmed that ice crystals are nucleated in the immediate vicinity of the bubble. 

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... 

Fig. 2.7 Acoustic cavitation-induced polymerisation of a monomer to generate polymer latex particles using 20 kHz ultrasonic horn [61]. The shear forces generates emulsion droplets. Radicals generated within cavitation bubbles react with monomer molecules to generate monomer radicals that diffuse into monomer droplets to initiate polymerisation process...

There are cavitation erosion tests (21,22) that can be used to assess the resistance of plastics to this form of wear, but there are few laboratories performing these tests and fewer use plastics as test specimens. The original ASTM cavitation erosion test required affixing the test specimen on the end of an ultrasonic horn. This would not work well on plastics. The second generation test used a stationary specimen placed near the tip of a vibrating ultrasonic horn. This test could be used on plastics. The latest test uses a high- pressure water jet to create a cavitation field. 

On a laboratory scale, generally an ultrasonic probe (horn) and an ultrasonic cleaner are used. The ultrasonic field in an ultrasonic cleaner is not homogeneous. Sonication extraction uses ultrasonic frequencies to disrupt or detach the target analyte from the matrix. Horn type sonic probes operate at pulsed powers of 400-600 W in the sample solvent container. Ultrasonic extraction works by agitating the solution and producing cavitation in the... 

Fig. 3.15. Sonoelectrochemical cell used to ultrasonicate a rotating disc electrode. A titanium amplifying horn, B cavitational plume, C Pt disc working electrode, D Ag/AgCI reference electrode, E auxiliary electrode, F fine porosity glass frit, G coolant inlet, H coolant outlet. (Reproduced with permission of the American Chemical Society.)...

McLean and Mortimer [187] have studied the variations in HO free radical production during the sonication of aqueous solutions at different powers at 970 kHz. A typical curve is given in Figure 36. From this it is clear that a threshold exists for radical production, after which there is a linear correlation with acoustic power up to a limiting value which probably corresponds with surface cavitation . Acoustic power was calibrated with a radiation balance and a PVDF hydrophone. Repeatability on experiments performed on the same day was less than 15%, but day-to-day variations could be as much as 50%, probably mainly due to small uncontrolled changes in the alignment of the reaction chamber (a test tube dipped in a water tank) with the ultrasonic source which was an acoustic horn. 

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