Transducers liquid-driven

Liquid-driven transducers (i.e. a liquid whistle) can be used to produce efficient homogenization. The majority of the chemical effects observed using whistle-type transducers for the sonication of non-homogeneous reactions can be attributed mainly to the generation of very fine emulsions leading to increase in the interfacial phenomena rather than the ultrasonic irradiation itself. 

Liquid-driven transducers are effectively liquid whistles where a liquid is forced across a thin blade which causes the blade to vibrate (Figure 10.2). The induced vibration then creates cavitation in the liquid. An analogy for this action is the cavitation which is produced by a ship s propeller as it rapidly cleaves through water. 

The transducers operate at a fixed frequency of irradiation emitting radiations through a fixed area of irradiation. Thus, the type of transducer coupled with the total area of irradiation and the operating frequency are the key factors in the efficient design of the sonochemical reactors. The three main types of transducers are gas driven, liquid driven and electromechanical transducers out of which the electromechanical transducers are by far the most versatile and widely used. 

In sonochemistry, physical and chemical changes are brought about by the usage of power ultrasound. Three types of transducers are used to generate power ultrasound gas driven transducers, liquid driven... 

Damped oscillations no yes liquids and some dry products. Employs oscillating dement which is normally a vibrating fork or paddle driven mechanically (Fig. 6.33a) or by a piezoelectric crystal vibrating at its resonant frequency. When immersed in the material there is a frequency or amplitude shift due to viscous damping which is sensed usually by a reluctive transducer (Section 6.3.3). 

In the most common bath design, the ultrasonic transducer is attached to the underside of the metallic base of the bath and the strongest power level occurs directly above the transducer. However, the ultrasonic power profile in the bath liquid varies in a non-uniform manner with the distance from the base. This has been ascribed to the fact that US is driven through the liquid as waves, so there are points where the irradiation amplitude is maximal and others where it is zero. Maxima are always desirable and can be calculated as multiples of the half-wavelength of sound X) in the medium. Such distances can be calculated from the equation ... 

Fig. 5.18 Principle of a growing drop experiment according to Passerone et al. (1991) S - motor driven syringe, C - capillary, DPT - differential pressure transducer, LI and L2 - the two liquids...

Fig. 5.19 Principle of a drop pressure experiment according to MacLeod Radke (1993) C - capillary, M - motor driven syringe for liquid 1, LI - liquid 1, L2 - reservoir of liquid 1, PC - computer, V - video camera with objective, VR - video recorder, PT - pressure transducer, SAC - signal amplifier and converter...

Calculations and experiments have been used to develop a crevice reactor with a reduced amplitude of the sound-transmitting face, thus reducing cavitational erosion, but a similar energy density regarding the sonicated liquid volume. The sketch of the reactor can be seen in Fig. 8.1.19. The reactor is made up of an inner sound source. This inner wall is operated at 44kHz and oscillates like a breathing wall. The outer transducers are driven at 25 kHz. The total power uptake of both sound sources is 3kW for a liquid volume of 1.86 L. 

R, one equipped with force transducers and held fixed, and the other is then driven rotationally at some constant angular velocity (j>. The torque T and axial force T at steady state are recorded. For a small gap angle a the shear rate is y = /a and is the same everywhere in the liquid. The following relationships then apply [9, 23] ... 

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