Low frequency power ultrasound (20 kHz to 1 MHz) is more and more used in chemical or electrochemical applications. The main effect of ultrasound is due to the increase of mass transfer nearby the electrode. The major effects are observed when the electrode is positioned in the axis of the transducer, for distances lower than 3 cm. However, the intense cavitation which occurs in this zone as well as the presence of the electrode are at the origin of a extremely perturbed hydrodynamics behavior which it is indispensable to characterize to optimize the sonoelectrochemical processes into development. The purpose of this study is to measure the speed of the hydrodynamics flow generated by ultrasound according to the distance between the electrode and the transducer, in various geometrical configurations (horn diameters and cell designs). In the same time, visualizations and speed determinations are carried out by optical techniques (laser tomography and Particle Image Velocimetry). Results are compared with those obtained by electrochemical techniques which consist in measuring the mass transfer coefficient of the reversible couple Fe_II/Fe_III at a millimeter-length platinum electrode surface. Mass transfer coefficients are converted in a equivalent upward flow speed, according to the Levich model hypothesis, to allow a direct comparison with the velocity vector measures obtained by PIV. Two kinds of reactors with a comparable internal diameter (60 mm) were characterized. The first one is the Compton Cell, a small cylindrical reactor (90 mm height) which allows adapting transducers of diameter 6 and 12 mm. The second is the cylindrical reactor, developed in our laboratory with a higher height(190 mm) and which allows to work with a wider range of transducers (6, 12 and 25 mm in diameter) by a different system of fixation from that of the Compton Cell. The flow velocities measurements turn out very different from a reactor to the other one. In the Compton Cell we observe a strong attenuation of the velocities with the horn-electrode distance. For example, the velocity measured at 1 mm away from the transducer is of the order of 10 ms^-1, varying slightly with the ultrasound power for the 6 mm diameter horn. It is divided by 10 for only a distance of 15 mm. Curves can be easily simulated by a simple attenuation law. The increase of the transducer diameter (12 mm) leads to an increase of the flow velocity by a 3 times factor. In the cylindrical reactor, the order of magnitude of the velocities is quite similar, but the variations do not follow a simple attenuation law. A sinusoidal phenomenon is observed, which makes more complex the modelling and leads to less control of the stirring conditions.

Keywords: low frequency ultrasound - sonoelectrochemistry - velocity determination

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