Importance of acoustic shielding in sonochemistry

Abstract

It is well known that sonochemistry is less efficient at high acoustic intensities. Many authors have attributed this effect to decoupling losses and shielding of the acoustic wave. In this study we investigate both phenomena for a 20 kHz ultrasound field with an intensity ranging from 40 to 150 W/cm2. Visualization of the bubble cloud has demonstrated that the void fraction below the ultrasound horn increases more than proportional with increasing power input. Nevertheless, the energy coupling between the horn and the liquid remains constant; this implies that decoupling losses are not reinforced for larger bubble clouds. On the contrary, microphone measurements have shown that due to the larger bubble cloud a substantial part of the supplied energy is lost at high power inputs. In striving towards more efficient sonochemistry, reduction of shielding appears as one of the major challenges.

Synchrotron quantification of ultrasound cavitation and bubble dynamics in Al–10Cu melts

“…The ultrasonic processing parameters used in this work are summarized in Table I. As a result, an output pressure of 4.5 MPa is effectively generated on the tip of the sonotrode in the melt, calculated using the analytical model in [23]. …”

“…The captured bubbles in each frame in this work might have already experienced a few hundred oscillations if not imploding since the exposure time used in this work is 25 ms, followed by attaching to the container surface. This cavitation activity could be considered as 'stable cavitation' in terms of lifetime according to Leighton's definition [43], in which case the observed cavitation bubbles exist at relatively low ultrasonic intensities (the acoustic intensity is 800 W/cm 2 at the tip of the transducer but this intensity quickly dissipates inside the cavitation zone and attenuates in the melt [23]), and oscillate for many acoustic cycles with increasing radius before reaching their maximum size followed by their dissolution, implosion or flotation if not touching the container.…”

Dynamics of two interacting hydrogen bubbles in liquid aluminum under the influence of a strong acoustic field

“…A 20 mm preheated sonotrode was immersed at a depth h = 20 mm below the free surface. The melt was treated with ultrasound at a power of 3.5 kW, corresponding to a pressure of 1.0 MPa below the radiating horn [32] and a forcing frequency of 17.7 kHz. The frequency emissions have been measured with a calibrated cavitometer [30] with the tip immersed 5 cm below the horn (Figure 4a) and outside the crucible with an ultrasound microphone (Figure 4b).…”

Effect of Ultrasonic Irradiation on the Treatment of Poly-Aromatic Substances (PAHs) from a Petrochemical Industry Wastewater

“…Gases with lower thermal conductivity (18.4 mW/mol.K) values such as N 2 (g) increased the temperature up to 49 • C inside the cavitation bubble upon collapse because they allowed less heat to the surroundings such as 37 • C (Dewulf, 2001;Van Lersel, 2008). The ratio of specific heat plays a role in determining the maximum size of the cavitation bubble.…”