ABSTRACT
Since the last decades, extensive work have been done on the numerical modeling of mono-frequency sonoreactors, we here consider the modeling of dual-frequency sonoreactors. We first present the basic features of the CAMUS code (CAvitating Medium under UltraSound), for mono-frequency excitation. Computation at low, medium and high frequency are presented. Extension of the numerical tool CAMUS is also presented: Caflisch equations are modified to take into account the dual-frequency excitation of the sound. We consider 28-56, 28-100 and 28-200 kHz sonoreactors. Fields of cavitation bubble emergence are quite different from the ones under mono-frequency. Study of spatio-temporal dynamics of cavitation bubbles in a 28-56 kHz sonoreactor is also considered. Taking into account the pressure field induced by the dual-frequency wave propagation, we compute the Bjerknes force applied on the cavitation bubble that is responsible for the bubble migration. A two phase flow approach allows to compute the bubble migration.
ABSTRACT
The propagation of ultrasound through a liquid induces the growth of inceptions and germs into bubbles. In a low frequency reactor, fragmentary transient bubbles emerge due to the acoustic driving. They violently collapse in one cycle and fragment into many smaller bubbles than in turn cavitate. This violent collapse is responsible for the mechanical effects of ultrasounds effects. The latter bubbles gather in a ball-shaped cloud and migrate to pressure antinodes. During their migration, their nonexplosive collapses mainly contribute to activate chemical reactions by producing OH. radicals. Mathematical modelling is performed as a new approach to predict the bubbles field. Through numerical simulation, we determinate emergence sites of mechanically active cavitation bubbles. Calculus are compared with aluminium foil degradation. The modelling of bubble migration allow us to have an insight on the privileged sites of the chemical reactions. Validation of the modelling is made through direct comparison with chemiluminescence photo. All experiments and computations are made in a 28.2 kHz sonoreactor.
ABSTRACT
The use of high frequency ultrasound in chemical systems is of major interest to optimize chemical procedures. Characterization of an open air 477 kHz ultrasound reactor shows that, because of the collapse of transient cavitation bubbles and pulsation of stable cavitation bubbles, chemical reactions are enhanced. Numerical modelling is undertaken to determine the spatio-temporal evolution of cavitation bubbles. The calculus of the emergence of cavitation bubbles due to the acoustic driving (by taking into account interactions between the sound field and bubbles' distribution) gives a cartography of bubbles' emergence within the reactor. Computation of their motion induced by the pressure gradients occurring in the reactor show that they migrate to the pressure nodes. Computed bubbles levitation sites gives a cartography of the chemical activity of ultrasound. Modelling of stable cavitation bubbles' motion induced by the motion of the liquid gives some insight on degassing phenomena.
