Extensive investigation of geometric effects in sonoreactors: Analysis by luminol mapping and comparison with numerical predictions.

This investigation focuses on the influence of geometric factors on cavitational activity within a 20kHz sonoreactor containing water. Three vessels with different shapes were used, and the transducer immersion depth and liquid height were varied, resulting in a total of 126 experiments conducted under constant driving current. For each one, the dissipated power was quantified using calorimetry, while luminol mapping was employed to identify the shape and location of cavitation zones. The raw images of blueish light emission were transformed into false colors and corrected to compensate for refraction by the water-glass and glass-air interfaces. Additionally, all configurations were simulated using a sonoreactor model that incorporates a nonlinear propagation of acoustic waves in cavitating liquids. A systematic visual comparison between luminol maps and color-plots displaying the computed bubble collapse temperature in bubbly regions was conducted. The calorimetric power exhibited a nearly constant yield of approximately 70% across all experiments, thus validating the transducer command strategy. However, the numerical predictions consistently overestimated the electrical and calorimetric powers by a factor of roughly 2, indicating an overestimation of dissipation in the cavitating liquid model. Geometric variations revealed non-monotonic relationships between transducer immersion depth and dissipated power, emphasizing the importance of geometric effects in sonoreactor. Complex features were revealed by luminol maps, exhibiting appearance, disappearance, and merging of different luminol zones. In certain parametric regions, the luminol bright regions are reminiscent of linear eigenmodes of the water/vessel system. In the complementary parametric space, these structures either combine with, or are obliterated by typical elongated axial structures. The latter were found to coincide with an increased calorimetric power, and are conjectured to result from a strong cavitation field beneath the transducer producing acoustic streaming. Similar methods were applied to an additional set of 57 experiments conducted under constant geometry but with varying current, and suggested that the transition to elongated structures occurs above some amplitude threshold. While the model partially reproduced some experimental observations, further refinement is required to accurately account for the intricate acoustic phenomena involved.

Simulations of a full sonoreactor accounting for cavitation.

In spite of the increasing interest in ultrasound processing applications, industrial scale-up remains limited, in particular by the unavailability of predictive computer tools. In this study, using a previously published model of cavitating liquids implementable as a non-linear Helmholtz equation, it is shown that a full sonoreactor can be modelled and simulated. The model includes the full transducer and the vibrations of the vessel walls, using the physics of elastic solids and piezo-electricity. The control-loop used by the generator to set the optimal frequency is also accounted for. Apart from the geometry, the unique input of the model is the current feeding the transducer whereas the dissipated electrical power, transducer complex impedance and working frequency are available as outputs. The model is put to the test against experiments realized in different geometries, varying either the input current or the transducer immersion depth. Despite the overestimation of the power dissipated in the liquid, the evolution of the acoustic load in both cases is reasonably well reproduced by simulation, which partially validates the method used.

Generation of subharmonics in acoustic resonators containing bubbly liquids: A numerical study of the excitation threshold and hysteretic behavior.

In this paper we study the generation and behavior of subharmonics in a bubbly liquid confined in an acoustic resonator, through numerical simulations carried out at finite-amplitude acoustic pressure. Several configurations in terms of resonator length and driving frequency are considered here. Our results show that these frequency components, created from a higher-frequency signal at the source (ultrasound), are due to the nonlinearity of the medium at high acoustic-pressure amplitude and to the configuration of the resonator (geometry and boundaries). We also show that they have an amplitude-threshold dependence, which is in concordance with the literature. The response of these subharmonics to different sequences of pressure amplitudes also reveals the hysteretic nature of the bubbly liquid.

Crystallization of α-glycine by anti-solvent assisted by ultrasound.

Crystallization of α-glycine by addition of an anti-solvent (ethanol) assisted by ultrasound is studied. The experiments of crystallization are conducted at 303.15 K in a solution of 150 ml with continuous agitation by a magnetic rod. Ultrasound is then applied at powers ranging from 8 to 41 W thanks to an ultrasonic horn at 20 kHz. The supersaturation ratio (S) is followed throughout all the experiment. At the end of the experiment, the suspension is filtered, the solid is washed with ethanol and dried at 333.15 K. The resulting crystals are characterized by their final size distributions measured by laser granulometry, their morphologies observed by scanning electronic microscope (SEM) and their crystalline structures by differential scanning calorimetry (DSC). The influence of ultrasonic power (continuous 13, 28 and 40 W or pulsed modes), measured by calorimetry method, is studied for different addition rates (0.05 to 0.36 g of ethanol/min). Ultrasound permits to reduce the metastable zone width and to decrease the size of crystals due to an increase of the nucleation rate. The rate of de-supersaturation is higher in presence of ultrasound, inducing a higher nucleation rate, a higher growth rate or both. At 40 W, the decrease of supersaturation is faster, and the crystallization is finished in 40 min instead of 80 min (at 13 and 28 W) or 120 min without ultrasound. The use of pulsed ultrasound (50 on/50 off) is interesting from an economic point of view because similar results are obtained: comparable size distributions and resembling concentration profiles.

20 kHz sonoelectrochemical degradation of perchloroethylene in sodium sulfate aqueous media: influence of the operational variables in batch mode.

A preliminary study of the 20 kHz sonoelectrochemical degradation of perchloroethylene in aqueous sodium sulfate has been carried out using controlled current density degradation sonoelectrolyses in batch mode. An important improvement in the viability of the sonochemical process is achieved when the electrochemistry is implemented, but the improvement of the electrochemical treatment is lower when the 20 kHz ultrasound field is simultaneously used. A fractional conversion of 100% and degradation efficiency around 55% are obtained independently of the ultrasound power used. The current efficiency is also enhanced compared to the electrochemical treatment and a higher speciation is also detected; the main volatile compounds produced in the electrochemical and sonochemical treatment, trichloroethylene and dichloroethylene, are not only totally degraded, but also at shorter times than in the sonochemical or electrochemical treatments.

Analytical expressions for primary Bjerknes force on inertial cavitation bubbles.

The primary Bjerknes force is responsible for the quick translational motion of radially oscillating bubbles in a sound field. The problem is classical in the case of small-amplitude oscillations, for which an analytical expression of the force can be easily obtained, and predicts attraction of sub-resonant bubbles by pressure antinodes. But for high-amplitude sound fields the bubbles undergo large-amplitude nonlinear oscillations, so that no analytical expression for the force is available in this case. The bubble dynamics is approximated on physical grounds, following the method of Hilgenfeldt [J. Fluid Mech. 365, 171 (1998)], but carefully accounting for surface tension. The analytical expression of the maximum radius of the bubble is recovered, the time of maximum expansion is noticeably refined, and an estimation of the collapse time is found. An analytical expression for the time-varying bubble volume is deduced, and the Bjerknes force is obtained in closed form. The result is valid for any shape of the sound field, including purely standing or purely traveling waves, and is ready to use in a theoretical model of bubble cloud evolution. In addition, the well-known sign inversion of the Bjerknes force for large standing waves is recovered and the inversion threshold in the parameter space is obtained analytically. The results are in good agreement with numerical simulations and allow a quantitative assessment of the effects of physical parameters. It is found that either reduction of the surface tension or increase in the static pressure should produce a widening of the bubble-free region near high-amplitude pressure antinodes.

Growth by rectified diffusion of strongly acoustically forced gas bubbles in nearly saturated liquids.

The growth or dissolution of small gas bubbles (R0<15 microm) by rectified diffusion in nearly saturated liquids, subject to low frequencies (20 kHz