Combining COMSOL modeling with acoustic pressure maps to design sono-reactors

Scaled-up and economically viable sonochemical systems are critical for increased use of ultrasound in environmental and chemical processing applications. In this study, computational simulations and acoustic pressure maps were used to design a larger-scale sono-reactor containing a multi-stepped ultrasonic horn. Simulations in COMSOL Multiphysics showed ultrasonic waves emitted from the horn neck and tip, generating multiple regions of high acoustic pressure. The volume of these regions surrounding the horn neck were larger compared with those below the horn tip. The simulated acoustic field was verified by acoustic pressure contour maps generated from hydrophone measurements in a plexiglass box filled with water. These acoustic pressure contour maps revealed an asymmetric and discrete distribution of acoustic pressure due to acoustic cavitation, wave interaction, and water movement by ultrasonic irradiation. The acoustic pressure contour maps were consistent with simulation results in terms of the effective scale of cavitation zones (∼10 cm and <5 cm above and below horn tip, respectively). With the mapped acoustic field and identified cavitation location, a cylindrically-shaped sono-reactor with a conical bottom was designed to evaluate the treatment capacity (∼5 L) for the multi-stepped horn using COMSOL simulations. In this study, verification of simulation results with experiments demonstrates that coupling of COMSOL simulations with hydrophone measurements is a simple, effective and reliable scientific method to evaluate reactor designs of ultrasonic systems.     

Designing and characterizing a multi-stepped ultrasonic horn for enhanced sonochemical performance

The commonly used ultrasonic horn generates localized cavitation below its converging tip resulting in a dense bubble cloud near the tip and limiting diffusion of reactive components into the bubble cloud or reactive radicals out of the bubble cloud. To improve contact between reactive components, a novel ultrasonic horn design was developed based on the principles of the dynamic wave equation. The horn, driven at 20 kHz, has a multi-stepped design with a cone-shaped tip increasing the energy-emitting surface areas and creating multiple reactive zones. Through different physical and chemical experiments, performance of the horn was compared to a typical horn driven at 20 kHz. Hydrophone measurements showed high acoustic pressure areas around the horn neck and tip. Sonochemiluminescence experiments verified multiple cavitation zones consistent with hydrophone readings. Calorimetry and dosimetry results demonstrated a higher energy efficiency (31.3%) and a larger hydroxyl radical formation rate constant (0.36 μM min⁻¹) compared to typical horns. In addition, the new horn degraded naphthalene faster than the typical horn tested. The characterization results demonstrate that the multi-stepped horn configuration has the potential to improve the performance of ultrasound as an advanced oxidation technology by increasing the cavitation zone in the solution.     

Visualization and optimization of cavitation activity at a solid surface in high frequency ultrasound fields

Despite the increasing use of high frequency ultrasound in heterogeneous reactions, knowledge about the spatial distribution of cavitation bubbles at the irradiated solid surface is still lacking. This gap hinders controllable surface sonoreactions. Here we present an optimization study of the cavitation bubble distribution at a solid sample using sonoluminescence and sonochemiluminescence imaging. The experiments were performed at three ultrasound frequencies, namely 580, 860 and 1142 kHz. We found that position and orientation of the sample to the transducer, as well as its material properties influence the distribution of active cavitation bubbles at the sample surface in the reactor. The reason is a significant modification of the acoustic field due to reflections and absorption of the ultrasonic wave by the solid. This is retraced by numerical simulations employing the Finite Element Method, yielding reasonable agreement of luminescent zones and high acoustic pressure amplitudes in 2D simulations. A homogeneous coverage of the test sample surface with cavitation is finally reached at nearly vertical inclination with respect to the incident wave.     

Pretreatment of defatted wheat germ proteins (by-products of flour mill industry) using ultrasonic horn and bath reactors: Effect on structure and preparation of ACE-inhibitory peptides

The ultrasonic horn and bath reactors were compared based on production of angiotensin-converting-enzyme (ACE) inhibitory peptides from defatted wheat germ proteins (DWGP). The DWGP was sonicated before hydrolysis by Alcalase. Degree of hydrolysis, ACE-inhibitory activity, surface hydrophobicity, fluorescence intensity, free sulfhydryl (SH), and disulfide bond (SS) were determined. The highest ACE-inhibitory activity of DWGP hydrolysate was obtained at power intensity of 191.1 W/cm² for 10 min in the ultrasonic horn reactor. The fixed frequency of 33 kHz and the sweep frequency of 40 ± 2 kHz resulted in the maximum ACE-inhibitory activity. The combined irradiation of dual fixed frequency (24/68 kHz) produced significant increase in ACE-inhibitory activity compared with single frequency (33 kHz). The ultrasonic probe resulted in significant higher ACE-inhibitory activity compared with ultrasonic bath operating at single or dual fixed and sweep frequencies. The changes in conformation of the DWGP due to sonication were confirmed by the changes in fluorescence intensity, surface hydrophobicity, SH_f and SS contents and they were found in conformity with the ACE-inhibitory activity in case of the ultrasonic horn reactor but not in bath reactor.     

Ultrasonic pretreatment of corn slurry for saccharification: A comparison of batch and continuous systems

The effects of ultrasound on corn slurry saccharification yield and particle size distribution was studied in both batch and continuous-flow ultrasonic systems operating at a frequency of 20 kHz. Ground corn slurry (28% w/v) was prepared and sonicated in batches at various amplitudes (192–320 μm_{peak-to-peak (p–p)}) for 20 or 40 s using a catenoidal horn. Continuous flow experiments were conducted by pumping corn slurry at various flow rates (10–28 l/min) through an ultrasonic reactor at constant amplitude of 12 μm_p–p. The reactor was equipped with a donut shaped horn. After ultrasonic treatment, commercial alpha- and gluco-amylases (STARGEN^TM 001) were added to the samples, and liquefaction and saccharification proceeded for 3 h. The sonicated samples were found to yield 2–3 times more reducing sugars than unsonicated controls. Although the continuous flow treatments released less reducing sugar compared to the batch systems, the continuous flow process was more energy efficient. The reduction of particle size due to sonication was approximately proportional to the dissipated ultrasonic energy regardless of the type of system used. Scanning electron microscopy (SEM) images were also used to observe the disruption of corn particles after sonication. Overall, the study suggests that both batch and continuous ultrasonication enhanced saccharification yields and reduced the particle size of corn slurry. However, due to the large volume involve in full scale processes, an ultrasonic continuous system is recommended.     

Towards an understanding and control of cavitation activity in 1 MHz ultrasound fields

Various industrial processes such as sonochemical processing and ultrasonic cleaning strongly rely on the phenomenon of acoustic cavitation. As the occurrence of acoustic cavitation is incorporating a multitude of interdependent effects, the amount of cavitation activity in a vessel is strongly depending on the ultrasonic process conditions. It is therefore crucial to quantify cavitation activity as a function of the process parameters. At 1 MHz, the active cavitation bubbles are so small that it is becoming difficult to observe them in a direct way. Hence, another metrology based on secondary effects of acoustic cavitation is more suitable to study cavitation activity. In this paper we present a detailed analysis of acoustic cavitation phenomena at 1 MHz ultrasound by means of time-resolved measurements of sonoluminescence, cavitation noise, and synchronized high-speed stroboscopic Schlieren imaging. It is shown that a correlation exists between sonoluminescence, and the ultraharmonic and broadband signals extracted from the cavitation noise spectra. The signals can be utilized to characterize different regimes of cavitation activity at different acoustic power densities. When cavitation activity sets on, the aforementioned signals correlate to fluctuations in the Schlieren contrast as well as the number of nucleated bubbles extracted from the Schlieren Images. This additionally proves that signals extracted from cavitation noise spectra truly represent a measure for cavitation activity. The cyclic behavior of cavitation activity is investigated and related to the evolution of the bubble populations in the ultrasonic tank. It is shown that cavitation activity is strongly linked to the occurrence of fast-moving bubbles. The origin of this “bubble streamers” is investigated and their role in the initialization and propagation of cavitation activity throughout the sonicated liquid is discussed. Finally, it is shown that bubble activity can be stabilized and enhanced by the use of pulsed ultrasound by conserving and recycling active bubbles between subsequent pulsing cycles.     

Modeling cavitation in a rapidly changing pressure field – Application to a small ultrasonic horn

Ultrasonic horn transducers are frequently used in applications of acoustic cavitation in liquids. It has been observed that if the horn tip is sufficiently small and driven at high amplitude, cavitation is very strong, and the tip can be covered entirely by the gas/vapor phase for longer time intervals. A peculiar dynamics of the attached cavity can emerge with expansion and collapse at a self-generated frequency in the subharmonic range, i.e. below the acoustic driving frequency. The term “acoustic supercavitation” was proposed for this type of cavitation Žnidarčič et al. (2014) [1].We tested several established hydrodynamic cavitation models on this problem, but none of them was able to correctly predict the flow features. As a specific characteristic of such acoustic cavitation problems lies in the rapidly changing driving pressures, we present an improved approach to cavitation modeling, which does not neglect the second derivatives in the Rayleigh–Plesset equation. Comparison with measurements of acoustic supercavitation at an ultrasonic horn of 20 kHz frequency revealed a good agreement in terms of cavity dynamics, cavity volume and emitted pressure pulsations.The newly developed cavitation model is particularly suited for simulation of cavitating flow in highly fluctuating driving pressure fields.     

Sonodisruption of re-assembled casein micelles at different pH values

Casein solutions with different pH values were sonicated at a frequency of 35 kHz and increasing acoustic powers. As the sonication power increased, turbidity of solutions and particle diameter decreased at any given pH value, suggesting particles disruption due to the ultrasonic treatment. The magnitude of decrease in re-assembled micelles diameter was greater at a higher pH, indicating an interaction between pH and sonication power in sonodissociation. This interaction is attributed to a looser structure of micelles at higher pH values which increases the efficiency of ultrasonic disruption and not directly to the increased cavitation efficiency. We argue that increased cavitation efficiency with increasing sonication power, which enhances shear forces is the most likely reason for sonodisruption of re-assembled casein micelles.     

Understanding the ultrasound field of high viscosity mixtures: Experimental and numerical investigation of a lab scale batch reactor

In this work, mixtures of increasing viscosity (from 0.9 to ≈720 mPas) are sonicated directly using an ultrasonic horn at 30 kHz to investigate the effect of viscosity on the ultrasound field both from an experimental and numerical point of view. The viscosity of the mixtures is modified by preparing water-polyethylene glycol solutions. The impact of the higher viscosity on the acoustic pressure distribution is studied qualitatively and semi-quantitatively using sonochemiluminescence. The velocity of light scattering particles added in the mixtures is also explored to quantify acoustic streaming effects using Particle Image Velocimetry (PIV). A numerical model is developed that is able to predict cavitationally active zones accounting for both thermoviscous and cavitation based attenuation. The results show that two cavitation zones exist: one directly under the horn tip and one around the part of the horn body that is immersed in the liquid. The erosion patterns on aluminum foil confirm the existence of both zones. The intensity of the cavitationally active zones decreases considerably with increasing viscosity of the solutions. A similar reduction trend is observed for the velocity of the particles contained in the jet directly under the tip of the horn. Less erratic flow patterns relate to the high viscosity mixtures tested. Finally, two numerical models were made combining different boundary conditions related to the ultrasonic horn. Only the model that includes the radial horn movements is able to qualitatively predict well the location of the cavitation zones and the decrease of the zones intensity, for the highest viscosities studied. The current findings should be taken into consideration in the design and modelling phase of horn based sonochemical reactors.     

Generation and control of acoustic cavitation structure

The generation and control of acoustic cavitation structure are a prerequisite for application of cavitation in the field of ultrasonic sonochemistry and ultrasonic cleaning. The generation and control of several typical acoustic cavitation structures (conical bubble structure, smoker, acoustic Lichtenberg figure, tailing bubble structure, jet-induced bubble structures) in a 20–50 kHz ultrasonic field are investigated. Cavitation bubbles tend to move along the direction of pressure drop in the region in front of radiating surface, which are the premise and the foundation of some strong acoustic cavitation structure formation. The nuclei source of above-mentioned acoustic cavitation structures is analyzed. The relationship and mutual transformation of these acoustic cavitation structures are discussed.     

Sonochemical characterisation of ultrasonic dental descalers

An ultrasonic dental descaling instrument has been characterised using sonochemical techniques. Mapping the emission from luminol solution revealed the distribution of cavitation produced in water around the tips. Hydroxyl radical production rates arising from water sonolysis were measured using terephthalate dosimetry and found to be in the range of μmol min⁻¹, comparable with those from a sonochemical horn. Removal of an ink coating from a glass slide showed that cleaning occurred primarily where the tip contacted the surface but was also observed in regions where cavitation occurred even when the tip did not contact the surface. Differences in behaviour were noted between different tip designs and computer simulation of the acoustic pressure distributions using COMSOL showed the reasons behind the different behaviour of the tip designs.     

Ultrasonic processing of dairy systems in large scale reactors

High intensity low frequency ultrasound was used to process dairy ingredients to improve functional properties. Based on a number of lab-scale experiments, several experimental parameters were optimised for processing large volumes of whey and casein-based dairy systems in pilot scale ultrasonic reactors. A continuous sonication process at 20 kHz capable of delivering up to 4 kW of power with a flow-through reactor design was used to treat dairy ingredients at flow rates ranging from 200 to 6000 mL/min. Dairy ingredients treated by ultrasound included reconstituted whey protein concentrate (WPC), whey protein and milk protein retentates and calcium caseinate. The sonication of solutions with a contact time of less than 1 min and up to 2.4 min led to a significant reduction in the viscosity of materials containing 18% to 54% (w/w) solids. The viscosity of aqueous dairy ingredients treated with ultrasound was reduced by between 6% and 50% depending greatly on the composition, processing history, acoustic power and contact time. A notable improvement in the gel strength of sonicated and heat coagulated dairy systems was also observed. When sonication was combined with a pre-heat treatment of 80 °C for 1 min or 85 °C for 30 s, the heat stability of the dairy ingredients containing whey proteins was significantly improved. The effect of sonication was attributed mainly to physical forces generated through acoustic cavitation as supported by particle size reduction in response to sonication. As a result, the gelling properties and heat stability aspects of sonicated dairy ingredients were maintained after spray drying and reconstitution. Overall, the sonication procedure for processing dairy systems may be used to improve process efficiency, improve throughput and develop value added ingredients with the potential to deliver economical benefits to the dairy industry.     

Cavitation mapping by sonochemiluminescence with less bubble displacement induced by acoustic radiation force in a 1.2 MHz HIFU

An acoustic radiation force counterbalanced appliance was employed to map the cavitation distribution in water. The appliance was made up of a focused ultrasound transducer and an aluminum alloy reflector with the exactly same shape. They were centrosymmetry around the focus of the source transducer. Spatial–temporal dynamics of cavitation bubble clouds in the 1.2 MHz ultrasonic field within this appliance were observed in water. And they were mapped by sonochemiluminescence (SCL) recordings and high-speed photography. There were significant differences in spatial distribution and temporal evolution between normal group and counterbalanced group. The reflector could avoid bubble directional displacement induced by acoustic radiation force under certain electric power (⩽50 W). As a result, the SCL intensity in the pre-focal region was larger than that of normal group. In event of high electric power (⩾70 W), most of the bubbles were moving in acoustic streaming. When electric power decreased, bubbles kept stable and showed stripe structure in SCL images. Both stationary bubbles and moving bubbles have been captured, and exhibited analytical potential with respect to bubbles in therapeutic ultrasound.     

Electromechanical,acoustical and thermodynamical characterization of a low-frequency sonotrode-type transducer in a small sonoreactor at different excitation levels and loading conditions

The paper reports and compares the results of the electromechanical, acoustical and thermodynamical characterization of a low-frequency sonotrode-type ultrasonic device inside a small sonoreactor, immersed in three different loading media, namely, water, juice and milk, excited at different excitation levels, both below and above the cavitation threshold. The electroacoustic efficiency factor determined at system resonance through electromechanical characterization in degassed water as the reference medium is 88.7% for the device in question. This efficiency can be reduced up to three times due to the existence of a complex sound field in the reactor in linear driving conditions below the cavitation threshold. The behaviour of the system is more stable at higher excitation levels than in linear operating conditions.During acoustical characterization, acoustic pressure is spatially averaged, both below and above the cavitation threshold. The standing wave patterns inside the sonoreactor have a stronger influence on the variation of the spatially distributed RMS pressure in linear operating conditions. For these conditions, the variation of ±1.7 dB was obtained, compared to ±1.4 dB obtained in highly nonlinear regime. The acoustic power in the sonoreactor was estimated from the magnitude of the averaged RMS pressure, and from the reverberation time of the sonoreactor as the representation of the losses. The electroacoustic efficiency factors obtained through acoustical and electromechanical characterization are in a very good agreement at low excitation levels. The irradiated acoustic power estimated in nonlinear conditions differs from the dissipated acoustic power determined with the calorimetric method by several orders of magnitude.The number of negative pressure peaks that represent transient cavitation decreases over time during longer treatments of a medium with high-power ultrasound. The number of negative peaks decreases faster when the medium and the vessel are allowed to heat up.     

An objective comparison of commercially-available cavitation meters

With a number of cavitation meters on the market which claim to characterise fields in ultrasonic cleaning baths, this paper provides an objective comparison of a selection of these devices and establishes the extent to which their claims are met. The National Physical Laboratory’s multi-frequency ultrasonic reference vessel provided the stable 21.06 kHz field, above and below the inertial cavitation threshold, as a test bed for the sensor comparison. Measurements from these devices were evaluated in relation to the known acoustic pressure distribution in the cavitating vessel as a means of identifying the mode of operation of the sensors and to examine the particular indicator of cavitation activity which they deliver. Through the comparison with megahertz filtered acoustic signals generated by inertial cavitation, it was determined that the majority of the cavitation meters used in this study responded to acoustic pressure generated by the direct applied acoustic field and therefore tended to overestimate the occurrence of cavitation within the vessel, giving non-zero responses under conditions when there was known to be no inertial cavitation occurring with the reference vessel. This has implications for interpreting the data they provide in user applications.     

Effects of argon sparging rate,ultrasonic power,and frequency on multibubble sonoluminescence spectra and bubble dynamics in NaCl aqueous solutions

The sonoluminescence spectra from acoustic cavitation in aqueous NaCl solutions are systematically studied in a large range of ultrasonic frequencies under variation of electrical power and argon sparging. At the same time, bubble dynamics are analysed by high-speed imaging. Sodium line and continuum emission are evaluated for acoustic driving at 34.5, 90, 150, 365, and 945 kHz in the same reactor vessel. The results show that the ratio of sodium line to continuum emission can be shifted by the experimental parameters: an increase in the argon flow increases the ratio, while an increase in power leads to a decrease. At 945 kHz, the sodium line is drastically reduced, while the continuum stays at elevated level. Bubble observations reveal a remarkable effect of argon in terms of bubble distribution and stability: larger bubbles of non-spherical shapes form and eject small daughter bubbles which in turn populate the whole liquid. As a consequence, the bubble interactions (splitting, merging) appear enhanced which supports a link between non-spherical bubble dynamics and sodium line emission.     

Dependence of cavitation,chemical effect,and mechanical effect thresholds on ultrasonic frequency

Cavitation, chemical effect, and mechanical effect thresholds were investigated in wide frequency ranges from 22 to 4880 kHz. Each threshold was measured in terms of sound pressure at fundamental frequency. Broadband noise emitted from acoustic cavitation bubbles was detected by a hydrophone to determine the cavitation threshold. Potassium iodide oxidation caused by acoustic cavitation was used to quantify the chemical effect threshold. The ultrasonic erosion of aluminum foil was conducted to estimate the mechanical effect threshold. The cavitation, chemical effect, and mechanical effect thresholds increased with increasing frequency. The chemical effect threshold was close to the cavitation threshold for all frequencies. At low frequency below 98 kHz, the mechanical effect threshold was nearly equal to the cavitation threshold. However, the mechanical effect threshold was greatly higher than the cavitation threshold at high frequency. In addition, the thresholds of the second harmonic and the first ultraharmonic signals were measured to detect bubble occurrence. The threshold of the second harmonic approximated to the cavitation threshold below 1000 kHz. On the other hand, the threshold of the first ultraharmonic was higher than the cavitation threshold below 98 kHz and near to the cavitation threshold at high frequency.     

Enhancement of cavitation activity and particle removal with pulsed high frequency ultrasound and supersaturation

Megasonic cleaning as applied in leading edge semiconductor device manufacturing strongly relies on the phenomenon of acoustic cavitation. As the occurrence of acoustic cavitation is incorporating a multitude of interdependent effects, the amount of cavitation activity in the cleaning liquid strongly depends on the sonication conditions. It is shown that cavitation activity as measured by means of ultraharmonic cavitation noise can be significantly enhanced when pulsed sonication is applied to a gas supersaturated liquid under traveling wave conditions. It is demonstrated that this enhancement coincides with a dramatic increase in particle removal and is therefore of great interest for megasonic cleaning applications. It is demonstrated that the optimal pulse parameters are determined by the dissolution time of the active bubbles, whereas the amount of cavitation activity depends on the ratio between pulse-off and pulse-on time as well as the applied acoustic power. The optimal pulse-off time is independent of the corresponding pulse-on time but increases significantly with increasing gas concentration. We show that on the other hand, supersaturation is needed to enable acoustic cavitation at aforementioned conditions, but has to be kept below values, for which active bubbles cannot dissolve anymore and are therefore lost during subsequent pulses. For the applicable range of gas contents between 100% and 130% saturation, the optimal pulse-off time reaches values between 150 and 340 ms, respectively. Full particle removal of 78 nm-diameter silica particles at a power density of 0.67 W/cm² is obtained for the optimal pulse-off times. The optimal pulse-off time values are derived from the dissolution time of bubbles with a radius of 3.3 μm and verified experimentally. The bubble radius used in the calculations corresponds to the linear resonance size in a 928 kHz sound field, which demonstrates that the recycling of active bubbles is the main enhancement mechanism. The optimal choice of the pulsing conditions however is constrained by the trade-off between the effective sonication time and the desire to have a sufficient amount of active bubbles at lower powers, which might be necessary if very delicate structures have to be cleaned.     

Numerical simulation of liquid velocity distribution in a sonochemical reactor

Ultrasonically induced flow is an important phenomenon observed in a sonochemical reactor. It controls the mass transport of sonochemical reaction and enhances the reaction performance. In the present paper, the liquid velocity distribution of ultrasonically induced flow in the sonochemical reactor with a transducer at frequency of 490 kHz has been numerically simulated. From the comparison of simulation results and experimental data, the ultrasonic absorption coefficient in the sonochemical reactor has been evaluated. To simulate the liquid velocity near the liquid surface above the transducer, which is the main sonochemical reaction area, it is necessary to include the acoustic fountain shape into the computational domain. The simulation results indicate that the liquid velocity increases with acoustic power. The variation of liquid height also influences the behavior of liquid velocity distribution and the mean velocity above the transducer centre becomes a maximum when the liquid height is 0.4 m. The liquid velocity decreases with increasing the transducer plate radius at the same ultrasonic power.     

Interaction behaviors at the interface between liquid Al–Si and solid Ti–6Al–4V in ultrasonic-assisted brazing in air

Power ultrasonic vibration (20 kHz, 6 μm) was applied to assist the interaction between a liquid Al–Si alloy and solid Ti–6Al–4V substrate in air. The interaction behaviors, including breakage of the oxide film on the Ti–6Al–4V surface, chemical dissolution of solid Ti–6Al–4V, and interfacial chemical reactions, were investigated. Experimental results showed that numerous 2–20 μm diameter-sized pits formed on the Ti–6Al–4V surface. Propagation of ultrasonic waves in the liquid Al–Si alloy resulted in ultrasonic cavitation. When this cavitation occurred at or near the liquid/solid interface, many complex effects were generated at the small zones during the bubble implosion, including micro-jets, hot spots, and acoustic streaming. The breakage behavior of oxide films on the solid Ti–6Al–4V substrate, excessive chemical dissolution of solid Ti–6Al–4V into liquid Al–Si, abnormal interfacial chemical reactions at the interface, and phase transformation between the intermetallic compounds could be wholly ascribed to these ultrasonic effects. An effective bond between Al–Si and Ti–6Al–4V can be produced by ultrasonic-assisted brazing in air.