Application of high-power ultrasound to enhance fluid/solid particle separation processes

The separation of fine particles from gases or liquids is a topic of permanent industrial attention. The use of ultrasonic energy to assist conventional separation techniques seems to be very promising. The adequate applications of high-intensity ultrasonic fields may contribute to improve the efficiency and capacity of the separation methods presently used. The specific mechanisms to ultrasonically enhance separation processes basically depend on the medium to be treated. In gas suspensions, where very fine particles have to be removed, ultrasonic action involves agglomeration of particles in order to increase their size and, consequently, to improve collection efficiency of conventional filters. In liquid suspensions, agglomeration is, in general, less efficient than in gases. Nevertheless, the ultrasonic energy is useful to dewater fine-particle high-concentration suspensions such as slurries and sludges. This paper deals with the application of acoustic energy to assist fluid/solid separation processes in gas and liquid suspensions and presents some theoretical and experimental results in specific applications.     

Macrosonics in industry: 4. Chemical processing

Acoustic irradiation can result in increased inter-phase mass and heat transfer rates. The second-order acoustic effects of cavitation, interfacial instability, radiation pressure and acoustic streaming are responsible for the enhancement in these rate processes. The application of sonic and ultrasonic energy in industrial processing is reviewed. A number of units using acoustic energy to enhance rates of conventional unit processes, for example, drying, solid-liquid extraction, etc, are described. In addition, new applications in waste water treatment and oil-water emulsion fuels are described. The development of newer, more efficient generators should lead to a greater use of acoustic energy for large-scale industrial processing.     

Investigation of Ti-6Al-4V alloy acoustic softening

High power ultrasonic vibration is widely used for improving manufacturing processes such as machining and metal forming. High frequency mechanical vibration affects material properties and friction forces in contacting surfaces. Flow stress reduction under superimposed ultrasonic vibration is called as acoustic softening. The amount of this parameter should be determined for ultrasonic assisted metal forming processes. For determination of this parameter for workhorse Ti-6Al-4V alloy, experimental setup was designed and fabricated. Then tensile test under longitudinal ultrasonic vibration was performed for different ultrasonic powers. Results show that ultrasonic vibration has considerable effect on plastic behavior of the alloy and decreases flow stress. Also, increasing ultrasonic power leads to higher acoustic softening. Yield stress reduction up to 9.52%, ultimate stress reduction up to 4.55% and elongation up to 13% were obtained at 340 W ultrasonic power. After applying ultrasonic vibrations and its termination, hardness of specimens were measured in which increase up to 9% was observed.     

Ultrasonic drying of foodstuff in a fluidized bed: Parametric study

The application of high power ultrasound for dehydration of porous materials may be very effective in processes in which heat-sensitive materials such as foodstuffs have to be treated. In fact, high-intensity ultrasonic vibrations are capable of increasing heat and mass transfer processes in materials. The application of ultrasonic energy can be made alone or in combination with other kind of energy such as hot-air. In this case, ultrasound helps in reducing temperature or treatment time. The aim of this work is to study the effect of air flow rate, ultrasonic power and mass loading on hot-air drying assisted by a new power ultrasonic system. The drying chamber is an aluminium vibrating cylinder, which is able to create a high intensity ultrasonic field in the gas medium. To that purpose the chamber is driven at its centre by a power ultrasonic vibrator at 21.8 kHz. Drying kinetics of carrot cubes and lemon peel cylinders were carried out at 40 degrees C for different air velocities, with and without ultrasound. The results show that the effect of ultrasound on drying rate is affected by air flow rate, ultrasonic power and mass loading. In fact, at high air velocities the acoustic field inside the chamber is disturbed and the effect of ultrasound on drying kinetics diminishes.     

Grain refining of Ti-6Al-4V alloy fabricated by laser and wire additive manufacturing assisted with ultrasonic vibration

The formation of the coarse columnar crystal structure of Ti-6Al-4V alloy in the process of additive manufacturing greatly reduces the mechanical performance of the additive manufactured parts, which hinders the applications of additive manufacturing techniques in the engineering fields. In order to refine the microstructure of the materials using the high intensity ultrasonic via the acoustic cavitation and acoustic flow effect in the process of metal solidification, an ultrasonic vibration technique was developed to a synchronous couple in the process of Laser and Wire Additive Manufacturing (LWAM) in this work. It is found that the introduction of high-intensity ultrasound effectively interrupts the epitaxial growth tendency of prior-β crystal and weakens the texture strength of prior-β crystal. The microstructure of Ti-6Al-4V alloy converts to fine columnar crystals from typical coarse columnar crystals. The simulation results confirm that the acoustic cavitation effect applied to the molten pool created by the high-intensity ultrasound is the key factor that affects the crystal characteristics.     

Stability analysis of the rotor of ultrasonic motor driving fluid directly

The stability of the rotor of ultrasonic motor driving fluid directly is a key to its applications and control. This paper introduced the acoustic streaming and acoustic viscous stress near the boundary layer. Following this, the effect of acoustic viscous force on the stability of the rotor of ultrasonic motor driving fluid directly was presented in detail. The result showed that this system can be equivalent to a mass-spring and the spring constant can be used to weigh the stability of the rotor. By this model and relevant experiments, factors that affect the stability of the rotor such as the driving frequency, the rotor's weight and radius, the saturated acoustic streaming velocity, the mode number of stator vibration, the fluid's height and type are investigated and useful guidelines for design and application are obtained.     

高强度聚焦超声治疗子宫肌瘤中水囊耦合的影响评估*

水囊作为一种特殊的辅助器件在高强度聚焦超声治疗子宫肌瘤的过程中发挥着重要作用,但它同时也会让声通道变得更加复杂,对超声治疗效率和超声引导影像均造成负面影响。基于实际临床场景,利用k-Wave声学仿真软件建立高强度聚焦超声辐照靶组织及其超声引导过程的仿真模型,在有无水囊两个场景下,对治疗效率和监控影像质量进行定量评估。结果表明,加入水囊后,声波的聚焦性变差,焦域处的最高温度降低;通过分析超声图像的分辨率、对比度、对比噪声以及背景信噪比,发现加入水囊后超声图像的质量降低。建立的数值仿真模型能够初步评估水囊对治疗效率和监控影像质量的影响,可作为一种评估体系用于优化水囊参数,如水囊材料、厚度、形状、内部溶液成分等,为下一步实验探索对治疗效率和影像质量影响最小的水囊耦合方式提供评估手段。     

Investigation on ultrasonic volume effects: Stress superposition,acoustic softening and dynamic impact

Conventional high power ultrasonic vibration has been widely used to improve manufacturing processes like surface treatment and metal forming. Ultrasonic vibration affects material properties, leading to a flow stress reduction, which is called ultrasonic volume effect. The volume effect contains multi-mechanisms such as stress superposition due to oscillatory stress, acoustic softening by easier dislocation motion and dynamic impact leading to extra surface plastic deformation. However, most researches ignored the stress superposition for the convenience of measurement, and few studies considered ultrasonic dynamic impact since the relatively low ultrasonic energy in macro scale. The purpose of this study is to investigate the characteristics and mechanisms of different ultrasonic volume effects in micro-forming. A 60 kHz longitudinal ultrasonic-assisted compression test system was developed and a series of ultrasonic-assisted compression tests at different amplitudes on commercially pure aluminum A1100 in micro-scale were carried out combining the surface analysis by SEM, EDX and micro-hardness test. Three different ultrasonic volume effects, stress superposition, acoustic softening and dynamic impact, were confirmed in the ultrasonic-assisted compression tests. In order to quantitatively predict stress superposition, a hybrid model for stress superposition is developed considering the elastic deformation of experimental apparatus in practice, the evolution of the modeling results fitted well with the experimental results. With low ultrasonic amplitude, stress superposition and acoustic softening occurred because vibrated punch contacted with the specimen all the time during compression. However, with higher amplitude, due to the extra surface plastic deformation by larger ultrasonic energy, forming stress was further reduced by the ultrasonic dynamic impact. A possible method to distinguish the effects of dynamic impact and acoustic softening is to analyze the waveform of the oscillatory stress in the process. In the case of ultrasonic dynamic impact effect, a higher amount of oxidation was observed on the specimen surface, which could be the result of local heating by surface plastic deformation and surface friction when the vibrated punch detached from the specimen. The findings of this study provide an instructive understanding of the underlying mechanisms of volume effects in ultrasonic-assisted micro-forming.     

Finite element modeling of heating phenomena of cracks excited by high-intensity ultrasonic pulses

A three-dimensional thermo-mechanical coupled finite element model is built up to simulate the phenomena of dynamical contact and frictional heating of crack faces when the plate containing the crack is excited by high-intensity ultrasonic pulses.In the finite element model,the high-power ultrasonic transducer is modeled by using a piezoelectric thermal-analogy method,and the dynamical interaction between both crack faces is modeled using a contact-impact theory.In the simulations,the frictional heating taking place at the crack faces is quantitatively calculated by using finite element thermal-structural coupling analysis,especially,the influences of acoustic chaos to plate vibration and crack heating are calculated and analysed in detail.Meanwhile,the related ultrasonic infrared images are also obtained experimentally,and the theoretical simulation results are in agreement with that of the experiments.The results show that,by using the theoretical method,a good simulation of dynamic interaction and friction heating process of the crack faces under non-chaotic or chaotic sound excitation can be obtained.     

Selective aggregation by ultrasonic standing waves through gas nuclei on the particle surface

Gas nuclei in water are usually too small to be directly observed. They will grow into bubbles under the negative pressure, which is called cavitation (heterogeneous cavitation). In this study, the gas nuclei in the hydrophilic and hydrophobic silica particle suspension were investigated using the transient cavitation threshold measured by a high-intensity focused ultrasound (HIFU). The transient cavitation bubbles were also observed by a high-speed camera. The results showed that the nuclei only exist on the surface of hydrophobic particles. Furthermore, the aggregation experiments revealed that the aggregates were only formed in the hydrophobic silica suspension by ultrasonic standing waves (USW) at 200 kHz. This distinct difference was mainly due to the formation of gas nuclei on hydrophobic silica particles, which grew and coalesced into stable bubbles under the 200 kHz USW. The aggregation process in suspension was observed by a CCD camera. Moreover, the cavitation thresholds and acoustic radiation forces were analyzed to explain the mechanism of the acoustic aggregation. This study showed a very promising acoustic method for the selective aggregation of hydrophobic particles, which might be efficiently used in the mineral separation industry.