Quantitative calculation of reaction performance in sonochemical reactor by bubble dynamics

In order to design a sonochemical reactor with high reaction efficiency, it is important to clarify the size and intensity of the sonochemical reaction field. In this study, the reaction field in a sonochemical reactor is estimated from the distribution of pressure above the threshold for cavitation. The quantitation of hydroxide radical in a sonochemical reactor is obtained from the calculation of bubble dynamics and reaction equations. The distribution of the reaction field of the numerical simulation is consistent with that of the sonochemical luminescence. The sound absorption coefficient of liquid in the sonochemical reactor is much larger than that attributed to classical contributions which are heat conduction and shear viscosity. Under the dual irradiation, the reaction field becomes extensive and intensive because the acoustic pressure amplitude is intensified by the interference of two ultrasonic waves.     

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.     

Acoustic radiation from a drop due to its nonlinear vibrations

It is shown that the intensity of acoustic radiation from a vibrating drop depends mainly on the monopole and dipole components appearing only in the second order of smallness in vibration amplitude. The intensity of the quadrupole acoustic radiation generated by the vibration fundamental mode in the first order of smallness in amplitude turns out to be much weaker. This is associated with the fact that, if the acoustic wavelength is much larger than the drop characteristic size, their ratio becomes a governing small parameter, being lesser than the ratio of the drop vibration amplitude to the drop linear size. Analytical estimates of the amplitudes of monopole, dipole, and quadrupole components of the velocity field associated with the acoustic field of the drop.     

Measuring fluid and slurry density and solids concentration non-invasively

Staff at Pacific Northwest National Laboratory have developed a highly sensitive, non-invasive, self-calibrating, on-line sensor to measure the density, speed of sound, and attenuation of ultrasound for a liquid or slurry flowing through a pipeline; the approach can also be applied for measurements made in vessels. The sensor transducers are mounted directly upon the stainless steel wall and the pipeline wall becomes part of the measurement system. Multiple reflections within the stainless steel wall are used to determine the acoustic impedance of the liquid, where the acoustic impedance is defined as the product of the density and the speed of sound. The probe becomes self-calibrating because variations in the pulser voltage do not affect the measurements. This feature leads to the stability of the measurements and the instrument requires much less time and effort to calibrate. Further, the calibration remains constant in time, because it does not depend upon the pulser voltage remaining at a given value. By basing the measurement upon multiple reflections, the sensitivity of the measurement is significantly increased. For slurries with wt% solids concentration of 1% or less, high sensitivity is gained by analyzing attenuation measurements obtained from multiple paths through the slurry. For slurries with higher concentrations of solids, sufficient sensitivity is obtained by analyzing data from a simple transmission. Data are presented that show probe performance for each of these cases: very dilute and highly concentrated kaolin clay slurries.     

Dependence of the characteristics of bubbles on types of sonochemical reactors

Computer simulations of bubble oscillations in liquid water irradiated by an ultrasonic wave have revealed that the characteristic of bubbles depends on types of sonochemical reactors: a horn-type reactor and a standing-wave type reactor. When the acoustic amplitude is large at 20 kHz, the bubble content is mostly water vapor even at the end of the bubble collapse and the temperature inside a bubble at the collapse is relatively low. On the other hand, when the acoustic amplitude is relatively low, the bubble content is mostly noncondensable gas at the end of the bubble collapse and the bubble temperature is relatively high. In a horn-type sonochemical reactor, the former type of bubbles are dominant because many bubbles exist near the horn-tip where the acoustic amplitude is large, while in a standing-wave type reactor the latter type of bubbles are dominant because the Bjerknes force gathers bubbles at a region where acoustic amplitude is relatively low.     

Sonochemical activity in ultrasonic reactors under heterogeneous conditions

Due to its physical and chemical effects, ultrasound is widely used for industrial purposes, especially in heterogeneous medium. Nevertheless, this heterogeneity can influence the ultrasonic activity. In this study, the effect of the addition of inert glass beads on the sonochemical activity inside an ultrasonic reactor is investigated by monitoring the formation rate of triiodide, and the ultrasonic power is measured by calorimetry and by acoustic radiation. It was found that the sonochemical activity strongly depends on the surface area of the glass beads in the medium: it decreases above a critical area value (around 10⁻² m²), partly due to wave scattering and attenuation. This result is confirmed for a large range of frequencies (from 20 to 1135 kHz) and glass beads diameters (from 8-12 µm to 6 mm). It was also demonstrated that above a given threshold of the surface area, only part of the supplied ultrasonic power is devoted to chemical effects of ultrasound. Finally, the acoustic radiation power appears to describe the influence of solids on sonochemical activity, contrary to the calorimetric power.     

Coupled FEM/BEM for control of noise radiation and sound transmission using piezoelectric shunt damping

In this paper, we present a coupled finite element/boundary element method (FEM/BEM) for control of noise radiation and sound transmission of vibrating structure by passive piezoelectric techniques. The system consists of an elastic structure (with surface mounted piezoelectric patches) coupled to external/internal acoustic domains. The passive shunt damping strategy is employed for vibration attenuation in the low frequency range. The originality of the present paper lies in evaluating the classically used FEM/BEM methods for structural–acoustics problems when taking account smart systems at the fluid–structure interfaces.     

Estimation of absolute sound pressure in a small-sized sonochemical reactor

A small-sized sonochemical reactor in which the absolute value of the sound pressure amplitude can be estimated from the vibration velocity of the transducer was investigated. The sound pressure distribution in the reactor and the relationship between the vibration velocity and the sound pressure amplitude were derived through Helmholtz wave equation. The reactor consists of a bolt-clamped Langevin transducer and a rectangular cell with a tungsten reflector. A 3λ/4-standing-wave-field was generated in the reactor to simplify the sound pressure distribution. The sound pressure distribution was measured from the optical refractive index change of water using a laser interferometer. The experimental and theoretical results showed a good agreement in the absolute value of the sound pressure amplitude, and it was confirmed that the sound pressure in the sonochemical reactor can be estimated from the input current of the vibrator.     

Acoustic Parameters of the Bottom In Lake Baikal

A technique for estimating the effective acoustic parameters of the bottom in shallow water areas under ice cover has been developed. The methodology compares the experimental and simulated dependences of the sound field amplitude on depth at a distance from the source about an order of magnitude greater than the depth of the water area. The effective parameters are the values of the sound speed in the bottom, density, and attenuation coefficient of acoustic waves, which provide maximum agreement with experimental data in the calculations. The methodology was tested in a field experiment on Lake Baikal and can be recommended for developing autonomous acoustic monitoring systems.

     

Parametric study of acoustically-driven microbubble cavitations in a sonochemical reactor

The bubble cavitation along a solid wall is investigated with a three-dimensional model based on the indirect boundary element method. Kinetic energy and Kelvin impulse are calculated in order to quantify the strength of cavitation. The influences of acoustic wave amplitude and frequency and liquid properties on the strength of cavitation are investigated. This study was carried out in order to better understand the relation between microscale processes and macroscale parameters in a sonochemical reactor used for impregnation of fabrics with nanoparticles.     

Simulation of the spatial distribution of the acoustic pressure in sonochemical reactors with numerical methods: A review

Numerical methods for the calculation of the acoustic field inside sonoreactors have rapidly emerged in the last 15 years. This paper summarizes some of the most important works on this topic presented in the past, along with the diverse numerical works that have been published since then, reviewing the state of the art from a qualitative point of view. In this sense, we illustrate and discuss some of the models recently developed by the scientific community to deal with some of the complex events that take place in a sonochemical reactor such as the vibration of the reactor walls and the nonlinear phenomena inherent to the presence of ultrasonic cavitation. In addition, we point out some of the upcoming challenges that must be addressed in order to develop a reliable tool for the proper designing of efficient sonoreactors and the scale-up of sonochemical processes.     

A prediction of the acoustic field of a vibrating box by FEM-BEM

I.Mathematicalmodelofacousticfie1dByapplyingtheSommerfeldradiationcondihonThus,anacousticfic1disdescribcdbyHelmholtzformula.Byconsideringaficldpointinthearea,Helmholtzformulaisgivenbythefollowingthreeintegra1equations:(1)Theexternalintcgra1fOrmu1a'(2)Thcintcrnalintegralformu1af(3)Thesurfaceintegralformu1afwhcreP.isthesoundpressureonthcSsurface;rthedistancefrompointqtothee1ementsurfacedSiqtheficldpoint,tuthefrcqucncyoftheacousticradiation;pthedensityofthesurroundingmedium,vsthenormalvibrati…     

经颅超声聚焦方法和声场特性研究

基于高分辨的CT数据建立了非均匀颅骨仿真模型,该模型引入了颅骨的声衰减系数,深入研究和分析了声波时间反转法和超声相控阵法在颅脑中的聚焦方法及效果。颅骨具有较强的声波衰减特性,使用时间反转聚焦时需要进行幅度补偿,对于0.7MHz的频率信号,幅度补偿后的时间反转聚焦声场主瓣宽度窄、旁瓣低,焦点处声场比无幅度补偿的时间反转法提高8.86dB,比超声相控阵聚焦法提高7.89dB,具有很好的空间聚焦精度和聚焦效率。研究了颅骨衰减系数、声场焦点位置、声波频率、换能器阵列位置和方位等参数对聚焦声场的影响,结果表明,幅度补偿时间反转法比相控阵法具有更低的旁瓣,且高频时的聚焦效果比相控阵好,相控阵聚焦对换能器阵列的位置和方位比较敏感,而时间反转经颅超声聚焦对声传播路径和入射角具有更高的鲁棒性。      

动态应变场下相移光栅光谱特性及实验研究

光纤传感技术以其抗电磁干扰、高精度、易组网等优势在航空航天及地下工程领域的声发射传感中得到了广泛应用。针对光纤光栅声发射监测系统,目前研究侧重于均匀光纤光栅的动态应变场传感特性,基于相移光纤光栅(phase-shifted fiber Bragg grating, PS-FBG)这一新型光纤光栅器件,重点研究声发射激励产生的动态应变场下相移光纤光栅的光谱特性变化规律。首先利用传输矩阵理论,建立PS-FBG的动态应变传感模型,利用指数衰减的余弦函数模拟声发射激励产生的动态应变场。通过数值仿真,详细研究PS-FBG光谱对动态应变场幅度、采样时刻、衰减系数、声发射频率及波长的响应特性。结果表明,PS-FBG反射谱形状随声发射频率与采样时刻呈现周期性变化,具体表现为透射窗口所对波长产生周期性漂移,而带宽变化不明显;随着动态应变场幅值越大,PS-FBG反射谱的谐振峰向两侧逐渐增多,透射窗口所对波长漂移很小;一定范围内衰减系数对PS-FBG光谱影响较大;而当声发射波长为0.1～2 L时,对PS-FBG光谱产生较大影响,此时光谱畸变较严重,在此范围外,光谱形状不发生明显变化。最后,搭建连续信号激励下的动态应变场实验平台,分析不同幅度a与频率f变化下PS-FBG的光谱特性,实验结果与仿真结果较好吻合。表明PS-FBG光谱对不同声发射激励产生的动态应变场响应不同,呈现一定规律性,该研究为基于相移光纤光栅传感技术的声发射传感提供理论指导。     

Effect of dissolved gas on efficacy of sonochemical reactors for microbial cell disruption: Experimental and numerical analysis

In the present work the effect of dissolved gases on the extent of ultrasonically induced microbial cell disruption has been explored using a mathematical model and it has been validated by experimental data from literature. Degassing experiments are carried out and a degassing kinetics model for horn type ultrasonic device is presented. An overall model combining hydrodynamic and kinetics of cell disruption for horn type reactor is then proposed. The model includes several important operational parameters such as stress generated by the cavity, cell wall strength, dissolved gas concentration, degassing due to sonication, acoustic streaming generated due to sonication and attenuation of ultrasound in water. Model basically realizes in categorizing the volume of sonochemical reactor as active cavitation zone (ACZ) and inactive cavitation zone (ICZ). All the transformations are seen to occur only in ACZ. The two regions, i.e. ACZ and ICZ are assumed to behave as two mixed flow reactor arranged in closed loop. Suggestions have been also made for efficient design and scale up of ultrasonic devices for microbial cell disruption. The same model can be extended for other applications like particle size reduction, nano particle synthesis, leaching, emulsification with the knowledge of critical rate controlling parameter.     

Sound waves generated due to the absorption of a pulsed electron beam in gas

The results of an experimental investigation of acoustic vibrations (their frequency, amplitude, and attenuation coefficient) generated in a gas mixture as a result of the injection of a high-current pulsed electron beam into a closed reactor are presented. It is shown that the change in the phase composition of the initial mixture under the action of the electron beam leads to a change in the frequency of the sound waves and to an increase in the attenuation coefficient. By measuring the change in frequency, it is possible to evaluate with sufficient accuracy (about 2%) the degree of conversion of the initial products in the plasmochemical process. Relations describing the dependence of the sound energy attenuation coefficient on the size of the reactor and on the thermal and physical properties of the gases under study are derived. It is shown that a simple experimental setup measuring the parameters of acoustic waves can be used for monitoring the plasmochemical processes initiated by a pulsed excitation of a gas mixture.     

INTERACTION BETWEEN A LIQUID LAYER AND VIBRATING PLATES: APPLICATION TO THE DISPLACEMENT OF LIQUID DROPLETS

Various experiments have been performed to study the interaction of a liquid layer and vibrating plates. A liquid layer deposited on a vibrating plate exhibits a deformation of the surface with a high amplitude of vibration (larger than 1 μm at 30kHz). Furthermore, a water droplet placed on the vibrating plate moves towards an antinode of vibration. These non-linear phenomena are explained by the action of acoustic radiation pressure. An application to the displacement of droplets is presented.     

动态应力场作用下线性啁啾光栅光谱特性研究

目前柔性结构损伤检测需多个传感器,且需要探测器具有极高的采集频率。光纤光栅具有动态响应快、易实现分布式测量等特点,为解决上述问题提供了有效的途径,提出采用线性啁啾光栅(LCFBG)实现动态应力场探测。首先,用传输矩阵理论建立了LCFBG反射光谱应变传感模型,用衰减正弦函数模拟沿LCFBG分布的动态应力场。通过仿真实验,详细研究了LCFBG反射光谱对不同振幅、不同衰减系数与不同传播速度动态应力场的响应特性。实验结果表明,LCFBG反射光谱的反射率、波长变化与光谱形状均与动态应力场上述参数有关,但是LCFBG反射光谱对动态应力场不同参数的响应规律不同。在一定范围内,LCFBG反射光谱的最大反射率随动态应力场幅值与速度的增大而增大,最终趋于一稳定值,但其随阻尼系数增大而减小。最后,研制了以LCFBG为敏感元件的传感器,并构建了动态应力场实验平台,实验结果与仿真实验数据基本一致。提供了一种通过实时采集LCFBG全光谱信息探测动态应力场的新方法。     

304不锈钢点蚀声发射监测研究

为研究304不锈钢在高于常温条件下的点蚀声发射特性,对70℃下6%氯化铁溶液中304不锈钢点蚀过程进行了声发射监测。采用参数和波形分析相结合的方法处理信号,并通过点蚀形貌观察进行验证。结果表明声发射撞击和能量随时间逐渐增加,在某一时段达到峰值,随后下降并维持平稳状态。信号波形主要由幅度、能量较大的低频段(<100kHz)弯曲波和幅度、能量较小的高频段(>100 kHz)扩展波构成。研究结果对304不锈钢高于常温条件下声发射点蚀监测具有一定意义。      

An improved sonochemical reactor

The design and optimization of sonochemical apparatus are still open to advancement. Under high-intensity ultrasound reaction rates and yields are mainly influenced by the characteristics of transducer and reactor. Several useful improvements are introduced and described. In order to achieve uniformity of the acoustic field and optimal acoustic streaming in every part of the reaction vessel (a Teflon tube), the reactor can be made to rotate eccentrically around the horn axis and the probe to move alternatively up and down by a pre-determined excursion at a chosen speed. Continuous high-power irradiation is feasible without any time limit because the whole probe system is refrigerated by an oil forced-circulation circuit connected to a chiller. The apparatus can control a number of important reaction parameters: modified atmosphere, reaction temperature, tunable frequency and constant amplitude. Excellent performance was observed on several reactions, such as the chemical modification of chitosan, a poorly soluble biopolymer.