多层介质中超声相控阵声场仿真*

由于良好的声束偏转与聚焦特性,超声相控阵已经广泛应用于多层固体介质的缺陷检测。当超声束经过多层介质时,由于反射、透射以及模式转换的存在,多种声束存在于这种结构中,使得声场分析变得复杂。为了提高多层介质检测的准确性,有必要对超声声场的分布规律进行深入地了解。该文结合高斯声束等效点源模型以及射线追踪法,给出了相控阵声源在多层固体介质中激发声场的仿真方法,并且模拟计算了一维线型相控阵在楔块-铝-黄铜-钢四层固体介质中的辐射声场。通过对不同延时法则的计算,实现了声波在这种复杂介质中的偏转与聚焦,进而研究了不同焦点处聚焦声场的分布。结果表明:相控阵方法能使聚焦点处的声场幅值增大,能量集中,提高了检测分辨率;不同聚焦点处声场聚焦效果不同,实际检测时应根据检测区域结构及位置特点,合理放置相控阵换能器。与瑞利积分法的比较表明,该文的仿真方法适用于多层介质相控阵声场的计算。     

Ultrasonic phased array controller for hyperthermia applications

Multiple and mechanically scanned ultrasound transducer systems have demonstrated the efficacy of using ultrasound to produce deep localized hyperthermia. The use of ultrasonic phased arrays has been proposed as an alternative to these systems. A phased array offers a more flexible approach to heating tumours in that the size, shape, and position of its focal region can be altered during the course of treatment in order to achieve the desired temperature distribution. This added flexibility comes at the cost of increased complexity of the hardware necessary to drive the transducer because each element requires its own amplifer with both phase and amplitude control. In order for phased arrays with large numbers of elements to be feasible for hyperthermia applications, the complexity of this circuitry must be minimized. This paper describes a circuit design which simplifies the electronics required to control a phased array transducer system for hyperthermia applications. The design is capable of controlling virtually any type of phased array transducer operating at frequencies less than 2 MHz. The system performance was verified through beam profile measurements using a 48-element tapered phased array transducer.     

Optimal ultrasonic array focusing in attenuative media

This paper presents a parametric study on the efficiency of ultrasound focusing in an attenuative medium, using phased arrays. Specifically, an analytical model of ultrasound wave focusing in a homogeneous, isotropic and attenuative fluid with point sources is presented. Calculations based on the model have shown that in an attenuative medium, an optimum frequency exists for the best focusing performance for a particular size of aperture and focal distance. The effect of different f numbers on the focusing performance in the attenuative medium is further investigated. The information obtained from the analytical model provides insights into the design and installation of a phased transducer array for energy efficient wave focusing.     

球形腔聚焦换能器的非线性声场建模

球形腔聚焦换能器是一种特殊形式的聚焦换能器。为理论证实球形腔聚焦换能器能突破传统超声聚焦在聚焦精度和聚焦增益上的限制,采用Westervelt非线性方程并结合时域有限差分法,建立了球形腔聚焦换能器的非线性声场的数值模型。数值计算了直径为120 mm的0.6 MHz球形腔聚焦换能器的非线性声场,并与传统球壳形聚焦换能器进行了对比。当激励声压为100 kPa时,球形腔聚焦换能器与同尺寸壳形聚焦换能器相比,焦点正声压增益提高约8.5倍,且焦域精度更高,-6 dB聚焦区域在z方向减小约20倍,达到次波长尺度。研究表明球形腔聚焦换能器在高强度聚焦超声精细治疗上具有潜在的应用前景。      

Numerical approaches to simulating nonlinear ultrasound fields generated by diagnostic-type transducers

Two theoretical approaches for simulating nonlinear focused ultrasound fields generated by a diagnostic convex array are compared. The first model is based on the three-dimensional Westervelt equation and describes the full structure of the array field with high accuracy. However, it requires great computational resources and is technically difficult. The second model is based on an axially symmetric form of the parabolic KZK equation for estimating the strength of nonlinear effects in the focal region of a beam, which reduces the computational time by a factor of several hundreds. To establish the boundary conditions to the KZK model, the radius and the focal length of a circular piston source are defined such that the simulated field on the beam axis in the linear case fits the real structure of the field in the focal region. It is shown that the parabolic model can be used to accurately describe the spatial and temporal structure of the field generated by a diagnostic transducer in the focal region of the beam along its axis and in the plane of the beam’s electronic focusing.     

Investigation of design parameters in ultrasound reactors with confined channels

This paper presents a three-dimensional numercial simulation of sonochemical degradation upon cavitational activity. The model relates the simulation of the acoustic pressure distribution to the sonochemical reaction rate. As a case study, the thermal degradation of carbon tetrachloride during sonication is studied in a tubular milliscale reactor. The model is used to optimize the reactor diameter, ultrasound frequency and power dissipated to the ultrasound transducers. The results indicate that multiple transducers at a moderate power level are more efficient than one transducer with high power level. Furthermore, the average cavity volume fraction is proposed as a reaction independent parameter to estimate the optimal reactor design. Within the results obtained in this paper, it appears possible to optimise reactor design based on this parameter.     

Characterization of HIFU transducers designed for sonochemistry application: Acoustic streaming

Cavitation distribution in a High Intensity Focused Ultrasound sonoreactors (HIFU) has been extensively described in the recent literature, including quantification by an optical method (Sonochemiluminescence SCL). The present paper provides complementary measurements through the study of acoustic streaming generated by the same kind of HIFU transducers. To this end, results of mass transfer measurements (electrodiffusional method) were compared to optical method ones (Particle Image Velocimetry). This last one was used in various configurations: with or without an electrode in the acoustic field in order to have the same perturbation of the wave propagation. Results show that the maximum velocity is not located at the focal but shifted near the transducer, and that this shift is greater for high powers. The two cavitation modes (stationary and moving bubbles) are greatly affect the hydrodynamic behavior of our sonoreactors: acoustic streaming and the fluid generated by bubble motion. The results obtained by electrochemical measurements show the same low hydrodynamic activity in the transducer vicinity, the same shift of the active focal toward the transducer, and the same absence of activity in the post-focal axial zone. The comparison with theoretical Eckart’s velocities (acoustic streaming in non-cavitating media) confirms a very high activity at the “sonochemical focal”, accounted for by wave distortion, which induced greater absorption coefficients. Moreover, the equivalent liquid velocities are one order of magnitude larger than the ones measured by PIV, confirming the enhancement of mass transfer by bubbles oscillation and collapse close to the surface, rather than from a pure streaming effect.     

基于超声相控阵的环焊缝缺陷检测方法研究

针对传统超声探头焦距固定,检测位置的改变就要更换相应焦距的探头而影响检测效率的问题,提出一种基于超声相控阵换能器的环焊缝缺陷检测方法。而超声相控阵具有电子偏转和电子聚焦特性,能在不移动的情况下发射偏转聚焦超声束,有效地解决了上述问题。首先基于超声相控线阵换能器的声场特点,采用数值分析方法,研究了影响声束偏转聚焦性能的几个主要参数。然后给出了与超声相控阵换能器相连接的多通道数据采集系统结构。介绍了单通道声信号的硬件结构及相应的信号处理方法,实现了对换能器中单个阵元的精确延时的控制。实验结果表明,优化设计的超声相控线阵换能器具有较高的检测精度和检测效率。     

Dependence of sonochemical luminescence on various sound fields

To understand the effect of the sound field on sonochemical luminescence, the exact sound pressure must be determined in each field. In this study it was determined by the Shlieren method, which measures the sound pressure without mixing the sound fields. We compared the efficiency of the sonochemical luminescence in three different ways: changing the diameter of the transducer, combining two transducers to obtain crossed propagating directions and surrounding the sound field by a glass cylinder. In the last case cylinders with various sizes were studied. We found that (i) at the same sound pressure, the larger transducer induces stronger luminescence per unit volume, (ii) driving two transducers produces stronger luminescence than the sum of each transducer and (iii) a glass cylinder surrounding the sound field induces stronger luminescence.     

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.     

Design and calibration of a single-transducer variable-frequency sonication system

The design of a novel single-transducer variable-frequency sonication system capable of operating at constant acoustic power over the range 20-500 kHz is described. The system employs a mass-loaded sandwich transducer arrangement and a series of transformers to provide an accurate impedance matching circuit. Approximately 0-5 W of acoustic power are produced by the system at typical operating frequencies of 20, 40, 150, 200, 300, and 450 kHz. As a first test of the single-transducer variable-sonication system we have re-examined the frequency dependence of the sonochemical oxidation of potassium iodide. Previous investigators have monitored the frequency dependence using a multi-transducer system to obtain the different frequencies required. In accordance with the earlier findings, we have observed an eightfold increase in the rate of potassium iodide oxidation at 300 kHz compared to 20 kHz, as well as an inversion in the rate of oxidation for argon and air-saturated solutions at 300 kHz. Possible reasons for the rate variations are discussed.     

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。      

Numerical simulations of partial elements excitation for hemispherical high-intensity focused ultrasound phased transducer

The hemispherical phased transducer maximizes the coverage of the skull and the ultrasonic energy per unit area of the skull is minimized, thereby reducing the risk of skull burns, but the transducer has a small focal area adjustment range, increasing the focal length of treatment is an urgent question for this type of transducer. In this paper, a three-dimensional high-intensity focused ultrasound (HIFU) transcranial propagation model is established based on the human head structure. The finite difference time domain (FDTD) is combined with the Westervelt acoustic wave nonlinear propagation equation and Penne's biological heat conduction equation for numerical simulation of the sound pressure field and temperature field. Forming a treatable focal area in a small-opening hemispherical transducer with a small amount of numerical simulation calculation focusing at a set position to determine the minimum partial excitation area ratio of focusing. And then, applying these preliminary results to a large-opening diameter hemispherical transducer and the temperature field formed by it or full excitation is studied. The results show that the focus area with the excitation area ratio of less than 22% moves forward to the transducer side when the excitation sound is formed. When the excitation area ratio is greater than or equal to 23%, it focuses at the set position. In the case of partial incentives, using 23% of the partial array, the adjustable range of the treatable focal area formed in the three-dimensional space is larger than that of the full excitation.     

Baeyer-Villiger oxidation of cyclohexanone to epsilon-caprolactone in airlift sonochemical reactor

Baeyer-Villiger oxidation of cyclohexanone to epsilon-caprolactone was studied in a new type reactor--the airlift loop sonochemical reactor. The reactor plays a synergistic effect of sonochemsity and higher oxygen transfer rate. The influences of ultrasound intensity, reaction temperature, the molar ratio of benzaldehyde to cyclohexanone and oxygen gas flow rate on the conversion and selectivity of cyclohexanone were investigated and discussed. Under ultrasound, the amount of benzaldehyde can be reduced from 75% to 67%. Ultrasound not only intensified the rates of reactions but also increased the yield of product. The optimized operation conditions are listed as follows: the reaction temperature is 30 degrees C, the molar ratio of cyclohexanone to benzaldehyde is 1:2, the oxygen gas flow rate is 1.15 cm s(-1), and ultrasonic irradiations 2h at 40 kHz, 2.25 W cm(-2). Under the optimum operation conditions, the average molar yield of epsilon-caprolactone comes up to 87.7%.     

Effects of gas sparging and mechanical mixing on sonochemical oxidation activity

The effects of air sparging (0–16 L min⁻¹) and mechanical mixing (0–400 rpm) on enhancing the sonochemical degradation of rhodamine B (RhB) was investigated using a 28 kHz sonoreactor. The degradation of RhB followed pseudo first-order kinetics, where sparging or mixing induced a large sonochemical enhancement. The kinetic constant varied in three stages (gradually increased → increased exponentially → decreased slightly) as the rate of sparging or mixing increased, where the stages were similar for both processes. The highest sonochemical activity was obtained with sparging at 8 L min⁻¹ or mixing at 200 rpm, where the standing wave field was significantly deformed by sparging and mixing, respectively. The cavitational oxidation activity was concentrated at the bottom of the sonicator when higher sparging or mixing rates were employed. Therefore, the large enhancement in the sonochemical oxidation was attributed mainly to the direct disturbance of the ultrasound transmission and the resulting change in the cavitation-active zone in this study. The effect of the position of air sparging and mixing was investigated. The indirect inhibition of the ultrasound transmission resulted in less enhancement of the sonochemical activity. Moreover, the effect of various sparging gases including air, N₂, O₂, Ar, CO₂, and an Ar/O₂ (8:2) mixture was compared, where all gases except CO₂ induced an enhancement in the sonochemical activity, irrespective of the concentration of dissolved oxygen. The highest activity was obtained with the Ar/O₂ (8:2) mixture. Therefore, it was revealed that the sonochemical oxidation activity could be further enhanced by applying gas sparging using the optimal gas.     

Acoustic power measurement using an optical heterodyne method

An optical heterodyne method is presented for measuring the Raman-Nath parameter of ultrasound. A light beam passes through a phase grating induced by ultrasound and then is recombined with another beam of light at a shifted frequency. The interference distribution of the two light beams is measured by a photodetector placed in the Fresnel region. The Raman-Nath parameter is determined from the amplitudes of frequency components in the beat signal and is independent of observation position. A theoretical analysis is given which includes pulsed ultrasound. Acoustic power determined using the technique is shown to agree fairly well with the radiation force method. Acoustic fields from a linear array transducer are also investigated. A problem involved in focused ultrasound is discussed.     

Recent advances in organic synthesis utilizing ultrasound

This review describes the recent applications of ultrasound in organic synthesis. Oxidation processes, N-C and C-C bond formation (encompassing the Wittig, Ullman and acyloin reactions), cycloaddition reactions (including the first example of an ultrasonically promoted Diels-Alder reaction), bond cleavage reactions and biological examples are discussed. The important area of sonochemical switching is reviewed and comments are made on the relationship between ionic and radical sonochemical reactions.     

Realization of cavitation fields based on the acoustic resonance modes in an immersion-type sonochemical reactor

Different modes of cavitation zones in an immersion-type sonochemical reactor have been realized based on the concept of acoustic resonance fields. The reactor contains three main components, namely a Langevin-type piezoelectric transducer (20 kHz), a metal horn, and a circular cylindrical sonicated cell filled with tap water. In order to diminish the generation of cavitation bubbles near the horn-tip, an enlarged cone-shaped horn is designed to reduce the ultrasonic intensity at the irradiating surface and to get better distribution of energy in the sonicated cell. It is demonstrated both numerically and experimentally that the cell geometry and the horn position have prominent effects on the pressure distribution of the ultrasound in the cell. With appropriate choices of these parameters, the whole reactor works at a resonant state. Several acoustic resonance modes observed in the simulation are realized experimentally to generate a large volume of cavitation zones using a very low ultrasonic power.     

Sonochemical and sonocatalytic degradation of monolinuron in water

The degradation of the phenylurea monolinuron (MLN) by ultrasound irradiation alone and in the presence of TiO(2) was investigated in aqueous solution. The experiments were carried out at low and high frequency (20 and 800 kHz) in complete darkness. The degradation of MLN by ultrasounds occurred mainly by a radical pathway, as shown the inhibitory effect of adding tert-butanol and bicarbonate ions to scavenge hydroxyl radicals. However, CO(3)(-) radicals were formed with bicarbonate and reacted in turn with MLN. In this study, the degradation rate of MLN and the rate constant of H(2)O(2) formation were used to evaluate the oxidative sonochemical efficiency. It was shown that ultrasound efficiency was improved in the presence of nanoparticles of TiO(2) and SiO(2) only at 20 kHz. These particles provide nucleation sites for cavitation bubbles at their surface, leading to an increase in the number of bubbles when the liquid is irradiated by ultrasound, thereby enhancing sonochemical reaction yield. In the case of TiO(2), sonochemical efficiency was found to be greater than with SiO(2) for the same mass introduced. In addition to the increase in the number of cavitation bubbles, activated species may be formed at the TiO(2) surface that promote the formation of H(2)O(2) and the decomposition of MLN.     

基于FPGA的高强度聚焦超声相位控制系统设计*

多阵元相控阵换能器具有焦距可调和可实现经颅聚焦等优势,近来受到众多研究者的关注,其相位控制系统是决定多阵元相控换能器能否应用于临床的关键技术之一。在输出信号稳定的同时提高高强度聚焦超声相位控制系统精度是设计过程中的重点与难点。本文基于现场可编程门阵列设计了一种高强度聚焦超声相位控制系统。实测结果表明,多通道延时分辨率为1 ns,延时误差小于1 ns,可满足多阵元高强度聚焦超声治疗相控聚焦换能器延时精度的需要。