Ultrasound pressure distributions generated by high frequency transducers in large reactors

The performance of an ultrasound reactor chamber relies on the sound pressure level achieved throughout the system. The active volume of a high frequency ultrasound chamber can be determined by the sound pressure penetration and distribution provided by the transducers. This work evaluated the sound pressure levels and uniformity achieved in water by selected commercial scale high frequency plate transducers without and with reflector plates. Sound pressure produced by ultrasonic plate transducers vertically operating at frequencies of 400 kHz (120 W) and 2 MHz (128 W) was characterized with hydrophones in a 2 m long chamber and their effective operating distance across the chamber’s vertical cross section was determined. The 2 MHz transducer produced the highest pressure amplitude near the transducer surface, with a sharp decline of approximately 40% of the sound pressure occurring in the range between 55 and 155 mm from the transducer. The placement of a reflector plate 500 mm from the surface of the transducer was shown to improve the sound pressure uniformity of 2 MHz ultrasound. Ultrasound at 400 kHz was found to penetrate the fluid up to 2 m without significant losses. Furthermore, 400 kHz ultrasound generated a more uniform sound pressure distribution regardless of the presence or absence of a reflector plate. The choice of the transducer distance to the opposite reactor wall therefore depends on the transducer plate frequency selected. Based on pressure measurements in water, large scale 400 kHz reactor designs can consider larger transducer distance to opposite wall and larger active cross-section, and therefore can reach higher volumes than when using 2 MHz transducer plates.     

Oscillating bubble concentration and its size distribution using acoustic emission spectra

New method has been proposed for the estimation of size and number density distribution of oscillating bubbles in a sonochemical reactor using acoustic emission spectra measurements. Bubble size distribution has been determined using Minnaert's equation [M. Minnaert, On musical air bubbles and sound of running water, Philanthr. Mag. 16 (1933) 235], i.e., size of oscillating bubble is inversely related to the frequency of its volume oscillations. Decomposition of the pressure signal measured by the hydrophone in frequency domain of FFT spectrum and then inverse FFT reconstruction of the signal at each frequency level has been carried out to get the information about each of the bubble/cavity oscillation event. The number mean radius of the bubble size is calculated to be in the range of 50-80mum and it was not found to vary much with the spatial distribution of acoustic field strength of the ultrasound processor used in the work. However, the number density of the oscillating bubbles and the nature of the distribution were found to vary in different horizontal planes away from the driving transducer surface in the ultrasonic bath. A separate set of experiments on erosion assessment studies were carried out using a thin aluminium foil, revealing a phenomena of active region of oscillating bubbles at antinodal points of the stationary waves, identical to the information provided by the acoustic emission spectra at the same location in the ultrasonic bath.     

Ultrasounds: an industrial solution to optimise costs, environmental requests and quality for textile finishing

Ultrasounds are widely used at industrial scale for cleaning of mechanical pieces for example. Potential applications exist for finishing of textiles. This work aimed to improve traditional textile finishing processes thanks to ultrasound. The technical objective was to develop specific applicators of ultrasonic energy which could be adapted on jigger, a widespread textile finishing machine. Laboratory studies have allowed to define the conditions for application of ultrasounds and check their effects on fibre structure, validated by trials in dynamic conditions. Ultrasound technology makes it possible to intensify the phenomena of diffusion and washing by the effect of cavitation and improves effectiveness of traditional washing treatments. Industrial ultrasound processes need further optimisation on industrial machines.     

Washing off intensification of cotton and wool fabrics by ultrasounds

Wet textile washing processes were set up for wool and cotton fabrics to evaluate the potential of ultrasound transducers (US) in improving dirt removal. The samples were contaminated with an emulsion of carbon soot in vegetable oil and aged for three hours in fan oven. Before washing, the fabrics were soaked for 3 min in a standard detergent solution and subsequently washed in a water bath. The dirt removal was evaluated through colorimetric measurements. The total color differences ΔE of the samples were measured with respect to an uncontaminated fabric, before and after each washing cycle. The percentage of ΔE variation obtained was calculated and correlated to the dirt removal. The results showed that the US transducers enhanced the dirt removal and temperature was the parameter most influencing the US efficiency on the cleaning process. Better results were obtained at a lower process temperature.     

A theoretical model for ice primary nucleation induced by acoustic cavitation

The modelling and simulation of ice nucleation triggered by acoustic cavitation was addressed in this study. The objective was to evaluate the number of nuclei generated by a single gas bubble and afterwards by a multi-bubble system as function of the acoustic pressure (ultrasound wave amplitude) and supercooling level (liquid temperature). According to our calculations, the nucleation could be initiated with moderated acoustic pressure amplitude (around one bar) even at low supercooling levels (around few degrees). These results may provide a sound basis for the control of ice crystal size and morphology which is a key issue in industrial freezing and freeze-drying processes.     

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.     

Application of ultrasound to textiles washing in aqueous solutions

The ultrasound was applied to textile washing as a mechanical action for soil removal. The polyester fabric was soiled with carbon black or oleic acid as a model contaminant, and washed with the original fabric in aqueous solutions without and with alkali or surfactant by applying ultrasound, shaking or stirring action. The detergency and soil redeposition were evaluated from the change in the surface reflectance of artificially soiled fabrics and the original fabric due to washing. In comparison with shaking and stirring actions, ultrasound was found to remove the particulate and oily soils efficiently in a short time and at low bath ratio. With increasing ultrasound power, the detergency of both soils increased and exceeded that obtained with Wascator, a horizontal axis drum type washer. Using three standard fabrics for determining mechanical action during washing, it was shown that ultrasound washing caused little mechanical damage to the fabric. However, the soil redeposition was frequently observed for ultrasonic washing, especially at low bath ratio.     

Investigation of industrial tea-leaf-fibre waste material for its sound absorption properties

The sound absorption of an industrial waste, developed during the processing of tea leaves has been investigated. Three different layers of tea-leaf-fibre waste materials with and without backing provided by a single layer of woven textile cloth were tested for their sound absorption properties. The experimental data indicate that a 1 cm thick tea-leaf-fibre waste material with backing, provides sound absorption which is almost equivalent to that provided by six layers of woven textile cloth. Twenty millimeters thick layers of rigidly backed tea-leaf-fibres and non-woven fibre materials exhibit almost equivalent sound absorption in the frequency range between 500 and 3200 Hz.     

Ultrasound fields in attenuating media

For medical ultrasonic imaging and for nondestructive testing, the attenuation of pressure waves and the resulting shift in wave velocity are important features in commonly used transmission media such as biological tissue. An algorithm for the numerical evaluation of pressure field distributions generated by ultrasonic transducers is presented. The attenuation and dispersion of the sound transmission medium are taken into consideration. The sound fields are computed numerically for continuous wave as well as pulse excitation. The transducer has plane or gently curved geometry and is embedded in a plane rigid baffle. The numerically determined pressure fields are presented as 3D plots, as gray-scale images for a fixed time stamp (like a snapshot), or as isobars regarding the maximum values over time for each local point in the area under investigation. The algorithm described here can be utilized as a tool for design of ultrasound transducers, especially array antennas.     

液体材料超声处理过程中声场和流场的分布规律研究

针对合金熔体等液体材料的超声处理过程,选取水作为透明模型材料,采用数值模拟计算和示踪粒子实验方法,研究了20和490 kHz两种频率超声作用下水中的声场和流场分布.结果表明,增大变幅杆半径能够提高水中声压水平,扩大空化效应的发生区域.当超声频率为20 kHz时,水中声压最大值出现在超声变幅杆下端面处,且声压沿传播距离的增大而显著减小.如果超声频率增加至490 kHz,水中的声压级相比于20 kHz时明显提高,且声压沿着超声传播方向呈现出周期性振荡特征.两种频率超声作用下水中的流场呈现相似的分布特征,且平均流速均随着变幅杆半径增大表现出先升高后降低的趋势.变幅杆半径相同时,20 kHz频率超声作用下水中的平均流速高于490 kHz频率超声.采用示踪粒子图像测速技术实时观察和测定了水中的流速分布,发现其与计算结果基本一致.     

Ultrasound-membrane hybrid processes for enhancement of filtration properties

Enhancing permeation in microfiltration (MF) hollow fiber membrane process was studied under ultrasound (US) by means of reflection technique. The US effect on the enhanced membrane process depended on the position of membrane module with or without a semi-cylindrical shaped reflection plate placed behind the membrane module. Evidence was presented that the membrane process influenced the detected sonic pressure distribution in the US bath. The correlation of sound pressure intensity and luminol fluorescence intensity suggested that violent collapse of cavity bubbles supported the enhanced membrane permeability of MF module in the US bath.     

中频小型矢量水听器设计研究

中频小型同振式矢量水听器采用低密度复合材料作为矢量通道外壳、压电加速度计作为内部振动传感器，以拾取水下声场中的矢量信息。与以往同振式矢量水听器设计不同的是每个矢量通道中只放置一只压电加速度计，这样，不仅减小了矢量水听器的体积，降低了水听器的整体平均密度，而且消除了以往同振式矢量水听器设计中矢量通道采用两只配对传感器而引起的相位不一致给矢量水听器定向带来的影响。而声压通道采用PVDF压电薄膜作为敏感元件，以拾取水下声场中的标量信息。     

Nonlinear generation of airborne sound by waves of ultrasonic frequencies

The results of research into the design and optimization of laboratory sources of intense airborne ultrasound are reported. Two types of sources are studied: multielement arrays of small-size piezoelectric radiators and single membrane transducers of a capacitor type. The measured characteristics of the ultrasound fields and the audible sound fields generated in air due to the nonlinear interaction of high-frequency waves are presented. Applications of nonlinear acoustic problems in air are discussed.     

Micromachined ultrasound transducers with improved coupling factors from a CMOS compatible process

For medical high frequency acoustic imaging purposes the reduction in size of a single transducer element for one-dimensional and even more for two-dimensional arrays is more and more limited by fabrication and cabling technology. In the fields of industrial distance measurement and simple object recognition low cost phased arrays are lacking. Both problems can be solved with micromachined ultrasound transducers (MUTs). A single transducer is made of a large number of microscopic elements. Because of the array structure of these transducers, groups of elements can be built up and used as a phased array. By integrating parts of the sensor electronics on chip, the cabling effort for arrays can be reduced markedly. In contrast to standard ultrasonic technology, which is based on massive thickness resonators, vibrating membranes are the radiating elements of the MUTs. New micromachining technologies have emerged, allowing a highly reproducible fabrication of electrostatically driven membranes with gap heights below 500 nm. A microelectronic BiCMOS process was extended for surface micromechanics (T. Scheiter et al., Proceedings 11th European Conference on Solid-State Transducers, Warsaw, Vol. 3, 1997, pp. 1595-1598). Additional process steps were included for the realization of the membranes which form sealed cavities with the underlying substrate. Membrane and substrate are the opposite electrodes of a capacitive transducer. The transducers can be integrated monolithically on one chip together with the driving, preamplifying and multiplexing circuitry, thus reducing parasitic capacities and noise level significantly. Owing to their low mass the transducers are very well matched to fluid loads, resulting in a very high bandwidth of 50-100% (C. Eccardt et al., Proceedings Ultrasonics Symposium, San Antonio, Vol. 2, 1996, pp. 959-962; P.C. Eccardt et al., Proceedings of the 1997 Ultrasonics Symposium, Toronto, Vol. 2, 1997, pp. 1609-1618). In the following it is shown how the BiCMOS process has been modified to meet the demands for ultrasound generation and reception. Bias and driving voltages have been reduced down to the 10 V range. The electromechanical coupling is now almost comparable with that for piezoelectric transducers. The measurements exhibit sound pressures and bandwidths that are at least comparable with those of conventional piezoelectric transducer arrays.     

基于声光折射的聚焦超声焦点声压检测

提出了一种基于声光折射对聚焦超声焦点声压进行非侵入式检测的方法。当一束直径小于声波长的平行光束入射穿过聚焦超声的焦点时,通过研究焦点声压与光线偏转的具体关系,建立了光线最大偏转距离与焦点声压变化的关系模型,从而计算出焦点峰值声压。为了对理论模型进行验证,采用凹球面型聚焦换能器进行实验研究。通过与采用光纤水听器测量的结果进行对比,证明理论模型的可行性。结果表明实验得到的光斑图像与理论分析的结果一致,且用该方法测得的焦点峰值声压与光纤水听器测量的结果相比,相对误差小于15%,证明该方法具有可行性,能够定量检测焦点峰值声压。模型的提出也为将声光折射效应用于整个聚焦声场的定量检测提供了实验依据和理论依据。     

浅海低频声场中目标深度分类方法研究

分析了浅海甚低频声场中的垂直双水听器的互谱特性,尤其关注了声场中只存在两阶简正波时声压互谱的有功分量.数值计算表明：当两个水听器的深度布放合理时,声压互谱有功分量的正负号分布与水平距离无关,选择合适的水听器深度布放组合,可以用于水面目标、水下目标的双择判决;声速分布对该方法的使用影响不大,海底存在吸收将影响算法的作用距离,但在有效作用距离内,该算法仍有较好的稳健性.运用Pekeris波导有效深度的概念建立了声压互谱有功分量正负号分布特性的近似理论分析并可预报双水听器合理的布放深度.理论分析揭示了双水听器布放深度之和应等于波导有效深度,并且在有效深度限制的波导范围内,整个声场的声压互谱有功分量的正负号分布被分为三个水平区域.理论分析加深了对浅海低频声场特性的理解,对该算法的实际使用有指导意义. 关键词： 目标深度分类 浅海低频声场 声压互谱 有功分量     

基于纹影法的聚焦超声声场重建算法研究

为了从聚焦超声声场纹影图像直接重建声场声压分布图像,首先根据水中声波与光波的作用规律,利用Zernike相衬技术得到纹影系统中空间声压分布与纹影图像中光强的关系,再通过纹影系统获得聚焦超声声场实时图像,最后根据纹影系统的物理特性经过反投影重建算法重建出凹球壳聚焦超声换能器的空间声压分布。分析可知,理论声焦域横向与声轴大小分别为0.15 mm、1.4 mm,重建声场电功率为12 W时横向最接近为0.25 mm,30 W时声轴最接近为1.35 mm。与球壳换能器的理论声压分布进行对比的结果表明,该方法具有一定可行性,可以用于聚焦超声换能器的声场分布检测。     

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.     

Tuning the type of nitrogen on N-RGO supported on N-TiO2 under ultrasonication/hydrothermal treatment for efficient hydrogen evolution – A mechanistic overview

Efficient hydrogen production through water splitting has been the challenging task to be achieved in the present context of energy crisis. Among the various catalysts employed, nitrogen doped Titanium dioxide/Reduced graphene oxide (N-TiO₂/RGO) nanocomposite has been established to be a promising photocatalytic material for this purpose. However, nuances of doping nitrogen on TiO₂ and the type of nitrogen (pyridinic, pyrrolic and graphitic) stabilized on RGO responsible for facilitating the H₂ production has not yet been addressed mechanistically. In the present investigation, an attempt has been made to synthesise N-Titanium dioxide/N-Reduced graphene oxide (NTNG) nanocomposite under ultrasonication followed by hydrothermal treatment. A stainlesssteel ultrasonic bath, of 6.5 L tank size (LxBxH) 300 × 150 × 150 mm, was used for ultrasonic treatments. The transducers located at the bottom of the ultrasonic bath generate a frequency of 40 kHz with maximum power of 200 W. A mechanism has been proposed including the nuances of formation and the stabilisation of each type of nitrogen on N-RGO as a function of ultrasonication time. The present work supports the stabilization of a given type of nitrogen on RGO through keto enol tautomerism. XPS and FTIR studies have been undertaken to identify the different types of nitrogen doping and the presence of functional groups respectively. XRD, UV–Vis DRS and PL investigations have been made to establish morphological profile and band gap structure of the nanocomposite. It was observed that pyrrolic type nitrogen stabilized on N-RGO augments the efficiency of photocatalytic activity through hydrogen production by water splitting.     

Modeling of three-dimensional pressure fields in sonochemical reactors with an inhomogeneous density distribution of cavitation bubbles. Comparison of theoretical and experimental results

During the last 50 years extensive experimental investigation has been carried out on the chemical effects of ultrasound, but limited work has been reported on modeling. This paper presents a new model in which a numerical calculation of the three-dimensional linear sound pressure field distribution in a commonly used sonoreactor containing three transducers is carried out. In this model the inhomogeneous three-dimensional time-dependent wave equation was solved using the finite difference approach. The modeled results are then compared with the experimentally measured values, and the agreement, in general, is found to be good. Further, our modeling studies have an advantage, since they clearly describe the continuous sound pressure field structure, unlike previously reported results in which some information is missing due to limited intermittent measured points.