超声空化及其声流结构实验研究*

超声空化及其声流效应在医学、化工和能源等领域得到广泛应用。本文采用高速摄像和粒子图像测速系统分别研究了超声场下的空化形态和声流场结构的时空演化规律。实验研究了50W,100W,200W和250W等四种不同输入功率对18kHz的超声变幅杆附近空化及其声流场的影响。研究结果表明：（1）在变幅杆下端面处观察到由大量空化气泡均匀分布组成的倒置锥形空泡结构,并且锥形空泡结构为稳态流动结构。（2）在超声变幅杆附近产生了两种不同的声流形式,第一种是变幅杆底端的射流型声流,第二种是变幅杆两侧的回旋流。此外,通过研究空泡与声流场中最大速度点之间的空间对应关系,发现声流是因为空泡流动带动而产生的。（3）空间位置和输入功率能显著影响射流型声流的流场结构,但是对回旋流的影响十分微弱。     

弱酸试纸法测量功率超声珩磨空化声场的研究*

为了简便、直观地测量功率超声珩磨磨削区的空化声场强度及分布情况,提出了利用弱酸PH试纸测量磨削区空化声场的方法。利用超声空化效应产生的弱酸空化泡在PH试纸表面溃灭后,形成深浅和分布不同的变色区域,间接地表征油石表面空化声场的强弱和分布规律。通过对比不同超声频率、测试距离和时间,得出了最佳测试距离和时间。结果表明,当超声频率为18.6 kHz,距离为10 mm时,测得油石表面的空化声场强度和分布最佳。该方法可形象地评价功率超声珩磨磨削区空化声场的强度和分布情况,具有一定的实际应用价值。     

超声反射成像测井的有限元分析*

利用有限元方法研究了有限长换能器激发的超声波在套管井多层介质中的传播规律。数值模拟了换能器长度及套管曲率对反射回波的影响。随着换能器长度增加,反射回波频谱谱陷个数由单一向多个变化;套管共振透射窗的反射波相对能量先逐渐减小后振荡变化,较小长度的换能器对套管与水泥环界面的声阻抗区分能力优于较大长度的换能器。随着套管曲率的增大,套管共振透射窗的反射波由单一模式的厚度振动纵波向其他模式波变化。数值模拟结果对超声脉冲反射声波测井仪器的换能器结构优化设计起到了良好的指导作用。     

功率超声与清洗技术

 超声波是物理学的一个分支,它是指频率在２０ｋＨｚ以上的声波．由于它具有良好的束射性、很高的强度和很强的穿透能力等特点,所以应用十分广泛．超声波的应用可以分为两大类：一类是检测超声;另一类是功率超声．检测超声是利用超声波的性质来对物质进行各种检验和测量,如Ｂ超、流量和液位测量等;而功率超声则是利用超声波振动形式的能量使物质的一些物理、化学和生物性质或状态发生改变。利用超声波进行清洗,是功率超声一种最广泛的应用,它起始于五十年代初,其特点是清洗速度快、质量高、易于实现自动化．特别适用于清洗表面形状复杂的工件,如对于精密工件上空穴、狭缝、凹槽、微孔及暗洞处,通常的洗刷方法难以奏效.     

超声清洗槽内空化强度的测量

对洗涤槽内超声空化的相对强度及其分布进行了研究。采用铝箔腐蚀法,应用图像、数据处理软件对试验结果进行处理,最后对实验结果进行了讨论。     

双频超声激励空化泡动力学耦合因素研究*

研究双超声各因素对空化气泡动力学影响,找出最佳的耦合效果。对Rayleigh-Plesset方程模型进行修改,并利用MATLAB提供的Runge-Kutta算法进行数值求解,考察频率、相位差,声强的不同耦合对双超声激励空化泡动力学过程的影响。结果表明:在其他条件相同的情况下,双超声与单超声相比,空化泡运动过程中的最大半径较大,空化泡半径收缩的变化率也较大,同时可知,采用相同频率低频超声组合的空化强度最强;保持双超声初相位相同,避免两者反相,以及超声声强组合平均分配等,有利于提高空化效果。当采用不同频率组合激励时,要根据实验条件选择合适的双超声频率组合、相位差和声强的分配,才能起到协同强化作用。     

单泡超声空化仿真模型的建立及其动力学过程模拟*

为了模拟单泡超声空化的动力学特性,建立了单泡超声空化的有限元仿真模型,基于流体动力学控制方程和流体体积分数模型,利用有限元分析软件模拟了超声驱动下水中单泡的空化动力学过程。结果表明:单泡随时间的演化规律是先缓慢膨胀到最大后迅速塌缩;泡内压强与气体密度变化与单泡体积变化成反比;在膨胀阶段,泡外压强与气体密度沿着泡的径向向外递减;在压缩阶段,泡外在声压垂直方向的压强与气体密度要大于声压激励方向的压强和气体密度。该文分析结果将为超声空化动力学过程模拟及研究提供参考。     

声表面波气体传感器研究进展*

基于声表面波技术的气体传感器包括采用敏感膜和结合气相色谱两种方式。比较而言,采用敏感膜的声表面波气体传感器体积小、功耗低,适应小型化毒气报警器的发展要求,但可检测的气体种类少、灵敏度低、存在交叉干扰问题;声表面波与气相色谱联用的气体分析仪灵敏度高、可检测气体种类多、很好地解决交叉干扰问题,特别适合于复杂大气背景条件下的气体成分分析。本文从传感器响应机理分析与物理功能结构两方面出发介绍了两类声表面波气体传感器的研究进展情况。     

超高泵浦功率下的Xe(6p[1/2]0, 6p[3/2]2, 和6p[5/2]2)原子的能量转移过程

本文研究了Xe(6p[1/2]₀, 6p[3/2]₂, and 6p[5/2]₂)原子在聚焦条件下的动力学过程. 激发能级的原子密度在聚焦条件下会显著地增加,因此两个高激发态原子之间的energy-pooling碰撞的概率也会增加. 这种energy-pooling碰撞主要有三种类型. 第一种类型为energy-pooling碰撞导致的电离. 一旦将激发激光聚焦,就可以从侧面的窗口观察到非常明显的电离现象,不论激发能级是6p[1/2]₀、6p[3/2]₂或6p[5/2]₂能级. 这种电离的产生机理是energy-pooling电离或者一个Xe^*原子再吸收一个光子产生电离. 第二种类型为跨越较大能极差的energy-pooling碰撞. 当激发能级为6p[1/2]₀能级的情况下,两个6p[1/2]₀原子碰撞会产生一个5d[3/2]₁原子和一个6s''[1/2]₀原子. 第三种类型为跨越较小能级差的energy-pooling碰撞. 以5个二次产生的6p能级为上能级的荧光强度都变得更强,并且这些荧光的上升沿都变得更陡峭. 产生这些6p原子的主要机理是energy-pooling碰撞并非简单的碰撞弛豫. 基于理想气体原子之间的碰撞概率公式,推导出两个6p[1/2]₀原子的energy-pooling碰撞速率为6.39x10⁸s^-1. 此外,6s原子在聚焦条件下的密度也会增加. 因此所有的荧光曲线会因为辐射俘获效应而出现非常严重的拖尾.     

超声对近壁微气泡溃灭过程的影响

近壁微气泡溃灭特性的深入研究对靶向给药和基因治疗等技术具有较好的指导作用。该文基于数值模拟技术,采用有限体积法结合流体体积模型对超声作用下的近壁微气泡溃灭特性进行了研究,分析了超声对近壁微气泡溃灭动力学过程的影响。结果表明气泡溃灭最大射流速度与近壁距离无量纲参数γ在1.1～1.6范围内时成正比,与超声频率在10～60 Hz范围内时成正比,与气泡初始半径在50～100μm范围内时成反比;近壁气泡的二次溃灭最大射流速度大于一次溃灭,二次溃灭的作用更加明显。超声参数对近壁气泡溃灭过程存在较大影响,该研究为超声在医学上的应用提供了依据。     

Physical and chemical effects of acoustic cavitation in selected ultrasonic cleaning applications

Acoustic cavitation in a liquid medium generates several physical and chemical effects. The oscillation and collapse of cavitation bubbles, driven at low ultrasonic frequencies (e.g., 20 kHz), can generate strong shear forces, microjets, microstreaming and shockwaves. Such strong physical forces have been used in cleaning and flux improvement of ultrafiltration processes. These physical effects have also been shown to deactivate pathogens. The efficiency of deactivation of pathogens is not only dependent on ultrasonic experimental parameters, but also on the properties of the pathogens themselves. Bacteria with thick shell wall are found to be resistant to ultrasonic deactivation process. Some evidence does suggest that the chemical effects (radicals) of acoustic cavitation are also effective in deactivating pathogens. Another aspect of cleaning, namely, purification of water contaminated with organic and inorganic pollutants, has also been discussed in detail. Strong oxidising agents produced within acoustic cavitation bubbles could be used to degrade organic pollutants and convert toxic inorganic pollutants to less harmful substances. The effect of ultrasonic frequency and surface activity of solutes on the sonochemical degradation efficiency has also been discussed in this overview.     

The influence of pressure on the acoustic cavitation in saturated CO2-expanded N,N-dimethylformamide

CO₂-expanded organic solvent is a kind of important fluid medium and has broad applications in chemical industry, environmental protection and other fields. Ultrasonic cavitation in gas expanded liquids (GXLs) is conducive to enhancing mass transfer and producing many exciting phenomena. In this paper, the ultrasonic cavitations and streaming in the saturated CO₂-expanded liquid N, N-dimethylformamide (DMF) at 4.2 MPa and 5.2 MPa are observed by a high-speed camera. The cavitation intensity and time trace of pressure pulses are recorded using a PZT hydrophone. The influences of gas–liquid equilibrium pressure and ultrasonic power on the cluster dynamics of transient and stable cavitation are examined. The excess molar enthalpies required for CO₂ dissociation from DMF are calculated by Peng-Robinson equations of state and the change of surface free energy of CO₂-expanded DMF is predicted. The results show that the excess enthalpy of the mixture is one of the key factors to control ultrasonic cavitation at high pressurized conditions, while the surface tension is the key factor for low pressure. As the increase of applied ultrasonic power, the formation and collapsing frequency of bubble clusters increases, and the amplitude and cyclic frequency of pressure pulse are enhanced. The transient cavitation intensity increases as it reaches a maximum value at a certain ultrasonic power and then decreases. The change trends of stable cavitation intensity under different pressures are basically same. It can be concluded from the evidence that ultrasonic cavitation in CO₂-expanded DMF is affected by the combined effect of compression and substitution: compression depresses the nucleation and growth of bubbles, while the high solubility of CO₂ in DMF is conducive to the generation of bubbles in cavitation.     

超声清洗装置空化噪声谱分析法空化噪声级测量

空化强度是用以衡量液体介质中空化活动的剧烈程度,同时空化效应在超声清洗中起关键作用,因此,测量超声清洗槽中的空化强度便可了解其中空化活动的情况.当发生空化时,液体介质中会产生成分复杂空化噪声,对空化噪声谱进行分析和计算得到空化噪声级,据此可判断空化强度.实验测得结果表明:超声清洗装置内稳态空化分布广泛、均匀,瞬态空化分...     

超声空化状态对苯酚降解的影响

给出不同空化状态下超声波降解苯酚溶液的实验结果,比较了相应的声压级频谱和合成声强。研究了苯酚溶液的浓度,二阶铁盐,超声辐射时间对苯酚降解率的影响,讨论了不同空化状态下的声压级频谱特征。     

Studies of a novel sensor for assessing the spatial distribution of cavitation activity within ultrasonic cleaning vessels

This paper describes investigations of the spatial distribution of cavitation activity generated within an ultrasonic cleaning vessel, undertaken using a novel cavitation sensor concept. The new sensor monitors high frequency acoustic emissions (>1 MHz) generated by micron-sized bubbles driven into acoustic cavitation by the applied acoustic field. Novel design features of the sensor, including its hollow, cylindrical shape, provide the sensor with spatial resolution, enabling it to associate the megahertz acoustic emissions produced by the cavitating bubbles with specific regions of space within the vessel. The performance of the new sensor has been tested using a 40 kHz ultrasonic cleaner employing four transducers and operating at a nominal electrical power of 140 W under controlled conditions. The results demonstrate the ability of the sensors to identify 'hot-spots' and 'cold-spots' in cavitation activity within the vessel, and show good qualitative agreement with an assessment of the spatial distribution of cavitation determined through erosion monitoring of thin sheets of aluminium foil. The implications of the studies for the development of reliable methods of quantifying the performance of cleaning vessels are discussed in detail.     

Numerical modelling of acoustic pressure fields to optimize the ultrasonic cleaning technique for cylinders

Fouling build up is a well-known problem in the offshore industry. Accumulation of fouling occurs in different structures, e.g. offshore pipes, ship hulls, floating production platforms. The type of fouling that accumulates is dependent on environmental conditions surrounding the structure itself. Current methods deployed for fouling removal span across hydraulic, chemical and manual, all sharing the common disadvantage of necessitating halting production for the cleaning process to commence. Conventionally, ultrasound is used in ultrasonic baths to clean a submerged component by the generation and implosion of cavitation bubbles on the fouled surface; this method is particularly used in Reverse Osmosis applications. However, this requires the submersion of the fouled structure and thus may require a halt to production. Large fouled structures such as pipelines may not be accommodated. The application of high power ultrasonics is proposed in this work as a means to remove fouling on a structure whilst in operation. The work presented in this paper consists of the development of a finite element analysis model based on successful cleaning results from a pipe fouled with calcite on the inner pipe wall. A Polytec 3D Laser Doppler Vibrometer was used in this investigation to study the fouling removal process. Results show the potential of high power ultrasonics for fouling removal in pipe structures from the wave propagation across the structure under excitation, and are used to validate a COMSOL model to determine cleaning patterns based on pressure and displacement distributions for future transducer array design and optimization.     

The influence of air content in water on ultrasonic cavitation field

Cavitation is a complex physical phenomenon affected by many factors, one of which is the gas dissolved in the medium. Researchers have given some efforts to the influence of gas content on sonoluminescence or some specific chemical reactions in and around the bubble, but limited work has been reported about the influence on the ultrasonic cavitation field distribution. In this work, the intensity distribution of the ultrasound field in a cleaning tank has been measured with the hydrophone. After analysed and visualised by MATLAB software, it was found that the cavitation intensity distribution in degassed water was much better than that in tap water. And further study proved that degassing process can improve the cavitation effect dramatically both in intensity and scope. Finally, the cavitation fields in mediums with different gas content were measured and the specific influence of air content on cavitation field was discussed.     

一种保证超声清洗机性能的换能器大功率测试方法

本文较详细地叙述了生产超声清洗机过程中，对换能器频率，频带宽度，阻抗等参数实施一种简单而实用的大功率测试法，据此进行筛选来提高超声清洗机的性能。     

不同环境温度下工艺参数对扩散吸收式制冷机的性能影响

外界环境温度对扩散吸收制冷循环有重要影响,以四台相同结构的扩散吸收式制冷机为基础,用实验的手段研究了不同环境温度下,氨水溶液灌装浓度、充注压力、输入功率等工艺参数对扩散吸收式制冷机性能的影响。得出了实验样机在各工况下的最优工艺参数;研究发现随环境温度的升高,样机的最佳灌装浓度下降,而最佳输入功率和最适宜充注压力升高。可为扩散吸收式制冷机根据环境温度选择各工艺参数提供一定的依据。     

Experimental investigation of the effect of ambient pressure on laser-induced bubble dynamics

The effect of ambient pressure on the dynamics of laser-induced bubbles was investigated by a fiber-optic diagnostic technique based on probe beam deflection (PBD). The experimental criterion for judging the maximum bubble radius is modified to the average value of the detecting distances at which the characteristic waveform signals appear. The ambient pressure affects the maximum radius and collapse of bubble strongly. The experimental results indicate that the maximum bubble radius and the collapse time both decrease nonlinearly while the ambient pressure increases linearly, and the decreasing velocities of them are smaller at a larger ambient pressure. The predicted value of collapse time has a good agreement with experiment at larger ambient pressure.