超声波降解有机物溶液的气泡动力学研究

在超声波降解有机物溶液过程中,超声空化产生的高温高压以及空化泡振荡产生的激波在有机物溶液的降解中发挥重要作用.本文通过对超声波作用下气泡动力学的研究,讨论了超声波声压、频率、气泡初始半径等参量对有机物溶液降解效率的影响.研究发现,存在使降解效率极大的声压和频率。在空化稳定的情况下,存在一个使降解效率极大的气泡初始半径,降解效率随着黏滞系数的增大而减小。研究还发现,双频超声作用的空化效果比单频超声作用时强,与双频超声作用下有机物溶液降解率较大这一实验结果一致。      

低频超声空化场中柱状泡群内气泡的声响应

本文在气泡群振动模型的基础上,考虑气泡间耦合振动的影响,得到了均匀柱状泡群内振动气泡的动力学方程,以此为基础分析了低频超声空化场中柱形气泡聚集区内气泡的非线性声响应特征.气泡间的耦合振动增加了系统对每个气泡的约束,降低了气泡的自然频率,增强了气泡的非线性声响应.随着气泡数密度的增加,气泡的自然共振频率降低,受迫振动气泡受到的抑制增强.数值分析结果表明:1)驱动声波频率越低,气泡的初始半径越小,气泡数密度变化对气泡最大半径变化幅度的影响越大;2)气泡振动幅值响应存在不稳定区,不稳定区域分布与气泡初始半径、驱动声波压力幅值、驱动声波频率等因素有关.在低频超声波作用下,对初始半径处在1—10μm之间的空化气泡而言,气泡初始半径越小,气泡最大半径不稳定区分布范围越大,表明小气泡具有更强的非线性特征.因此,气泡初始半径越小,声环境变化对空化泡声响应稳定性影响越显著.     

双泡超声空化计算分析

将由速度势叠加原理得到的双泡超声空化动力学微分方程归一化,通过matlab语言编程计算,分析了水中空化泡的线度、双泡间距、声压幅值、声波频率等因素对空化过程的影响. 在双泡超声空化动力学微分方程中引入双频超声,探讨了双泡双频超声问题. 研究表明泡的线度是决定空化特性的主要因素,声压幅值对空化特性的影响最大,其次是超声波的频率;双泡间的相互作用影响空化特性,这种影响随双泡间距的增大而减弱;双频超声对双泡空化特性的影响有限,这种影响在两超声分量的声压幅值相等时较强. 关键词： 超声空化 双泡 双频超声     

流体控制方程的超声空化泡动力学模拟*

本文基于流体动力学控制方程和VOF模型,在FLUENT 14.5软件环境下对超声空化泡进行数值模拟。首先研究了超声空化泡一个周期内的形态变化,并且利用空化泡形态变化的最大面积、最小面积、膨胀时间、收缩时间等数值结果分析超声参数对空化效果的影响。同时探究了双频超声作用下空化泡运动的变化,计算结果表明:在其他条件相同的情况下,在1~5MPa范围内,超声声压幅值为3MPa时空化效果最好;当超声频率大于20kHz时,空化效果随着超声频率的增大而降低。对于频率相同的双频超声,较声压幅值为其两倍的单频超声有更好的空化效果;对于频率不同的双频超声,空化效果受到频率差的影响。     

双泡耦合声空化动力学过程模拟

为了对双泡耦合的声空化过程进行模拟,本文从流体动力学控制方程和流体体积分数模型出发,在Fluent软件中构建双泡耦合超声空化三维有限元仿真模型,对超声波驱动下流体中双泡耦合声空化动力学过程进行数值模拟,并通过对空化气泡周围声场的变化进行分析研究双泡耦合声空化的非线性动力学特性.结果显示:在超声波驱动下,球形气泡先缓慢扩张,扩张到最大半径后迅速收缩直至溃灭;耦合双气泡间存在相互作用力,使得空化气泡的扩张受到抑制、气泡收缩时间增长;空化气泡在收缩阶段的能量转换能力增强,相比单气泡声空化,耦合双气泡溃灭时气泡内部的压强更大.本文分析结果将为超声空化泡群的动力学过程模拟提供参考.     

声场作用下两空化泡相互作用的研究

建立了声场作用下两空化泡泡壁的运动方程,得出了双空化泡的共振频率,振动半径及空化噪声声压．由频率方程,振动半径和声压方程可以看出两气泡的运动情况与单气泡的运动情况有着明显的不同．共振频率,共振振幅及声压与两气泡之间的间距有关．在一定的简化条件下,运用MATLAB语言对共振频率,共振振幅及空化噪声声压进行了数值求解,发现共振频率和共振振幅随空泡间距的增大而增大,空化噪声声压随距离增大先增大后减小． 关键词： 超声 空化 频率 声压     

超声波作用下泡群的共振声响应

从球状泡群气泡动力学方程出发, 考虑泡群间次级声辐射的影响, 得到了声场中两泡群共同存在时气泡振动的动力学方程, 并以此为基础探讨声波驱动下双泡群振动系统的共振响应特征. 由于泡群间气泡间的相互作用, 系统存在低频共振和高频共振现象, 两不同共振频率的数值与泡群内气泡的本征频率相关. 泡群内气泡的本征频率又受到初始半径、泡群大小和泡群内气泡数量的影响. 气泡自由振动和驱动声波的耦合激起泡群内气泡的受迫振动, 气泡初始半径、气泡数密度和驱动声波频率等都会影响泡群内气泡的振动幅值和初相位. 关键词： 气泡群 共振 声响应 超声空化     

球状泡群内气泡的耦合振动

振动气泡形成辐射场影响其他气泡的运动, 故多气泡体系中气泡处于耦合振动状态. 本文在气泡群振动模型的基础上, 考虑气泡间耦合振动的影响, 得到了均匀球状泡群内振动气泡的动力学方程, 以此为基础分析了气泡的非线性声响应特征. 气泡间的耦合振动增加了系统对每个气泡的约束, 降低了气泡的自然共振频率, 增强了气泡的非线性声响应. 随着气泡数密度的增加, 振动气泡受到的抑制增强; 增加液体静压力同样可抑制泡群内气泡的振动, 且存在静压力敏感区(1–2 atm, 1 atm=1.01325×10⁵ Pa); 驱动声波对气泡振动影响很大, 随着声波频率的增加, 能够形成空化影响的气泡尺度范围变窄. 在同样的声条件、泡群尺寸以及气泡内外环境下, 初始半径小于5 μm 的气泡具有较强的声响应. 气泡耦合振动会削弱单个气泡的空化影响, 但可延长多气泡系统空化泡崩溃发生的时间间隔和增大作用范围, 整体空化效应增强.     

不同超声作用方式对葛根有效部位提取率的影响

研究不同的超声作用方式对葛根总黄酮的提取率的影响.采用单频,双频（包括槽式双频以及槽式＋探头式双频）,及三频等不同的处理方法,对超声作用参数,如时间,声强及处理量的多少进行研究,采用两个超声特性参数即能量效率和空化产量来对比不同容积、频率的超声作用效果.目前的研究表明：多频超声耦合时,可以获得较高的能量效率和空化产量.     

考虑水蒸气蒸发和冷凝的球状泡群中泡的动力学特性

为了深入探究空化泡群中气泡的动力学特性,建立了超声驱动下考虑水蒸气的蒸发和冷凝的泡群中泡的动力方程.基于该方程,研究了泡群中泡的位置、泡的数量、泡的初始半径对其动力学特性的影响,探究了超声作用下球状泡群中气泡半径、能量、温度、压力和气泡内水蒸气分子数的变化规律.结果表明:泡群中泡的运动受到周围气泡的抑制作用;泡群中泡的初始半径大小对泡群中泡的半径、能量、温度、压力和气泡内水蒸气分子数有显著影响;泡群中泡的位置距离泡群中心越远,泡的膨胀半径越大;随着泡群中泡的数目增加,泡的振幅减小;超声频率增加,泡群中泡的空化效应减弱;超声声压增加,泡群中泡的空化效应增加.研究结果为超声空化泡群的研究提供了理论参考.     

Effect of ultrasonic frequency and surfactant addition on microcapsule destruction

In a previous study, we found that cavitation bubbles cause the ultrasonic destruction of microcapsules containing oil in a shell made of melamine resin. The cavitation bubbles can be smaller or larger than the resonance size; smaller bubbles cause Rayleigh contraction, whereas larger bubbles are not involved in the sonochemical reaction. The activity in and around the bubble (e.g., shear stress, shock wave, microjet, sonochemical reaction, and sonoluminescence) varies substantially depending on the bubble size. In this study, we investigated the mechanism of the ultrasonic destruction of microcapsules by examining the correlations between frequency and microcapsule destruction rate and between microcapsule size and cavitation bubble size. We evaluated the bubbles using multibubble sonoluminescence and the bubble size was changed by adding a surfactant to the microcapsule suspension. The microcapsule destruction was frequency dependent. The main cause of microcapsule destruction was identified as mechanical resonance, although the relationship between bubble size and microcapsule size suggested that bubbles smaller than or equal to the microcapsule size may also destroy microcapsules by applying shear stress locally.     

Difference frequency and its harmonic emitted by microbubbles under dual frequency excitation

Vibro-acoustography is an elasticity imaging method that uses two ultrasound beams of slightly different frequency to excite an object and detects the resulting acoustic emission (AE) at the difference frequency. This method is especially sensitive to bubbles due to their nonlinearity. This study explores the harmonic acoustic emission (HAE) at twice the difference frequency emitted from bubbles. A perturbation method based on the dynamic bubble equation is used to derive the AE and HAE from a single bubble excited by dual frequency waves. Simulation shows that HAE is generated only by microbubbles whose resonant frequencies match the incident ultrasound frequencies. In contrast, AE is more sensitive to resonance at the difference frequency, which is relevant to sub-millimeter bubbles. This finding was confirmed by experiments where HAE was produced from Optison microbubbles, but not from larger air bubbles which are off resonance at the incident ultrasound frequency. In conclusion, harmonic acoustic emission is present for microbubbles. It is very sensitive to the size of the bubble and may be used for selective detection of microbubbles.     

Influence of mixing and ultrasound frequency on antisolvent crystallisation of sodium chloride

Ultrasound is known to promote nucleation of crystals and produce a narrower size distribution in a controlled and reproducible manner for the crystallisation process. Although there are various theories that suggest cavitation bubbles are responsible for sonocrystallisation, most studies use power ultrasonic horns that generate both intense shear and cavitation and this can mask the role that cavitation bubbles play. High frequency ultrasound from a plate transducer can be used to examine the effect of cavitation bubbles without the intense shear effect. This study reports the crystal size and morphology with various mixing speeds and ultrasound frequencies. The results show high frequency ultrasound produced sodium chloride crystals of similar size distribution as an ultrasonic horn. In addition, ultrasound generated sodium chloride crystals having a more symmetrical cubic structure compared to crystals produced by a high shear mixer.     

High-speed imaging of ultrasound driven cavitation bubbles in blind and through holes

The interest in application of ultrasonic cavitation for cleaning and surface treatment processes has increased greatly in the last decades. However, not much is known about the behavior of cavitation bubbles inside the microstructural features of the solid substrates. Here we report on an experimental study on dynamics of acoustically driven (38.5 kHz) cavitation bubbles inside the blind and through holes of PMMA plates by using high-speed imaging. Various diameters of blind (150, 200, 250 and 1000 µm) and through holes (200 and 1000 µm) were investigated. Gas bubbles are usually trapped in the holes during substrate immersion in the liquid thus preventing their complete wetting. We demonstrate that trapped gas can be successfully removed from the holes under ultrasound agitation. Besides the primary Bjerknes force and acoustic streaming, the shape oscillations of the trapped gas bubble seem to be a driving force for bubble removal out of the holes. We further discuss the bubble dynamics inside microholes for water and Cu²⁺ salt solution. It is found that the hole diameter and partly the type of liquid media influences the number, size and dynamics of the cavitation bubbles. The experiments also showed that a large amount of the liquid volume inside the holes can be displaced within one acoustic cycle by the expansion of the cavitation bubbles. This confirmed that ultrasound is a very effective tool to intensify liquid exchange processes, and it might significantly improve micro mixing in small structures. The investigation of the effect of ultrasound power on the bubble density distribution revealed the possibility to control the cavitation bubble distribution inside the microholes. At a high ultrasound power (31.5 W) we observed the highest bubble density at the hole entrances, while reducing the ultrasound power by a factor of ten shifted the bubble locations to the inner end of the blind holes or to the middle of the through holes.     

Forced linear oscillations of microbubbles in blood capillaries

A theoretical investigation of the forced linear oscillations of a gas microbubble in a blood capillary, whose radius is comparable in size to the bubble radius is presented. The natural frequency of oscillation, the thermal and viscous damping coefficients, the amplitude resonance, the energy resonance, as well as the average energy absorbed by the system, bubble plus vessel, have been computed for different kinds of gas microbubbles, containing air, octafluropropane, and perflurobutane as a function of the bubble radius and applied frequency. It has been found that the bubble behavior is isothermal at low frequencies and for small bubbles and between isothermal and adiabatic for larger bubbles and higher frequencies, with the viscous damping dominating over the thermal damping. Furthermore, the width of the energy resonance is strongly dependent on the bubble size and the natural frequency of oscillation is affected by the presence of the vessel wall and position of the bubble in the vessel. Therefore, the presence of the blood vessel affects the way in which the bubble absorbs energy from the ultrasonic field. The motivation of this study lies in the possibility of using gas microbubbles as an aid to therapeutic focused ultrasound treatments.     

Resonance frequency of microbubbles: effect of viscosity

The transmitted frequency at which a gas bubble of millimeter or submillimeter size oscillates resonantly in a low-viscosity liquid is approximately equal to the undamped natural frequency (referred to as the Minnaert frequency if surface tension effects are disregarded). Based on a theoretical analysis of bubble oscillation, this paper shows that such an approximation cannot be validated for microbubbles used in contrast-enhanced ultrasound imaging. The contrast-agent microbubbles represent either encapsulated bubbles of size less than 10 microm or free (nonencapsulated) bubbles of submicron size. The resonance frequency of the microbubbles deviates significantly from the undamped natural frequency over the whole range of microbubble sizes due to the increased viscous damping coefficient. The difference between these two frequencies is shown to have a tremendous impact on the resonant backscatter by the microbubbles. In particular, the first and second harmonics of the backscattered signal from the microbubbles are characterized by their own resonance frequencies, equal to neither the microbubble resonance frequency nor the undamped natural frequency.     

Exploring the effects of pulsed ultrasound at 205 and 616 kHz on the sonochemical degradation of octylbenzene sulfonate

Compared to continuous wave (CW) ultrasound, pulsed wave (PW) ultrasound has been shown to result in enhanced sonochemical degradation of octylbenzene sulfonate (OBS). However, pulsed ultrasound was investigated under limited pulsing conditions. In this study, pulse-enhanced degradation of OBS was investigated over a broad range of pulsing conditions and at two ultrasonic frequencies (616 and 205 kHz). The rate of OBS degradation was compared to the rate of formation of 2-hydroxyterephthalic acid (HTA) following sonolysis of aqueous terephthalic acid (TA) solutions. This study shows that sonication mode and ultrasound frequency affect both OBS degradation and HTA formation rates, but not necessarily in the same way. Unlike TA, OBS, being a surface active solute, alters the cavitation bubble field by adsorbing to the gas/solution interface of cavitation bubbles. Enhanced OBS degradation rates during pulsing are attributed to this adsorption process. However, negative or smaller pulse enhancements compared to enhanced HTA formation rates are attributed to a decrease in the high-energy stable bubble population and a corresponding increase in the transient bubble population. Therefore, sonochemical activity as determined from TA sonolysis cannot be used as a measure of the effect of pulsing on the rate of degradation of surfactants in water. Over relatively long sonolysis times, a decrease in the rate of OBS degradation was observed under CW, but not under PW conditions. We propose that the generation and accumulation of surface active and volatile byproducts on the surface and inside of cavitation bubbles, respectively, during CW sonolysis is a contributing factor to this effect. This result suggests that there are practical applications to the use of pulsed ultrasound as a method to degrade surface active contaminants in water.