用碘释放法研究双束超声辐照的脉冲空化峰

本文使用双束同频脉冲超声水平正交辐照,通过磺释放法检测,发现双束同频水平正交辐照亦可观察到明显的空化峰现象及声化学产额增长。     

超声空化的电化学检测及混响场空化效应的研究

本文提出并设计了超声空化的电化学检测法，实现了通过分析样品的电学量变化来检测声空化，通过对混响场空化效应的研究，定性分析解释了混响场中空化饱和现象? ?     

低压大流量自激脉冲空化射流试验研究

采用自行设计的自激脉冲空化射流喷嘴和试验测试系统进行了一系列低压大流量空化射流试验,得到了一组喷嘴结构参数,使得喷嘴在入口压力为0.95~1.13 MPa时能产生较为明显的脉冲效果。通过试验获得了随入口压力升高腔室内部流场变化规律,并对试验中喷嘴腔室内部压力突降现象进行分析。试验表明自激脉冲空化射流的打击力是连续射流的1.3~1.6倍。     

低压大流量自激脉冲空化射流试验研究EI北大核心CSCD

采用自行设计的自激脉冲空化射流喷嘴和试验测试系统进行了一系列低压大流量空化射流试验,得到了一组喷嘴结构参数,使得喷嘴在入口压力为0.95~1.13 MPa时能产生较为明显的脉冲效果。通过试验获得了随入口压力升高腔室内部流场变化规律,并对试验中喷嘴腔室内部压力突降现象进行分析。试验表明自激脉冲空化射流的打击力是连续射流的1.3~1.6倍。     

声空化泡对声传播的屏蔽特性*

该文介绍了声空化液体中声波被反常吸收的现象,即驱动声压越大,吸收越强,远场声压越低。研究给出其物理机理是高声压导致强空化,空化泡吸收驱动能量辐射高次谐波,高频声波更易被液体吸收,最终形成更低的远场声压。为了克服空化屏蔽,改善声空化的均匀性,提出了改变工作液体的空化阈值的思路。并就简单的双层液体系统进行了计算和实验,结果证实这种思路的正确性。     

关于表述空化场强弱术语的讨论

声空化是声学中特别是超声学中一个极其重要的物理现象。然而,尽管对于声空化的研究已经有着极长的历史,但仍然有一些极基础的理论问题有待解决。如何来描述或表征声空化状态的强弱程度,就是这样一类的问题。该文对所谓的“空化强度”概念提出质疑,并对影响声空化现象的一些因素做出分析,相应地指出解决问题的一些建议,以期人们对这个无论在理论研究和实际应用方面都有重要意义的问题深入研究。     

自激脉冲喷嘴发生机理数值模拟

本文揭示了自激脉冲喷嘴产生自激振荡的根本原因。由于腔内产生了低于汽化压力的负压,引起低压区液体的汽化,加上射流不稳定剪切层的作用,最终形成大小周期性变化的轴对称空化气囊;对脉冲喷嘴腔内流场进行了二维非定常数值模拟,通过对腔内压力和流场的分析,验证了上述结论;试验研究通过对腔内压力的数据采集及对脉冲现象的同步观察,进一步说明了本文结论的正确性。     

超声滚压中的空化现象*

在超声滚压加工中引入切削液后可能会产生空化现象,由此产生的微射流和冲击波对超声表面强化将有积极作用。为研究超声滚压加工中空化现象是否存在及空化效应在超声滚压中的作用,本文首先分析了超声滚压中的空化阈值,然后进行了染色法试验和超声滚压后试样氧元素能谱分析,最后通过超声滚压加工对比试验研究了空化效应对加工后材料表面粗糙度和显微硬度的影响。研究发现,超声滚压加工中的声压幅值远大于空化阈值,满足空化存在的必要条件;超声滚压中发生了明显的卡纸染色现象,引入切削液后工件超声滚压加工表面氧元素含量显著提高,表明超声滚压中发生了空化现象。超声滚压加工中的空化效应能进一步降低工件表面粗糙度和提高表面显微硬度,有利于提高工件表面强化质量。本研究为空化效应在超声滚压中的积极利用提供了依据。     

超声空化现象影响因素的实验研究

超声空化在许多不同的学科和工业生产中有着广泛的应用。超声空化的应用与声场的分布及空化的机理密切相关,精准地反映空化场和空化机理是超声空化技术实际应用的关键。该文通过分析采集的声信号和金属箔膜空蚀法对空化区域随液位发生变化的现象进行研究,并利用Matlab对金属箔膜空蚀程度量化。实验发现,超声波会在液面与实验箱体底部形成驻波场。在某一液体温度下,随着液位高度的变化,超声空化现象的出现具有周期性。并且,在同一液位下,当超声功率改变时,空化区域强度分布情况随之改变。小功率时各空化区域空化强度分布均匀,当功率增大到一定时,会出现空化屏蔽现象。该研究为超声清洗设备的改良提供了借鉴,对进一步认识和利用超声空化效应具有重要意义。     

伴随水力空化现象的对流换热数值研究

以水为介质,建立了液体流动的混合物多相流模型及空化模型,运用CFD方法对水平圆管内伴随有水力空化现象的受迫对流换热过程进行了数值研究,详细分析了管道入口压力、入口温度和限流孔与管道直径比等因素对水力空化及对流换热过程的影响规律。数值模拟结果表明,空化现象出现在圆管喉部(限流孔)壁面附近区域;与相同流量下无空化时的传热相比,在发生空化现象的区域,传热壁面被蒸汽所覆盖导致传热急剧恶化,而在远离空化发生区域的下游位置,由于空化的扰流作用使得加热壁面与流体之间的传热得到明显改善。     

Acoustic microcavitation: its active and passive acoustic detection

In this work acoustic microcavitation in water is studied primarily at 0.75 MHz and 1% duty cycle. To detect cavitation, two kinds of acoustic detectors are used. The first one is an unfocused, untuned 1-MHz receiver transducer that serves as a passive detector. The other one is a focused 30-MHz transducer that is used in pulse-echo mode and is called the active detector. Cavitation itself is brought about by a focused PZT-8 crystal driven in pulse mode. The active detector is arranged confocally with respect to the cavitation transducer. Both the interrogating pulse and the cavitation pulse arrive simultaneously at the common focus, which is the region of cavitation. With the test chamber filled with clean water, no cavitation is observed, even when the cavitation transducer is driven to give its peak output of 22 bar peak negative. Cavitation is, however, observed when polystyrene microparticles are added to the host water. Our view of how these smooth, spherical, monodispersed microparticles give rise to cavitation is described with some estimates. An attempt has been made to understand whether the presence of "streaming" affects the thresholds, and it has been found that the active detector field affects the cavitation process.     

脉冲超声激励下SonoVue微泡的瞬态空化特性

将SonoVue微泡从临床疾病诊断拓展至治疗引起了诸多研究人员的兴趣。为了平衡治疗效率和生物安全性,深入理解声学参数和SonoVue微泡瞬态空化的关系至关重要。本研究自行制备仿体容器放置SonoVue微泡,使用1 MHz发射换能器激励其产生空化效应,另一个7.5 MHz的聚焦换能器接收声信号,经放大及高速数据采集后送上位机处理。通过深入分析信号的时频域特征,我们提出以宽带信号的能量及其随时间变化曲线的半高宽来表征瞬态空化的剂量(ICD)和相对持续时间(ICP),并确定:瞬态空化的发生和ICD依赖于峰值负声压,但ICP随峰值负声压的增加而减小;脉冲重复频率和脉冲持续时间都和ICD及ICP正相关;且脉冲持续时间的影响较大。这些结果有望为SonoVue微泡的治疗应用提供理论支持。      

Acoustic microcavitation: enhancement and applications

Experimental investigation into acoustic microcavitation is extended to include "cavitation activity" in addition to the threshold measurements using acoustic detectors. The primary setup incorporates two detectors: an unfocused, untuned 1-MHz transducer, which serves as a passive detector, and a focused 30-MHz transducer used in pulse-echo mode as the active detector. Cavitation itself is brought about by a focused piezoelectric transducer driven in pulse mode. The active detector is arranged confocally with respect to the cavitation transducer. Both the interrogating pulse and the cavitation pulse arrive simultaneously at the common focus which is the region of cavitation. Cavitation is conducted primarily at 0.75 MHz and 1% duty cycle in clean water using microparticles to seed the events. Cavitation activity appears to be directly proportional to the number density of the particles present in the cavitation medium. The fact that the active detector affects the cavitation process can be further exploited to seek interesting applications leading possibly to submicronic particle counting and testing of surface characteristics of silica particles used in liquid chromatography.     

Modeling cavitation nucleation from laser-illuminated nanoparticles subjected to acoustic stress

In an earlier work by Farny et al. [ARLO 6, 138-143 (2005).] it was demonstrated that the acoustic cavitation threshold in a tissue mimicking gel phantom can be lowered from 4.5 to ～1 MPa by "seeding" the optically transparent phantom with light absorptive gold nanoparticles and irradiating these absorbers with nanosecond pulses of laser light at intensities less than 10 mJ/cm(2). As a follow-up study, a three-stage numerical model was developed to account for prenucleation heating, the nucleation and formation of the vapor cavity, and the resulting vapor bubble dynamics. Through examination of the radius-time evolution of the cavity, the combined thresholds for laser radiant exposure and acoustic peak pressure required to induce inertial cavitation are deduced. It is found that the threshold pressure decreases when laser exposure increases; but the rate depends on exposure levels and the size of the particle. Investigations of the roles of particle size and laser pulse length are performed and optimum choices for these parameters determined in order to obtain inertial cavitation at the lowest possible acoustic pressure and laser intensity.     

Study of a peak in cavitation activity from HIFU exposures using TA fluorescence

Methods of measuring the sound field and focal region of a 1.05 MHz high intensity focused ultrasound (HIFU) are described in this paper. 1.05 MHz pulsed HIFU with intensity 2400 W/cm(2) with a 1:1 duty cycle ("on" phase equaled "off" phase) was used to irradiate terephthalic acid (TA). Pulse periods of 0.5 ms, 1 ms, 3.3 ms, 10 ms, 15 ms, 33 ms, 0.1s and 1s were used. The irradiation time was 2 min. To indicate the intensity of inertial cavitation activity, the fluorescence intensity of hydroxyterephthalic acid (HTA) was measured. The result shows that the cavitation activity of pulsed HIFU peaks at a pulse period of 10 ms, cavitation activity is significantly greater for pulse periods from 2 to 20 ms than for others.     

Comparison of electrohydraulic lithotripters with rigid and pressure-release ellipsoidal reflectors. I. Acoustic fields.

The most common lithotripter, a Dornier HM-3, utilizes an underwater spark to generate an acoustic pulse and a rigid ellipsoidal reflector to focus the pulse on the kidney stone to be comminuted. The pulse measured in water with a PVDF membrane hydrophone at the external focus of the ellipsoid was a 1-microsecond positive-pressure spike followed by a 3-microsecond negative-pressure trough. When we replaced the rigid reflector in our experimental lithotripter with a pressure-release reflector, the pulse was a 1.6-microsecond trough followed by a 0.6-microsecond positive spike. The waveforms are nearly time inverses (i.e., their spikes and troughs are reversed). The frequency spectra, the maximum peak positive pressures P+ (42 MPa, rigid and 43 MPa, pressure-release), and the maximum peak negative pressures P- (-12 MPa and -14 MPa) are comparable. The maximum P- occurred 20 mm closer to the reflector than did the maximum P+, for both reflectors. However, the spatial maxima of the peak pressures (P+ and P-) produced by the pressure-release reflector were located 20 mm nearer to the reflector than those produced by the rigid reflector. Qualitative explanation of the waveforms and the location of pressure maxima as well as comparison to previous theoretical and experimental results is given. The alternate waveform produced by the pressure-release reflector may be a tool in determining the role of cavitation in lithotripsy because cavitation is highly sensitive to waveform.     

Time-resolved measurements of picosecond optical breakdown

Picosecond optical breakdown was investigated in order to assess its potential for performing highly localized incisions for laser surgery. Measurements of breakdown were performed using single 40-ps Nd: YAG laser pulses in distilled water. Novel optical pump-probe techniques were developed to characterize the transient spatial and temporal dynamics of the plasma, shock wave, and cavitation phenomena which are associated with the breakdown. The maximum cavity radius and the shock wave zone are shown to scale as the cube root of the pump pulse energy over almost three orders of magnitude. For pulse energies close to the threshold energy of 8 J, the shock range was 100–200 m and the cavity radius was 140 m. Complementary experiments were performed with 10-ns pulse durations. Since picosecond pulses have high peak intensities with low pulse energies, a significant enhancement in localizability may be achieved. The implications for ophthalmic microsurgery are discussed.     

Cavitation threshold measurements for microsecond length pulses of ultrasound

The acoustic cavitation threshold of an aqueous solution has been measured at megahertz frequencies as a function of pulse width and pulse repetition frequency for various combinations of these quantities. The fluid tested was a 0.1M KOH-H3BO3 buffer solution with pH 10.9, which contained luminol, was saturated with argon, and filtered to 25 mu. The presence of cavitation was detected by a photomultiplier tube that required the emission of visible light that was both larger in magnitude and longer in duration than a preset criterion. It was observed that the cavitation threshold of water under pulse conditions decreases both when the pulse width is fixed and the pulse repetition frequency is increased, and when the pulse repetition frequency is fixed and the pulse width is increased. Acoustic cavitation thresholds measured in aqueous solutions are significantly less than those acoustic pressures associated with instruments that are currently in widespread use in medicine.     

Ultrasound-enhanced diffusion through isolated frog skin

The effect of ultrasound on the transport of oxygen across excised frog abdominal skin has been studied. Samples were mounted in an exposure chamber in which the Ringer's solution on one side was saturated with oxygen while the other side of the skin had a low initial oxygen concentration. They were treated with ultrasound at 1, 1.5 and 2 W cm⁻² SATA c.w., respectively, and increases in the rate of oxygen transport were observed at all intensities. These increases ranged from 38 ± 4% at 1 W cm⁻² to 55 ± 8% at 2 W cm⁻². Variation in the pulse lengths from 25 to 200 ms and a constant average intensity did not affect the rate of transport significantly provided that the temporal intensity was constant. Since the peak acoustic pressure within the pulse increased with decreasing pulse length and increasing acoustic pressure increases the probability of cavitation occurring, the mechanism responsible for this phenomenon is probably not cavitation.