超声波对池沸腾换热影响

对不同超声强度和辐射距离条件下过冷池沸腾换热特性进行了实验研究.超声波有效强化了沸腾起始段换热,对高热流密度沸腾传热也有一定的强化作用.超声辐射距离越近,强度越大,强化传热效果越好.对单相对流和沸腾起始区传热,超声波强化传热机理为空化作用;高热流密度沸腾时超声波对强化传热的主要机理是声流作用.得到了传热实验关联式.     

微波辅助对生物溶液超滤过程的影响

为验证微波场强化植物提取的"膜超滤"假说机理,以聚醚砜超滤膜为研究对象,以膜通量为考察指标,结合膜和黄芪饮片显微结构的扫描电镜(SEM)表征,讨论了微波辐射对黄芪水提液超滤过程的影响。结果表明,驱动压力相同,微波辅助超滤30 min时,膜通量下降到初始值的57.36%,且膜表面观察不到凝胶层;而常规超滤30 min,膜通量已下降到初始值的30.29%,且膜表面覆盖一层约20μm的凝胶层。说明微波辐射能够减弱浓差极化现象,促进膜超滤过程中的物质传递。     

超声波对脉动热管传热影响的实验研究

本文通过实验研究了超声波作用下不同操作温度对脉动热管传热性能的影响。实验结果表明,超声波对脉动热管的传热性能有强化作用,但超声波的强化传热作用会随着热负荷的升高而逐渐减小;超声波能有效减小脉动热管的热阻,且其减小量与操作温度有关,当操作温度由20℃增加至40℃时,超声波对脉动热管传热性能的强化作用会逐渐增强,当操作温度由40℃增加至60℃时,超声波对脉动热管的强化作用逐渐减弱。     

超声波强化传热的链式反应机理与模拟研究

利用超声外场作用实现强化传热的技术越来越受到人们的关注。本文开展超声波强化传热技术的作用机理分析及数值模拟研究。在理论方面,提出了超声波强化传热机理的新解释——"空化链式反应";在数值模拟方面,建立了超声波强化换热的数值模型,验证了超声波对换热的强化作用。理论分析显示超声空化核心的链式破裂具有减薄边界层、增强换热的效果;模拟给出了不同波动压力和频率下的强化换热情况,当驱动压力为7 kPa和频率为20 kHz时,超声波对流体传热的强化效果最好。     

铝酸钠溶液的鲁米诺增强声致发光研究

用荧光光谱分析仪测定了在超声作用下的铝酸钠Luminol溶液的荧光光谱.并对不同超声功率、不同组成的铝酸钠溶液的声致荧光光谱进行了研究,发现在超声空化时,铝酸钠Luminol溶液可以产生声致荧光;在相同功率的超声波作用下,浓度较高,铝酸钠溶液中Na2O/Al2O3摩尔比较低的铝酸钠溶液的声致荧光强度较弱.同时讨论了声致荧光与超声强化铝酸钠溶液结晶过程的关系.     

超声波强化膜分离的研究进展

本文综述了超声波强化膜分离的机理，国内外超声波强化膜分离的研究现状以及影响超声波强化膜分离的因素(如声场参数、操作参数、料液性质等)；同时评述了该技术存在的问题。现有的研究成果表明：如何在利用超声波提高膜分离效率的同时，减小超声波对有机溶质及膜稳定性的影响，应是近期的研究重点。     

Dielectric Analysis of Dynamic Fouling Behavior on Surface of Polyethersulfone Composite Ultrafiltration Membrane

基于界面极化与静电场理论建立了聚醚砜复合超滤膜表面污染层与溶液体系的介电解析模型. 应用参数敏感性分析和介电测量研究了体系中浓差极化层与滤饼层的厚度与电导率对超滤体系介电弛豫的影响机制. 结果表明超滤过程中浓差极化层能快速建立,而滤饼层的形成是一个增长与剥蚀的动态过程;并且浓差极化层与滤饼层的电学性质是影响超滤体系介电谱的关键因素. 比较介电测量与模拟结果证实了建立的超滤体系介电解析模型能够有效可靠地描述与解释超滤体系产生介电驰豫的机制.     

超声波强化多孔纤维干燥过程试验研究

为了探究超声波场对多孔纤维干燥过程热湿迁移的影响,对有无超声波作用下多孔纤维的干燥过程进行了试验研究。试验结果表明:无超声波作用时,多孔纤维干燥包括恒速和降速两个阶段,且其转化临界点在干基含湿率为0.5左右;施加超声波场之后,干燥过程只有降速阶段,且随着功率的增加超声波的强化作用逐渐增强,功率和干燥速率呈现非线性关系;随着干燥过程的进行,含湿量降低,超声波的强化作用逐渐削弱;为了减少能耗当干基含湿率降低至0.5时即可终止干燥。     

超声波对管内外流体间热交换过程的强化作用

本文就超声波对热交换过程的强化作用作了实验研究和理论探讨,该项研究工作与以往其他学者所作研究工作不同之处在于探讨了超声波对管内外流体间热交换过程的强化作用,实验所用超声波频率由14.7至163千赫。其强度达7瓦/厘米²,在超声波频率为21.4千赫、强度为7瓦/厘米²之作用工况下管内外冷热水间传热系数之最大平均增长率达80％。     

晶体管超声波发生器的研制

本文在分析超声波发生器现状的基础上,提出了研制晶体管超声波发生器应引起重视的几点原则,并着重介绍了按此原则研制完成的超声波发生器。同时,又对用这种发生器作为基本单元所构成的二种程式的超声波发生器的原理框图和试验结果作了简述。这类发生器配套而成的超声设备的可靠性和效率问题,在文末亦作了初步的讨论。     

Ultrasound-enhanced membrane-cleaning processes applied water treatments: influence of sonic frequency on filtration treatments

Ultrasound (US) cleaning technique was applied to remove fouling of ultrafiltration (UF) and microfiltration (MF) membranes which were used to treat peptone and milk aqueous solutions, respectively. Membrane operations were performed by cross-flow filtration with 60 kPa operating pressure in an US field. The US employed had 28, 45 and 100 kHz frequency with 23 W/cm(2) output power. For each polymeric membrane made of polysulfone UF and cellulose MF, cleaning experiments were carried out with and without US after fouling. The fouled UF and MF membranes showed volume flux decline, but the membrane property was recovered by US irradiation. It was found in 28 kHz frequency that water cleaning was effective for recovery of declined condition due to fouling. Also, US-enhanced permeability of membranes was discussed in both membrane systems. We observed that US decreased the fouling condition in both membrane systems when US was irradiated before fouling. It was found that 28 kHz frequency US could enhance formation of the fouled layer in both filtration systems of peptone and milk solution.     

Intensification of ultrasonic-assisted crude oil demulsification based on acoustic field distribution data

Water removal is an essential step during crude oil production due to several problems such as increased transportation costs and high corrosion rate due to dissolved salts. Indirect low frequency ultrasonic energy (US), using baths, has been recently proposed as an effective alternative for crude oil demulsification. However, the reactor position during sonication and its influence on the demulsification efficiency for crude oil has not been evaluated. In this sense, the aim of this study was to develop an automated system based on an open source hardware for mapping the acoustic field distribution in an US bath operating at 35 kHz using a hydrophone. Data acquired with this system provided information to evaluate the demulsification efficiency in the different positions of the US bath and correlate it with the acoustic intensity distribution. The automated 3D-mapping system revealed a higher acoustic intensity in the regions immediately above the transducers (ca. 0.6 W cm⁻²), while the other regions presented a relatively lower intensity (ca. 0.1 W cm⁻²). Experimental data demonstrated that reactors positioned in the most intense acoustic regions provided a much higher efficiency of demulsification in comparison with the ones positioned in the less intense acoustic field regions. Demulsification efficiency up to 93% was obtained with 15 min of sonication (100% amplitude) using few amount of chemical demulsifier. Hence, this work demonstrated that the information acquired with the developed mapping system could be used for inducing a higher efficiency of demulsification only by finding the more suitable position of reactor in the US bath, which certainly will help development of appropriate reactors design when looking for such approach.     

Cleaning efficiency enhancement by ultrasounds for membranes used in dairy industries

Membrane cleaning is a key point for the implementation of membrane technologies in the dairy industry for proteins concentration. In this study, four ultrafiltration (UF) membranes with different molecular weight cut-offs (MWCOs) (5, 15, 30 and 50 kDa) and materials (polyethersulfone and ceramics) were fouled with three different whey model solutions: bovine serum albumin (BSA), BSA plus CaCl₂ and whey protein concentrate solution (Renylat 45). The purpose of the study was to evaluate the effect of ultrasounds (US) on the membrane cleaning efficiency. The influence of ultrasonic frequency and the US application modes (submerging the membrane module inside the US bath or applying US to the cleaning solution) were also evaluated. The experiments were performed in a laboratory plant which included the US equipment and the possibility of using two membrane modules (flat sheet and tubular). The fouling solution that caused the highest fouling degree for all the membranes was Renylat 45. Results demonstrated that membrane cleaning with US was effective and this effectiveness increased at lower frequencies. Although no significant differences were observed between the two different US applications modes tested, slightly higher cleaning efficiencies values placing the membrane module at the bottom of the tank were achieved.     

An experimental study on the coalescence process of binary droplets in oil under ultrasonic standing waves

The coalescence process of binary droplets in oil under ultrasonic standing waves was investigated with high-speed photography. Three motion models of binary droplets in coalescence process were illustrated: (1) slight translational oscillation; (2) sinusoidal translational oscillation; (3) migration along with acoustic streaming. To reveal the droplets coalescence mechanisms, the influence of main factors (such as acoustic intensity, droplet size, viscosity and interfacial tension, etc) on the motion and coalescence of binary droplets was studied under ultrasonic standing waves. Results indicate that the shortest coalescence time is achieved when binary droplets show sinusoidal translational oscillation. The corresponding acoustic intensity in this case is the optimum acoustic intensity. Under the optimum acoustic intensity, drop size decrease will bring about coalescence time decrease by enhancing the binary droplets oscillation. Moreover, there is an optimum interfacial tension to achieve the shortest coalescence time.     

An acoustic switch

The benefits derived from the development of acoustic transistors which act as switches or amplifiers have been reported in the literature. Here we propose a model of acoustic switch. We theoretically demonstrate that the device works: the input signal is totally restored at the output when the switch is on whereas the output signal nulls when the switch is off. The switch, on or off, depends on a secondary acoustic field capable to manipulate the main acoustic field. The model relies on the attenuation effect of many oscillating bubbles on the main travelling wave in the liquid, as well as on the capacity of the secondary acoustic wave to move the bubbles. This model evidences the concept of acoustic switch (transistor) with 100% efficiency.     

Assessing the effect of a barrier between two rooms subjected to low frequency sound using the boundary element method

The boundary element method (BEM) has been used to compute the acoustic wave propagation through a single vertical panel, which separates two rooms, made of concrete, when one of the rooms is excited by a steady-state, spatially sinusoidal, harmonic line load pressure at low frequencies. This work focuses on how the connection of the panel to the ceiling affects the acoustic insulation provided by the wall. Perfect double-fixed partitions and acoustic barrier-type structures with differently-sized gaps between the ceiling and the barrier are studied. The BEM model is formulated in the frequency domain and takes the air-solid interaction fully into account. Insulation dips are localised in the frequency domain and identified with dips associated with both the wall's natural dynamic vibration modes and with those associated with the air in the rooms. The influence of the wall's thickness on acoustic insulation is also analysed. The computed results obtained with the acoustic barrier type structure are compared with those obtained by a rigid model. The importance of the rooms' surface conditions is assessed, modelling the rooms with cork.     

Tandem shock waves to enhance genetic transformation of Aspergillus niger

Filamentous fungi are used in several industries and in academia to produce antibiotics, metabolites, proteins and pharmaceutical compounds. The development of valuable strains usually requires the insertion of recombinant deoxyribonucleic acid; however, the protocols to transfer DNA to fungal cells are highly inefficient. Recently, underwater shock waves were successfully used to genetically transform filamentous fungi. The purpose of this research was to demonstrate that the efficiency of transformation can be improved significantly by enhancing acoustic cavitation using tandem (dual-pulse) shock waves. Results revealed that tandem pressure pulses, generated at a delay of 300 μs, increased the transformation efficiency of Aspergillus niger up to 84% in comparison with conventional (single-pulse) shock waves. This methodology may also be useful to obtain new strains required in basic research and biotechnology.     

Acoustic impedance of perforated panels covered by membranes

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Investigation of the influence of humidity on the ultrasonic agglomeration of submicron particles in diesel exhausts

Removing very fine particles in the 0.01-1 micro m range generated in diesel combustion is important for air pollution abatement because of the impact such particles have on the environment. By forming larger particles, acoustic agglomeration of submicron particles is presented as a promising process for enhancing the efficiency of the current filtration systems for particle removal. Nevertheless, some authors have pointed out that acoustic agglomeration is much more efficient for larger particles than for smaller particles. This paper studies the effect of humidity on the acoustic agglomeration of diesel exhausts particles in the nanometer size range at 21 kHz. For the agglomeration tests, the experimental facility basically consists of a pilot scale plant with a diesel engine, an ultrasonic agglomeration chamber a dilution system, a nozzle atomizer, and an aerosol sampling and measuring station. The effect of the ultrasonic treatment, generated by a linear array of four high-power stepped-plate transducers on fumes at flow rates of 900 Nm(3)/h, was a small reduction in the number concentration of particles at the outlet of the chamber. However, the presence of humidity raised the agglomeration rate by decreasing the number particle concentration by up to 56%. A numerical study of the agglomeration process as a linear combination of the orthokinetic and hydrodynamic agglomeration coefficients resulting from mutual radiation pressure also found that acoustic agglomeration was enhanced by humidity. Both results confirm the benefit of using high-power ultrasound together with humidity to enhance the agglomeration of particles much smaller than 1 micro m.     

Thermal effect on the acoustic behavior of an axisymmetric lined duct

This paper deals with the effect of the temperature and the frequency on the acoustic behavior of lined duct partially treated with usual material used in acoustic insulation.First, the effect of frequencies and temperature on the acoustic impedance of usual materials used in lined duct such as glass or rock wools in order to reduce acoustic level is investigated.Secondly, the variational formulation of the acoustic duct problem taking into account velocity and temperature effects is established. Then, a numerical model is derived which permits to compute the reflection and the transmission coefficients of such duct for different temperatures and several flow velocities. The acoustic power attenuation is then computed from these coefficients and the effect of the temperature and flow velocities on this energetic quantity is evaluated.The numerical results are obtained for three configurations of a lined duct treated for different temperature ranges and several velocities. Numerical coefficients of transmission and reflection as well as the acoustic power attenuation show the relative influence of temperature.