超声珩磨作用下两空化泡动力学特性

为了探讨超声珩磨作用下磨削区的空化机理,基于速度势叠加原理,考虑超声珩磨速度和珩磨压力,建立了磨削区两空化泡的动力学模型. 数值模拟了磨削区空化泡初始半径、两空化泡间距、超声声压幅值、珩磨压力、珩磨头转速对磨削区两空化泡动力学特性的影响. 研究表明,考虑两空化泡之间的相互作用时,要想获得良好的空化效果,可将两空化泡初始半径之比控制在3 倍以内;选择较高的超声波声压幅值与较低的珩磨压力,并且使超声波声压幅值与珩磨压力和液体静压力之差介于0.66~1.89MPa 之间;增大珩磨头转速空化泡溃灭也略有加速;通过试验测量材料表面粗糙度的方法间接验证了理论分析的合理性.      

DYNAMICAL BEHAVIORS OF DOUBLE CAVITATION BUBBLES UNDER ULTRASONIC HONING

为了探讨超声珩磨作用下磨削区的空化机理,基于速度势叠加原理,考虑超声珩磨速度和珩磨压力,建立了磨削区两空化泡的动力学模型. 数值模拟了磨削区空化泡初始半径、两空化泡间距、超声声压幅值、珩磨压力、珩磨头转速对磨削区两空化泡动力学特性的影响. 研究表明,考虑两空化泡之间的相互作用时,要想获得良好的空化效果,可将两空化泡初始半径之比控制在3 倍以内;选择较高的超声波声压幅值与较低的珩磨压力,并且使超声波声压幅值与珩磨压力和液体静压力之差介于0.66~1.89MPa 之间;增大珩磨头转速空化泡溃灭也略有加速;通过试验测量材料表面粗糙度的方法间接验证了理论分析的合理性.     

超声驱动下激励参数对单泡空化振动的影响

根据考虑了液体可压缩性的改进的微气泡动力学方程,采用改进的初始半径对单泡超声空化现象进行了数值计算研究.结果表明,微气泡振动对一些参量很敏感:微气泡振动半径与初始半径的比值随振动频率的增大而减小;提高声场声压会加剧气泡崩塌程度,但过高的声压又不能使微气泡崩塌;微气泡崩塌速率随气泡初始半径的增加而增大,在一定范嘲内能保证空化泡稳定振动,在初始半径为1.6μm处空化程度最强,如果继续增大初始半径则空化程度减弱、甚至消失;微气泡崩塌程度随黏滞系数和表面张力的增大而减弱,过大的黏滞系数和表面张力会使微气泡崩塌难以发生.计算结果与他人的实验数据相比,发现液体的可压缩性使单泡空化强度增强,对最佳空化区域范围的确定有较大的影响.     

nfluence of exciting parameters on the vibration of single cavitation bubble driven by intensive sound

根据考虑了液体可压缩性的改进的微气泡动力学方程,采用改进的初始半径对单泡超声空化现象进行了数值计算研究. 结果表明,微气泡振动对一些参量很敏感：微气泡振动半径与初始半径的比值随振动频率的增大而减小;提高声场声压会加剧气泡崩塌程度,但过高的声压又不能使微气泡崩塌;微气泡崩塌速率随气泡初始半径的增加而增大,在一定范围内能保证空化泡稳定振动,在初始半径为1.6\,$\mu$m 处空化程度最强,如果继续增大初始半径则空化程度减弱、甚至消失;微气泡崩塌程度随黏滞系数和表面张力的增大而减弱,过大的黏滞系数和表面张力会使微气泡崩塌难以发生. 计算结果与他人的实验数据相比,发现液体的可压缩性使单泡空化强度增强, 对最佳空化区域范围的确定有较大的影响.     

空化水动力学非定常特性研究进展及展望

空化作为一种重要的复杂水动力学现象,具有明显的三维流动特征与剧烈的非定常特性,在水力机械、船舶推进器、水利工程中广泛存在,且通常会带来不利的影响,长期以来一直是水动力学领域研究的重点与难点课题之一.本文首先从实验测量和数值模拟两个角度,综述了空化水动力学非定常特性研究的发展概况, 分析了当前存在的问题.在空化实验研究中,主要介绍了空化水洞、空化流场测量以及多物理场同步测量等方面所取得的进展.在数值模拟方法中, 对目前的空化模型和湍流模型进行了分类介绍,并重点讨论了大涡模拟、验证和确认等在空化流模拟中的应用.之后以附着型空化为主, 同时兼顾云状空泡、空蚀、涡空化等,梳理了其研究中存在的几个关键科学问题,包括空化演变、空化流动的三维结构、失稳机制、空化不稳定性及其与低频压力脉动的联系、空化与旋涡的相互作用、空化与弹性水翼的流固耦合、空化对尾流场影响等.最后展望了空化水动力学的研究方向和未来发展趋势.      

空化水动力学非定常特性研究进展及展望

空化作为一种重要的复杂水动力学现象,具有明显的三维流动特征与剧烈的非定常特性,在水力机械、船舶推进器、水利工程中广泛存在,且通常会带来不利的影响,长期以来一直是水动力学领域研究的重点与难点课题之一.本文首先从实验测量和数值模拟两个角度,综述了空化水动力学非定常特性研究的发展概况,分析了当前存在的问题.在空化实验研究中,主要介绍了空化水洞、空化流场测量以及多物理场同步测量等方面所取得的进展.在数值模拟方法中,对目前的空化模型和湍流模型进行了分类介绍,并重点讨论了大涡模拟、验证和确认等在空化流模拟中的应用.之后以附着型空化为主,同时兼顾云状空泡、空蚀、涡空化等,梳理了其研究中存在的几个关键科学问题,包括空化演变、空化流动的三维结构、失稳机制、空化不稳定性及其与低频压力脉动的联系、空化与旋涡的相互作用、空化与弹性水翼的流固耦合、空化对尾流场影响等.最后展望了空化水动力学的研究方向和未来发展趋势.     

Research progresses and prospects of unsteady hydrodynamics characteristics for cavitation

空化作为一种重要的复杂水动力学现象,具有明显的三维流动特征与剧烈的非定常特性,在水力机械、船舶推进器、水利工程中广泛存在,且通常会带来不利的影响,长期以来一直是水动力学领域研究的重点与难点课题之一.本文首先从实验测量和数值模拟两个角度,综述了空化水动力学非定常特性研究的发展概况, 分析了当前存在的问题.在空化实验研究中,主要介绍了空化水洞、空化流场测量以及多物理场同步测量等方面所取得的进展.在数值模拟方法中, 对目前的空化模型和湍流模型进行了分类介绍,并重点讨论了大涡模拟、验证和确认等在空化流模拟中的应用.之后以附着型空化为主, 同时兼顾云状空泡、空蚀、涡空化等,梳理了其研究中存在的几个关键科学问题,包括空化演变、空化流动的三维结构、失稳机制、空化不稳定性及其与低频压力脉动的联系、空化与旋涡的相互作用、空化与弹性水翼的流固耦合、空化对尾流场影响等.最后展望了空化水动力学的研究方向和未来发展趋势.     

超声波作用下气泡的非线性振动

Flynn方程在引入泡内气体的压缩性修正后能够更好地描述超声波作用下液体内气泡的非线性振动。我们以此为基础通过数值分析得到了不同初始半径的气泡在同一声场中的振动位移时间图像和相轨迹,发现气泡的运动行为对其初始状态有很强的依赖关系,在频率为26.5 kHz,幅度为1.35 atm的声波作用下,初始半径小于1 μm的气泡作小幅受迫振动,大于200 μm的气泡作小幅准本征振动,不具备空化气泡特征。声场中的小气泡对声波强度变化的反应更为激烈,当增加声强度时,可将一定范围内的小幅受迫振动气泡转化为空化气泡,并且,当驱动声波压力幅值增加时,初始半径越小的气泡的最大位移增加幅度越大。驱动声波频率同样影响气泡的振动。随着声波频率的升高,空化气泡的初始半径取值区间越来越小,空化振荡也越来越弱。本文还通过高速摄影系统对换能器作用于未除气的自来水所引起的变化进行了实验研究,结果表明,超声波作用下液体内的气泡场是一个混合场,场内气泡尺寸呈一定的分布状态,不仅有空化气泡,还有毫米级的大气泡。气泡的运动行为直接影响空化效果。超声空化场是一个复杂的物理场,场内除了有气泡的振动外,还有气泡间的相互吸引、碰撞和结合。      

初始压力对多孔介质中气体水合物生成的影响

利用自制的一维天然气水合物生成与开采模拟实验系统,实验研究多孔介质中天然气水合物生成时不同初始压力对生成量、生成时间的影响.分别用相同气水比注入、相同注气量不同注水量、相同注水量不同注气量三种方式来控制初始压力.结果表明:在砂粒粒径300μm～500μm,盐水质量浓度2%,系统温度为2℃、初始压力为5MPa～9MPa的条件下进行水合物的等容生成实验时,初始压力越大,生成的水合物量越多,水合物开始生成的时间越早;但初始压力越大,实验系统中水合物生成最终稳定所需的时间越长.本实验系统采用的三种不同的控制初始压力的方式都可以得到上述结果.由此,可以为今后室内进行天然气水合物的生成实验提供科学指导.     

机械密封三维稳态模型及密封性能分析

针对接触式机械密封,建立了包括轴向模式和角向模式的三维稳态力学模型,考虑密封环端面接触及液膜空化,提出了数值求解方法,计算了在稳定工况下的泄漏率、液膜厚度、液膜压力和接触压力的分布形式,分析了动环初始安装误差,端面锥度等因素对接触式机械密封端面三维位置关系的影响.结果表明:初始安装误差、端面锥度对接触式机械密封的性能影响较大.所建数学模型和相关计算方法对接触式机械密封的优化设计具有一定的指导意义.     

Photodegradation of benzene by TiO_2 nanoparticles prepared by flame CVD process

Photodegradation of benzene at ppb levels by mixed-phase TiO 2 nanoparticles, synthesized by the oxidation of TiCl 4 in propane/air turbulent flame chemical vapor deposition (CVD) process, is investigated experimentally by using a tubular photoreactor with thin TiO 2 films coated on the reactor wall by sedimentation. Effects of inlet benzene concentration from 10 to 300 g/m 3 , rutile mass fraction from about 20 to 50% and photoluminescence (PL) intensity of TiO 2 nanoparticles on degradation degree are exa...     

Non-invasive measurement of void fraction and liquid temperature in microchannel flow boiling

Past thermometry research for two-phase microfluidic systems made much progress regarding wall temperature distributions, yet the direct measurement of fluid temperature has received little attention. This paper uses a non-invasive two-dye/two-color fluorescent technique to capture fluid temperature along with local liquid fraction in a two-phase microflow generated by injecting air into a heated microchannel. The fluorescent emission of Rhodamine 110 and Rhodamine B, measured with photodiodes, is used to obtain local liquid temperature (±3°C) and void fraction (±2% full-scale) over a temperature range from 45 to 100°C. Arrays of these sensors can significantly expand the set of measurable flow parameters to include bubble/slug frequency, size, velocity, and growth rates in addition to mapping the local liquid temperature and void fraction.     

TiO2-loaded activated carbon fiber： Hydrothermal synthesis,adsorption properties and photo catalytic activity under visible light irradiation

TiO2-loaded activated carbon fibers （ACF） were prepared by a hydrothermal method. The samples were characterized by scanning electron microscopy （SEM）, X-ray diffraction （XRD）, Fourier transform infrared （FTIR） spectrometry and UV-vis diffuse reflectance spectra （DRS）. SEM images showed that the TiO2 nanoparticles were deposited on the surface of ACF, and the particle size and loading amount of TiO2 were varied by changing the initial concentration of tetrabutyl titanate （TBOT）. The results of an ash experiment showed that the loading amounts of TiO2 were 18.4%, 43.3%, 52.5%, 75.1%, and 91.1% for initial concentrations of TBOT of 0.07,014, 0.21,0.28, and 0.35 tool/L, respectively, Physical interactions played an important role in the formation of TiO2/ACF composite fibers that absorb UV and visible light. Compared with those of ACF, improved adsorption and photocatalytic activity toward Rhodamine B （RhB） were observed for TiO2/ACF composite fiber. The Rhodamine B could be removed efficiently by TiO2/ACF composite fibers, and the TiO2 loading amount had a significant effect on the photocatalytic activity of TiO2/ACF composite fibers.     

Whole field measurement of temperature in water using two-color laser induced fluorescence

 A technique is described that measures the instantaneous three-dimensional temperature distribution in water using two-color laser-induced fluorescence (LIF). Two fluorescent dyes, Rhodamine B and Rhodamine 110, are used as temperature indicators. A laser light sheet scanned across the entire measurement volume excites the fluorescent dye, and an optical system involving a color beam splitter gives the intensity distribution of the individual fluorescent dyes on two separate monochrome CCD cameras. The ratio of these fluorescence intensities at each point of the image is calibrated against the temperature to eliminate the effect of the fluctuation of illuminating light intensity. A stable thermally stratified layer was measured by this system to evaluate the total accuracy of the measurement system. The random error of the measurement was ±1.4 K with 95% confidence. Measurements of thermal convection over a heated horizontal surface show temperature iso-surfaces having typical structures such as plumes, ridges and thermals. Received: 1 October 1997/Accepted: 23 March 1998     

TiO2-loaded activated carbon fiber: Hydrothermal synthesis,adsorption properties and photo catalytic activity under visible light irradiation

TiO₂-loaded activated carbon fibers (ACF) were prepared by a hydrothermal method. The samples were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectrometry and UV–vis diffuse reflectance spectra (DRS). SEM images showed that the TiO₂ nanoparticles were deposited on the surface of ACF, and the particle size and loading amount of TiO₂ were varied by changing the initial concentration of tetrabutyl titanate (TBOT). The results of an ash experiment showed that the loading amounts of TiO₂ were 18.4%, 43.3%, 52.5%, 75.1%, and 91.1% for initial concentrations of TBOT of 0.07, 014, 0.21, 0.28, and 0.35 mol/L, respectively. Physical interactions played an important role in the formation of TiO₂/ACF composite fibers that absorb UV and visible light. Compared with those of ACF, improved adsorption and photocatalytic activity toward Rhodamine B (RhB) were observed for TiO₂/ACF composite fiber. The Rhodamine B could be removed efficiently by TiO₂/ACF composite fibers, and the TiO₂ loading amount had a significant effect on the photocatalytic activity of TiO₂/ACF composite fibers.     

Experimental study of heat flux in mixed convective flow over solid waves

In this experimental study, we address transport processes in a mixed convective flow over a heated wavy surface. Therefore, we combine digital particle image velocimetry (DPIV) and two-color planar laser induced fluorescence (PLIF) to simultaneously measure the velocity and temperature field. For this, we propose to use the dye combination Rhodamine B and Rhodamine 110, both excited with the Nd:YAG laser also used for the PIV measurements. We investigate the influence of mixed convection over a wavy surface on the velocity field, turbulence statistics, the temperature field and the heat flux. By computing these quantities we find a correlation between the maximum in the Reynolds stress profiles and the components of the heat flux vector, thus regions of maximum momentum and scalar transport coincide. In addition, we apply a proper orthogonal decomposition (POD) to extract the most dominant flow structures in a measurement plane above the wavy surface. This first POD mode is identified as streamwise-oriented, counter-rotating vortices whose spanwise scaling is also correlated with the maximum of heat flux.     

2D temperature measurements in the wake of a heated cylinder using LIF

A technique is described to measure the instantaneous 2D temperature distribution in the wake of a heated cylinder using `laser-induced fluorescence'. Rhodamine B, a fluorescent dye, is used as a temperature indicator. The relation between fluorescence intensity and temperature is determined by means of calibration experiments in the temperature range of 20–35 °C with an accuracy of ±0.1 °C. The temperature distribution behind the heated cylinder is well visible and can be measured with a high spatial resolution. Corrections for variation in laser energy and intensity distribution in the laser sheet have to be made to further improve the accuracy of the measuring method. Received: 3 January 2001/Accepted: 18 May 2001     

Experimental investigation of a scaled-up passive micromixer with uneven interdigital inlet and teardrop obstruction elements

Micromixers are vital components in micro total analysis systems. It is desirable to develop micromixers which are capable of rapidly mixing two or more fluids in a small footprint area, while minimizing mechanical losses. A novel planar scaled-up passive micromixer is experimentally investigated in this study. The design incorporates a 7-substream uneven interdigital inlet which supplies two liquid species in a parallel arrangement and promotes diffusion along the side walls. Forty-eight staggered teardrop-shaped obstruction elements located along the channel length combined with 32 side walls protrusions increase the two-fluid interfacial area while converging the flow due to periodic reductions in cross-sectional area. The scaled-up micromixer has a mixing channel length of 110 mm with a mixing channel height and width of 2 and 5 mm, respectively. Experimental investigations are carried out at four locations along the channel length and at Reynolds numbers equal to 1, 5, 10, 25, 50, and 100, where the Reynolds number is calculated based on total two-fluid flow and the mixing channel hydraulic diameter. Flow visualization is employed to study flow patterns, while induced fluorescence (IF), using de-ionized water and low concentration Rhodamine 6G solutions, provides mixing efficiency data. Results show a change in dominant mixing mechanism from mass diffusion to mass advection, with a critical Reynolds number of 25. At high Reynolds numbers, the formation of additional lamellae is observed, as is the formation of Dean vortices in the vicinity of the teardrop obstructions. Of the tested cases, the highest outlet mixing efficiency, 68.5%, is achieved at a Reynolds number of 1, where mass diffusion dominates. At low Reynolds numbers, superior mixing efficiency is due primarily to the implementation of the uneven interdigital inlet. A comparable mixing length is proposed to allow for reasonable comparison with published studies.     

Condensation from steam—air mixtures in a horizontal annular flow channel

Experiments were performed on the condensation of steam from steam-air mixtures in annular flow at a cooled inner tube. The range of investigation was varied for laminar and turbulent flow for 1.5 × 10³ Re 1.3 × 10⁴ and inlet concentrations 0.59 p_steam/p_total 0.95. The measurements, performed at an open test loop at p_total ≈ 0.96 bar, allowed local heat and mass transfer coefficients to be evaluated for various inlet lengths in the 2 m long annulus. The steam concentration was measured locally inside the annulus with a newly developed dew-point probe. The heat flux was measured locally using the temperature gradient in the cooled inner tube.

Near the inlet region the experiments showed a slightly higher heat flux at the bottom of the tube compared to the top, although it is expected to be smaller there owing to a thicker liquid film. Far downstream from the inlet region the heat transfer at the top was higher than at the bottom. The reasons for these effects are discussed, yielding a better understanding of the thermal and fluid processes involved in condensation from vapor-gas mixtures. The measured data allow the development of correlations for predicting the local Nusselt and Sherwood numbers in a horizontal annular-flow chanbel.     

Effect of salt spray corrosion on air-side hydrophilicity and thermal-hydraulic performance of copper-fin heat exchangers

The effect of salt spray corrosion on the air-side hydrophilicity and the thermal-hydraulic performance of copper-fin heat exchangers were experimentally investigated. Artificial accelerated method of salt spray corrosion on the copper-fin heat exchangers was used for simulating the actual corroded heat exchangers. The experimental results show that, the contact angles increase with the increase of salt spray corrosion hours, which results in the degradation of the hydrophilicity of copper fin. The air-side heat transfer coefficients decrease and pressure drops increase with the increase of corrosion hours. The effect of salt spray corrosion on the heat transfer coefficients and pressure drops become more obvious with the increase of inlet air velocity. The heat transfer coefficients of the corroded copper-fin heat exchangers decrease by 4.4–34.0% and the pressure drop increase by 5.2–26.1% comparing with those of the uncorroded copper-fin heat exchanger at the inlet air velocity ranging from 0.5 to 2.0 m/s.