Characterization of diabatic two-phase flows in microchannels: Flow parameter results for R-134a in a 0.5 mm channel

An optical measurement method for two-phase flow pattern characterization in microtubes has been utilized to determine the frequency of bubbles generated in a microevaporator, the coalescence rates of these bubbles and their length distribution as well as their mean velocity. The tests were run in a 0.5 mm glass channel using saturated R-134a at 30 °C (7.7 bar). The optical technique uses two laser diodes and photodiodes to measure these parameters and to also identify the flow regimes and their transitions. Four flow patterns (bubbly flow, slug flow, semi-annular flow and annular flow) with their transitions were detected and observed also by high speed video. It was also possible to characterize bubble coalescence rates, which were observed here to be an important phenomena controlling the flow pattern transition in microchannels. Two types of coalescence occurred depending on the presence of small bubbles or not. The two-phase flow pattern transitions observed did not compare well to a leading macroscale flow map for refrigerants nor to a microscale map for air–water flows. Time averaged cross-sectional void fractions were also calculated indirectly from the mean two-phase vapor velocities and compared reasonably well to homogeneous values.     

Recent developments in axial tomography for heat transfer and fluid flow studies

The need for three-dimensional, nonintrusive field measurements in the area of heat transfer and fluid flows grows very rapidly, and at this time there are very few experimental techniques that can be used for such a purpose. Axial tomography is a promising technique for accurate quantitative measurements for a variety of heat transfer and fluid flow problems. It has already been tested and practiced in a number of applications, including medical scanning technology. The technique is based on Radon's original work and reconstructs a three-dimensional field from its two-dimensional projections (integrated measurements) obtained at different view angles. This review highlights some of the most important methods used in tomography and their applications to experimental heat transfer and fluid flow studies.     

Particle tracking method for measurements of turbulence properties in a curved channel

The turbulence properties of a curved channel flow have been measured by particle tracking method. The results reveal some characteristics of the structure for wall turbulence.     

Limited-view optical tomography technique and its application to temperature field measurement in flow of hot air

In this paper, a limited-view optical tomography technique is presented, which contains an orthographic holography system, an image processing system and the simultaneous algebraic reconstruction technique (SART). Using this technique, the temperature field of a cross section in the flow of hot air from a nozzle is measured. The measured results are satisfactory. The project supported by the National Natural Science Foundation and the State Science and Technology Commission of China     

Experimental investigation of splash and crown formation during single drop impact on wetted surfaces

Experimental results concerning crown formation during liquid drop impact on wetted surfaces are reported. Different liquids and numerous impact conditions are investigated. In particular, crown-splash (C-S) and deposition-crown (D-C) limits are determined on the basis of the experimental observations. These limits converge for dimensionless film thickness thinner than 0.03, leaving the outcome of crown formation unobserved. The sole Weber number and dimensionless film thickness cannot explain the phenomenon. It appears that all these data can be described using a combination of Weber and Ohnesorge numbers versus dimensionless film thickness.     

Optimizing experimental design for coupled porous media flow studies

It is important in specific cases to know if and when coupling occurs during the flow of immiscible fluids in and through porous media. Unless special laboratory design precautions are taken, however, even small errors in the measurement of the experimental variables can adversely camouflage the very effects that are under investigation. Here a novel remedy is proposed that involves the use of a least squares optimization algorithm for the selection of the most favorable constraint conditions to be imposed.     

几种应变检测的光力学测试方法及其比较

应变测量技术在工程应用中有重要意义.一是它可以直接用来获得应力场,可以用来评价材料的安全性和可靠性;二是应变梯度很大的部位往往又是材料破坏的起始点,所以应变测量又可以用于进行材料的无损检测.当前几种光力学方法受到了重视,即云纹干涉法、电子剪切散斑干涉法和数字相关散斑法,对上述方法的原理、光路设计、特点作了介绍,以便这些传统方法的进一步推广,文中还特别介绍了一种作者近来在国内率先开展的微压痕应变花法.这一方法可以方便用于金属等材料上,可以较精确定量研究测试物体的应变而且对刚体位移不敏感,应用起来更加直接和方便,更有推广和应用前景.本文有利于针对研究对象的特点,选用更加合适的应变测量方法.     

Measurement and simulation of the two-phase velocity correlation in sudden-expansion gas-particle flows

In this paper the present authors measured the gas-particle two-phase velocity correlation in sudden expansion gas-particle flows with a phase Doppler particle anemometer(PDPA) and simulated the system behavior by using both a Reynolds-averaged Navier-Stokes(RANS) model and a large-eddy simulation(LES).The results of the measurements yield the axial and radial time-averaged velocities as well as the fluctuation velocities of gas and three particle-size groups(30 μ m,50 μ m,and 95 μ m) and the gas-particle velocity correlation for 30 μ m and 50 μ m particles.From the measurements,theoretical analysis,and simulation,it is found that the two-phase velocity correlation of sudden-expansion flows,like that of jet flows,is less than the gas and particle Reynolds stresses.What distinguishes the two-phase velocity correlations of sudden-expansion flow from those of jet and channel flows is the absence of a clear relationship between the two-phase velocity correlation and particle size in sudden-expansion flows.The measurements,theoretical analysis,and numerical simulation all lead to the above-stated conclusions.Quantitatively,the results of the LES are better than those of the RANS model.     

磁性敏感器用于冲击变形测量

一种新型的非接触式磁性位移敏感器,兼有高灵敏度(50-80mV/mm)、大线性范围(15-25mm)以及响应快(4kHz)等优点.用于测量20mm以内的冲击动态变形获得成功.     

一种阻容串联式柔性力传感器

本文研制了一种柔性电容电阻串联式力传感器,用来实现提高柔性力传感器测量精确度和增大测量灵敏度的目的。该传感器利用电容电阻串联电路特点,提出了一种双电容串联电阻的传感器结构方案。传感器两侧为电容结构,两电容之间是一导电橡胶弹性层,电学上起电阻的作用。相当于两电容之间串联一个电阻,外力作用时导电橡胶电阻的变化与两电容的变化共同构成最终输出电容值的变化。理论和实验都证明该方法有助于提高传感器的测量灵敏度。通过一系列实验测试了传感器的输出特性,实验结果证明传感器输出电容平均灵敏度由未串联导电橡胶时的0.03pf/N提高到串联后的0.55pf/N。此外,相较于直接测量电阻变化,测量电容输出的相对误差更小,输出信号稳定时间缩短。     

埋入式光纤布拉格光栅传感器低温力学传感性能的基础实验研究

在绕制、冷却和强磁场等极端运行条件下,高温超导磁体内部将会产生复杂的应力和应变,其内部最大应变可达到数千微应变,极易引起超导性能的退化,造成结构的破坏,因此,高温超导磁体结构内部的力学状态直接关系到运行的安全性和可靠性。而国内外针对高温超导磁体结构内部应变测量的技术尚处于初步研究阶段。基于光纤布拉格光栅(FBG)应变测量技术,结合钇钡铜氧(YBCO)高温超导带材结构,本文提出了一种埋入式FBG传感测量技术,即利用低温黏接剂等特殊材料将FBG传感器封装至带材的一侧,且几乎不影响带材尺寸,待绕制成磁体结构后,FBG传感器自然地埋入至线圈层间,以测量结构内部的形变行为等。结合YBCO高温超导线圈固化材料及固化技术,研制了4种不同的埋入式FBG应变传感器,并对传感器进行了测试对比和精度分析,给出了相应的实验标定曲线,以探讨FBG传感器在制备过程中粘贴工艺、低温环境等对实验结果的影响。结果表明：在低温胶以及环氧树脂的正确封装和良好固化情况下,所研制的埋入式FBG传感器在室温和液氮温区下具有较高的测量精度,可应用于低温环境下双层超导带材结构内部的应变监测等;所研制的埋入与准分布式FBG传感器在低温...     

Swirl effects on variable-density jet mixing in confined flows

An experimental investigation on swirl effects on inhomogeneous confined jet mixing in a combustor configuration is reported. The confined swirling flow was simulated by a swirler with a central jet mounted in a cyclindrical tube. Helium and air jets set at different velocities were injected into the confined swirling air flow. The resulting flow fields due to two vane swirlers with constant vane angles of 35° and 66° were compared. Results show that the 35° vane swirler produces a solid-body rotation core with a slope about twice that created by the 66° vane swirler. It is the behavior of this solid-body rotation core that determines jet mixing rather than the swirler vane angle. Consequently, the coaxial jet decays much faster, the mixing is more intense, and the turbulence intensities are higher for the 35° vane swirler. In view of these results, combustor designers should be more concerned with behavior of the solid-body rotation core produced by the swirler, instead of the swirler vane angle.     

电动力学中衰减机械系统的非线性振荡方法

衰减机械系统的非线性振荡可用来研究长约瑟夫逊结的电动力学方程式，而这方程式等同于弱衰减机械系统的非线性振荡。本文应用的方法是将控制方程线性化及结合谐波平衡法（线性谐波平衡法）而产生色散关系，再把平均法应用在弱非线性的耗散系统中得到非常准确的瞬变反应。在此提出的方法不仅考虑能量耗散，而且利用简单的线性代数等式关系来代替冗长及复杂的分析近似解。     

光纤传感振动检测系统及其实验研究

实际工程结构中的振动检测是十分重要的,也是十分复杂的,常用的结构振动检测传感器易受工程现场恶劣环境的影响,而光纤传感器具有小巧、抗电磁干扰、灵敏度高、适合长期监测等优点.本文建立了基于马赫-曾德(Mech-Zehnder)干涉原理的光纤传感振动检测系统,研制了运用先进的数字信号处理技术采集和处理数据的专用软件,并在典型结构件--钢制悬臂梁结构上进行了外加信号作用下的强迫振动检测和冲击载荷作用下的自由衰减振动检测,测量了该结构件的频率及振幅,其结果与同时进行的成熟的电测结果相近,说明光纤传感器用于结构件的振动测量是可靠的.本文为光纤传感器应用于实际工程的振动检测提供了新的技术装置,具有工程应用前景.     

一种测量纯摩擦力的柔性传感器及应变敏感条的研制

研制了一种可以测量纯摩擦力的柔性力学量传感器,作用在传感器上的正压力对摩擦力测量的影响不超过满量程的1.8％.传感器灵敏度高(186με/N),测量范围在几个牛顿(N)的量级.传感器的敏感元件为柔性敏感条,由电阻应变片和超弹性硅橡胶组成,其继承了超弹性材料的柔软性与电阻应变片技术成熟的特点.实验证明,柔性敏感条具有输入...     

弹道靶线圈法测速的研究

本文介绍了一种新的用于弹道靶实验的测速方法.该方法用线圈作为测速的探测装置,结合自行设计的计时装置——单次双路测速仪,组成一套测速设备.实验表明,该设备装置简单,造价低,抗干扰能力强,对模型无破坏作用,测量精度高.本方法在目前国内弹道靶实验中尚属首次使用.     

A qualitative study on the pulsatile flow phenomenon in a dense fly ash pneumatic conveyor

Understanding of the dynamic particulate flow structures within a dense gas-fly ash pneumatic conveyor must be improved in order to better aid its design guidance.The complex pulsatile movement of the gas-fly ash mixture dominates the flow performance within the pipeline,and historically,non-invasive measurement devices such as the electrical capacitance tomography（ECT） were often used to sufficiently capture the flow dynamics.However,inadequate studies have been conducted on the pulsatile flow phenomenon,which directly relate to the gas-fly ash two-phase flow performance.This paper aims to investigate the pulsatile flows using an ECT device.Initially,pulsatile flow patterns under various experimental conditions were obtained through ECT.Pulses within a flow were then characterised into pulse growth and decay segments,which represent the superficial fluidisation and deaeration processes during conveying.Subsequently,structural and statistical analyses were performed on the pulse growth and decay segments.Results suggested that the increasing air mass flow rate led to the decrease of the superficial fluidisation/deaeration magnitude,however,the increase of the superficial fluidisation/deaeration durations.Also,the air mass flow rate was indicated as the dominant factor in determining the pulsing statistical parameters.This research provides fundamental insights for further modelling the dense fly ash pneumatic flows.     

可压缩流动问题笛卡尔网格模拟方法研究进展与展望

计算网格是复杂流动问题精细化模拟的先决条件,会对数值结果的准确性和可靠性产生重要影响.然而,高质量计算网格生成需要费时费力的人机交互过程且严重依赖于工程师的个人经验,使得现阶段计算流体力学(CFD)自动化程度处于较低水平.笛卡尔网格具备生成简单、内存要求低、计算效率高及自动性强等特点,使其能够极大减少网格生成过程中的人工成本,因而成为CFD社区的研究热点之一.对此,以笛卡尔网格可压缩流模拟为主线,结合课题组多年的研究成果对其中涉及的关键技术和国内外发展现状展开综述.首先简要概述了笛卡尔网格自适应方法及其数据结构,随后系统阐述了切割单元、浸入边界、重叠笛卡尔网格和混合笛卡尔网格等方法的发展现状及存在的问题,最后从自适应加密策略、动态并行技术、高保真计算方法、先进物理模型及实际工程应用等多个维度深入探讨了笛卡尔网格方法涉及的关键技术与未来发展趋势.通过全面回顾和深入分析笛卡尔网格方法的研究现状和发展动态,试图为读者提供一个清晰及全面的认识,并为相关领域的研究提供有益的参考和启示.