Bubble Velocity and Size Measurement with a Four‐Point Optical Fiber Probe

The possibility to measure the velocity and size of individual bubbles in a high‐void fraction bubbly flow is investigated by using a four‐point optical fiber probe. The air bubbles have an initial spherical equivalent diameter ranging from 4 to 10 mm and the void fraction is up to 0.3. Firstly, single bubble experiments show that intrusiveness effects, i.e. bubble deformations due to the probe, are negligible provided that the bubble approaches the probe at the axis of the central fiber. A selection criterion is utilized for multiple bubble experiments. A good compromise can be found between the required accuracy, the duration of the measurements and the number of validated bubbles required for reliable statistical averaging. In an air‐water high‐void fraction vertical bubbly pipe flow, the void fraction obtained with the instrument is found to be in good agreement with both local single‐fiber probe measurements, and with the volume average void fraction obtained from pressure gradient measurements. The area average volumetric gas flow rate, based on the bubble velocity and void fraction as measured with the four‐point probe, agree with the measured gas flow rate. Also, the liquid velocity is measured by means of a laser‐Doppler anemometer, to investigate the slip velocity. The results show that reliable and interesting measurements can be obtained by using a four‐point optical fiber probe in high void fraction flows.     

Numerical simulation and experimental validation of multiphysics field coupling mechanisms for a high power ICP wind tunnel

We take the established inductively coupled plasma(ICP) wind tunnel as a research object to investigate the thermal protection system of re-entry vehicles. A 1.2-MW high power ICP wind tunnel is studied through numerical simulation and experimental validation. The distribution characteristics and interaction mechanism of the flow field and electromagnetic field of the ICP wind tunnel are investigated using the multi-field coupling method of flow, electromagnetic, chemical, and thermodynamic field. The accuracy of the numerical simulation is validated by comparing the experimental results with the simulation results. Thereafter, the wind tunnel pressure, air velocity, electron density, Joule heating rate, Lorentz force, and electric field intensity obtained using the simulation are analyzed and discussed. The results indicate that for the 1.2-MW ICP wind tunnel, the maximum values of temperature, pressure, electron number density, and other parameters are observed during coil heating. The influence of the radial Lorentz force on the momentum transfer is stronger than that of the axial Lorentz force. The electron number density at the central axis and the amplitude and position of the Joule heating rate are affected by the radial Lorentz force. Moreover, the plasma in the wind tunnel is constantly in the subsonic flow state, and a strong eddy flow is easily generated at the inlet of the wind tunnel.     

Laser-induced thermal-acoustic velocimetry with heterodyne detection

Laser-induced thermal acoustics (LITA) was used with heterodyne detection to measure simultaneously and in a single laser pulse the sound speed and flow velocity of NO>(2) -seeded air in a low-speed wind tunnel up to Mach number M =0.1 . The uncertainties of the velocity and the sound speed measurements were ~0.2 m/s and 0.5%, respectively. Measurements were obtained through a nonlinear least-squares fit to a general, analytic closed-form solution for heterodyne-detected LITA signals from thermal gratings. Agreement between theory and experiment is exceptionally good.     

The generation of wind waves on clean water and in the presence of an oil film

The conditions of the resonant excitation of waves on a liquid surface by a horizontal air flow that has a decreasing velocity in the direction of motion were established, such that steady waves occurred when the period of the escape of a chain of eddies that is generated in a viscous layer of an air flow coincided with the period of natural oscillations, which is determined by the dispersion relationship for a group of waves. The dependence of the lengths of steady waves on the air-flow velocity over the surface of clean water and water with a light oil film was obtained. The resulting model was tested experimentally.     

Multifilament Model of PET Melt Spinning and Prediction of As‐spun Fiber's Quality

Based on the multifilament model with cross air blowing proposed by Dutta (1987) and the assumption that the quench air temperature around the filament obeys an exponential distribution, a multifilament model suitable for the annular air blowing condition of PET staple fiber melt spinning is proposed. The quench air velocity, quench air temperature, filament velocity, filament temperature, etc. at different positions were predicted and the relation between birefringence and the important quality index of as‐spun fiber, Eys1.5 (elongation corresponding to 1.5 times the yielding stress in a stress‐strain curve) was obtained through experiment. The as‐spun fiber properties of PET staple and its variability were predicted and the effects of spinning conditions and spinneret design on as‐spun fiber properties were discussed and verified.     

Dissolved air effects on three-phase hydrodynamic cavitation in large scale Venturi- Experimental/numerical analysis

Hydrodynamic cavitation (HC) in the Venturi nozzle, apart from the harmful influence on the devices, can be used to improve a range of industrial processes, such as biofuel generation, emulsion preparation, and wastewater treatment. The present investigation deals with the influence of dissolved air in Venturi cavitating flow based on numerical and experimental approaches. The experimental campaigns have been done in a closed-loop water tunnel equipped with a Venturi test section. The post-processing techniques such as Fast Fourier Transform (FFT), Power Spectral Density (PSD), temporal/spatial Grey Level distribution and mean value grey level distribution are employed to analyse the experimental observations and measurement. The URANS numerical method is modified based on the Density Corrected-Based Model (DCM) to be more adaptable for flows with high differences in density. The results approve the remarkable effect of dissolved air on the configuration of the cavity, its evolution process, and transient/averaged characteristics. It is observed that the incipient point and ratio of sheet cavity length to cloud cavity length are changed. Furthermore, the flow velocity inside of the sheet and cloud cavities is different; as well as, the higher content of dissolved air leads to slower flow velocity inside the cloud cavity. In addition, the shedding frequency is significantly reduced in case of higher level of air content.     

激光诱导炽光法与消光法诊断烧蚀羽烟消光性能

为了研究流场中碳纤维增强环氧树脂复合材料在激光辐照时产生的烧蚀羽烟对入射激光的屏蔽效应,通过对朗伯-比尔定律进行分析,得到了评价羽烟消光性能的平均质量消光系数的表达式,其与羽烟场浓度和激光透过率相关。采用激光诱导炽光法(LII)和激光消光法,搭建了羽烟消光性能联合诊断实验平台,使待测激光落于LII的激发光平面上,通过同步采集待测激光的透过率和LII信号,获得激发光平面上羽烟浓度场和激光消光比,得到羽烟在不同气流速度下的平均质量消光系数。实验得到气流速度为7,10,20m/s时羽烟对1064nm激光的归一化质量消光系数分别为2.51,1.08,1.00。实验发现,质量消光系数受到气流速度影响,当气流速度较低时质量消光系数曲线波动幅度大,且曲线均值较大;当气流速度较高时质量消光系数趋于稳定且均值较小。     

基于光纤光栅的高灵敏度流速传感器

利用光纤光栅压强传感机构和汾丘里管设计了一种基于光纤光栅的流速传感器,并推导了光纤光栅中心波长漂移量与流速的关系式。实验表明,该传感器具有较高的灵敏度,稳定性较好,光纤光栅的中心波长随流速的增加而不断向短波方向漂移,而带宽几乎没有变化,实验和理论符合得较好。该流速传感器的动态感测范围为51.0~148.2 mm/s,在该范围内,至少可感测到0.3 mm/s的流速变化,这是目前所报道的最优值。优化光纤光栅压强传感机构及汾丘里管的参量,可测量其它速度段的流速,并可进一步提高传感灵敏度。     

高温空气煤粉直燃技术的数值模拟

本文通过数值计算的方法,对高温空气煤粉直燃燃烧器的多种运行工况进行了数值试验研究,分析了煤粉气流入口速度、煤粉浓度和高温空气速度等主要因素对煤粉气流着火的影响,模拟结果还可以反映出高温空气无油点火燃烧器内的流动、燃烧和传热情况。与试验数据的对比分析表明,数值模拟结果与试验数据趋势一致。研究结果对此燃烧器的结构及运行参数的优化提供了指导。     

Experimental study of different control methods for hypersonic air inlets

An experimental study of different control methods for hypersonic air inlets aimed at ensuring reliable starting of these apparatuses and improving their operating characteristics in the range of Mach numbers 2 to 8 is reported. Conditions for boundary-layer separation and possibilities for preventing this separation by using modified diffuser configurations and/or perforation bleedage are examined. An air-inlet model was tested for operation in an intermittent wind tunnel and in a blow-down wind tunnel respectively in the Mach-number ranges 2 to 6 and 5 to 8. Distributions of static and total air pressures on the walls of the model and in several cross sections were measured, together with air flow coefficients and total-pressure recovery coefficients. Perforation bleedage is shown to offer an efficient means to facilitate air-inlet starting. Perforation bleed has enabled a more than two-fold increase in the air flow coefficient on the model with sidewalls. A perforation-bleed panel installed closer to the air-inlet throat proved to be more efficient. The possibility of sudden starting of the air-inlet apparatus was checked in the intermittent wind tunnel; it was shown that, here, sudden starting could be realized. The data obtained in the intermittent wind tunnel proved to be consistent with data obtained in the blow-down wind tunnel with up to 150-ms blowdown time. This work was supported by the International Scientific and Engineering Center (Contract No. 887) and by MBDA, France.     

Stability characteristics and flowfields of turbulent non-premixed swirling flames

A simple, yet representative, burner geometry is used for the investigation of highly swirling turbulent unconfined, non-premixed, flames of natural gas. The burner configuration comprises a ceramic faced bluff-body with a central fuel jet. The bluff-body is surrounded by an annulus that delivers a swirling primary flow of air. The entire burner assembly is housed in a wind tunnel providing a secondary co-flowing stream of air. This hybrid bluff-body/swirl burner configuration stabilizes complex turbulent flames not unlike those found in practical combustors, yet is amenable to modelling because of its well-defined boundary conditions. Full stability characteristics including blow-off limits and comprehensive maps of flame shapes are presented for swirling flames of three different fuel mixtures: compressed natural gas (CNG), CNG–air (1:2 by volume) and CNG–H₂ (1:1 by volume).

It is found that with increased fuel flow, flame blow-off mode may change with swirl number, S_g. At low swirl, the flame remains stable at the base but blows off in the neck region further downstream. At higher swirl numbers, the flames peel off completely from the burner's base. Swirling CNG–air flames are distinct in that they only undergo base blow-off. In the low range of swirl number, increasing S_g causes limited improvement in the blow-off limits of the flames investigated and (for a few cases) can even lead to some deterioration over a small intermediate range of S_g. It is only above a certain threshold of swirl that significant improvements in blow-off limits appear. Six flames are selected for further detailed flowfield and composition measurements and these differ in the combination of swirl number, primary axial velocity through the annulus, U_s, and bulk fuel jet velocity, U_j. Only velocity field measurements are presented in this paper. A number of flow features are resolved in these flames, which resemble those already associated with non-reacting swirling flows of equivalent swirl obtained with the present burner configuration. Additionally, asymmetric flowfields inherent to some flames are revealed where the fluidic centreline of the flow (defined in the two-dimensional (U–W velocity pair) velocity field by the ?ω? = 0 tangential velocity contour), meanders strongly on either side of the geometric centreline downstream by about one bluff-body diameter. Flow structures revealed by the velocity data are correlated to flame shapes to yield a better understanding of how the velocity field influences the flames physical characteristics.     

基于光纤光栅传感器的体外血管模型应力测量北大核心CSCD

血管中的血液流动会对血管壁产生周向应力,周向应力与血管的结构及功能变化密切相关,测量体外血管模型的周向应力是生物力学研究中较重要的问题。提出了利用光纤光栅测量血管周向应力的方法,基于微流控技术利用钢针模具建立了集成光纤光栅的三维圆形血管模型,通过仿真研究了不同流动速度与应力的关系。仿真结果得到流速在8 mm/s~75 mm/s范围内,应力随速度的改变呈线性变化。利用光纤光栅传感器测量了流体流动时产生的周向应力,根据实验得到了光栅波长改变量与速度的关系,流速范围在8 mm/s~75 mm/s之间变化时,速度引起波长的变化为0.173 nm。结合仿真实验得到了应力与光栅波长改变量的关系,为血液流动时产生周向应力的体外测量提供了新思路。     

A rotating arc gas pump for circuit breaking and other applications

A rotating arc circuit breaker is described which uses an auxiliary current source to generate the magnetic field for driving the arc. Test results obtained using optical fiber measurement systems have shown that there are three main arcing phases. Initially the arc rotates at an essentially constant but low velocity, subsequently its velocity oscillates between this and much higher values, and finally the are plasma may become diffuse in nature. Test results obtained with dielectric strength probes have indicated that a unidirectional flow of arc heated gas is generated. The flow is away from the moving contact of the interrupter so promoting good dielectric strength in this critical contact region. The combination of the optical fiber and dielectric probe results indicates two possible modes of gas pumping represented, respectively, by a fan and a piston-type action of the arc. Simplified analytical models for both modes are developed with predictions obtained showing good agreement with the experimental results. Discussion of experimental results suggests that the transition from oscillatory velocity changes to diffuse arcing represents an important parameter for scaling the geometries of future interrupters and arc heaters     

矩形腔体流场模拟及噪声研究

用大涡模拟方法对低速湍流引起的矩形腔体内流动进行了模拟,并应用FW-H声学类比方程分析了由流动诱发的气动噪声.数值模拟观察到了涡结构的脱体及腔体内部的自激振荡过程,通过分析得出了由流动诱发噪声的声压-频率曲线.研究发现在流速30 m/s时,流动噪声声压级在60 dB以下,348.48 Hz及其高次谐波是噪声的主要来源,流场与声场表现出耦合关系,辐射声场具有明显的方向性.腔体噪声的风洞实验研究得到了与数值模拟吻合的结果.     

激光诱导荧光技术在高焓空气氮原子测量中的应用

高超声速飞行器再入大气层时,受到激波的压缩和激波层内粘性阻滞作用,周围绕流流场的空气温度在4 000～15 000 K之间,使空气中的氧、氮分子发生离解,从而出现高温气体效应,形成高焓化学非平衡流。飞行器表面防热材料基本都含有大量的碳元素,通常情况下主要是氧原子与碳发生反应,但在焓值大于18 MJ·kg-1的情况下,氮原子与飞行器表面碳反应的无量纲烧蚀因子BC_N＞0.172 5,此时碳在高焓空气介质中的无量纲质量烧蚀因子BC_air＞0.345,因此,碳的氮化烧蚀变得非常剧烈,和氧化烧蚀相当;同时离解的氮原子也会在飞行器表面发生催化复合反应放出大量的热,使飞行器表面承受更多的热载荷。因此,分析高焓化学非平衡流流场中的氮原子具有现实意义。在地面模拟设备中建立高焓化学非平衡流场,对氮原子进行测量技术,可以很好的对其进行研究。双光子吸收激光诱导荧光（TALIF）技术作为一种非接触测量,在不干扰流场环境的情况下,可以直接获得氮原子的浓度分布。对流场氮原子激发荧光,通过布置在风洞试验段外与流场和激光形成的平面呈垂直方向的ICCD获取到二维氮原子的荧光信号。为确保荧光图像的清晰及视场合适,选择了Nikon f=50 mm F/1.4镜头作为前级收光设备。实验成像采用50次曝光的累计结果,以消除气流扰动及激光能量抖动造成的不确定性。实验中,在理论激发波长附件进行测试,优化选择出206.717 nm作为正式实验中脉冲激光的最佳激发波长。在最佳激光波长条件下,从小到大调整激发能量,获得了该环境下的氮原子激发的非饱和线性区为1.8 mJ以下。正式实验过程中激光能量为1.6 mJ,处于线性区。对所获取的荧光图像进行处理,提取激光中心线上的荧光强度进行分析,发现无论是亚声速还是超声速,荧光强度沿径向都呈驼峰状分布,与之前氧原子测试结果比较,发现流场中氮分子还未完全离解,这符合实验风洞流场特性。     

水泵吸水池内部流动PIV试验的深入分析

ＰＩＶ技术是一种新型的流动量测手段，使用这种技术能够容易的得出流场中的速度分布情况，在此基础上进行深入分析，还可以得到许多十分重要的流动参数。这里，对水泵吸水池内部流动的ＰＩＶ试验进行深入分析，得出如下的流动参量：吸水口处的环量分布、测量面上的涡度分布、交叉面处的三维速度分布，另外，对试验中所拍摄的图片进行分析，可以得出吸入涡中气核大小的分布情况．     

Flow field analysis by large-area interrogation in particle image velocimetry

The evaluation of flow records obtained by particle image velocimetry (PIV) employs the interrogation of certain regions in the flow image. Basic relations are derived for the case that the velocity is not constant within the region of interrogation. The influence of a distribution of velocities on the Young's fringe system and its Fourier transform, the autocorrelation of the flow record, are discussed. The considerations show that there is an easy access to statistical parameters of the flow by carrying out large-field interrogation. The autocorrelation function will reproduce the probability density function of the velocity, and there is no need for a large number of time-consuming small-area interrogations. Furthermore, the effects of velocity variations in ordinary fringe analysis are discussed. Examples are given from synthetic PIV records and measurements in a wind tunnel.     

Passive nonlinear reshaping towards parabolic pulses in the steady-state regime in optical fibers

The passive nonlinear reshaping in normally dispersive optical fibers in the steady-state regime is studied numerically. It is found that normal dispersion and self-phase modulation are able to provide pulse reshaping towards a parabolic pulse profile at the distances exceeding the optical wave breaking length. However, as compared to the similariton formation in active fibers the resulted pulse shape in passive fibers is strongly depended on the initial pulse parameters and nonlinear and dispersive fiber properties as well. The influence of initial pulse shape, initial chirp, third-order dispersion and loss on the parabolic pulse formation is studied consistently, and estimation of practical conditions which are needed for parabolic pulses formation in a passive fiber is provided.     

Application of “POLIS” PIV system for measurement of velocity fields in a supersonic flow of the wind tunnels

Measurement results on the mean velocity fields and fields of velocity pulsations in the supersonic flows obtained by means of the PIV measurement set “POLIS” are presented. Experiments were carried out in the supersonic blow-down and stationary wind tunnels at the Mach numbers of 4.85 and 6. The method of flow velocity estimate in the test section of the blow-down wind tunnel was grounded by direct measurements of stagnation pressure in the setup settling chamber. The size of tracer particles introduced into the supersonic flow by a mist generator was determined; data on the structure of pulsating velocity in a track of an oblique-cut gas-dynamic whistle were obtained under the conditions of self-oscillations.