低含气率气液两相流涡街特性研究

气液两相涡街现象广泛存在于生产实际中,由于相间作用,研究变得较为复杂。通过数值模拟和实流试验,对水平管低含气率情况下气相对两相涡街的影响进行研究。建立三维计算流体力学（Computational fluid dynamics, CFD）仿真模型,采用VOF多相流模型和RNG k-e 湍流模型,模拟气液两相钝体绕流。仿真与试验结果在定量频率上具有良好的一致性。结合仿真与试验结果,从近尾迹流场与涡拓扑、斯特劳哈尔数两方面,对不同含气率两相涡街特性进行对比研究。结果表明,在稳定涡街范围内,随着气相含率增大,对涡街起重要作用的滞止区长度增大,导致斯特劳哈尔数线性减小;同时,涡街周期性和信号质量变差,涡街能量降低,这是由于涡中心吸入密度小、速度大的气泡,造成涡的旋转能量降低,进而影响涡街的稳定性。     

毛细管空调与传统空调的空气调节性能的对比试验

以能源与动力工程实验中心节能减排实验室为背景,对毛细管空调系统与传统空调进行制冷及除湿性能对比实验。监测室内温湿度分布情况;结果显示:毛细管辐射空调运行时,在人体主要活动范围内温度平均在25.5~27.3℃,相对湿度在56.8%~65.0%;对比毛细管辐射空调运行时,传统空调温度分布均匀性较差,温度在25.0~26.3℃之间,垂直方向上随着高度升高逐渐降低,水平方向上各测点的温度分布不均。这种温度分布使得人体有忽冷忽热的感觉,各测点相对湿度波动较明显,但是垂直和水平方向的相对湿度值相差在5%以内,较毛细管辐射空调供冷相对湿度区间较小。     

Image processing techniques for high-speed videometry in horizontal two-phase slug flows

Two-phase flow measurements are very common in industrial applications especially in oil and gas areas. Although some works in image segmentation have analyzed gas–liquid slug flow along vertical pipes, few approaches have focused on horizontal experiments. In such conditions, the detection of the Taylor bubble is challenging due the great amount of small bubbles in the slug area and, thus, requires a special treatment in order to separate gas from liquid phases. This article describes a new technique that automatically estimates bubble parameters (e.g. frequency, dimension and velocity) through video analysis of high-speed camera measurements in horizontal pipes. Experimental data were obtained from a flow test section where slug flows were generated under controlled conditions. Image processing techniques such as watershed segmentation, top-hat filtering and H-minima transform were applied to detect and estimate bubble contour and velocities from the observed images. Finally, the estimated parameters were compared to theoretical predictions, showing good agreement and indicating that the proposed technique is a powerful tool in the investigation of two-phase flow.     

Analysis of vacuum melting, ultrasonic, and radiographic techniques for gas porosity evaluation in die castings

Parts manufactured using die-casting processes may have significant limitations, mainly due to the tendency for air to be trapped during die-cavity filling. The detection and accurate measurement of porosity due to trapped air are essential issues for improving the quality of manufactured parts. This paper focuses on a comparison of radiographic, ultrasonic inspection, and vacuum melting techniques for analyzing the porosity level due to trapped air in parts manufactured using die-casting processes. To this end, aluminum alloy parts manufactured in a horizontal cold chamber die-casting machine using different injection velocities and mold configurations are analyzed. For the radiographic and ultrasonic techniques, a methodology is established to determine the level of porosity in the part, while, for the vacuum melting technique, a test procedure is established and an expression is obtained to relate the quantity of gas trapped in the part with the pressure increments measured as the part is melted in a vacuum furnace. The advantages and limitations of these techniques to evaluate porosity levels in die-cast parts are discussed in detail. Finally, the possibilities of using these techniques to select operating conditions that reduce the amount of trapped air during the filling stage are analyzed.     

Three-dimensional computational fluid dynamic analysis of the conventional PEM fuel cell and investigation of prominent gas diffusion layers effect

A full three-dimensional, single phase computational fluid dynamics model of a proton exchange membrane fuel cell (PEMFC) with both the gas distribution flow channels and the membrane electrode assembly (MEA) has been developed. A single set of conservation equations which are valid for the flow channels, gas-diffusion electrodes, catalyst layers, and the membrane region are developed and numerically solved using a finite volume based computational fluid dynamics technique. In this research some parameters such as oxygen consumption, water production, velocity distribution, ohmic losses, liquid water activity and fuel cell performance for straight (base case) and prominent gas diffusion layers were investigated in more detail. The numerical simulations reveal that prominent gas diffusion layer improves the transport of the reactant gases through the porous layers; it is due to increase of the mentioned fuel cell efficiency, and prominent gas diffusion layers yield appreciably higher current density. Finally the numerical results of proposed CFD model (base case) are compared with the available experimental data that represent good agreement.     

Experiments on foam texture under high pressure in porous media

Foam flooding has become one of the most effective methods to improve oil recovery in high water-cut oilfields. In order to obtain images of the texture of foam in the cores under high pressure, this paper developed an experimental real-time image acquisition device for detecting the foam's texture as the bubbles flowed in the porous media. Image processing and statistic analysis methods for describing the foam's texture were also established. Experimental results showed that the bubble distribution became homogeneous and stabilized, and the pressure difference between two ends of the core increased gradually during the process of foam flooding. In the initial stage, the bubbles presented a flake-like distribution. The liquid phase flowed continuously, as did the gas phase. As the displacement continued, the volume of a single bubble lessened, while the number of bubbles increased. When the bubbles reached a steady state, the pressure difference between the two ends of the core was stable, and the bubbles presented a uniform distribution. A larger injection rate, a higher core permeability and a larger gas–liquid ratio all can cause a larger-than-average bubble diameter, poor stability and a decreased degree of homogeneity.     

Experimental study on bubble motion in a rectangular bubble column using high-speed video observations

A two-phase flow in a rectangular bubble column of 100×20 mm cross-section and 1.5 mm height was studied using a high-speed video system. Series of images were taken at different elevations at a frequency of 500 Hz. The images were processed using a bubble recognition algorithm. In such a way, an individual bubble in the gas swarm could be tracked. The time-averaged velocity profiles and the turbulent diffusion coefficients were derived as a function of the superficial gas velocity.The lateral displacement of bubbles travelling over a certain vertical distance was transformed into a probability density distribution in order to measure the turbulent diffusion coefficient of the gaseous phase. The shape of the distributions obtained was found to fit well to the Gaussian standard distribution. The dispersion coefficients were observed to grow proportionally to the square root of the vertical distance. The diffusion coefficients were calculated from the proportionality factor and were compared with some correlations published in the literature. The experiments were performed for superficial gas velocities ranging from 1 to 6 mm/s. The bubbles were generated either by a porous sparger or a set of capillaries placed at the bottom of the column. The measurements were taken at different heights between 1 and 1.2 m where the bubble cloud was occupying the entire cross-section.     

多尺度气泡尺寸分布数字图像测量方法研究

鼓泡塔是一种广泛应用于能源和环境领域的多相流反应器,鼓泡塔中气泡的大小和浓度对于研究鼓泡塔中"三传一反"过程具有重要意义。采用高速摄像法和数字图像处理技术开展了鼓泡塔中内多尺度气泡尺寸分布测量研究,针对气泡识别过程中密集气泡易发生重叠的问题,提出基于曲率计算的凹点匹配与圆周拟合的重叠气泡分割与轮廓重构算法。搭建了鼓泡塔反应器实验装置,针对星型、均匀和方形3种不同进气孔形态的气泡分布器开展了实验研究,分析了不同尺度气泡的尺寸分布规律。试验结果表明:该算法不仅能够有效地从图像中提取轮廓清晰完整的气泡,而且能够对图像粘连重叠的气泡进行准确分割,从而可精确地获得多尺度气泡尺寸分布。随着气体流量的增加,小气泡的数量急剧增加,同时产生更大的气泡;气泡的最大直径和Sauter平均直径均随气体流量的增加而增大,且两者的比值基本保持不变,即分布器形式对气泡尺寸分布均匀性有影响,方形分布器产生气泡最均匀,气含率相对其他两种分布器更高。实验结果证明了图像分割与轮廓重构方法在气液两相流中气泡参数在线测量的可行性。     

Modified UNet with attention gate and dense skip connection for flow field information prediction with porous media

Porous media flow phenomena are commonly found in nature and industry. Understanding the patterns can improve industrial production and efficiency. Traditional numerical methods can predict flow field information within porous media exactly, even on pore scale. However, once the pore structures of porous media are changed, the traditional numerical methods have to start over again, which implies poor extension and low efficiency for realistic application. To address the challenge, we propose a new deep learning method for image segmentation based on convolutional neural networks that can quickly predict the saturation variation and flow velocity profile of multiphase flow directly from the structure of porous media. In the present method, the UNet network structure is improved by replacing parallel splicing with different levels of dense skip connection to extract flow information at different depths. When processing flow topological information, the introduction of attention mechanism allows effective focus on adjusting the flow field edges and front shapes to improve accuracy. Depending on the pore structures information, the saturation of the porous media also can be evaluated. This result shows that the flow field information (saturation and velocity) of porous media can be quickly predicted by deep learning methods. Compared with the computational fluid dynamics (CFD) method, this approach has a better prediction efficiency for porous media. The machine learning method is a good prediction tool for porous media with different porosities and pore structures.     

Experimental study of bubbly swirling flow in a vertical tube using ultrasonic velocity profiler (UVP) and wire mesh sensor (WMS)

Two-phase air-water bubbly swirling flow through a pipe is a complex turbulent flow and its prediction is still challenging. The present paper describes the experimental investigation of the air-water bubbly swirling flow in vertical co-current flow. Swirling flow is induced by a twisted tape in a 20 mm inner diameter pipe. The flow is investigated using Ultrasonic velocity profiler (UVP), which allows the measurement of liquid and gas velocities simultaneously. Furthermore, simultaneous measurement of void fraction is performed using Wire mesh sensor (WMS). The experimental results reveal that swirling flow has significant impact on bubbles’ distribution. In low liquid flow rate, the average bubble velocity is fairly uniform along the radial position and void fraction increases in the near wall region. However, increasing liquid flow rate at constant gas flow rate leads to increase in void fraction in the core region, this is mainly due to drift velocity which is affected by centrifugal force. Experimental findings and parametric trends based on the effects of swirling flow are summarized and discussed.     

Design of a three-dimensional current sensor with measuring upwelling

A new three-dimensional flow velocity sensor which can measure upwelling was proposed. This sensor was composed of horizontal flow sensor and vertical flow sensor, and it can measured horizontal flow and tiny upwelling separately. The structure of horizontal flow and vertical flow velocity measuring device were analyzed and optimized based on static simulation, static calculation and an experiment. The results show that the sensor can measure both the small upwelling and the horizontal flow velocity in principle. Based on the root of energy loss, the amplifying structure was optimized. After optimization, the magnification factor was increased by 436.84%. According to experimental results, mechanical analysis of cross beams and static simulation, there was a good linear relationship between the flow resistance around the sphere and the output strain of the horizontal velocity measuring device, and it can be used to measure the horizontal flow velocity.     

Three-dimensional numerical analysis of proton exchange membrane fuel cell

A full three-dimensional, non-isothermal computational fluid dynamics model of a proton exchange membrane fuel cell (PEMFC) with both the gas distribution flow channels and the membrane electrode assembly (MEA) has been developed. A single set of conservation equations which are valid for the flow channels, gas-diffusion electrodes, catalyst layers, and the membrane region are developed and numerically solved using a finite volume based computational fluid dynamics technique. In this research some parameters such as Oxygen consumption and fuel cell performance according to the variation of porosity, thickness of gas diffusion layer, and the effect of the boundary conditions were investigated in more details. Numerical results shown that the higher values of gas diffusion layer porosity improve the mass transport within the cell, and this leads to reduce the mass transport loss. The gas diffusion layer thickness affects the fuel cell mass transport. A thinner gas diffusion layer increases the mass transport, and consequently the performance of the fuel cell. Furthermore, the study of boundary conditions effects showed that by insulating the bipolar surfaces, hydrogen and oxygen consumption at the anode and cathode sides increase; so that the fuel cell performance would be optimized. Finally the numerical results of proposed CFD model are compared with the available experimental data that represent good agreement.     

3D and 2D experimental views on the flow field of gas-evolving electrode cold model for electrolysis magnesium

In the magnesium electrolysis process, chlorine gas bubbles release at the surfaces of anode and affect electrolyte flow patterns. This paper presents an experimental apparatus to simulate the flow field induced by chlorine gas evolution at the gas-evolving electrodes of magnesium electrolysis cell. The three-dimensional flow structures were determined by using volumetric three-component velocimetry (V3V) technique, which has the ability to capture the out-of-plane velocity component. The three-dimensional flow structures in the region with a depth about 120 mm can be obtained. To achieve this, approximately 15,000 three-dimensional velocity vectors were detected in the flow measurements and constituted the three-dimensional flow field, which eliminated the perspective error caused by the out-of-plane motion in Particle Image Velocimetry (PIV) method. In experiments, comparisons are made between the V3V and PIV results. The in-of-plane velocities data obtained by V3V technique have the same trend with the PIV results, and V3V provides more details in the third direction for the flow field accurately.     

Numerical modeling on the formation process of keyhole-induced porosity for laser welding steel with T-joint

A mathematical model of laser welding steel with a T-joint was developed in this paper to investigate the formation process of keyhole-induced porosity, helping to understand the mechanism of porosity formation in laser welding steels with heavy section. Solidification model and adiabatic bubble model were coupled in this model, which could more approximately reflect the formation process of bubble and its evolution into porosity. The volume-of-fluid (VOF) method was taken to track free surfaces of keyhole and porosity. The numerical results showed that the unstable keyhole during the laser welding process induced the collapse of keyhole and then resulted in bubbles in the molten pool. These bubbles moved following with the fluid flow in the molten pool, where some bubbles could escape out of molten pool under the competition of flow and solidification speed. But some bubbles captured by a solidified wall during the migration process in the molten pool would evolve into porosities. It was also found that some bubbles formed adjacent to a fusion line were easier to be captured by a solidification surface, which could give explanation for some porosities occurring close to the fusion line. A good agreement of simulation and experimental results proved the reliability of this mathematical model, while the mechanism of porosity formation was better clarified with this model.     

Analysis of externally pressurized porous gas journal bearings —II

A theoretical analysis of load-carrying capacity generated by vibration of the journal (the squeeze-film effect) of an externally pressurized porous gas journal is made. The flow of gas in the porous media is in three dimensions and obeys Darcy's law. The solution has been obtained by the perturbation method. The journal is assumed to vibrate either by translatory motion or by rotational motion around the transverse axis. The dimensionless squeeze load, moment, load- and moment-phase angles are given for various squeeze numbers, feeding parameters and a porosity number.     

基于FLUENT的孔板消减气流脉动的数值模拟

在输流管道内的适当位置添加孔板作为一种简单有效的消减管道气流脉动的方法,已经在生产实践中得到广泛应用,但是对于孔板开孔率和开孔数这两个主要设计参数的共同研究很少。针对这一问题,现以弯管为例,采用数值模拟软件Fluent建立管道三维定常流动模型,分别计算单孔不同开孔率孔板下以及不同开孔数相同开孔率下管道内部的脉动状况,并与不加孔板时的情况进行了对比。通过数值模拟,表明添加适当尺寸孔板能够起到一定的脉动消减作用,同时得出在开孔率为30%和开孔个数为1的情况下,穿过孔板的气流不均匀度由23.12%下降到0.60%。     

Effects of different porous beds on turbulent characteristics in an open channel above the porous bed

An analysis of the turbulent characteristics of the flow in an open channel with horizontal porous beds using two-dimensional (2D) Particle Image Velocimetry (PIV) is presented. Hydraulic characteristics such as velocity distributions, turbulent intensities and Reynolds stress are investigated at a fine resolution using the PIV. Experiments were carried out, with four types of porous bed with the same porosity ε=0.70 and the same porous thickness (3 cm): (a) porous filter, (b) rods bundle, (c) grass vegetation and (d) gravel bed. Turbulent characteristics are measured above the porous bed for the same different water heights (h΄=3, 5, 7 and 9 cm) and for the same slope S=1.5‰. Main emphasis is given to the effects of relative porous thickness s΄/h (s΄=porous thickness, h=total flow height) on the flow characteristics over the porous beds. The relative thickness s΄/h varies between 0.250 and 0.500. The discharge ranges between 3.0 and 24.6 lt/s. Measurements of mean velocity and turbulence characteristics indicate the effects of the bed material used on the flow characteristics. The presence of rods bundle influences with different way the turbulent characteristics of the flow as regards the other porous beds.     

Concentration measurements of bubbles in a water column using an optical tomography system

Optical tomography provides a means for the determination of the spatial distribution of materials with different optical density in a volume by non-intrusive means. This paper presents results of concentration measurements of gas bubbles in a water column using an optical tomography system. A hydraulic flow rig is used to generate vertical air-water two-phase flows with controllable bubble flow rate. Two approaches are investigated. The first aims to obtain an average gas concentration at the measurement section, the second aims to obtain a gas distribution profile by using tomographic imaging. A hybrid back-projection algorithm is used to calculate concentration profiles from measured sensor values to provide a tomographic image of the measurement cross-section. The algorithm combines the characteristic of an optical sensor as a hard field sensor and the linear back projection algorithm.     

Comparison between wire-mesh sensor and ultra-fast X-ray tomograph for an air–water flow in a vertical pipe

A comparison between ultra-fast X-ray CT and a wire-mesh sensor is presented. The measurements were carried out in a vertical pipe of 42 mm inner diameter, which was supplied with an air–water mixture. Both gas and liquid superficial velocities were varied. The X-ray CT delivered 263 frames per second, while the wire-mesh sensor was operated at a frequency four times higher. Two different gas injectors were used: four orifices of 5 mm diameter for creating large bubbles and gas plugs and a sintered plate with a pore size of 100 μm for generating a bubbly flow. It was found that the wire-mesh sensor has a significantly higher resolution than the X-ray CT. Small bubbles, which are clearly shown by the wire-mesh sensor, cannot be found in the CT images, because they cross the measuring plane before a complete scan can be performed. This causes artifacts in the reconstructed images, instead. Furthermore, there are large deviations between the quantitative information contained in the reconstructed tomographic 2D distributions and the gas fractions measured by the sensor, while the agreement is very good when the gas fraction is obtained by a direct evaluation of the X-ray attenuation along the available through-transmission chords of the tomography set-up. This shows that there is still potential for an improvement of the image reconstruction method. Concerning the wire-mesh sensor it was found that the gas fraction inside large bubbles is slightly underestimated. Furthermore, a significant distortion of large Taylor bubbles by the sensor was found for small liquid velocities up to 0.24 m/s. This effect vanished with growing superficial water velocity.     

Tilt stiffness and damping of externally pressurized porous gas journal bearings

The dynamic behaviour of an externally pressurized porous gas journal bearing is analysed by assuming one-dimensional flow through the porous bushing. A periodic disturbance (angular displacement) about the transverse axis is imposed on the bearing and the resulting dynamic pressure distribution is determined by small perturbations of the Reynolds equation. A finite difference method is used to determine the dynamic pressure. Design data for tilt stiffness and damping as a function of squeeze number, feeding parameter, supply pressure and porosity parameter are calculated numerically using a digital computer and are presented in tables and figures.