Bubble motion in inclined pipes

Highly nonlinear free-surface flows in vertical, inclined, and horizontal pipes are analyzed. The problem of bubble motion in a vertical pipe is closely related to the Rayleigh-Taylor instability problem. Inclined pipe flows are intensively studied as related to gas and oil transportation. A new theory of motion of large bubbles in pipes is developed. As distinct from previous approaches, which relied on semiempirical methods or numerical fitting, analytical methods of potential theory and complex analysis are used. A careful comparison of 2D and 3D solutions is presented. It is shown that a higher dimensionality may not correspond to a higher bubble velocity. For the first time, free-surface flows in inclined pipes are analyzed by means of direct numerical simulation, which makes it possible to develop a new approach to the Rayleigh-Taylor instability problem (bubbles with wedge-and cone-shaped noses).     

气力提升系统气液两相流数值模拟分析

污水处理、油田采油、液态金属冷却反应堆和磁流体动力转换器等领域采用气力提升系统有其显著优势.由于不同液体介质与气体介质密度对气力提升系统性能影响较大,因此本文基于Fluent仿真软件,采用欧拉模型、k-ω剪切应力输运湍流模型数值模拟了氮气-水、氮气-煤油、氮气-水银及空气-水、氩气-水、氮气-水下气力提升系统内气液两相流动行为,分析了系统稳定时提升立管内气相体积分数、提升液体流量、提升效率、提升管出口处液体径向速度的变化规律.研究结果表明:1)氮气-水、氮气-煤油、氮气-水银系统中,提升管内液体介质密度越大,提升管内气相体积分数越小、提升液体流量越大、提升效率越高;2)空气-水、氩气-水、氮气-水系统中,提升管内气体介质密度越大,提升管内气相体积分数越小、提升液体流量越大、提升效率峰值越小;3)提升管出口处提升液体径向速度随气体充入量的不断增加而整体波动升高,最终管轴中心附近液体速度较大,管壁附近液体速度较小.本文研究成果为污水处理、气举采油、液态重金属冷却核反应堆和磁流体动力转换器等应用领域的气力提升技术的优化提供科学的理论基础.     

有气流扰动下管流油水混合物粘度实验测量与计算模型

采用局部即时取样方法对水平管内油气水三相流动情况下各种混合比例的复杂混合物的流动粘度进行了实验研究,实验工质采用46号机械油、自来水和空气。以实验数据为基础提出了考虑流动参数变化影响的反相点预测关联式。考虑到管内油水两相的混合发展过程,以局部即时取样的实验数据为基础,提出了一个气流扰动下管内流动条件下油水混合物粘度的预测关系式,该模型考虑了油水两相本身的物性以及流动因素的影响。指出考虑流动参数影响的粘度预测模型能大大提高油气水三相流动情况下油水混合物实测粘度的预测精度。     

Flow visualization of relaminarization phenomena in curved pipes and the related measurements

Flow visualization results for secondary flow phenomena at the exit of 90° and 180° bends and helically coiled pipes (1, 2 and 5 turns), (radius of curvatureR _c=381 mm, inside pipe diameterd=37.5 mm, curvature ratiod/2R _c=0.049) and in the downstream straight pipe (l/d=30) are presented to study the stabilizing (relaminarization) effect in curved pipes with fully developed entry turbulent air flow and the destabilizing (re-transition from laminar to turbulent flow) effect in the downstream straight region. The entry Reynolds numbers areRe=2200, 3200, 4300 and 5300). The related measurement results using a hot-film anemometer are presented for developing profiles of the time-mean streamwise velocity distribution and the axial turbulence intensity field in the 180° return bend and in the downstream straight pipe for Reynolds numbersRe=3200, 4300, 6300 and 8200. The time traces showing the output of the hot-film sensor are also presented for developing fluctuating velocity field in the 180° bend and in the downstream straight pipe for the same Reynolds number range. Some aspects of the relaminarization phenomena in curved pipes and the re-transition phenomena from laminar to turbulent flow in the downstream straight pipe are clarified by the present experimental investigation.     

Flow on blades of wells turbine for wave power generation

Wells turbine has the cascade whose stagger angle is 90°, namely the blades are perpendicular to the axial velocity. Good performance is required from 0° to 90° angle of attack because the turbine is operated in the oscillating airflow produced with wave energy. Furthermore, very interesting and complex flows are experimentally observed by the oil film method for large angles of attack where the performance is strongly influenced, especially, the self-starting. This paper tries to analyze the mechanism of these three-dimensional flows around the turbine with the flow visualization and the numerical analysis, focusing on the off-design condition.     

Influence of bubbly clusters on the characteristics of the two-phase gas-liquid flow in a flat channel

Results of experimental investigation of a bubbly gas-liquid flow in horizontal and weakly inclined (from −20° to +20°) flat channel are presented. These measurements were carried out within the 0.2–1 m/s range of superficial velocities and volumetric gas flow rate ratio of up to 0.2. The hydrodynamic structure was measured by the electrochemical method with application of wall shear stress and conductivity microprobes. During the experiments signals of shear stress on the upper channel wall and local gas flow rate ratio were recorded completely. After numerical treatment of recorded signals the profiles of local gas flow rate ratio were obtained, average shear stress and its relative mean square pulsations on the upper channel wall were determined. It is shown that under the studied regimes the bubbles are grouped into clusters, and the bubbly flow is presented by alternation of bubbly clusters and single-phase liquid with separate bubbles and without them. Average wall shear stress and absolute shear stress pulsations in the range of bubbly clusters and beyond them were determined. Histograms of probability density distribution were obtained for the wall shear stress on the upper wall. It is shown that average shear stress and absolute pulsations in clusters are significantly higher than those in the flow zone free from bubbles. The work was financially supported by the Russian Foundation for Basic Research (No. 07-08-00405a).     

垂直管内油水两相流局部相分布特性实验研究

应用双头电导探针测量系统,对垂直上升管内油水两相分散流局部相分布特性进行了系统测量。得到了油水两相分散流的局部含油率分布类型图。研究结果表明低折算水速和低折算油速条件下,局部含油率在实验段截面上呈抛物线型局部分布特征,局部最大值出现在实验段中心区域。随折算油速增大,油滴受到横向力如升力的作用,逐渐向实验段壁面区域迁移,形成局部含油率的壁面峰值分布特性。当折算水速大于0．8 m／s时,局部含油率在实验段截面上呈均匀分布。     

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.     

Mass transfer in a two-phase flow in a curvilinear channel

In this paper, the results of an experimental investigation of the distribution of local coefficients of mass transfer on a pipe wall for a flow of a two-phase gas-liquid mixture with a turn of 90° are presented. The mass transfer coefficients were measured by using an electrodiffusion method. It has been shown that the distribution of local mass transfer coefficients on the inner surface of a curvilinear channel is nonuniform both in length and along the perimeter and depends on the content of gas in the two-phase flow.     

Rotation effect on near-wall turbulence statistics and flow structures

Turbulent flow in an axially rotating pipe, involving complicated physical mecha- nism of turbulence, is a typical problem for the study of rotating turbulent flow. The pipe rotation induces two effects on the flow. One is the stabilizing effect due to the centrifu- gal and Coriolis forces, which accounts for the relaminarization of the turbulence[1—3] and the reduction of the friction coefficient at the pipe wall. The behavior is also related to the wall streaks inclining to the azimuthal di…     

气液两相流中的声速研究

气液两相流在工业各领域中广泛存在,而声速是描述其声学性质的一个重要参数。本文从流体的体积弹性模量的定义出发,推导了气液两相流中的声速随含气率的变化关系式,即混合流体的Wood声速公式,将其声速的部分计算结果和其他作者的实验数据进行了比较,吻合良好。并通过COMSOL有限元模拟软件得到不同气体分布下圆管谐振腔最低阶模式的共振频率,间接数值模拟研究了含气率对声速的影响。模拟结果与理论计算结果一致,当气液两相流中含气率较低时,声速随含气率的增大急剧减小。本研究结果为确定声速与气液两相流中的含气率间的关系提供了参考依据。     

A bouncing droplet on an inclined hydrophobic surface

Abstract  

A trajectory of a water droplet of volume 0.4 μL impacting and bouncing on the hydrophobic surface (143°) inclined with 12° is presented. The photograph was taken with a 35 mm format digital SLR camera in a long-time exposure of 2.5 s illuminated with a couple of continuous point lighting. Also, a selected comparison is shown for snapshots of a bouncing millimetric droplet, having volume of 10 μL, inclined angle of 15° and contact angle of 147°, during the first impact between the experimental results which were recorded with a high-speed camera at 1,222 frames/s and the 3D computation (using a Volume-of-Fluid method on 330 × 70 × 35 computational grids with dt = 1.0 × 10⁻⁴ s) which was carried out with FLUENT software. At the impact the droplet is found to deform and then dissipate the energy due to the surface tension.     

COUNTERCURRENT FLOODING IN VERTICAL-TO-INCLINED PIPES

Countercurrent flooding data have been obtained using air and water for vertical-to-downwardly inclined pipes containing elbows of varying angles. Experiments were performed with six different test sections, all having an inner diameter of 51 mm and a 1-m-tong vertical tube connected to an inclined or horizontal tube

The flooding data for 112.5° and 135° elbow angles were almost identical and showed that these geometries required the largest gas flow rates for flooding among all the geometries tested. The flooding gas velocities for the 157.5° elbow were slightly less than those of the 112.5° and 135° elbows but greater than those of the vertical pipe without any elbow and vertical-to-horizontal pipes at low to moderate liquid flow rates

In all vertical-to-inclined pipes, flooding was initiated in the inclined section at about 15 to 50 cm downstream of the elbow. Due to the countercurrent flow of gas, the liquid stream just downstream of the elbow became highly agitated and a frothy mixture was carried upstream by gas at flooding. At moderate to high liquid flow rates, the liquid was deflected off at the elbow to form a turbulent, jetlike stream that partially broke up into droplets, These droplets were, at the onset of flooding, entrained and carried over by the gas stream

Comparison of the data with the slugging correlation at low liquid flow rates and with the liquid entrainment/carryover model at high liquid flow rates suggested that these mechanisms are likely responsible for flooding in vertical-to-inclined pipes.     

Flow characteristics of a single stage EHD gas pump in circular tube

Characteristics of flow induced by electrohydrodynamic (EHD) gas pumps in circular pipe have been experimentally evaluated. Two tube diameters (61.8 mm and 127.8 mm) and two electrode gap distances (25 mm and 50 mm) have been considered. The gas pumps use eight evenly spaced emitting electrodes which are flush mounted on the tube wall. As such, flows induced by the pumps have a profile with a higher velocity near the wall and a lower velocity at the tube center. Experiments are conducted using positive corona discharge with voltage varying from 17.5 kV to 30 kV. The results show that the volume flow rate increases with the applied voltage but approaches an asymptotic value before sparkover takes place. From the present results, several important implications for the practical engineering applications are presented.     

Application of PIV to over-expanded supersonic flows: Possibilities and limits

Two-dimensional velocity distributions outside a Mach 2.0 supersonic nozzle have been investigated using a digital particle im age velocimetry (PIV). Mean velocities , vor ticity field and volume dilatation field were obtained from PIV images using 0 .33 μm titanium dioxide (TiO₂) particle. The seeding particle of larger size , 1.4 μrn Ti0₂, was also used for the experimental comparison of velocity lag downstream of shock waves. The results have been compared and analyzed with schlieren photographs for the locations of shock waves and over-expanded shock structure to inspect possibilities and limits of a PIV technique to over-expanded supersonic flows. It is found that although the quantitative velocity measurement using PIV on over-expanded supersonic flows with large velocity and pressure gradients is limited, the locations of normal shock and oblique shock waves can be resolved by the axial/radial velocity fields, and over-expanded shock structure can be predicted by vorticity field and volume dilatation field which are acquired from the spatial differential of the velocity field.     

The runaway effect in a Lorentz gas

The Lorentz gas of charged particles in a constant and uniform electric field is studied. The gas flows through the medium of immobile, randomly distributed scatterers. Particles with velocity v suffer collisions with frequency proportional to ¦v¦ ⁿ . Forn < 0 runaway of the gas is forced by the field: the mean velocity of the flow increases without bounds. By a simple physical argument an integral relation is established between the probability of collisionless motion and the velocity distribution. It is then shown that whenn < –1 a fraction of particles moves as if the scattering centers were absent. The detailed discussion of this uncollided runaway is presented. Some qualitative features of the velocity distribution are illustrated on rigorous solutions in one dimension.     

Ultrasonic velocity dispersion in methylcyclohexane

Ultrasonic velocity dispersion in methylcyclohexane was studied in the frequency range of 0.5 to 5.0 MHz at 60°, 65°, 70°, 75° and 80°C using a variable path interferometer. Necessary corrections for the diffraction of the ultrasonic beam were applied. The velocity data were used to evaluate the relaxation parameters pertaining to the interconversion of the axial and equatorial isomers. The value of the difference in enthalpy, difference in entropy and the activation energy are 11.2±4.0 kJ mole⁻¹, 28.0±8.0 J mole⁻¹K⁻¹ and 43.7±4.0 kJ mole⁻¹ respectively. These values are in good agreement with values derived by Piercy and Subrahmanyam from their ultrasonic absorption measurements.     

Velocity effects on pollutant diagnostics with tunable lasers for doppler-broadened lines

The effect of velocity distortions in high-speed gas flows must be allowed for in diagnostic studies using tunable lasers.We present the results of theoretical studies on the distortion of a Doppler-line profile produced by directed motion in a conical source flow. This type of flow is conviniently defined by the ratio of axial velocity at the observation station to the critical velocity and by the axial temperature. In addition, we present results for an idealized flow with constant flow velocity and temperature in the observation plane.Deviations from the Gaussian Doppler contour increase, under otherwise constant conditions, with increasing source-flow divergence angle, decreasing temperature, and increasing flow speed. Even under moderate condition (e.g. maximum semi-divergence angle of 30°, axis temperature T_l = 300 K, and axial flow speed divided by the speed of light = q_l/c = 1 × 10^-5), the Gaussian profile is grossly distorted because of occurrence of velocity components towards and away from the observer.Our results suggest that it will be very difficult to determine gas compositions from high-resolution spectroscopic observations (e.g. laser-absorption measurements) unless the velocity field is determined independently. Sensitivity losses in line-center and derivative spectroscopy may be estimated from our data for representative flow conditions.     

Laser doppler anemometry in hydrodynamic testing

Classical methods and the laser-Doppler-anemometry (LDA) application for the test section calibration of a water-cavitation tunnel is presented. The results show that the LDA is the best method for calibration and the classical methods satisfy the needs of standard tests. The advantages of the LDA are illustrated by determination of the pressure coefficient C _p for a hydrofoil of a high-speed axial pump under the stationary and nonstationary conditions, as well. The velocity vector distribution is measured around the central hydrofoil of a straight grid for angles of attack δ = 0° and 25° and undisturbed velocity v _∞ = 5.32 m/s. The results of the LDA measurements are used as a basis for the definition of the boundary conditions for numerical flow simulation and C _p calculation by the Fluent program. A flow visualization is made by aniline dyes and air bubbles.     

Novel processing tools for automated doppler picture veocimetry (DPV) evaluation

A unique technique, the Doppler Picture Velocimetry (DPV), for measuring and visualizing velocities especially in hypersonic gas flows is presented. By means of a Michelson interferometer (MI) the Doppler shifted light scattered by tracers is transformed into an image showing Fizeau fringes. The image fringe distribution provides information on the Doppler frequency shift which is related to the velocity of the particles crossing a light sheet plane. To overcome former disadvantages of DPV, the optical set-up as well as the processing scheme have been improved significantly. Two schemes have been tested in recent times for fringe processing: 1) an alternative process, the Least Square Estimators (LES) and 2) an algorithm based on a technique using Fast Fourier Transformations (FFT) to examine the DPV images in the frequency domain. The new DPV algorithms now allow an automated calculation of the velocity profiles from the Doppler pictures without manual fringe tracing as it had to be done in the past. Both methods are compared by means of Mach 6 flows around a wedge and a sphere produced in the ISL high energy shock tunnel STA. TiO₂ particles are illuminated by a laser light sheet which was perpendicularly arranged to the main flow direction. Light observation via the MI from the side (90° to illumination) informs on the vertically oriented velocity through the DPV images.