Experimental study for the stratified to slug flow regime transition mechanism of gas-oil two-phase flow in horizontal pipe

Theoretical relations that predict the transition from a stratified pattern to a slug pattern, including a one-dimensional wave model that contains less empiricism than the commonly used Taitel-Dukler model, and the ideal model for stratified flow for the gas-liquid flow in horizontal pipes are presented. Superficial velocities of each phase, as the onset of slugging occurs, were predicted, and theoretical analysis was conducted on the stratified to slug flow regime transition. The friction, existing between the fluid and pipe wall, and on the interface of two phases, was especially taken into account. A theoretical model was applied to an experiment about air-oil two-phase flow in a 50 mm horizontal pipe. The effect of pipe diameter on the transition was also studied. The results show that this approach gives a reasonable prediction over the whole range of flow rates, and better agreement has been achieved between predicted and measured critical parameters.     

Theoretical and experimental investigation of the onset of slugging in horizontal stratified air-water countercurrent flow

An experimental and theoretical study has been carried out to derive the wave height and transition criterion from wavy to slug flow in horizontal air-water countercurrent stratified flow conditions. A theoretical formula for the wave height in a stratified wavy flow regime has been developed using the concept of total energy balance over a wave crest to consider the shear stress acting on the interface of two fluids. From the limiting condition of the formula for the wave height, a criterion for transition from a stratified wavy flow to a slug flow has been derived. A series of experiments have also been conducted changing the non-dimensional water depth and the flow rates of air in a horizontal pipe and a duct. Comparisons between the measured data and the predictions of the present theory show that the agreement is within ±10%.     

Study of two-phase flow regime identification in horizontal tube bundles under vertical upward cross-flow condition using wavelet transform

A wavelet-transform based approach for flow regime identification in horizontal tube bundles under vertical upward cross-flow condition was presented. Tests on two-phase flow pattern of R134a were conducted under low mass velocity and flow boiling conditions over ranges of mass flux 4–25 kg/m²·s, vapor quality 0.02–0.90. Time series of differential pressure fluctuations were measured and analyzed with discrete wavelet transform. Different time-scale characteristics in bubbly flow, churn flow and annular flow were analyzed. The wavelet energy distributions over scales were found to be appropriate for flow regime identification. Based on the wavelet energy distribution over characteristic scales, a criterion of flow regime identification was proposed. The comparison with experiment results show that it is feasible to use the discrete wavelet transform as the tool of flow regime identification in horizontal tube bundles under vertical upward cross-flow condition. __________ Translated from Journal of Shanghai Jiaotong University, 2007, 41(3): 337–341, 346 [译自: 上海交通大学学报]     

Simulation of horizontal slug-flow pneumatic conveying with kinetic theory

Wavelike slug-flow is a representative flow type in horizontal pneumatic conveying. Kinetic theory was introduced to establish a 3D kinetic numerical model for wavelike slug gas-solid flow in this paper. Wavelike motion of particulate slugs in horizontal pipes was numerically investigated. The formation and motion process of slugs and settled layer were simulated. The characteristics of the flow, such as pressure drop, air velocity distribution, slug length and settled layer thickness, and the detailed changing characteristics of slug length and settled layer thickness with air velocity were obtained. The results indicate that kinetic theory can represent the physical characteristics of the non-suspension dense phase flow of wavelike slug pneumatic conveying. The experiment in this paper introduced a new idea for the numerical calculation of slug-flow pneumatic conveying. Translated from Journal of Engineering Thermophysics, 2006, 27(1): 75–78 [译自: 工程热物理学报]     

Optimization of Venturi tube design for pipeline pulverized coal flow measurements

A Venturi flow meter was designed to accurately measure the mass flow rate of pulverized coal in power plant pipelines. Numerical simulations of the dilute turbulent gas-solid two-phase flow in a horizontal Venturi tube were used to study the effects of Venturi tube geometry on the pressure distribution in the mixture. The results show that Venturi tube geometry strongly influences the metering of the dilute gas-particle two-phase flow. The geometry can be optimized to improve the precision of the measurement and ensure stable measurements. Furthermore, the geometries of three types of Venturi tubes were optimized for an experimental study of pulverized coal mass flow rate measurements. __________ Translated from Journal of Tsinghua University (Sci & Tech), 2007, 47(5): 666–669 [译自: 清华大学学报( 自然科学版)]     

Two-phase co-current flow in inclined pipe

Data are presented on horizontal and slightly inclined flows at +5 and −5° for a 0.058 m inner diameter (i.d.) pipe with the co-current air–water system.The prediction capabilities of existing flow regime maps were shown to be inadequate. However, the transitions for stratified ripple to role wave, for slug to blow-through-slug, for film plus droplet to stratified, and the modified maps for stratified type to slug flows all gave good prediction performance with horizontal and slightly inclined flows.The largest liquid hold-up occurred in upward flow except at high gas rates and low liquid rates where the downflow condition gave the highest liquid hold-up. The lowest liquid hold-up occurred in downward flow at low gas flow rates and horizontal flow at high gas flow rates. Hold-up prediction proved to be flow regime dependent.The inclined total average pressure drop data crossed over the horizontal data from higher to lower values with increasing gas rate at a gas rate of just under V?_SG = 10 m s⁻¹.Below this gas rate the horizontal pipe gave the lowest pressure drop while above this gas rate the upwardly inclined pipe gave the lowest pressure drop. A pressure loss minimum occurred at V?_SG = 10 m s⁻¹ for upward flows. Below V?_SG = 10 m s⁻¹ the pressure loss for downward flow was virtually independent of gas rate being mainly due to hydrostatic head. As the gas flow approached V?_SG = 50 m s⁻¹ there was very little effect of inclination on the pressure loss.Pressure drop was successfully predicted although the accompanying hold-up prediction was not always reliable.     

Two instability criteria for the stratified flow in horizontal pipe at cocurrent flow conditions

This paper presents a theoretical approach of two instability criteria from the stratified to nonstratified flow in horizontal pipes at cocurrent flow conditions. The new theoretical instability criteria for the stratified and nonstratified flow transition in horizontal pipe have been developed by hyperbolic equations in two-phase flow. These criteria, onset of slugging criterion and critical flow condition criterion, at cocurrent flow condition correspond to imaginary and zero characteristics which occur when the hyperbolicity of a stratified two-phase flow are broken, respectively. Through comparison between results predicted by the present theory and the Kukita et al.'s experimental data, it is shown that they are in good agreement with experimental data.     

Numerical analysis of aerodynamic noise radiated from cross flow fan

The flow field in a cross flow fan was simulated by solving the 2-D unsteady Reynolds-averaged Navier-Stokes equations. The calculated pressure fluctuations of the blades, the vortex wall, and the rear wall were then used as noise sources to calculate the sound field. The Ffowcs Williams-Hawkings (FW-H) equation was employed to predict the noise field caused by these sources. The predictions show that the rear wall and the vortex wall sources contribute significantly to the total noise and that both the predicted aerodynamic performance and noise agree well with the experimental results. __________ Translated from J Tsinghua Univ (Sci & Tech), 2007, 47(2): 236–239 [译自: 清华大学学报(自然科学版)]     

Flow characteristics of single-phase flow in narrow annular channels

The characteristics of single-phase flow in narrow annular channels were analyzed and theoretical model was proposed. Based on the present model, the theoretical calculation was performed to predict the flow characteristics for the developed flow in narrow annuli with the gap sizes of 1.0, 1.5 and 2.0 mm, respectively. The results were in good agreement with the experimental data. In addition, the gap size of narrow annuli has great impact upon the flow characteristics. The decrease of gap size reduces friction factor. The higher the Reynolds number, the more remarkable the effect of gap size upon friction coefficient during single-phase flowing through narrow annular channels. The effect of gap size upon friction coefficient is dependent on the Reynolds number, and will decrease with the decrease of the Reynolds number. __________ Translated from Atomic Energy Science and Technology, 2007, 41(5): 575–579 [译自 : 原子能科学技术]     

Numerical simulation of three-dimensional gas/liquid two-phase flow in a proton exchange membrane fuel cell

Investigation into the formation and transport of liquid water in proton exchange membrane fuel cells (PEMFCs) is the key to fuel cell water management. A three-dimensional gas/liquid two-phase flow and heat transfer model is developed based on the multiphase mixture theory. The reactant gas flow, diffusion, and chemical reaction as well as the liquid water transport and phase change process are modeled. Numerical simulations on liquid water distribution and its effects on the performance of a PEMFC are conducted. Results show that liquid water distributes mostly in the cathode, and predicted cell performance decreases quickly at high current density due to the obstruction of liquid water to oxygen diffusion. The simulation results agree well with experimental data. Translated from J Tsinghua Univ (Sci & Tech), 2006, 46(2): 252–256 [译自: 清华大学学报]     

Lagrangian simulation of deposition of CO2 gas-solid sudden expansion flow

Freezing and blockage resulting from the deposition of solid CO₂ formed because of sudden expansion of the downstream pipe during the release of CO₂ through safety valves, will endanger the protected equipment. To overcome this problem, the characteristics of the CO₂ gas2solid sudden expansion flow are studied by using the disperse Lagrangian model. A comparison of the calculated deposition of the solid CO₂ with the experimental results shows that they are in reasonable agreement. The simulation results show that the size of the solid CO₂ formed should not be in the range of 0.04–0.07 mm (St number 3.2–9.8). This can be achieved by using an appropriate flow cross section of the safety valve. __________ Translated from Journal of Shanghai Jiaotong University, 2007, 41(3): 419–423 [译自: 上海交通大学学报]     

Two-phase Flow Patterns in a Square Mini-channel

This paper presents a new set of experimental data of air-water flow patterns in a channel with a cross-section of 1×1 mm2. The ranges of the gas and liquid superficial velocities are 0.1-10 m/s and 0.2~7 m/s, respectively. Bubble, bubble-slug, slug, and frothy flows are observed. The present data are compared with other data in mini-channels reported in literature, and also compared with those in normal channel at microgravity, in which the Bond number has the same order of magnitude. The slug-frothy boundary is in consistent with each other, but for the bubble-slug transition, a much smaller value for the transition quality in the drift-flux model is obtained in the present study than those predicted by the empirical relations for the case of microgravity. It's shown that the mini-scale modeling may not be an effective way to anticipate the bubble-slug transition of two-phase flow at microgravity.     

Cavitation in semi-open centrifugal impellers for a miniature pump

Cavitation in miniature pumps was investigated experimentally for two semi-open centrifugal impellers. Although both impellers had the same blade cross-section, one impeller had a two-dimensional blade, while the other had a leaned blade. The flows were also analyzed using a numerical model of the three-dimensional turbulent flow in the pumps near the peak efficiency point using the k-ɛ turbulence model and the VOF cavitation model. The average cavitation performance of each impeller was satisfactorily predicted by the numerical simulations. The results show that the miniature pumps have similar cavitation performances as an ordinary-size pump, with the cavitation performance of the semi-open impeller reduced by increased axial tip clearances. Also, both the hydraulic and cavitation performance of the semi-open impeller were improved by the leaned blade. The results also show that uniform flow upstream of the impeller inlet will improve the cavitation performance of a miniature pump. __________ Translated from J Tsinghua Univ (Sci & Tech), 2006, 46(8): 1451–1454 [译自: 清华大学学报(自然科学版)]     

一种基于差压波动图的段塞流识别方法

在内径为80mm的大型水平实验环道上进行了广泛的空气一水两相流实验,将采集的一定时间长度的分层、段塞、环状流型的差压波动信号,显示成二维图像。通过对30组差压波动数据分析发现：段塞流的信号区和背景区面积之比平均为0．026,分层流为0,53,环状流为0．35。段基流信号区占整个图像面积的比值远小于其他流型,因此该比值可以作为判别段塞流型的一个特征参数。该方法能有效利用于段塞流流型的快速自动检测。     

Two-phase flow patterns and transition characteristics for in-tube condensation with different surface inclinations

In-tube condensation of R-11 was experimentally investigated with various surface inclination angle between the direction of the vapor flow and the gravitational force, φ. The two-phase flow patterns were observed visually and the transition locations between different flow patterns were measured in a transparent test section. The vapor quality distribution along the test tube and the measured transition locations were used to prepare a flow pattern map. The experimental results indicated that the surface inclination, φ, strongly influenced the vapor and condensate distribution. Annular flow spanned the whole tube for φ = 0° at various vapor flow rates. Annular flow and stratified flow patterns were observed for φ = 45° and 60°. Annular flow, stratified flow, half-slug flow, and slug flow exist in sequence for φ = 90–120°. Annular flow, churn flow and slug flow occurred for φ = 180°. The possible prediction of flow patterns for in-tube condensation in a microgravity environment was discussed briefly.     

Investigation of flow boiling in horizontal tubes: Part I—A new diabatic two-phase flow pattern map

Several important modifications to the flow pattern map of Kattan-Thome-Favrat [J. Heat Transfer 120(1) (1998) 140-147] made, resulting in a significantly new version of the map. Based on the dynamic void fraction measurements described in [Int. J. Multiphase Flow 30 (2004) 125-137], the stratified-wavy region has been subdivided into three subzones: slug, slug/stratified-wavy and stratified-wavy. Furthermore, annular-to-dryout and dryout-to-mist flow transition curves have been added and integrated into the new flow pattern map, identified by distinct trends of the heat transfer coefficient as a function of vapor quality and by flow pattern observations to determine (and then predict) the inception and completion of dryout in horizontal tubes.     

Two-phase flow pattern in small diameter tubes with the presence of horizontal return bend

This study provides a qualitatively visual observation of the two-phase flow patterns for air-water mixtures inside 6.9, 4.95, and 3 mm smooth diameter tubes with the presence of horizontal return bend. The influence of the return bend on the two-phase flow patterns are investigated. For D=6.9 mm and at a mass flux of 50 kg m⁻² s⁻¹ having a quality less than 0.1, no influence on the flow patterns is seen at a larger curvature ratio of 7.1. However, were the curvature ratio reduced to 3, the flow pattern in the recovery region is temporarily turned from stratified flow into annular flow. The temporary flow pattern transition phenomenon from stratified flow to annular flow is not so pronounced with the decrease of tube diameter. It is likely that this phenomenon is related to the influence of surface tension and the reduction of developing length of the swirl flow. Based on the present flow visualization, three flow pattern maps are proposed to describe the effect of return bend on the transition of two-phase flow pattern.     

水平管内油气水三相泡状流-弹状流转换的研究

结合压差波动法识别流型，对水平管内油-气-水三相泡状流-弹状流转换特性进行了理论和实验研究，建立了泡状流向弹状流转变的界线方程，结果表明，泡状流向弹状流转变的主要因素不仅包括气相折算速度和液相折算速度，而且含油率与管径也起重要作用，计算结果与实验结果基本相吻合。     

不同纳米流体水平管内沸腾流型的模拟

对以纯水为基液的不同纳米流体管内流动沸腾进行了模拟。利用UDF编程定义流体的沸腾相变源项,将其导入FLUENT软件中分别模拟这四种流体在水平管内的流动沸腾过程。得到了四种流体流动沸腾的速度云图,以及流型分布云图。对比分析了四种流体的速度分布云图、四种流体从初始状态沸腾到1s时的流型分布云图以及四种流体层状流流型的特点。结果表明,四种流体在水平管内沸腾都会出现泡状流、弹状流、层状流以及波状流四种流型,并且沿管长方向含气量逐渐增加。在相同的时间内,纳米流体的沸腾比纯水更剧烈,而且,不同的纳米流体沸腾程度也不同。在相同的体积分数下,纳米颗粒的导热系数越大,其对应的纳米流体沸腾越剧烈。说明纳米颗粒的导热系数是影响纳米流体沸腾传热的主要因素。     

The flow characteristics of an upward gas‐liquid two‐phase flow in a vertical tube with a wire coil: Part 1. Experimental results of flow pattern,void fraction,and pressure drop

Experiments were carried out to investigate the flow pattern, average void fraction, and pressure drop of an upward air‐water two‐phase flow in vertical tubes of 25‐mm inside diameter with wire coils of varying wire diameter, pitch, and number of coils in cross section. Five kinds of flow patterns—bubble, slug, churn, semiannular, and annular flow—were defined based on the observation of flow behavior in the experiments. At higher water flowrates, the bubble‐to‐slug transition occurred at lower air flowrates in tubes with wire coils than in smooth tubes. The average void fraction was found by using the drift flux model. Further, the experimental results of the friction pressure drop were compared with the Lockhart‐Martinelli correlation. As a result, a correlation with the constant C in Chisholm's equation was obtained as a function of the wire coil pitch‐to‐diameter ratio. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(8): 639–651, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10066