Experimental validation of a simple model for gas-liquid slug flow in horizontal pipes

In this work, the simple submodel of Taitel and Barnea (Chem. Eng. Sci. 45 (1990) 1191) for horizontal gas-liquid slug flow has been reformulated. An additional source of pressure loss suggested by Cook and Behnia (Chem. Eng. Sci. 55 (2000) 4699) was incorporated in the model, thus improving its predictive accuracy. The proposed model was tested extensively against 12 pressure gradient and eight liquid holdup data sources for both air-water and air-oil slug flow in smooth and rough pipes over a wide range of operating conditions. The very good agreement between the predicted and experimental results substantiates the general validity of the model.     

油水两相变径管流动模拟研究

油水两相混合流动是输油管路中经常遇到的流动现象,其流动规律研究具有重要的科学价值和广泛的工程应用价值。利用FLUENT软件对油水两相流在变径管中的流动进行模拟发现,管路内压力、速度在管径突然变化的区域有明显变化,且随含油量的不同其变化程度也有所不同。通过模拟分析突变管内油水两相流动规律,为实际油品输送提供一定的参考依据。     

Experimental investigation on two-phase air/high-viscosity-oil flow in a horizontal pipe

The flow of very-viscous-oil and air through a horizontal pipe (inner diameter 22 mm) is experimentally studied. We first build and analyze the flow pattern map; a comparison between the air–water and the air–oil flow pattern maps shows a strong influence of the fluid properties. The experimental flow maps are compared with empirical and theoretical ones – Baker (1954), Mandhane et al. (1974), and Petalas and Aziz (1998) – showing a poor agreement. Experimental pressure gradients are also reported and compared with theoretical model, but also in this case the agreement is not very satisfactory. Finally, the elongated bubble velocity and length are measured and compared to model present in the literature. We conclude that the high viscosity of the liquid phase has a strong influence on the results and that the current models are not able to predict the flow features satisfactorily.     

双螺旋电容传感器水平油水两相流液相持率测量特性(英文)

This paper presents the characteristics of a double helix capacitance sensor for measurement of the liquid holdup in horizontal oil–water two-phase flow. The finite element method is used to calculate the sensitivity field of the sensor in a pipe with 20 mm inner diameter and the effect of sensor geometry on the distribution of sensitivity field is presented. Then, a horizontal oil–water two-phase flow experiment is carried out to measure the response of the double helix capacitance sensor, in which a novel method is proposed to calibrate the liquid holdup based on three pairs of paral el-wire capacitance probes. The performance of the sensor is analyzed in terms of the flow structures detected by mini-conductance array probes.     

Liquid holdup measurement with double helix capacitance sensor in horizontal oil-water two-phase flow pipes

This paper presents the characteristics of a double helix capacitance sensor for measurement of the liquid holdup in horizontal oil–water two-phase flow. The finite element method is used to calculate ...     

Characterization of the two-phase flow regimes and liquid dispersion in horizontal and vertical tubes by using coloured tracer and non-intrusive optical detector

The monitoring of heat exchangers on duty with respect to flow mal-distributions needs the development of inlet-outlet experimental techniques in order to perform a fault diagnosis. Within this framework and as a first attempt to solve this problem in the case of multi-phase heat exchangers, we propose to use a non-intrusive optical sensor associated to a liquid phase tracer experiment. In order to check the capabilities of this technique, we present here the results that we have obtained for the characterization of two phase flows in horizontal and vertical tubes. The difference between refractive index of the two phases allows estimating the void fraction on the section illuminated by the optical sensor and permits to characterize two-phase flow regime from the signal characteristics. Signal analysis and treatment of the absorbance variation due to the coloured tracer injected in the liquid phase permit to estimate the real liquid phase averaged velocity and consequently the real gas phase average velocity. We also calculated volumetric void fraction and compared it to usual correlations with a good agreement. Finally, the evolution of the experimental liquid phase Peclet number with the two-phase flow regime has been observed and qualitatively explained.     

Calculation of particle-wall adhesion in horizontal gas-solids flow using CFD

The horizontal pneumatic conveying of fine particles is simulated with Computational Fluid Dynamics (CFD) including the particle-wall adhesion. The simulation is performed with FLUENT, whereby the dispersed phase (quartz powder, d_P,50=3 μm) is described with the Lagrange approach. The continuous phase is resolved with the Realizable k-? model. Models not provided by FLUENT are implemented via user defined functions. A horizontal pipe with a length of 3 m and an inner diameter of 50 mm is used for the calculation. The influence of different wall treatments on pressure drop and particle-wall adhesion is shown. Furthermore, several parameters are varied (e.g., electrostatic charge of particles, air velocity). The results are evaluated with measured data.     

Holdup for two-phase flow of gas-non-newtonian liquid mixtures in horizontal and vertical pipes

The paper deals with the experimental investigations carried out to evaluate holdup for gas-non-Newtonian (pseudoplastic) liquid mixtures in vertical and horizontal flow in pipes. Correlations developed predict holdup in slug flow regime with acceptable statistical accuracy.     

Investigating the effect of pressure on a vertical two-phase upward flow with a high viscosity liquid

This article presents void fraction and pressure gradient data for sulfur hexafluoride (SF₆) with gas densities of 28 and 45 kg/m³ and oil (with viscosity 35 times that for water) in a 127 mm diameter pipe. The superficial velocities of gas ranged from 0.1 to 3 m/s and those for liquid from 0.1 to 1 m/s, respectively. Measurements of void fraction data were recorded using a capacitance wire mesh sensor (WMS) system, which permits the 3D visualization of the flow patterns. All the data were obtained with a data acquisition frequency of 1,000 Hz. A differential pressure transducer was used to measure the pressure drops along the length of the pipe. The WMS provide time and cross-sectionally resolved data on void fraction and from an analysis of its output, flow patterns were identified using the characteristic signatures of probability density function (PDF) plot of time series of void fraction. The PDF plots showed the single peak shapes associated with bubbly and churn flows but not the twin-peaked shape usually seen in slug flows. This confirms previous work in larger diameter pipes but with less viscous liquids. For the bubble and churn flows investigated, the pressure gradient was observed to decrease with an increase in gas superficial velocity. Nevertheless, there was an insignificant observed effect of pressure on void fraction below certain transitional flow rates, the effect however became significant beyond these values. In the present work, wisps appear to be smaller, which might be due to the different fluid properties of the working fluids employed. In addition, wisps are easily revealed as long as there is a transition between churn and annular flows regardless of the pressure. Experimental data on void fraction and pressure gradient are compared against existing data. Reasonably good agreements were observed from the results of the comparison.     

稠油-水二相水平管流表观粘度的实验研究

以渤海稠油和水为工质进行尝试实验,作出了水平不锈钢实验回路(内径为25.7 mm,长为52 m)内稠油-水二相管流的流型图,并对管流的表观粘度及影响因素进行了实验研究。着重归纳了含水体积分数、温度等因素对二相管流表观粘度的影响规律,并对比分析了与旋转粘度仪测得粘度值的差异,研究结论对油田现场的油水混输管线的设计与安全运行具有较好的指导意义。     

CFD modeling of turbulent reacting flow in a semi-batch stirred-tank reactor

For the mixing-sensitive reactions,both chemical kinetics and mixing conditions of the reactants determine the distributions of products.The direct quadrature method of moments combining with the interaction by exchange with the mean micro-mixing model (DQMOM-IEM) has been validated for the chemical reacting flows in microreactors.Quite encouraging simulation results offer great promise,but the applicability of this method is needed to be explored furthermore,such as in stirred reactors.In this work,the two-environment DQMOM-IEM model was created with C language and used to customize Fluent through the user-defined functions.The mixing effects on the course of parallel competing chemical reactions carried out in a semi-batch single-phase stirred reactor were predicted.The simulation results show that the rising feed velocity enlarges the volume of reaction zone and maximize the yield of the by-product,which also indicates that the feed stream is more difficultly dispersed into the main stream and the zone surrounding feedpipe exit with high turbulent kinetic dissipation rate cannot be efficiently used.     

Verification and validation of CFD simulations for local flow dynamics in a draft tube airlift bioreactor

Airlift reactors have been recognized as one of the promising photobioreactors for biomass/bio-energy production, where mixing has significant impact on the reactor performance. In recent years, using CFD simulations to track microorganism cells and to generate their trajectories in the reactor for reactor performance evaluations becomes more common. However, there is a lack of systematic and rigorous verifications and validations of the reliability of CFD models in particle tracking against experimental measurements in the open literature, which is vital for the faithful application of CFD in reactor design and scale-ups. In this work, we attempt to evaluate the reliability of using CFD simulations to generate trajectories of microorganisms in a draft tube column photobioreactor. A computationally promising CFD simulation model based on CFX5.7 was validated against a benchmark experimental database reported in [Luo and Al-Dahhan, 2008a], [Luo and Al-Dahhan, 2008b] and [Luo and Al-Dahhan, 2010]. This model was then used to generate typical trajectories of microorganisms in the studied airlift column, which was further validated against experimentally measured tracer trajectories. The results indicated that the CFD model reasonably predicted the recirculation of the microorganism around the draft tube, however over-estimated the cells' residence time in the wall regions. Proper treatment for the wall region such as griding and wall function is needed to better capture the movement of microorganism cells in such bioreactors.     

An experimental investigation of gas-liquid two-phase flow in single microchannel contactors

This paper describes two-phase flow pattern and pressure drop characteristics during the absorption of CO₂ into water in three horizontal microchannel contactors which consist of Y-type rectangular microchannels having hydraulic diameters of 667, 400 and , respectively. With the help of a high-speed photography system, flow patterns such as bubbly flow, slug flow (including two sub-regimes, Taylor flow and unstable slug flow), slug-annular flow, churn flow and annular flow were observed in these microchannels. The applicability of the currently available correlations for describing flow pattern transitions in microchannels has been examined. Generally, the predicting performance of these correlations deteriorates as the channel diameter further reduces. Toward solving this discrepancy, an empirical correlation based on the superficial Weber numbers was developed to interpret the transition from Taylor flow to unstable slug flow in three microchannels. Taylor bubble formation process in microchannels was found to be in the squeezing regime at lower superficial liquid velocities (Ca ranging from 0.0019 to 0.029) while the transition to the dripping regime was observed at the highest superficial liquid velocity of 1.0 m/s. Lengths of Taylor bubbles formed in the squeezing regime can be well represented by the scaling relation proposed by Garstecki et al. [Formation of droplets and bubbles in a microfluidic T-junction—scaling and mechanism of break-up. Lab on a Chip, 6, 437-446]. For flow patterns including slug-annular flow, annular flow and churn flow, a simple analysis based on the separated flow model has been performed in order to reveal the observed effect of the superficial liquid velocity on two-phase frictional multiplier in the present microchannels. Then, reasonable correlations for the prediction of two-phase frictional pressure drop under these flow patterns were suggested.     

Multiscale lattice Boltzmann modeling of two-phase flow and retention times in micro-patterned fluidic devices

Recent advances for fabricating micro-featured architectures such as posts or pillars in fluidic devices provide exciting opportunities for multiphase flow management. Here we describe a novel, multiscale modeling approach for two-phase flows in microfeatured architectures developed within the Shan and Chen Lattice Boltzmann method. In our approach a fine scale is used to resolve the true microfeatured architecture, with a coarser scale used to model the gross geometry of the device. We develop the basic features of the approach and demonstrate its applicability to modeling retention times of droplets of a dispersed phase in an array of microposts – an architecture used in microfluidic reactors, bioreactors, and biomedical devises. Additionally we show that it is feasible to model the microfeatured geometry in a piecewise manner which includes extrapolating dispersed phase flow characteristics in the entire system based on simulations in smaller subdomains.     

Prediction of hydrodynamic entrance length for single and two-phase flow in helical coils

The hydrodynamic entrance length in helical coils for single and two-phase bubbly flow was studied experimentally and numerically. Development region length and the detailed characteristics of fluid flow have been investigated by varying helical coil parameters such as tube diameter, coil diameter and void fraction. For the CFD simulation of the two-phase fluid flow, the Eulerian–Eulerian model was employed. To calculate the turbulent fluctuations, the SST^k − ω turbulence model has been used. The experimental and numerical simulation of the local parameters demonstrates that the hydrodynamic developing length (L/D) increases when Reynolds number increases in the single and two-phase flows. The obtained results show that the development entrance length increases with the increase of pipe diameter and decreases with the increase of coil diameter. Also, entrance length decreases while curvature ratio of helical coil and void fraction increase. A correlational equation has been suggested to predict the hydrodynamic entrance length as functions of various parameters of the helical coil.     

A two-phase Eulerian approach using relative permeability concept for modeling of hydrodynamics in trickle-bed reactors at elevated pressure

Most commercial trickle-bed reactors (TBRs) employed in hydroprocessing and other industrially relevant operations normally operate at elevated pressures. Two-phase pressure drop and liquid holdup are two foremost important hydrodynamic parameters to consider for analysis and design of a TBR, including those operating at higher pressures. Even after several decades of research efforts directed towards the development of TBR technology, know-how about the hydrodynamics of two-phase flow in a TBR especially operating at high-pressure conditions has been inadequate. In this study, an effort has been made to assess the complex hydrodynamics of high-pressure TBR through the development of a Computational Fluid Dynamics (CFD) based model to predict pressure drop and liquid saturation. A two-phase Eulerian CFD model envisaging the flow field as porous region has been utilized for evaluating these hydrodynamic parameters. Different combinations of relative permeability correlations in the closure terms have been exercised to realize the best fit. The comparisons between model predictions and numerous experimental data, collected from different independent sources under a varied set of operating conditions, lead to the favourable implementation of this less computationally intensive, yet first-principle based CFD model to forecast the two-phase hydrodynamics for high-pressure TBRs.     

Modeling of low viscosity oil-water annular flow in horizontal and slightly inclined pipes: Experiments and CFD simulations

To characterize the effect of pipe inclination, low viscosity, flow rate and inlet water cut on annular flow pattern, a low viscosity oil-water two-phase annular flow in horizontal and slightly inclined (+1°, +3° and +5°) pipes with diameter of 20 mm has been experimentally investigated. A modified VOF model based on the CFD software package FLUENT was used to predict the in-situ oil fraction and pressure drop. The experimental data indicate that annular flow appears at a medium-high water cut. The slip ratio increases with flow rate increase but decreases with increasing water cut. The changes are more significant as the degree of inclination increases. Pressure drop is strongly dependent on flow rate, as it increases rapidly as inlet flow rate increase. Good agreement between the experimental data and calculated results of slip ratio and pressure drop was obtained.     

水平管内油水两相流流型转换特性

以高黏度的油和水为工质,在内径为25.7 mm,长52 m的水平不锈钢油水两相流实验环道内对油水两相流流型及其转换特性进行了实验研究.根据实验结果定义了不同流动条件下出现的流型,绘制了流型图.对影响油水两相管流流型转换的各种因素进行了综合分析,利用量纲分析的方法得出了流型转换的准则关系式,并提出了一个较为准确的有关油水两相管流中反相临界含水率的计算相关式.     

Core-annular flow in horizontal and slightly inclined pipes: Existence, pressure drops, and hold-up

An analysis of high viscosity oil/water flow in horizontal and slightly inclined pipe is presented. We propose experimental flow maps – in particular we focus on the core-annular flow pattern boundary – pressure drops, and oil hold-up measurements. Experimental data are used to validate some models present in literature. Since hold-up measurement in liquid–liquid systems is particularly challenging and, therefore, only few data are available in literature, they are analyzed and commented in detail. The slip ratio computed by hold-up data is also provided and analyzed.