Slug Flow Regime Research Articles

Modelling mass transfer in liquid-liquid slug flow in a microchannel

Liquid-liquid slug flow regime is characterised by the presence of strong internal circulation which enhances mass transfer. In this work, we develop a mathematical model for conjugate mass transfer in the liquid-liquid slug flow regime in a microchannel. A Lagrangian approach is adopted where the behaviour of a unit cell in the channel is analysed in a moving reference frame. The system is analysed for two cases when (I) the slug is in direct contact with the channel wall and (II) there is a thin film of continuous fluid surrounding the slug. A novel contribution of this work is the extension of the stream function formulation to determine the flow structures in multiply connected domains made up of two liquids in a pressure driven flow. The primary objective of the work is to understand the influence of the flow patterns (which is determined by fluid properties and operating conditions) on the mass transfer of a solute from the slug to the continuous phase. Towards this, the species transport equation is solved numerically using the velocity field obtained. The reliability of the model developed is validated with experiments reported in the literature. This study gives us insights on the influence of the film on the hydrodynamics and its contribution to mass transfer. A key finding of this study is that the rate of mass transfer can be enhanced if the continuous phase has higher viscosity than the slug phase.

Experimental analysis of downward liquid-gas slug flow in slightly inclined pipes

Downward gas-liquid slug flow occurs in hilly-terrain pipelines and is frequently observed in oil and gas transportation lines. The onset of this kind of flow owes to instabilities generated by irregular pipe topography. Therefore, a solid understanding on the hydrodynamics of the slug flow is paramount in the design of crude oil production lines as well as in the project of equipment involved in oil and gas operations. The goal of this work is to analyze the mean values of the translational velocity, slug frequency, bubble and slug lengths in ducts with inclination angles from −4° down to −13° on experimental grounds. The analyses were performed at different gas-liquid volumetric flow rates for which the slug flow regime was observed. An existing experimental rig in the NUEM/UTFPR labs was used to collect data. A pair of wire-mesh sensors was used to evaluate the flow structure, and thence void fraction temporal series were obtained. From those temporal series, the experimental data were obtained and comparative analyses could then be made. The mean bubble (C0) and drifty velocity (C∞) coefficients were determined for the translational velocity. The mean bubble and slug lengths were compared for minimum and maximum values. A schematic of the unit cell behavior in downward slug flow is proposed.

NMPC integrated with optimization layer in offshore production

Abstract In offshore oil production, the occurrence of oscillatory flow regime that causes instabilities is very common, resulting in production losses, occasioned by shutdown, and environmental fine due to the violation of quality requirements. In this work, we evaluate the capability of a nonlinear model predictive controller (NMPC) integrated with an optimization layer to maximize production, minimize energy consumption and mitigate impacts caused by slug flow regimes present in offshore processes. The objective function used in the optimization problem is formulated with the oil and gas production as profit and energy consumption as cost. The degree of freedom of the optimization problem are the topside pressure in the processing plant and the gas-lift injection flow rate in each well. The constraints are the BSW (Basic and Water sediments) and TOG (Grease and Oil content) specifications, capacity of the compression unit and the limits of gas-lift injection, in each well and total flow. The simulated offshore production system encompasses the subsea well, the flowline, the riser, and the topside platform. The subsea part, with four wells, was modeled in OLGA simulator and integrated with a simulated platform in EMSO simulator. The control problem was formulated with nine manipulated variable: gas-lift injection and opening choke valve in each well, and interface level in the separator drum. The results show that the controller can reject the slug disturbances, keeping the BSW and TOG inside the limits and driving the process to the setpoints coming from the optimization layer.

CFD analysis of the evaporating two-phase flow in the slug flow regime of a cryogenic fluid in a microchannel

Flow boiling in a microchannel is investigated in the present work using the multiphase CFD. Cryogenic fluid nitrogen is used as a working fluid for the CFD simulations. A single bubble is patched in the upstream of the microchannel and the bubble profile, velocity and temperature profile and the heat transfer performance is evaluated. Effect of parameters, like wall heat flux and inlet flow rate or Reynolds number, are studied. It is found that the passage of the bubble significantly alters the parabolic velocity. The passage of the bubble and the evaporation at the interface also significantly reduces the wall temperature due to the enhanced heat transfer. Heat flux does not seem to have any effect on the average heat transfer coefficient, while the heat transfer coefficient increases with the inlet flow rate. The numerical framework employed to perform this study is the open source CFD package OpenFOAM with the volume of fluid interface capturing method.

Image Processing Technique for slug length measurement during Mixing and Separation of Two-phase Flows in T sections

Abstract Study of two phase flow regimes in a mini/micro channel finds its applications in micro-fluidic devices such as micro heat exchangers owing to its large heat transfer. Such systems have large surface to volume ratio. The present work detail the results obtained for bubbly and slug flow regimes formed in a T-junction for an air water mixture. Experiments are performed in two different tubes with internal diameters of 0.6mm and 2.5mm having a length of 100mm. Image processing techniques are employed using MATLAB software. The data will be helpful in designing fluid flow mini/micro channels commonly used in thermal MEMS.

Numerical simulation of two-phase separation in T-junction with experimental validation

Liquid carryover in T-junction due to splitting nature of two-phase flow causes serious issues for downstream equipment which is not designed to handle excessive liquid. In this paper, the phenomena of liquid carryover in T-junctions were analyzed using the Volume of Fraction (VOF) together with the k-ε turbulence model. T-junction separation efficiency was measured through mass flow rate fraction of air and water between the branch and main arm over a range of diameter ratios 0.6 to 1.0, water superficial velocity 0.186 to 0.558 m/s and air superficial velocity 4 to 8 m/s. The results showed simulation model was successfully validated with average deviation of less than 5% and can be used to predict phase split of slug flow in T-junction. The numerical model confirmed the significant influence of diameter ratio and superficial velocities of air and water on phase split. Reduced T-junction delivers better separation performance compared to regular T-junction. In slug flow regime, T-junction’s performance can be improved by either decreasing air velocity or increasing water velocity. A new dimensionless parameter, namely the area under the curve of separation efficiency (S), was proposed and proved as a qualified judging criteria for evaluating phase separation efficiency of T-junctions.

Mass transfer from a soluble wall into gas-liquid slug flow in a capillary tube

The mechanism of wall-liquid mass transfer of a solute in micro-scale systems has a huge relevance in many practical scenarios with particular interest for medical devices. A possible enhancement on this kind of phenomenon through the application of slug flow regime was studied with CFD techniques. Different flow conditions were considered to enable the inspection on the distinct hydrodynamics that may occur on the Taylor bubble surroundings in micro-scale. The VOF methodology was used to track the gas–liquid interface and the mass and hydrodynamic fields were simultaneously solved.The effects of the bubble passage on the mass transfer from a finite soluble wall to the flowing fluid were analyzed for each flow condition, and the corresponding mass transfer coefficients were quantified.Overall, this numerical work indicates that the flow due to the presence of one Taylor bubble leads to a moderate increase of the wall-liquid mass transfer coefficients. This increase can be enhanced if, instead of one, a continuous flow of bubbles is considered. The abrupt variation on the wall shear stress induced by the bubble movement is important to promote the referred increase.

Investigation of a microchannel heat and mass exchanger for absorption systems

An investigation of a microchannel heat and mass exchanger for vapor absorption systems is presented. A sheet with 88 microchannels of depth 0.5 mm and width 0.76 mm is investigated via visual and thermal measurements. The flow in the microchannels is predominantly in the slug-flow regime, with distinct vapor and liquid regions. High rates of heat transfer were observed in the absorber. However, poor liquid mass transfer rates limit the performance of the microchannel absorber. The performance of the microchannel absorber is compared with that of an absorber with serpentine microscale passages containing micro-pin fins. A physics-based model for the absorption of ammonia into a dilute solution of ammonia-water in a microchannel absorber with mixing sections is developed using insights from flow visualization. The model is validated by comparing its predictions with the data. The potential of surfactants to enhance absorber performance is also investigated.

Thermohydraulic characterization of flow boiling in a nanostructured microchannel heat sink with vapor venting manifold

Capillarity due to textures on microchannel surfaces promotes thin-film evaporation to allow high heat dissipation in the annular flow boiling regime. However, the advantage of textures is not realized at moderate heat fluxes in the slug flow regime wherein rapid vapor generation due to improved nucleation increases temperature fluctuations. Here we investigate the combination of liquid supply and vapor-venting strategies with surface modifications to minimize temperature fluctuations in the slug flow regime. We perform experiments with plain wall (PWM) and nanostructured (NSM) microchannels to show that poor vapor venting with conventional horizontal-inlet and horizontal-outlet (HH) manifold design stimulates temperature fluctuations of ≈±5°C , even at a nominal heat flux of ≈750kW/m2. Conversely, a novel vertical-inlet and vertical-outlet (VV) manifold design with jet impingement cooling at the inlet and enhanced vapor-suction at the outlet minimizes fluctuations. The VV manifold when complemented with the intrinsic merits of NSM, reduces temperature fluctuations to within ±0.5°C, up to a high heat flux of ≈1000kW/m2. These results suggest that surface modification techniques should be complimented with vapor venting strategies for heat transfer enhancement via flow stabilization in the slug flow regime.

Effect of gas–liquid ratio on the wall shear stress in slug flow in capillary membranes

AbstractThe membrane processes generally suffer from the fouling caused by the deposits at the membrane surface and membrane pores. In recent years, gas–liquid flow has been shown to be an effective method of membrane fouling prevention. Among the different flow regimes that occur during gas–liquid flow through membrane capillaries, slug flow has been observed to be the most effective for enhancement of permeate flux. The shearing caused by the fluid on the membrane wall is the primary factor causing fouling removal. Previous studies in the slug flow regime suggest that the major contribution to the wall shear stress comes from the bubble front and rear regions and the contribution from the constant film thickness region in the middle is negligible. Therefore, the average wall shear stress in the slug flow regime is dependent on the bubble length. In this work, two‐phase gas–liquid flow in a capillary has been modeled computationally to understand the effect of bubble length on the wall shear stress. Further, the effect of liquid viscosity on the wall shear stress has also been studied for capillary numbers ranging from 0.0037 to 0.098. The volume‐of‐fluid method has been employed to capture the gas–liquid interface between the two phases. The results have been compared with a simple phenomenological model for wall shear stress available in literature and found to be in good agreement for long Taylor bubbles.

Influence of a sudden expansion on slug flow characteristics in a horizontal two-phase flow: a pressure drop fluctuations analysis

Experimental investigations were conducted to study the evolution of air/water slug flow characteristics through a horizontal sudden expansion having a ratio of $$\sigma_{A}$$ = 0.444. A series of acquisition of differential pressure upstream and downstream were carried out leading to statistical as well as spectral analyses. The influences of both liquid and gas superficial velocities on flow behavior as well as standard deviation were studied. Substantial modifications of two-phase flow distribution were reported downstream the singularity with a quantitative reduction in standard deviation. Besides the fact that the slug flow regime didn’t persist downstream the singularity for low values of superficial phasic-velocities, the latter ( $$J_{\ell }$$ and $$J_{g}$$ ) seem to have no noticeable effects downstream the expansion. Reduction of slug frequency between upstream and downstream the singularity was also observed.

Comparison of Numerical Methods for Two-Fluid Model for Gas–Liquid Transient Flow Regime and Its Application in Slug Modeling Initiation

The formation of slug in the horizontal pipes due to the hydrodynamic instabilities has always been of great interest to many researchers. In this research, the effect of various numerical methods on the simulation of slug flow initiation using pressure-free two-fluid model has been investigated. The two-fluid model has been solved by conservative shock-capturing method. When the slug is formed, a strong discontinuity will be developed in the flow stream. Therefore, a numerical method should have the capability to predict this discontinuity with high accuracy and should not have oscillation near the discontinuity. There are three different models to simulate two-phase flow systems: homogeneous equilibrium model, drift-flux model, and two-fluid model. This research used two-fluid model for predicting the slug flow initiation through a pipe. Four different numerical two-fluid methods, namely Lax-Friedrichs, Rusanov, Richtmyer and flux-corrected transport (FCT) have been used in this research. Results show that FCT is the most accurate method for the prediction of the slug flow initiation among other methods where Rusanov and Lax-Friedrichs numerical methods were in the next steps, respectively. Due to the oscillatory nature near the discontinuity caused by formation of slug regime, the Richtmyer numerical method is not an appropriate method for modeling slug flow regime. Results also show that as the numerical diffusion of these methods reduces in the flow field, the slug flow initiation will be predicted with higher accuracy.

Flow regimes in gas–solid fluidized bed with vertical internals

In this work, the impact of the vertical internals on the flow regimes and their transition velocities has been studied in a 0.14m inside diameter gas–solid fluidized bed. The identification of the flow regimes was accomplished statistically (standard deviation) and chaotically (Kolmogorov entropy) by analyzing the pressure drop fluctuations. Circular configurations of vertical tubes with two different sizes (0.0254 and 0.0127m diameter), two kinds of solid particles of Geldart B type (glass beads and aluminum oxide), and a wide range of superficial gas velocities (0.15–1.2m/s) have been implemented in this study. Generally, it was demonstrated that the vertical internals have a significant effect on the flow regimes, transition velocities, and transition velocity ranges of each individual flow regime.However, such effect is a function of the physical properties of the used solid particles in which the turbulent transition velocity (Uc) decreased in the case of glass beads and increased in the case of aluminum oxide for both of the configuration designs of vertical internals used in the present work. In addition, the 0.0254m vertical internals type has been shown to be more efficient either in minimizing the turbulent transition velocity (Uc) and superficial gas velocity within the range of slugging flow regime and increasing the range of the superficial gas velocity within the range of bubbling flow regime or in reducing the pressure drop and pressure fluctuations inside the bed.

Formation mechanisms and lengths of the bubbles and liquid slugs in a coaxial-spherical micro mixer in Taylor flow regime

One of the most important problems of the efficiency of mini and micro devices (with diameter larger than ∼1 mm) for gas-liquid systems is the maintenance of regular Taylor (slug) flow regime. The paper describes a coaxial-spherical mixer (CS-mixer), which allows to create a slug flow regime in a wide range of phase flow rates. For CS-mixer a flow map was build up and superimposed to that for C-mixer.It was shown experimentally with 2 mm ID microchannel that the position of the nozzle for the injection of the gas relative to the spherical extension of CS-mixer significantly affects the lengths of the bubbles and the slugs generated in the micromixer. Two nozzle positions in the narrowing part of spherical extension were considered as optimal from the point of view of uniformity of slug flow.For nozzle position D (middle cross-section), which was studied more detailed in this research, equations for bubbles and slugs lengths were obtained. Values of bubbles and slugs lengths calculated by these equations were compared with data of other authors for different types of micro-mixers. It was shown that CS-mixer allows to control the length of the bubbles and to generate bubbles with smaller length. The size of the slugs produced by CS-mixer demonstrated weaker dependence on superficial liquid flow rate compared to the other types of micromixers. Unexpected (for other types of micromixers) behavior of two-phase flow in microchannel by use of CS-mixer was observed: with growing gas superficial velocity the length of the bubbles is decreasing. Such behavior was explained by particular effects of bubble detachment from the nozzle during its formation in the spherical extension of CS-mixer.

Gas-liquid two-phase flow in a square microchannel with chemical mass transfer: Flow pattern, void fraction and frictional pressure drop

The dynamics of the gas-liquid two-phase flow are of great significance for the heat and mass transfer performance in a microreactor. In this paper, the flow pattern, void fraction and frictional pressure drop of gas-liquid two-phase flow of CO2 and MEA/[Bmim][BF4] aqueous solution accompanied by the chemical reaction in a 400 μm square microchannel were investigated experimentally by a high-speed camera and pressure sensor. Three kinds of flow regimes were observed: slug-bubbly flow, slug flow and slug-annular flow. The void fraction and frictional pressure drop in the slug flow regime were mainly investigated, and the applicability of models without consideration of the mass transfer proposed in the literature was assessed. Considering the effect of chemical absorption, two correlations were proposed with the Hatta number for predicting the two-phase void fraction and Chisholm parameter C, which showed good prediction performance.

Hydrodynamics and mass transfer performance during the chemical oxidative polymerization of aniline in microreactors

The hydrodynamics and mass transfer performance in the chemical oxidative polymerization of aniline in capillary microreactors within the liquid-liquid slug flow regime were investigated. Higher reaction temperature, higher HCl concentration and larger volumetric flow rate ratio of the aqueous phase to the organic phase improved the mass transfer performance and thus led to higher yield of polyaniline. Internal recirculation in the dispersed phase slugs was captured by high-speed camera, which was beneficial for the mass transfer enhancement in the polymerization process. The polyaniline yield reached 82.1% in the microreactor at the residence time of 95 s and the temperature of 40 °C. The slug coalescence became significant with increasing Reynolds number, thus reducing the mass transfer rate. Moreover, both Hatta number and mass transfer enhancement factor were evaluated to further characterize the mass transfer mechanism and reflect the competition between the mass transfer and the reaction during the polymerization.

Addressing Two-Phase Flow Maldistribution in Microchannel Heat and Mass Exchangers

Multiphase flow phenomena in single micro and minichannels have been widely studied. Characteristics of two-phase flow through a large array of microchannels are investigated here. An air–water mixture is used to represent the two phases flowing through a microchannel array representative of those employed in practical applications. Flow distribution of the air and water flow across 52 parallel microchannels of 0.4 mm hydraulic diameter is visually investigated using high-speed photography. Two microchannel configurations are studied and compared, with mixing features incorporated into the second configuration. Slug and annular flow regimes are observed in the channels. Void fractions and interfacial areas are calculated for each channel from these observations. The flow distribution is tracked at various lengths along the microchannel array sheets. Statistical distributions of void fraction and interfacial area along the microchannel array are measured. The design with mixing features yields improved flow distribution. Void fraction and interfacial area change along the length of the second configuration, indicating a change in fluid distribution among the channels. The void fraction and interfacial area results are used to predict the performance of different microchannel array configurations for heat and mass transfer applications. Results from this study can help inform the design of compact thermal-fluid energy systems.

Experimental Simulation of Hydrodynamics and Heat Transfer in Bubble and Slug Flow Regimes in a Heavy Liquid Metal

For the confirmation of the claimed design properties of a reactor plant with a heavy liquid-metal coolant, computational and theoretical studies should be performed in order to justify its safety. As one of the basic scenarios of an accident, the leakage of water into the liquid metal is considered in the case of steam generator tube decompression. The most important in the analysis of such a kind of accidents are questions concerning the motion and heat exchange of steam bubbles in the steam generator and the probability of blocking the flow area owing to freezing coolant, since the temperature of boiling feedwater in the steam generator can become lower than the melting point of lead. In this paper, we present main approaches and relationships used for the simulation of the motion of gas bubbles and heat transfer between bubbles and liquid metal flow. A brief description of the HYDRA-IBRAE/LM computational code that can be used to analyze emergency situations in a liquid metal-cooled reactor facility is also presented. It should be noted that the existing experimental data on the motion and heat transfer of gas bubbles in a heavy liquid metal are insufficient. For this reason, in order to verify the HYDRA-IBRAE/LM code models, experiments have been performed on the cooling of liquid lead by argon and on the motion of gas bubbles in the Rose’s alloy. In particular, a change in the temperature of the coolant over time has been studied, and the void fraction of gas at different flow rates of gas has been measured. A detailed description of the experiments and a comparison of the results of the calculations with the experimental data are presented. The analysis of uncertainties made it possible to reveal the main factors that exert the greatest effect on the results of calculations. The numerical analysis has shown that the models incorporated into the HYDRA-IBRAE/LM code allow one to describe to a sufficient degree of confidence the process of cooling liquid lead melt when argon bubbles pass through it, simulating the flow of water into the liquid metal in the course of steam generator tube rupture.

Experimental investigation on the effect of diameter ratio on two-phase slug flow separation in a T-Junction

T-junctions are often used in offshore platforms to partially separate gas from produced fluids. Poorly designed T-junctions frequently produce very high liquid (oil and/or water) carryovers, causing major issues to the downstream equipment train which is not designed to handle excessive liquid. This paper reports the liquid carryover experiments in T-junctions using five different side to main arm diameter ratios under slug flow regime. The obtained phase separation curves can be divided into two component variables; liquid-carryover threshold and peak liquid carryover. The experiments demonstrate that with a decrease in diameter ratio both of these variables decrease. Yet, for superior multiphase flow separation, a high liquid carryover threshold and a low peak liquid carryover are required. Hence, the generally accepted rule that a reduction in diameter ratio improves the phase separation is revealed to be an over-extrapolated statement. The novel findings of this work are: 1) for optimum flow splitting under slug flow conditions, the diameter ratio should be kept between 1 and 0.67, while the diameter ratio 0.67 was found to be most suitable; 2) two correlations were developed for predicting two-phase slug flow separation in different diameter ratio T-junctions. These correlations offer beneficial guidance and clarifications for a number of oil and gas flowline and pipeline applications.

In-situ investigation of bubble dynamics and two-phase flow in proton exchange membrane electrolyzer cells

Gas bubble dynamics and two-phase flow have a significant impact on the performance and efficiency of proton exchange membrane electrolyzer cells (PEMECs). It has been strongly desired to develop an effective experimental method for in-situ observing the high-speed/micro-scale oxygen bubble dynamics and two-phase flow in an operating PEMEC. In this study, the micro oxygen bubble dynamic behavior and two-phase flow are in-situ visualized through a high-speed camera coupled with a specific designed transparent PEMEC, which uses a novel thin liquid/gas diffusion layer (LGDL) with straight-through pores. The effects of different operating conditions on oxygen bubble dynamics, including nucleation, growth, and detachment, and two-phase flow have been comprehensively investigated. The results show that temperature and current density have great effects on bubble growth rate and reaction sites while the influence of flow rate is very limited. The number, growth rate, nucleation site, and slug flow regime of oxygen gas bubbles increase as temperature and/or current density increases, which indicates that an increase in temperature and/or current density can enhance the oxygen production efficiency. Further, a mathematical model for the bubble growth is developed to evaluate the effects of temperature and current density on the bubble dynamics. A mathematical model has been established and shows a good correlation with the experimental results. The studies on two-phase flow and high-speed micro bubble dynamics in the microchannel will help to discover the true electrochemical reaction at micro-scale in an operating PEMEC.