Hydrodynamics Of Slug Flow Research Articles

Growth of solitary slugs in long pipes

We show that a pipe transporting gas and liquid can be operated at conditions where both stratified flow and slug flow are stable. In this bi-stable flow regime, it is possible to observe stratified flow or regular hydrodynamic slug flow, depending on the inlet conditions. However, it is also possible to get one single slug at a time in the pipe. The distinctive characteristic of such “solitary” slugs is that they grow continuously as they travel along the pipe and therefore, in long pipelines, they can reach lengths up to thousands of pipe diameters and flood the downstream process installations. We first show how the length of solitary slugs can be predicted using slug tracking simulations. Based on those transient simulation results, we provide an explanation for the slug growth and we propose a simple quasi-steady-state model allowing us to predict the slug length in good agreement with the transient simulations. Finally, we explain how we can determine the boundaries of the bi-stable flow regime in which solitary slugs occur.

A study of CO2 two-phase flow in a near horizontal pipe

In this article, the observations from two different experimental studies on CO2 two-phase flow regimes in a near horizontal pipe are analysed. These experiments show significant bubble entrainment in a stratified flow, and the bubble entrainment in liquid is found to be a dominant mechanism for the flow regime transition between the stratified and bubbly flow. The hydrodynamic slug flow is not observed in these experimental studies. The X-ray measurement of the experimental study in a near horizontal pipe of 44 mm internal diameter is processed to obtain the bubble volume fraction in the liquid layer, which is found to be a function of two-phase Froude number. This Froude number definition takes into account gas density and pipe inclination effects. A parametric study shows that the bubble entrainment into the liquid layer decreases significantly with an increase of pipe internal diameter.

3D numerical study of a single Taylor bubble rising along an inclined tube through Newtonian and non-Newtonian liquids

Slug flow is an important gas-liquid flow pattern characterized by large bullet-shaped bubbles (Taylor bubbles) and is widely found in industrial and natural applications. The hydrodynamics are complex and depend on tube dimensions and orientation, operating conditions, and physical properties. Therefore, detailed knowledge of slug flow hydrodynamics can provide insight into a robust and safe intensification of industrial processes in several fields.This work fills a gap in knowledge by producing new information about slug flow in inclined tubes. For the first time with complete conservative equations, CFD tools are applied to simulate 3D systems composed of isolated Taylor bubbles rising through stagnant liquids in tubes with different inclinations. This study addresses Newtonian liquids with different viscosities and non-Newtonian liquids with shear-thickening behavior.The effects of geometric and physical properties are assessed, and descriptions of bubble shape and velocity, liquid film thickness, velocity profiles around the bubble, and liquid wake characteristics are presented and discussed.The results allow to conclude that the tube inclination deeply impacts the flow by changing the bubble position, velocity, shape, and the way the liquid flows around it. The inclination effect coupled with different liquid rheology introduced even more complexity to the hydrodynamic features.

Effect of dilution on the performance of ionic liquids in milliflow solvent extraction applications: Towards integration of extraction, scrubbing and stripping operations with in-line membrane-based phase separation

The combination of ionic liquids (ILs) and millifluidic reactors is often suggested as a form of process intensification. The high viscosity of ILs has, however, raised concerns about hydrodynamics, mass-transfer enhancement and the ability to integrate several millifluidic operations. In the present study the performance of a quaternary ammonium hydrogensulfate IL, [A336][HSO4], in a millifluidic solvent extraction (SX) process was evaluated and compared with various, lower viscosity solutions of the IL in a diluent. Dilution of [A336][HSO4] significantly improved the slug flow hydrodynamics by reducing the film thickness, avoiding the loss of secondary recirculations inside the plugs and improving stability at elevated flow rates. The overall mass-transfer performance of the system was also improved upon dilution by a balancing of mass-transfer and reaction limitations. It was also observed that dilution reduced reagent consumption and improved the extraction selectivity. In addition, the decrease of the viscosity of the IL upon dilution facilitated the use of an in-line membrane-based phase separator. An integrated millifluidic set-up containing an extraction, scrubbing and stripping reactor was built using these in-line phase separators and applied to the SX purification of germanium from a synthetic zinc refinery residue leachate. The integrated reactor system was operated using a 30 vol% [A336][HSO4] dilution in p-cymene and successfully run for 4 h without any loss in separation performance or stability.

Slug Flow Hydrodynamics Modeling for Gas–Liquid Two-Phase Flow in a Pipe

Gas–liquid flow in a pipeline is a very common. Slug two-phase flow is dominated in the case of slightly upward flow (+0.25°) and considered to be the comprehensive flow configuration, and can be in close contact with all the other flow patterns. The models of different flow patterns can be unified. Precise prediction of the slug flow is crucial for proper design and operation. In this paper, we develop hydrodynamics unified modeling for gas–liquid two-phase slug flow, and the bubble and droplet entrainment is optimized. For the important parameters (wall and interfacial friction factors, slug translational velocity and average slug length), the correlations of these parameters are optimized. Furthermore, the related parameters for liquid droplet and gas bubble entrainment are given. Accounting for the gas–liquid interface shape, hydrodynamics models, i.e., the flat interface model (FIM) and the double interface model (DIM), of liquid film in the slug body are applied and compared with the experimental data. The calculated results show that the predictions for the liquid holdup and pressure gradient of the DIM agree with experimental data better than those of the FIM. A comparison between the available experimental results and Zhang’s model calculations shows that the DIM model correctly describes the slug dynamics in gas–liquid pipe flow.

Numerical investigation of gas–liquid and liquid–liquidTaylor flow through a circular microchannel with a sudden expansion

AbstractThis paper investigates a CFD‐based analysis for gas–liquid and liquid–liquid Taylor flows through a circular axisymmetric microchannel with a sudden enlargement. A series of simulations are conducted by exploring the influence of different superficial velocity ratios, apparent viscosities, and channel expansion on the hydrodynamics of slug flow. A concentric junction introduces dispersed airflow into a continuous flow of water for gas–liquid flow, and the junction introduces dispersed water into a continuous flow of dodecane for liquid–liquid flow. The air‐bubble and water‐slug evolution processes, slug breakup, and slug expansion are investigated. In all cases, the lengths of air bubbles and water slugs increase with increasing superficial velocity ratio, particularly before the expansion. For gas–liquid flow, the apparent viscosity ratio causes a fluctuating interface over the uniform film region. However, the water slug length is shorter and the film region is slightly thicker in liquid–liquid compared to gas–liquid flow. The numerical analysis developed in this paper is in good agreement with the existing correlations and experimental data in the literature.

Slug flow hydrodynamics of immiscible fluids within a rectangular microchannel towards size-controllable fabrication of dextran-based cryogel beads

Cryogel beads are novel adsorbents with potential applications in biotechnology, environmental and tissue engineering. However, the controllable preparation of cryogel beads with expected small and uniform diameters is a challenging work. The micro-flow-focusing method combining with the cryo-polymerization has been suggested as a new approach for the fabrication of cryogel beads. In order to form the uniform droplet, the slug flow is generated in the microchannel, so the slug flow hydrodynamics are crucial and fundamental for this process. In the present work, the dextran-methacrylate (Dex-MA-PEGDA) solution and ethyl acetate containing surfactant of Span 80 were used as the aqueous phase and the water-immiscible, respectively. The fabrication of dextran-based cryogel beads and the slug flow hydrodynamics in a rectangular microchannel were investigated experimentally. The flow regime map was proposed and the typical regime boundaries were measured and correlated, and the formation mechanisms of slugs were also discussed. The regime boundaries were correlated by Weber number and Capillary number, i.e., Weo = 58.16Caw0.92 for the boundary between the annular and the slug flow, and Weo = 4.12Caw0.09 for the slug and the bubbly flow, respectively. Several crucial parameters of the droplets generated by the slug flow, i.e., the aqueous slug length, the aqueous slug velocity and the aqueous droplet size, were measured and correlated. The Dex-MA-PEGDA cryogel beads were prepared successfully under different conditions and their diameters were correlated to the sizes of the droplets. Eventually, the uniform size distributions and supermacroporous microstructures of dextran-based cryogel beads were demonstrated.

Experimental investigation on the flow-induced vibration of a free-hanging flexible riser by internal unstable hydrodynamic slug flow

Unstable hydrodynamic slug flow is extensively encountered in marine risers, resulting in internal flow-induced vibration, which is quite different with the vortex-induced vibration caused by a cross flow. Experiments were performed in an air-water test loop to study the internal slug flow-induced vibration of a free-hanging flexible riser with aspect ratio (the ratio of the length to the diameter of a riser) of 158. A non-intrusive technique of high speed imaging method was employed to record the vibration displacements of the riser model and the flow regime of two-phase mixture in the riser simultaneously. The results show that an in-plane vibration is created by the internal slug flow, and the responses along the two directions in the plane are interdependent to each other. In the considered gas-liquid ratio (the ratio of air flow rate to water flow rate), the second-order mode dominates the response. The hydrodynamic behaviors of slug flow, including the gas-liquid ratio, liquid slug length and superficial velocity, affect the local pressure fluctuation and hence influence the vibration amplitude. The fluid-structure interaction is reflected from the same frequencies presenting in the vibration and pressure fluctuation.

Investigation of the hydrodynamics of slug flow in airlift pumps

Abstract A slug flow model considering the dispersed bubbles entrained from the tail of Taylor bubble (TB) and re-coalesced with the successive TB was proposed. Experiment was conducted to test the validity of this model by using a high-speed camcorder and particle image velocimetry (PIV). It was found that the model was valid for predicting the characteristics of slug flow in airlift pump within average error of 14%. Moreover, large pipe diameter was found to accelerate the rise velocity of TB and decreases void fraction in liquid slug by a small margin.

Dynamic behaviours of horizontal gas-liquid pipes subjected to hydrodynamic slug flow: Modelling and experiments

The dynamic behaviours of gas-liquid pipes are difficult to predict because of the complex flow characteristics of gas-liquid two-phase flow and fluid-structure interactions. In this paper, a dynamics model capable of describing the characteristics of hydrodynamic slug flow and fluid-structure interactions involving centrifugal force and Coriolis force is presented to investigate the dynamic behaviours of horizontal pipes subjected to hydrodynamic slug flow. The motion equation is solved using the finite element method. Experiments are performed to measure the characteristics of slug flow and dynamic behaviours of horizontal pipes and to validate the theoretical results. The motion of pipes under different two-phase flow parameters are simulated, and some interesting and unexpected results are presented. Finally, the cumulative effect of fluid-structure interactions and slug characteristics on the dynamics of the system is discussed. This study is helpful for understanding the complicated dynamic behaviours of gas-liquid pipes and promoting pipeline safety.

A three-phase solid-liquid-gas slug flow mechanistic model coupling hydrate dispersion formation with heat and mass transfer

Pipe blockage due to gas hydrate formation is a main concern in the oil and gas industry due to the revenue losses caused by either production impairments or interruptions, and to the high costs associated to the elimination of such blockages. Assuming a hydrate formation rate in the gas-water interface based on the system subcooling, the present work models the transition from two-phase liquid-gas to three-phase solid-liquid-gas flows when hydrates form. The multiphase flow is assumed to be within the slug flow pattern region, as this is the prevailing flow regime in offshore production scenarios. The model couples mass, momentum and energy balances for the slug flow unit cell. The hydrate phase is assumed as homogeneously dispersed in the water. The gas and water consumption rates due to hydrate formation are modeled as source terms in the mass balance equations. The exothermic characteristic of the hydrate formation is taken into account in the energy conservation equation. The model provides analytic expressions for temperature and pressure distributions along the pipeline. However, the unit cell geometry is solved by numerical integration and the model closure is achieved only when empirical correlations for the slug frequency, the unit cell translational velocity and the slug aeration are used. The results from the numerical simulations are presented for the same input parameters for cases with and without hydrate formation. The discussion focuses on the influence of hydrate formation in the slug flow hydrodynamics and heat transfer. The main mechanisms affecting the mixture temperature and pressure distributions, the mixture heat transfer coefficient, the superficial velocities of the phases, the liquid loading, the slug flow frequency and the unit cell geometry are presented.

Gas-liquid reaction and mass transfer in microstructured coiled flow inverter

Microstructured coiled flow inverter (MCFI) as a helically coiled tubular device with 90° alternating bends provides enhanced radial mixing due to secondary flow (Dean vortices) in different planes. Liquid-liquid mass transfer characterization in MCFI revealed higher mass transfer rates compared to other capillary setups. However, the influence of Dean vortices and 90° bends on gas-liquid mass transfer has not been investigated and described yet. Different reactor setups, i.e. MCFI, straight capillary, helical coil, and bend reactor were fabricated from FEP tubes (ID=1mm). Gas-liquid mass transfer performance was investigated for a gas-liquid reaction system, i.e. cobalt (II) catalyzed air oxidation of sodium sulfite. Two-phase slug flow hydrodynamics and pressure drop were experimentally characterized, while the influence of operating and geometrical reactor parameters on conversion was investigated for reactor setups. MCFI offers up to 14% higher conversion in comparison to other capillary setups. Mixing within the liquid phase is enhanced by the formation of Dean vortices and additional direction change via 90° bends, which contribute to the internal diffusion in the liquid phase.

Modeling the effects of hydrate wall deposition on slug flow hydrodynamics and heat transfer

Offshore oil and gas exploitation is usually accompanied of water (brine) and sand. High water cuts at high pressure scenarios (deep waters) and low temperature (cold waters) favor the formation of gas hydrates. Pipe blockage due to gas hydrates formation is a main concern in the oil and gas industry due to the profit losses caused by either production impairments or interruptions, and to the high costs associated to the elimination of such blockages. Hydrates form mainly at: (i) the water-gas interface, forming a suspension, or (ii) on the pipe inner wall, forming a deposit that constricts the flow. Both cases will affect flow hydrodynamics and heat transfer, and predicting how the flow will be affected is essential for improving exploitation efficiency and safety. Due to the gas and liquid volumetric flow rates that are typical of this kind of operation, the spatial distribution of the phases is often assumed to fall within the slug flow pattern region. Slug flows are characterized by the intermittent passage of elongated bubbles flowing over a liquid film, followed by liquid slug bodies that may contain dispersed bubbles in their interior. The present work introduces modifications on an existing mechanistic slug flow model to consider the effects brought by the deposition of hydrates on the pipe inner wall. The main differences herein considered are: (i) a cross sectional area reduction, which is related to higher pressure drops, and (ii) a conductive thermal resistance at the pipe wall, since the hydrate is an insulating material. Assuming a hydrate layer of constant thickness already deposited on the wall – that is, both the mass transfer and the heat generation associated to hydrate formation have already ceased – the slug flow model is used to verify the consequences of hydrate deposition to the slug flow hydrodynamics and heat transfer. Simulations for methane-water mixtures flowing along a 1.5-km length, 26-mm ID horizontal pipeline are presented so that the consequences to the pressure and temperature distributions, to the mixture heat transfer coefficient, to the unit cell geometry and to the slug flow frequency can be explained.

Experimental investigation of hydrodynamic slug mitigation potential of an intermittent absorber

The need to handle hydrodynamic slugs in a more efficient way becomes important as oil and gas activities shift deep offshore. This study describes the use of a vessel coupled to the pipeline-riser system upstream of the first stage separator for hydrodynamic slug attenuation. The experiments were carried out in a 2⿳ pipeline-riser system which comprises of a 40m long horizontal pipe connected to a 11m high vertical riser followed by a 3m horizontal topside section. Air and water were used as experimental fluids. Bifurcation maps and slug attenuation index (SAI) have been used to quantify increase in oil production and the slug attenuation potential of this concept. The device was observed to reduce the pressure fluctuations characterising hydrodynamic slug flow up to 22%. The device also provides additional benefits of stabilising the flow at higher valve opening (choke setting) and lower pressure compared to traditional choking. This in practice translates to increase in oil production. Special case of hydrodynamic slugs which exhibit overchoking induced slugging (OIS) was also observed to be relatively attenuated by the introduction of the absorber.

Hydrodynamics of slug flow in a vertical narrow rectangular channel under laminar flow condition

The hydrodynamics of gas–liquid two-phase slug flow in a vertical narrow rectangular channel with the cross section of 2.2mm×43mm is investigated using a high speed video camera system. Simultaneous measurements of velocity and duration of Taylor bubble and liquid slug made it possible to determine the length distributions of the liquid slug and Taylor bubble. Taylor bubble velocity is dependent on the length of the liquid slug ahead, and an empirical correlation is proposed based on the experimental data. The length distributions of Taylor bubbles and liquid slugs are positively skewed (log-normal distribution) at all measuring positions for all flow conditions. A modified model based on that for circular tubes is adapted to predict the length distributions in the present narrow rectangular channel. In general, the experimental data is well predicted by the modified model.

Study on hydrodynamic slug flow mitigation with wavy pipe using a 3D–1D coupling approach

Objective: A wavy pipe, constructed by connecting standard piping bends in series in one plane, was proposed to mitigate hydrodynamic slug flows in horizontal pipelines.Methods: A newly developed 3D–1D co-simulation method, i.e. STAR–OLGA coupling, was applied to model the wavy-pipe systems experiencing hydrodynamic slug flows. The flow characteristics of slug flow in the wavy-pipe systems were investigated, through which the effectiveness of the wavy pipe on slug mitigation and how the geometrical parameters of the wavy pipe affecting its performance were examined.Results: It has been found that the upstream liquid slug degenerates into a ‘liquid dense zone’ of a greater length downstream of the wavy pipe due to the gas penetration into the slug body. A reduction of the effective density of the ‘liquid dense zone’ from that of the upstream slug body is resultant showing the effectiveness of the wavy pipe on mitigating the adverse impacts of slug flow. The amount of gas penetration increases then decreases with the increasing wavy-pipe amplitude indicating that excessively higher amplitude is even less effective on introducing the gas phase into the slug body. Differently a wavy pipe with the same amplitude but a greater length, i.e. more bends, is more effective.Conclusions: It is concluded that the wavy pipe works by reducing the effective density of the liquid slug through introducing the gas phase into the slug body in the push-out process in each bend of the wavy pipe and the geometrical parameters need to be designed properly in specific applications.

Experimental study of the hydrodynamic behaviour of slug flow in a vertical riser

This paper presents an investigation of the hydrodynamics of slug flow in a vertical 67mm internal diameter riser. The slug flow regime was generated using a multiphase air–silicone oil mixture over a range of gas (0.42<USG<1.35m/s) and liquid (0.05<USL<0.38m/s) superficial velocities. Electrical capacitance tomography (ECT) was used to determine: the velocities of the Taylor bubbles and liquid slugs, the slug frequencies, the lengths of Taylor bubbles and the liquid slugs, the void fractions within the Taylor bubbles and liquid slugs and the liquid film thicknesses. A differential pressure transducer was used to measure the pressure drops along the length of the riser. It was found that the translational velocity of a Taylor bubble (the structure velocity) was strongly dependent on the mixture superficial velocity. As the gas superficial velocity, was increased, the void fraction and the lengths of the liquid slugs and the Taylor bubbles were observed to increase. The increase in gas superficial velocity causes an increase in the frictional pressure drop within the pipe, whilst the total pressure drop (which is a sum of the hydrostatic and frictional pressure drop) along the length of the riser decreases. In addition, the frequencies of the liquid slugs were observed to increase as the liquid superficial velocity increases, but to be weakly dependant on the gas superficial velocity. The manual counting method for the determination of slug frequency was found to be in good agreement with the power spectral density (PSD) computed method.

CFD Simulation of hydrodynamics of air sparging in a vertical tubular membrane

Computational fluid dynamics (CFD) simulation of the hydrodynamics of slug flow which is generated by air sparging in a vertical tubular membrane has been investigated. The results of simulation have been reported in the form of parameters such as shape, velocity profile, surface shear stresses and gas slug (Taylor bubble) rising velocities, and evaluated with experimental data which were presented in previous articles. This study showed that CFD modeling is able to accurately simulate the shape and velocity field around the gas slugs. Also the shear stress induced by slug flow passage and rising velocity of gas slugs for high-velocity liquid and low-velocity gas fit appropriately to values in reference data. Simulation results for gas slug rising velocity showed about 0.35–9% error in the different conditions investigated in respect to experimental data.

Modeling the hydrodynamics of slug flow in a minichannel for liquid-liquid two-phase system

The hydrodynamics of a Taylor flow in minichannels for a liquid-liquid system is modeled. As a result of solving an inverse problem, a relationship is derived for calculating the thickness of a film around the droplets of the dispersed phase from the velocity of the dispersed phase and the true volume fraction of the dispersed phase is calculated. The adequacy of the model is verified by experimental studies over a wide range of capillary numbers (Ca = 0.05–0.7).

Multiphase flow modeling of gas hydrates with a simple hydrodynamic slug flow model

The formation and accumulation of natural gas hydrates in deep subsea flow lines are one of the most challenging flow assurance problems. Typical high pressure/low temperature operation conditions in deep subsea facilities promote rapid formation of gas hydrates. Recent observations in flow loop experiments in gas/water systems suggested a relationship between hydrate volume and flow regime on pressure drop. In the current work, a simple hydrodynamic slug flow model, based on fundamental multiphase flow concepts, is coupled with a transient hydrate kinetics model to study the effect of hydrate formation on slug flow in gas/water systems.