Slug Flow Characteristics Research Articles

Numerical Study of Hydrodynamic Characteristics of Gas–Liquid Slug Flow in Vertical Pipes

Multiphase flows occur in wide applications including; nuclear, chemical, and petroleum industries. One of the most important flow regime encountered in multiphase flow is the slug flow which is often encountered in oil and gas production systems. The slugging problems may cause flooding of downstream processing facilities, severe pipe corrosion and the structural instability of pipeline and further induce the reservoir flow oscillations, and a poor reservoir management. In the present study, computational fluid dynamics simulation is used to investigate two phase slug flow in vertical pipe using the volume of fluid (VOF) methodology implemented in the commercial code ANSYS Fluent. The viscous, inertial, and interfacial forces have significant effect on the hydrodynamic characteristics of two-phase slug flow. These forces can have investigated by introducing a set of dimensionless numbers, namely; inverse viscosity number, Nf, Eotvos number, Eo, and Froude number, FrTB. The simulation accounts for the hydrodynamic features of two phase slug flow including; the shape of Taylor bubble, bubble profile, velocity and thickness of the falling film, wake flow pattern, and wall shear stress distribution. The CFD simulation results are in good agreement with previous experimental data and models available in literature.

Investigation and prediction of slug flow characteristics in highly viscous liquid and gas flows in horizontal pipes

Slug flow characteristics in highly viscous liquid and gas flow are studied experimentally in a horizontal pipe with 0.074m ID and 17m length. Results of flow regime map, liquid holdup and pressure gradient are discussed and liquid viscosity effects are investigated. Applicable correlations which are developed to predict liquid holdup in slug body for low viscosity flow are assessed with high viscosity liquids. Furthermore, a mechanistic model is developed for predicting the characteristics of slug flows of highly viscous liquid in horizontal pipes. A control volume is drawn around the slug body and slug film in a slug unit. Momentum equations with a momentum source term representing the significant momentum exchange between film zone and slug body are applied. Liquid viscosity effects are considered in closure relations. The mechanistic model is validated by comparing available pressure gradient and mean slug liquid holdup data produced in the present study and those obtained from literature, showing satisfactory capabilities over a large range of liquid viscosity.

Gas/shear-thinning liquid flows through pipes: Modeling and experiments

In chemical and oil industry gas/shear-thinning liquid two-phase flows are frequently encountered. In this work, we investigate experimentally the flow characteristics of air/shear-thinning liquid systems in horizontal and slightly inclined smooth pipes. The experiments are performed in a 9-m-long glass pipe using air and three different carboxymethyl cellulose (CMC) solutions as test fluids. Flow pattern maps are built by visual observation using a high-speed camera. The observed flow patterns are stratified, plug, and slug flow. The effects of the pipe inclination and the rheology of the shear-thinning fluid in terms of flow pattern maps are presented. The predicted existence region of the stratified flow regime is compared with the experimental observation showing a good agreement. A mechanistic model valid for air/power-law slug flow is proposed and model predictions are compared to the experimental data showing a good agreement. Slug flow characteristics are investigated by the analysis of the signals of a capacitance probe: slug velocity, slug frequency, and slug lengths are measured. A new correlation for the slug frequency is proposed and the results are promising.

Flow patterns of an air–water mixture at the exit of a micro T-junction

An experimental investigation of typical flow patterns in adiabatic gas–liquid mixture (air-water) generated in a micro T-junction varying the superficial velocity of the fluids is presented. The micro T-junction is made as intersection of two glass microchannels with a rectangular cross-section having a width of 300μm and a height of 256μm. The air–water mixture is obtained by means of an injection of air in perpendicular direction with respect to a straight stream of water. The air flow is broken in slugs having geometrical characteristics depending on the superficial velocity ratio imposed. Changing the air and water superficial velocity from 0.005m/s up to a maximum value of 0.15m/s, different kinds of intermittent slugs have been observed. In this way, a flow pattern map has been generated in order to reword the main characteristics of the intermittent slug flow generated after the T-junction as a function of the superficial air–water velocity ratio. In order to build the flow pattern map, 256 experimental runs characterized by different values of superficial velocity have been made. Different kinds of Taylor bubble flows have been observed during this experimental campaign: for a fixed low value of the air flow rate, by increasing the water flow rate it has been evidenced how the formation of the bubbles becomes faster and the length of the single bubble becomes shorter. A detailed discussion of the impact of the superficial air to water velocity ratio on the observed flow patterns is given.

A Mechanistic Slug-Liquid-Holdup Model for Different Oil Viscosities and Pipe-Inclination Angles

Summary Slug liquid holdup is one of the most important parameters of slug flow. It is closely related to the average liquid holdup and pressure gradient of slug flow in wells and pipelines. The mechanistic models of Barnea and Brauner (1985) and of Zhang et al. (2003a) are based on the turbulent liquid-slug assumption for low-viscosity oils. However, for high-viscosity oil, the liquid slug is laminar because of the low slug Reynolds number. In this study, a mechanistic slug-liquid-holdup model is developed for low- and high-oil-viscosity slug flows. The model is based on two shear mixings: shear mixing between the slug front and pipe wall and shear mixing between the slug body and liquid film. The model uses slug-flow characteristics that can be calculated by solving the continuity and momentum equations of slug flow. A data bank consisting of 418 slug-liquid-holdup measurements that were obtained from various authors is used to analyze and validate the model. In the data bank, liquid viscosity ranges from 0.0016 to 0.589 Pa·s (1.6 to 589 cp). Pipe-inclination angle ranges from −30° to upward vertical. Pipe inside diameter varies from 5.08 to 10 cm. Statistical evaluations are conducted and compared with predictions of other models, and significant improvement is observed in the performance of the new model.

Characteristics of slug flow in a narrow rectangular channel under inclined conditions

The characteristic parameters of slug flow in a narrow rectangular channel with the cross section of 3.25 mm × 43 mm were measured for various flow rates and channel inclinations. Measurements were carried out by a high speed video camera system and image processing technique. It is found that the Taylor bubble length increases with the gas flow rate and the inclination angle, and decreases with the liquid flow rate. The channel inclinations, the gas and liquid flow rates have significant influences on the thickness and velocity of the liquid film at the bottom of Taylor bubble. The nose position of Taylor bubble is dependent on the two-phase superficial velocity as well as the inclination angle. For the low two-phase superficial velocity, the Taylor bubble velocity and the drift velocity increase with the inclination angle and almost approximate the maximum value for the inclination angle around 30°, while the coefficient C0 decreases with the inclination angle. For the high two-phase superficial velocity, the influence of the inclination angle is insignificant.

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.

Immiscible liquid–liquid slug flow characteristics in the generation of aqueous drops within a rectangular microchannel for preparation of poly(2-hydroxyethylmethacrylate) cryogel beads

The microchannel liquid-flow focusing and cryo-polymerization is an efficient method for the preparation of cryogel beads with a narrow diameter distribution. In order to prepare cryogel beads with expected diameters, it is necessary to get insights in the liquid–liquid immiscible flow characteristics of the flow-focusing fluid and the monomer solution in microchannels. In this work, the slug flow behaviors of two immiscible liquids regarding the preparation of poly(2-hydroxyethylmethacrylate) (pHEMA) cryogel beads in a rectangular cross-junction microchannel were investigated experimentally by the high-speed imaging method. Correlations of the immiscible liquid–liquid slug flow parameters like the aqueous slug velocity and length, the aqueous slug nose and tail lengths, the water-immiscible slug length as well as the aqueous droplet size were obtained. The pHEMA cryogel beads were prepared under certain flow conditions and the bead sizes were measured by laser particle size analyzer. The obtained correlations were then employed to estimate the bead sizes and compared with those obtained experimentally. The results showed that the present correlations gave reasonable estimations of the mean bead diameters at various conditions and thus, could be useful and helpful in the preparation of cryogel beads with expected size distributions in rectangular microchannels.

Dynamical analysis of turbulent liquid phase in two-phase pipe flow

An experimental study was conducted to evaluate dynamic structures embedded in a liquid and turbulent characteristics of slug flow in a horizontal pipe. Initially, single liquid phase flow was considered at a flow rate of 120 l/m and later on slug flow was generated by the introduction of a gas phase at the rate of 40 l/m keeping the liquid phase flow at the same rate. Particle image velocimetry was employed to calculate the two-dimensional instantaneous velocity field of the liquid phase. The turbulent velocity fields and dynamics of the liquid phase flow were investigated by proper orthogonal decomposition analysis. The structures containing the most energy were successfully revealed. The energy distributions of the spatial and temporal modes were also obtained.

Characteristics of slug flow in a vertical narrow rectangular channel

In the view of modeling slug flow, a detailed understanding of its hydrodynamics is of great importance. Non-intrusive flow visualization using a high speed video camera system is applied to study characteristics of slug flow in a vertical narrow rectangular channel (3.25×43mm2). The characteristics of the Taylor bubble, the liquid film and the liquid slug are studied and compared with the models available in literature. It is shown that the slug flow in the present channel is somehow different from the classical slug flow in medium size channels. The gas and liquid flow rates have significant effects on the Taylor bubble length, the thickness and velocity of liquid film at the bottom of Taylor bubble. For the continuous slug flow, the drift velocity is larger than the terminate velocity of a single Taylor bubble in stagnant liquid; the velocity of Taylor bubble could be well predicted by the Nicklin et al. correlation. The minimum stable liquid slug length is in the range from 9 to 17 hydraulic diameters in fully developed turbulent flow. Correlations for calculating the length of Taylor bubble, the thickness and velocity of liquid film at the bottom of Taylor bubble are proposed by fitting the experimental data, respectively.

CFD modeling of hydrodynamic characteristics of slug bubble flow in a flat sheet membrane bioreactor

It has previously been shown experimentally that the use of intermittent slug bubbling through the periodic introduction of large bubbles is an effective strategy to control fouling in MBRs. As the current set of experimental results is limited, a numerical study on the properties of a single bubble rising in an industrial flat sheet MBR was undertaken. A three-dimensional simulation with the volume of fluid (VOF) method was implemented. The predicted bubble is of a typical spherical cap shape which is consistent with experimental observation. The variation of the predicted shear stress on the membrane surface also shows good agreement with previous electrochemical results. Compared with the film region, the shear stresses in the wake region of the bubble were more intense and covered a much larger area. Additionally, the dominant component of shear stress is parallel with the bubble rising direction. For a fixed 100mL bubble, the averaged shear stress attains a maximum value when the gap between the membranes is around 8mm. With increasing activated sludge viscosity, the averaged shear stress increased only slightly. A brief review of CFD modeling and associated experimental work on two-phase flow in various membrane modules is included.

작은 수직관을 흐르는 기-액 슬러그 유동의 유동특성

Some characteristics of nitrogen-water slug flow were optically measured, in vertical acrylic tubes of 2, 5 and 8 mm diameter. Bubble velocity, bubble and unit cell lengths were measured, by analyzing the light intensity signals from two sets of dot laser-infrared sensor modules mounted along the transparent tubes. Optical images of the bubbles were also taken and analyzed, to measure bubble shapes and liquid film thickness. It was found that the measured bubble velocities were in good agreement with the empirical models in the literature, except for those measured under high superficial velocity condition in the 2 mm tube. Bubble length was found to be the longest in the 2 mm tube, being 4 to 5 times those of the other tubes. Liquid film was found to have developed early in the 2 mm tube, which made the blunt shape of the bubble head. Liquid film thickness in the 8 mm tube was measured at almost twice those of the other tubes.

Characteristics of slug flow with inertial effects in a rectangular microchannel

Slug flow behavior and characteristics have been studied in a rectangular microchannel with Y-junction. Two simple bubble shape models are proposed to calculate the film thickness, gas hold-up and the specific surface area. The results show a significant effect of inertia on the bubble generation process. The liquid film thickness was greatly thickened by the inertial effect. The main forces on the rupture of the gas bubbles present a shifting from pressure (squeezing regime) to shearing stress and dynamical pressure of the liquid (shearing regime). The transition was found to mainly depend on the liquid velocity. A strong leakage flow of the liquid film around the gas bubbles was suggested as the liquid slug lengths decreased instead when liquid velocity increased. This leakage flow is also the reason of the over-prediction of the bubble velocity based on the stagnant film hypothesis.

Numerical study on mitigating severe slugging in pipeline/riser system with wavy pipe

The gas/liquid two-phase flow in pipeline/wavy-pipe/riser systems was investigated numerically with CFD. A CFD model of the pipeline/wavy-pipe/riser system was obtained by adding a wavy pipe to the model of the pipeline/riser system verified by the experimental data previously. The effects of the geometrical parameters and location of the wavy pipe on its performance of slug mitigation and flow characteristics in pipeline/wavy-pipe/riser systems were examined through the CFD models. With the increase of the amplitude or length of the wavy pipe, the slug in the pipeline/riser system becomes shorter. The optimum location of the wavy pipe in the pipeline exists for a pipeline/riser system and a wavy pipe at given operating conditions. The CFD modelling provides a feasible and flexible way to investigate the effectiveness of the wavy pipes on mitigating severe slugging in pipeline/riser systems.

A PLIC-VOF-Based Simulation of Water-Organic Slug Flow Characteristics in a T-Shaped Microchannel

A water-organic slug flow in a T-shaped microchannel was numerically studied due to its importance in the microreactor system. Various factors affecting the flow mode were studied, for example, channel width, fluid viscosity, interfacial tension, and inlet velocity. The volume of fluid (VOF) method was used to track the liquid-liquid interface, and the piecewise-liner interface construction (PLIC) technique was adopted to get a sharp interface. The interfacial tension was simulated with continuum surface force (CSF), model and the wall adhesion boundary condition was taken into consideration. The results show that strong vortexes appear in both phases at the meeting sites of main and lateral channels where an organic slug is producing. Inlet velocity influences the slug length and flow mode greatly. The ratio between the slug lengths of two phases in the main channel is almost equal to the ratio between their inlet velocities. If the slug is produced, the interfacial tension and organic viscosity have less effect on the slug length for 200 μm microchannel. The slug producing rate is much higher in a narrow channel than that in a wide channel.

Experimental study on near wall transport characteristics of slug flow in a vertical pipe

In this work, the wall shear stress and the mass transfer coefficient of the gas–liquid two-phase upward slug flow in a vertical pipe are investigated experimentally, using limiting diffusion current probes and digital high-speed video system. In experiments, the instantaneous and averaged characteristics of wall shear stress and mass transfer coefficient are concerned. The experimental results are compared with the numerical results in previous paper of the authors. Both experiment and numerical simulation show that the superficial gas and liquid velocities have an obvious influence on the instantaneous characteristics of the two profiles. The mass transfer coefficient has characteristics similar to the wall shear stress. The instantaneous wall shear stress and mass transfer coefficient profiles have the periodicity of slug flow. The averaged wall shear stress and mass transfer coefficient increase with increased superficial gas velocity. However, there is inconsistency in the variation trends of the averaged wall shear stress and mass transfer coefficient with superficial liquid velocity between experimental result and numerical simulation result, which can be attributed to the difference in flow condition. Moreover, the Taylor bubble length is also another impacting factor. The experimental and numerical results all shows that the product scale can not be damaged directly by the flow movement of slug flow. In fact, the alternative forces and fluctuations with high frequency acting on the pipe wall due to slug flow is the main cause for the slug flow enhanced CO2 corrosion process.

Visualisation research on characteristics of the cryogenic slug flow in vertical and inclined tubes

AbstractThe statistical parameters of Taylor bubbles in cryogenic slug flow along glass tube were studied experimentally for various inclination angles θ (0°–60° from the vertical direction) and four tube inner diameters D (14, 18, 25 and 32 mm) by using high speed digital camera. The distributions and mean values of initial formation position, and the length and velocity of Taylor bubbles along the tube at various inclination angles were obtained. Initial position of Taylor bubbles increases as tube inner diameter increases and the effect of tube diameter on Taylor bubble initial position becomes more obvious at θ ≥ 30°. Taylor bubble length shows an allometric decreasing trend with the increasing of tube inner diameter. The influence of inclination angle and axial position on the length and velocity of Taylor bubble in cryogenic slug flow agrees qualitatively with conventional air–water system. The maximum mean velocity of Taylor bubble occurs at 30° ≤ θ ≤ 45° and the minimum mean length of Taylor bubble occurs at 0° ≤ θ ≤ 20°. The Mean length of Taylor bubble increases along the tube for x/D ≤ 60. © 2011 Canadian Society for Chemical Engineering

Numerical simulation of thermofluid characteristics of two-phase slug flow in microchannels

A fundamental study of heat transfer characteristics of two-phase slug flow in microchannels is carried out employing the Volume-of-Fluid (VOF) method. Despite of the fact that numerical simulations of two-phase flows in microchannels have been attempted by many investigators, most efforts seem to have failed in correctly capturing the flow physics, especially those pertaining to the slug flow regime characteristics. The presence of a thin liquid film in the order of 10 μm around the bubble is a contributing factor to the above difficulty. Typically, liquid films have a significant effect on the flow field and heat transfer characteristics. In the simulations reported in this paper, the film is successfully captured and a very high local convective heat transfer coefficient is observed in the film region. A strong coupling between the conductive heat transfer in the solid wall and the convective heat transfer in the flow field is observed and characterized. Results showed that unsteady heat transfer through the solid wall in the axial direction is comparable to that in the radial direction. Results also showed that a fully developed condition could be achieved fairly quickly compared to single-phase flows. The fully developed condition is defined based on the Peclet number ( Pe) and a dimensionless length of the liquid slug. Local and time-averaged Nusselt numbers for slug flows are reported for the first time. It was found that significant improvements in the heat transfer coefficient could be achieved by short slugs where the Nusselt number was found to be 610% higher than in single-phase flows. The study revealed new findings related to slug flow heat transfer in microchannels with constant wall heat flux.

Prediction of Gas-Liquid Two-Phase Slug Flow Characteristics in Vertical Small Diameter Pipes by a One-Dimensional Two-Fluids Model

The purpose of this study is to improve a two-fluid model applicable to gas-liquid two-phase slug flows in small diameter pipes. Experimental data on void fraction and frictional pressure drop were obtained for vertical upward slug flow in 5 and 9 mm i.d. circular pipes, and those on the interfacial friction force were obtained by substituting the above data into an equation derived from the one-dimensional two-fluids model. The test liquids were a Poly-oxy-ethylene lauryl ether water solution, a 72 wt% glycerin water solution and a tap water at 30 °C, while the test gas was air at near atmospheric pressure. In order to study the effects of liquid properties, surface tension of the test liquids against air was varied from 0.042 to 0.071 N/m, viscosity was 0.797 to 19.6 mPa·s, and density was 996 to 1184 kg/m3. The range of volumetric flux of the liquid was 0.1 to 2.0 m/s, and that of the gas was 0.1 to 22 m/s. In addition, the drag coefficient data of the large bubble was determined from the data on the bubble diameter, the respective void fractions of the liquid slug section and the large bubble one, the interfacial friction force and the length ratio of the large bubble to the total of the liquid slug and the large bubble. These data were used to validate representative two-fluid model codes. Since the prediction by the codes did not fit well the present data, a new drag coefficient correlation for the large bubble was proposed using 5 dimensionless parameters. The predictions by the new correlation and familiar ones in literatures have been tested against the present data, and the applicability of the new one has been demonstrated.

Hydrodynamic and mass transfer characteristics of slug flow in a vertical pipe with and without dispersed small bubbles

In this work, we present a numerical study to investigate the hydrodynamic characteristics of slug flow and the mechanism of slug flow induced CO 2 corrosion with and without dispersed small bubbles. The simulations are performed using the coupled model put forward by the authors in previous paper, which can deal with the multiphase flow with the gas–liquid interfaces of different length scales. A quasi slug flow, where two hypotheses are imposed, is built to approximate real slug flow. In the region ahead of the Taylor bubble and the liquid film region, the presence of dispersed small bubbles has less impacts on velocity field, because there are no non-regular intensive disturbance forces or centrifugal forces breaking the balance of the liquid and the dispersed small bubbles. In the liquid slug region, the strong centrifugal forces generated by the recirculation below the Taylor bubble lead to the effect of heterogeneity, which makes the profile of the radial liquid velocity component sharper with higher volume fraction of dispersed small bubbles. The volume fraction has a maximum value in the range of r/ R = 0.5–0.6. Meanwhile, it is usually higher than 0.35, which means that larger dispersed bubbles can be formed by coalescences in this region. These calculated results are in good agreement with experimental results. The wall shear stress and the mass transfer coefficient with dispersed small bubbles are higher than those without dispersed small bubbles due to enhanced fluctuations. For short Taylor bubble length, the average mass transfer coefficient is increased when the gas or liquid superficial velocity is increased. However, there may be an inflection point at low mixture superficial velocities. For the slug with dispersed small bubbles, the product scales still cannot be damaged directly despite higher wall shear stress. In fact, the alternate wall shear stress and the pressure fluctuations perpendicular to the pipe wall with high frequency are the main cause for breaking the product scales.