Gas-liquid Slug Flow Research Articles

A Navier-slip model for liquid film thickness in gas–liquid slug flow in capillaries

ABSTRACT The motion of a long gas bubble in a continuous liquid phase in a small diameter channel results in a thin liquid film between the bubble and the channel wall. Generally, a free-slip (zero shear) boundary condition is applied at the gas–liquid interphase boundary which is applicable only at low gas velocities. However, at increased flow rates of gas phase, shear stress at the gas–liquid interface is non-negligible. Applying non-zero shear at the interface requires knowledge of velocity profile in the gas phase. In this work, we circumvent this problem by using a slip length like parameter to take into account the velocity gradient in the gas phase. The slip length is infinite for the free-slip condition at the interface and zero for the no-slip condition. An expression is derived for the film thickness in terms of slip length and capillary number and is compared with the experimental data and other correlations available in the literature.

Using machine vision algorithms for characterizing gas-liquid slug flows in vertical pipes

Slug flow, characterized by the distinctive interfacial structures of Taylor bubbles surrounded by liquid films and bridged by aerated liquid slugs, is a dynamically complex two-phase flow pattern exist in many oil and gas, and energy systems. Accurate and precise quantification of such complex flow behaviour is essential for optimal design, safe operation, and reliable modelling of these systems. Existing image-based measurement techniques mostly rely on offline image processing algorithms and are often limited to a narrow set of flow characteristics primarily focusing on Taylor bubbles. Such constraints not only impede real-time flow monitoring and regulation but also leave liquid slug characteristics unmeasured, resulting in an inability to accurately determine the flow characteristics and extract instantaneous void fraction signals. Present study examined the performance of adaptive thresholding (AT) and background subtraction (BS) algorithms in capturing slug flow characteristics. It was found that while the former excels in Taylor bubbles detection and the latter in small bubbles identification, neither individually addresses the accurate measurement of both flow structures' characteristics. This observation, along with the mentioned restrictions of existing algorithms are the main reason for developing the present combined machine vision-based algorithm. While unlocking the ability to extract instantaneous void fraction signals, this new approach facilitates online measurement of a wide range of key flow characteristics, including Taylor bubble length, velocity, void fraction, and surrounding liquid film thickness; liquid slug length and void fraction; and slug unit length, void fraction, and frequency. Parallel to the high-speed imaging, time-series void fraction data was collected using two capacitance sensors installed alongside the imaging area on the pipe, providing benchmark data essential for the validation of the new algorithm's accuracy. The comparisons demonstrated a high degree of accuracy and precision for the combined algorithm. Quantitatively, the new algorithm measured key unit cell characteristics with RMS errors ranging from 2 to 10 %, while the BS and AT algorithms exhibited wider RMS error ranges of 8–46 % and 2–53 %, respectively. This underscores the new algorithm's potential as a transformative tool for slug flow analysis.

Novel correlations for the liquid holdup in a gas-liquid slug flow

This paper introduces novel equations for calculating the slug unit liquid holdup (average liquid holdup) and the slug liquid holdup (slug holdup) in liquid-gas slug flow. The equations were tested across various pipe diameters, incline angles, fluid viscosities and densities, system pressures, and liquid and gas flow rates. The correlation utilizes two dimensionless numbers and displays a self-similar nonlinear symmetrical sigmoidal trend. The two parameters used are the ratio of the no-slip liquid holdup to the in-situ liquid holdup and the ratio of liquid superficial velocity to gas superficial velocity. When the superficial velocities ratio is high, all data converge to a single value of approximatly1 for the no-slip to in-situ liquid holdup ratio. The present unified correlations are simple to implement in any slug flow model. It employs defined parameters, and it has already been tested for varying pipe diameters, inclinations, and fluid viscosities that range from water to 962 cP. The presented correlations are unified and unique and not found in public literature. The suggested correlations exhibit superior performance compared to existing correlations and models.

Investigations of the Formation Mechanism and Pressure Pulsation Characteristics of Pipeline Gas-Liquid Slug Flows

The pipeline system is widely used in marine engineering, and the formation mechanism and flow patterns of two-phase slug flows are of great significance for the optimal design of and vibration prevention in a complex pipeline system. Aiming at the above problems, this paper proposes a modeling and solving method for gas-liquid slug flows. First, a VOF-PLIC-based coupling gas-liquid slug flow transport model is conducted. Second, to reduce the fuzzy boundary between the gas-liquid coupling interfaces, an artificial compression term is added to the transport equations, and the formation and evolution mechanism of severe slugging flow in piping systems is investigated. The pressure pulsation and gas content characteristics of the gas-liquid coupling process are explored. Research results found that the slugging phenomenon occurs at the gas-liquid interface, where liquid slugging frequency reaches its peak. The pipeline system has prominent periodic characteristics of the slugging phenomenon, and the period decreases when the gas-phase converted speed rises; pressure fluctuation amplitude increases, and the gas-phase velocity change is the inducing factor for the drastic change of pressure fluctuation. The research results can offer theoretical references for optimal designs of and vibration prevention in marine pipeline systems.

Velocity Measurement of Gas–Liquid Slug Flows Using Multichannel Ultrasonic Doppler Sensor System

Velocity Measurement of Gas–Liquid Slug Flows Using Multichannel Ultrasonic Doppler Sensor System

MODELING OF TWO-PHASE GAS-LIQUID SLUG FLOWS IN MICROCHANNELS

Even though numerical simulation of two-phase flows in microchannels has been attempted by many investigators, most efforts seem to have failed in correctly capturing the flow physics, especially the slug-flow regime characteristics. The presence of a thin liquid film in the order of 10 μm around the bubble (sometimes called gas pocket or gas slug) may be a contributing factor to the above difficulty. Typically, liquid films have a significant effect on the flow field. Thus, there is a strong motivation to employ numerical simulation methods to avoid some of the experimental difficulties. In this paper, the characteristics of two-phase slug flows in microchannels are calculated with the help of the volume-of-fluid (VOF) method. Formation of the slugs for different superficial velocities, capillary numbers, and gas volume fractions are investigated. The minimum mesh resolution required to capture the liquid film surrounding the gas bubble is reported employing a dynamic mesh adaptation methodology with interface tracking. Results are shown to be in good agreement with experimental data and empirical correlations.

Spatial distribution of void fraction in the liquid slug in vertical Gas-Liquid slug flow

Vertical slug flow is characterized by the intermittent passage of two structures, the Taylor bubble and the liquid slug. The Taylor bubble is an elongated gas bubble with a bullet shape that flows upward and fills most of the radial cross-section, leaving room for an annular downward flow of a liquid film. The liquid slug is composed of the liquid phase carrying small, scattered bubbles and flowing upwards. The liquid slug presents three main regions: the wake, the development and the fully developed one. The liquid slug has a high concentration of dispersed bubbles in the wake region because of the entrainment mechanism. The detachment point (Dt) is a stagnation point with almost no dispersed bubbles around it, occurring close to the wall in the wake region end. Experimental data for the gas fraction in three different pipe diameters (26, 40.8 and 50 mm) were gathered in the 14-meter-high vertical flow experimental facility of the NUEM/UTFPR MULTILAB. The gas fraction along the liquid slug was evaluated by using a capacitive wire-mesh sensor (WMS) for quantitative results and a high-speed camera was used to analyze qualitative results. A new correlation to predict the average volumetric gas fraction along the unit cell is proposed. Radial and axial gas fraction distribution were analyzed and the detachment point was identified. The Reynolds and Froude number influence on the Dt point was evaluated and a new correlation to predict the Dt location (LDt) is proposed.

Effect of the heat flow distribution and thermophysical properties of the pipe wall and fluid on the wall temperature response to a rising Taylor bubble

A novel non-intrusive thermal diffusion measurement (TDM) method has been proposed for measuring the phase flowrates of gas–liquid slug flows in our previous research. Since the structure of TDM was arbitrarily designed and needed to be optimized for accurate measurement, the effect of the heat flow distribution and the thermophysical properties of the pipe wall and fluid on the wall temperature response to a rising Taylor bubble (TB) is further investigated by using the CFD tool in this paper. The wall temperature and temperature rise are used to describe the wall temperature response, and their variation rules with heat flow distribution and the thermophysical properties of the pipe wall and fluid are presented and analyzed based on heat transfer. The simulation results indicate that the optimal heating length for a large wall temperature rise and small power consumption is 0.6D, and the optimal pipe material should be with small density and specific heat and a suitable thermal conductivity of about 15 W/(m·K). Moreover, the wall temperature response is obvious, especially for TBs moving in the liquid with large fluid thermal conductivity, density, and specific heat, and small viscosity. The paper can clarify the wall temperature response mechanism of the heated pipe wall and provide a theoretical basis for the development of the TDM method.

Mass transfer and modeling of deformed bubbles in square microchannel

Understanding of mass transfer in gas-liquid slug flow is imperative to design and optimize micro-reactors. There exist extensive studies on symmetric bubbles by the phase volume monitor technique, whereas deformed bubbles are rarely studied due to the limitation of volume calculation methods. In this work, CO2-water and N2-water two-phase flows were investigated in a square microchannel, obtaining annular flow, slug flow, and bubbly flow. A flow pattern map was then proposed and compared with the literature. A 3D slicing technique was developed to measure the volume and interfacial area of bubble, including symmetric bubbles and deformed bubbles, by slicing the bubble along the streamwise direction. Scaling laws of the important parameters that characterize the micro-reactors were proposed. Mass transfer coefficients kLa were quantified from the time-changing volume. The empirical correlation involving dimensionless numbers were fitted, which shows accurate predictive performance for mass transfer coefficients in this study and literatures. The bigger index of Reynolds number ReG indicated that gas flow condition is the main influencing factor during mass transfer process. To have a better universality, a new semi-theoretical model involving the ratio of the size of the liquid and gas phases LL/LG was developed based on the Pigford and Higbie penetration theory because experimental data confirms that the degree of bubble deformation is related to LL/LG. The semi-theoretical model shows a satisfactory agreement over the whole range of slug flow in this study.

Measurement of film fluctuations and aerated characteristics during gas-liquid slug-churn flow transition by wire-mesh sensor

Vertical upward gas-liquid slug flows are frequently encountered in chemical processes and petroleum industries. The measurement of the film fluctuations and the aerated characteristics is of great significance for uncovering the mechanism of slug-churn flow pattern transitions. In this study, a conductance wire-mesh sensor (WMS) measurement system is designed based on a Field Programmable Gate Array (FPGA) to visualize the structures of vertical gas-liquid flows. Liquid film flooding is a significant factor prompting the transition from slug to churn flow. Based on the WMS data, the 3D film structures are derived to indicate film instability during the flow pattern transition. Three types of film fluctuations in stable slug flow, unstable slug flow, and churn flow are presented. Liquid slug aeration is another important factor contributing to the slug-churn flow transition. The spatial distribution and the diameters of the gas bubbles in the liquid slug are detected by the WMS. The coalescence behavior of the bubbles is uncovered. Finally, mechanistic models based on the film flooding and slug aeration are constructed to predict the boundary of the flow pattern transition. The performance of the film flooding model and slug aeration model in predicting the onset of churn flow is evaluated.

Prediction of unsteady slug flow in a long curved inclined riser with a slug tracking model

An improved one-dimensional mechanistic model is presented for the prediction of unsteady gas-liquid slug flows in inclined curved pipes, using the slug tracking approach. The equations for mass and momentum conservation are applied to the slug body, liquid film, and elongated bubble regions constituting a slug unit cell. The proposed model can be applied to horizontal or inclined upward flows. Statements of mass conservation result in axial changes of the liquid and gas velocities in the liquid film and elongated bubble. The slug initiation at the inlet is modelled as a random process with slug length variations. Closure relationships for the bubble nose velocity, modified by the wake effect, and the slug frequency for slug initiation are employed. The discretized governing equations are solved fully implicitly, introducing numerical treatments associated with the outlet boundary conditions and the merging of slug units. Of practical interest is an upward gas-liquid slug flow in a catenary riser with a high aspect ratio (length over diameter) being an order of a thousand representing an offshore subsea pipe for the oil and gas production. By considering the pipe initially fully filled with the traveling liquid, the dynamic scenario of the pipe transporting successive slug units is simulated, capturing the continuing evolution of slug flow patterns along the pipe exhibiting the disappearances of liquid slugs due to the bubble coalescences. Spatio-temporal variations of the liquid holdup, the pressure and its gradient, the film and slug lengths, the slug frequency, the velocities of the slug front, bubble nose, liquid in the slug body and film, and of the gas in the elongated bubble are evaluated. The backward flow occurrence in the film zone near the outlet is also predicted due to the pipe inclination. Parametric investigations are performed by specifying the superficial liquid and gas velocities, and comparing the cases of catenary pipes (with variable inclinations) versus inclined and horizontal straight pipes (with fixed inclinations). Results highlight the important effect of gas-to-oil superficial velocity ratio (GOR) in combination with the pipe inclination and curvature effects. Fluctuations of slug flow properties appear to be considerably amplified and more intermittent when increasing the GOR. This observation is important towards regulating the practical flow rates for subsea oil and gas productions as well as designing flexible pipes subject to slug flow-induced vibrations.

Simultaneous measurement of velocity profile and liquid film thickness in horizontal gas–liquid slug flow by using ultrasonic Doppler method

Horizontal gas–liquid two-phase flows widely exist in chemical engineering, oil/gas production and other important industrial processes. Slug flow pattern is the main form of horizontal gas–liquid flows and characterized by intermittent motion of film region and slug region. This work aims to develop the ultrasonic Doppler method to realize the simultaneous measurement of the velocity profile and liquid film thickness of slug flow. A single-frequency single-channel transducer is adopted in the design of the field-programmable gate array based ultrasonic Doppler system. A multiple echo repetition technology is used to improve the temporal–spatial resolution for the velocity profile. An experiment of horizontal gas–liquid two-phase flow is implemented in an acrylic pipe with an inner diameter of 20 mm. Considering the aerated characteristics of the liquid slug, slug flow is divided into low-aerated slug flow, high-aerated slug flow and pseudo slug flow. The temporal–spatial velocity distributions of the three kinds of slug flows are reconstructed by using the ultrasonic velocity profile measurement. The evolution characteristics of the average velocity profile in slug flows are investigated. A novel method is proposed to derive the liquid film thickness based on the instantaneous velocity profile. The liquid film thickness can be effectively measured by detecting the position and the size of the bubbles nearly below the elongated gas bubble. Compared with the time of flight method, the film thickness measured by the Doppler system shows a higher accuracy as a bubble layer occurs in the film region. The effect of the gas distribution on the film thickness is uncovered in three kinds of slug flows.

Regulating the Gas–Liquid Slug Flow in Microchannels through High-Frequency Pulsatile Perturbations

The gas–liquid slug flow that is characterized with alternate formation of gas and liquid segments has been widely applied for chemical synthesis and analysis. For its applications, it is essential to precisely control the gas and liquid slug lengths. However, using the passive method, the bubble/droplet generation frequency and their sizes are influenced by multiple parameters including the channel dimension, fluid physical properties, the gas-to-liquid flow rate ratio, and so forth and hence lack control flexibility. In this work, an active approach using pulsatile perturbations to control the gas–liquid slug flow is demonstrated. A rotating valve is applied to produce a high-frequency pulsating air flow. Under its influence, either Taylor bubbles in a hydrophilic channel or liquid-in-gas droplets in a hydrophobic channel can be generated synchronously with the flow pulsation. Therefore, the gas and liquid slug lengths can be independently tuned via the air inlet pressure and the liquid flow rate, respectively. The maximum generation frequency is up to ∼1300 Hz. In addition, the effects of the pulsation frequency, the pulse duty ratio, and the liquid viscosity are also investigated. This technique provides an efficient method to regulate the gas–liquid slug flow in multiphase microreactors or microfluidic analytical systems.

INFLUENCE OF LOW-FREQUENCY EXTERNAL PULSATIONS OF LIQUID FLOW RATE ON GAS-LIQUID SLUG FLOW IN A RECTANGULAR MICROCHANNEL

This paper presents the results of an experimental study of the influence of the liquid flow rate of external pulsations on the gas-liquid flow characteristics of periodic slug flow in a microchannel. The experimental data were obtained using high-speed visualization of the flow of an ethanol-nitrogen mixture in a T-junction microchannel with a 390 × 150 μm rectangular cross section. A novelty of this work was the use of pulsations of the continuous liquid phase on the gas-liquid flow. The form of external pulsations was a sine wave with different frequencies (f = 1 and 2 Hz) and amplitude values (A = 8%, 20%, and 40%). The results obtained showed that using high-amplitude and low-frequency external pulsations destroys periodic slug flow and changes the monodispersity of bubbles and liquid slugs. The statistical characteristics of the flow were obtained based on the frequency and amplitude of the pulsations.

Rapid On-Line and Non-Invasive Flow Rate Measurement of Gas–Liquid Slug Flow Based Only on Basic Differential Pressure Fluctuations

The gas–liquid two-phase flow is widely encountered in many industrial applications, and its on-line and non-separation flow rate measurement for each phase has plagued the industry for many years. Based on the one-to-one correspondence between differential pressure fluctuations and the movement of Taylor bubbles (TBs) and liquid slugs (LSs), a novel method is proposed to measure the flow rate of gas–liquid slug flow in this article. Theoretical correlations between flow characteristics (velocity, length, and void fraction) and differential pressure fluctuations were quantitatively analyzed by experiments, and the gas–liquid ratio was found to be linearly related to the length ratio of TB and LS. The gas flow rate and liquid flow rate were predicted based on the measured flow characteristics. Experimental tests on air–water two-phase flow show that the proposed method and correlations were effective, and the average relative errors (AREs) in the gas flow rate and liquid flow rate are 5.50% and 5.62% with maximum values of 11.46% and 12.06%, respectively, under the conditions of 0.16–0.56 m/s of air superficial velocity and 0.23–0.48 m/s of water velocity. This method is based only on the differential pressure fluctuations and is on-line, non-separation, noninvasive, and radiation-free, which provides a new cost-effective solution for the flow rate measurement of industrial two-phase flows.

Flow and mixing characteristics of gas-liquid slug flow in a continuous Taylor-Couette flow reactor with narrow gap width

This work experimentally investigated flow characteristics and mixing properties of the two-phase flow in a continuous Taylor-Couette flow reactor (CTCR) with narrow gap width. In the device with a gap width of 0.5 mm, to the authors’ knowledge, the narrowest gap width device ever studied in CTCR systems, the present study revealed that slug flows with alternating gas and liquid phases, as observed in microchannels, appeared in a certain flow regime. This slug flow appeared mainly in the region of low rotational Reynolds and axial Weber numbers. The non-dimensional effective diffusion coefficient Deff/uL [-] of slug flow was also measured by a tracer response method. Deff/uL in the slug flow was on the order of 10−6 and it has been successfully found that ideal plug flow mixing characteristics could be achieved under this flow condition. Since the intensified and controllable mixing in the liquid phase due to Taylor vortex motion induced by the inner cylinder rotation is expected to be much better than the mixing in the liquid phase of slug flows in microchannels, this flow is expected to be one of the process intensification techniques that will outperform the performance of the conventional microreactors with microchannels.

Liquid holdup modeling analysis in horizontal gas–liquid slug flow

In slug flow, the wetted wall fraction and liquid holdup of the liquid film section are key parameters affecting the mass transfer and radial velocity distribution of the liquid film, the friction pressure drop, the momentum transfer between the two phases, and heat transfer characteristics. A modified wetted wall fraction and liquid holdup model based on the modified apparent rough surface (MARS) model is proposed in this study, which considers the friction coefficient and shear stress on the gas–wall, liquid–wall and gas–liquid interfaces and introduced the modified average interface velocity of the liquid film. Air–water slug flow experiments were in a 50 mm diameter acrylic glass pipe equipped with a high-speed camera. The edge detection operator was optimized to obtain the wetted wall fraction and liquid holdup at the liquid film section based on image analysis technology. A comparative analysis of the model performance shows that the mean absolute percentage error (MAPE) of the wetted wall fraction model is 9.4%, and the 96.1% relative deviations are within the ±20% error band. The MAPE of the liquid holdup model is 8.04%, and 93.4% relative error is within the ±25% error band. The modified MARS model has good prediction ability for the wetted wall fraction and liquid holdup of the gas–liquid slug flow.

Free vibration and stability analysis of inclined pipes conveying gas-liquid slug flow

In this paper, the free vibration and stability of inclined pipes conveying gas-liquid slug flow are investigated, by considering the intermittency of slug flow and gravity effects of the system. A new lateral motion model has been established based on the stable slug flow dynamics model and the Euler–Bernoulli beam vibration model and is then truncated by the Galerkin approach. The natural frequencies, critical gas velocities, and responses to flow conditions are comprehensively investigated to show the basic dynamic characteristics of inclined pipes. The results show that the gravity effects caused by the inclined angle have a significant effect on the vibration characteristics. The system is less stable with lower critical gas velocity and natural frequency with the increase of inclined angle, due to the gravity-induced compression. For a given inclined angle, the system dynamic responses to superficial gas/liquid velocities are directly related to the mass of liquid per unit length. In addition, the fluctuation of natural frequency caused by the intermittency of slug flow decreases with the increase of inclined angle. This study contributes to understanding the complex dynamic behavior of inclined pipes conveying slug flow and promoting pipeline safety.

Effect of hybrid wall contact angles on slug flow behavior in a T-junction microchannel: A numerical study

This study investigates the effects of hydrophilic-to-superhydrophobic discontinuous wall contact angles on gas-liquid slug flow in a T-junction microchannel. The two-phase volume-of-fluid model with dynamic contact angle are used by solving wall-adhesion equations, which determine the shape of the fluid’s interface at and close to the wall. The governing equations of the flow field are solved numerically using a finite volume method. The obtained results are first validated against previously reported experimental data. Subsequently, the effects of changes in the wall contact angles on the slug flow behavior are examined resulting in the new insights into the size, shape, and velocity of slug flow as a function of the wall contact angle. Using a hydrophilic-superhydrophobic hybrid surface leads to the formation of an asymmetric bubble and separation of the bubble from the hydrophobic part. It also causes adhesion to the hydrophilic part, a change in slug length, a reduction in pressure drop, and a variation in the heat transfer characteristics and continuous phase structure. It is shown that using a particular pattern significantly increases the thickness of the liquid wall film.

Characteristics of Gas-Liquid Slug Flow in Microchannel by Instantaneous Liquid Film Thickness Measurement

Characteristics of Gas-Liquid Slug Flow in Microchannel by Instantaneous Liquid Film Thickness Measurement