Slug Formation Research Articles

Slug Formation Analysis of Liquid–Liquid Two-Phase Flow in T-Junction Microchannels

Slug flow is a common flow pattern in the liquid–liquid two-phase flow in microchannels. It is an ideal pattern for mass transfer enhancement. Many factors influence the slug formation such as the channel geometries (channel widths, channel depth), flow rates of the two phase, and physical properties. In this paper, in order to investigate the liquid–liquid two-phase slug formation in a T-junction microchannel quantitatively, the volume of fluid (VOF) method is adopted to simulate the whole slug formation process. With the validated model, the effects of the disperse phase channel width, channel depth, and two-phase flow rate ratio on slug formation frequency and slug size (slug volume and slug length) are analyzed with dimensionless parameters. Dimensionless parameters include the disperse-to-continuous phase channel width ratio, height-to-width ratio, and two-phase flow rate ratio. Results show that both the channel geometry and two-phase flow rate ratio have a significant influence on slug formation. Compared with the conventional slug formation stages, a new stage called the lag stage emerges when the disperse phase channel width decreases to half of the continuous phase channel width. When the channel depth decreases to one third of the continuous phase channel width, the flow patterns become unstable and vary with the two-phase flow rate ratio. Moreover, empirical correlations are proposed to predict the slug formation frequency. The correlation between slug formation frequency and slug volume is quantified.

Effects of flow loop components in suppressing flow boiling instabilities in microchannel heat sinks

Present work reports an experimental investigation of flow boiling instabilities in three different configurations of a flow boiling loop. The three flow configurations i.e. loop having flexible tubing and connections, loop having stiffed tubing and loop having stiffed tubing with a throttling valve have been investigated and their performances are compared. Experiments are performed with segmented finned microchannels (SMC) to investigate the roles of above mentioned components in flow boiling instabilities. The array of segmented finned microchannels are fabricated on a copper substrate with a footprint area of 10 × 10 mm2 and experiments have been performed using deionized water at 20 °C subcooling. It has been observed that loop components external to the heat sink affect the heat transfer, pressure drop characteristic and assist in mitigating flow boiling instabilities. Based on the flow patterns, the slug ebullition cycle has been categorized into three distinct stages (a) slug formation (b) upstream compression and (c) flooded flushing. Cycle time and number of oscillations during upstream compression stage were reduced in the stiffed connections and throttling valve flow configurations. Heat transfer performance of the throttling valve configuration has been found to be the best among all three, followed by stiffed tubing configuration. The loop with flexible tubing connections demonstrated largest time period of slug ebullition cycle and also experienced large amplitude wall temperature and pressure oscillations. The combination of stiffed tube materials surrounding the heat sink and throttling valve at pump exit effectively reduce the flow boiling instabilities resulting in better thermal and hydraulic performance.

Characterization of slug formation towards the performance of air-liquid segmented flow

The increased attention towards continuous processes in, e.g. reaction or crystallization has put emphasis towards gas-liquid two-phase flow and especially towards slug flow. The segmentation leads to narrow residence time distributions under laminar flow conditions enabling the implementation of kinetically slow processes. Furthermore, mixing inside slugs is ensured without introduction of additional mixing devices. However, such internal mixing is affected by the slug length itself and its distribution formed during slug formation. Therefore, for uniform process conditions inside the slugs, they need to be as homogeneous as possible in length identifying slug formation as one of the crucial steps towards the design of slug flow reactors/crystallizers. In this study, the performance of slug flow with focus on slug lengths’ homogeneity is quantified by evaluating characteristic values of a slug length distribution. For that, an automated image based quantification method is developed. Various inlet conditions, including the air supply and the geometry of mixer, are investigated regarding their influence on slug length distribution. Different mixer materials are compared such that finally an improved setup towards homogeneity of slug flow is defined.

Simulation of Intermittent Flow Development in a Horizontal Pipe

In this study, detailed three-dimensional (3D) numerical simulations of intermittent multiphase flows were carried out to investigate the slug initiation process and various features of intermittent flows inside a horizontal pipe. Air and water are used as working fluids. The domain used for simulations is a 14.4 m long pipe with 54 mm inner diameter. The volume of fluid (VOF) model was used to capture the air/water interface and its temporal evolution. Using the developed computational fluid dynamics (CFD) model, the slug formation and propagation along horizontal circular pipe were successfully predicted and studied comprehensively. Slug length and the frequency of slug formation, as two main features of intermittent flow, were used to validate the model against experimental results and available correlations in the literature. Three-dimensional numerical simulation of intermittent flow proved to be a powerful tool in tackling limitations of experiments and providing detailed data about various features of the intermittent flow. The effect of gas and liquid superficial velocities on the liquid slug and elongated bubble length was explored. Moreover, the study revealed new findings related to the elongated bubble shape and velocity field in the slug unit.

Impact of dissolved salts on two-phase flow and boiling heat transfer in a natural circulation loop

Two-phase natural circulation and boiling experiments are conducted in this study with artificial seawater as a heat transfer fluid or process coolant. This experimental work is focused on understanding the role of dissolved salts in water on the steady state two-phase natural circulation flow rates. Mass flow rates were measured at different heat flux levels with artificial seawater, deionized water and tap water in the experimental loop. It was found that the average quasi-steady state mass flow rates were the same at any particular heat flux level regardless of the fluid being used. Artificial seawater coolant exhibited more stable natural circulation at higher heat flux as compared to the other two coolants. Mass flow measurements recorded at 2 s intervals show that magnitude of flow oscillations are much higher in case of tap water and deionized water. Also, high speed camera images revealed that the bubble departure diameters are smaller in seawater compared to tap or deionized water. This can be attributed to the increased local wettability due to the presence of dissolved salts in the fluid. Visual observations of the riser section showed that bubble coalescence and formation of slugs is inhibited in the seawater. These observations also explain the reduced pressure drop across test section when seawater was used as a coolant.

Flow Assurance Analyses for Optimum Pipeline Transportation of Gas for Use in CNG Vehicles in Nigeria

The objective of this work is to conduct flow assurance analyses for optimum transportation of gas through pipelines for use in CNG vehicles in Nigeria. Port Harcourt – Owerri – Onitsha expressway is used as case. The work is for a pipeline network that would be laid from a gas treatment plant about 10km off the Port Harcourt - Owerri – Onitsha expressway to the five CNG refueling stations installed along the route for refilling the CNG vehicles. Flow assurance analyses include analyses on: condensate drop-out prevention; hydrate formation prevention; erosion prevention; slug formation tendency; and leak detection. They are performed to ensure that solids or liquids do not deposit in the pipeline. Condensate drop-out prevention analysis is conducted to determine the conditions within which hydrocarbon liquids would not form in the pipeline during the transportation of the gas. Hydrate formation is checked to establish the temperatures and pressures at which hydrate would not deposit in the pipeline. Erosion prevention analysis is performed to determine the velocity profile above which erosion cannot occur in the pipe. Slug formation tendency is evaluated to ensure that slugs do not form in the pipe. Pipeline leak is also checked by generating pressure profile that would govern the flow of gas in the pipe and help determine any possible leak of gas from the pipeline. The results of the analyses indicate that: liquid drop-out will not occur in the pipe if the temperature of the gas does not go below 65.73°F; once the temperature and pressure of the gas remain outside the hydrate curve, hydrate will not form in the pipe; the velocity profile along the pipe length is far higher than the erosional velocity therefore the possibility of erosion occurring in the pipe is highly minimal; slugs cannot form in the pipe since the fluid remains in single phase throughout the transportation in the pipe; with the pressure profile generated; leak detection is made easy by installing five pressure gauges at different lengths of the pipe and recording the expected pressures at these lengths. It is further observed from the work that these flow assurance parameters and tests are highly indispensable in gas transportation as the results of the analyses help the engineer to apply all precautionary measures to ensure that solids or liquids do not deposit in the pipeline. Recommendations are also made for the optimum operating conditions based on the flow assurance analyses.

Experimental characterization of gas-liquid flows in splitting distributor for parallel micro-channels

The present work is focused on experimental investigations of gas-liquid flows in T-junction splitting distributor for parallel micro-channels. The tree type T-junction splitting distributor was designed with four successive T-junctions (T1, T2, T3 and T4) that are linked in series to split a single main channel into four parallel channels. The design strategy was adopted from Adamson et al. [1], Hoang et al. [2], and modified with a combination of micro and milli channel dimensions and reduction in width of the successive blocks by a factor of 20.5 to ensure the flow uniformity in terms of constant relative lengths (Lbubble/slug/Wchannel) of bubbles/slugs in all the blocks. Experiments were performed to study the effect of Qoil (=3, 6, 8, 10, 12 and 16 ml/min) at fixed Qair (=3 ml/min) on formation dynamics of bubbles/slugs at 1st T-junction and splitting mechanisms of bubbles/slugs at the successive T-junctions, the corresponding relative lengths of bubbles/slugs and flow regimes. The results obtained from the present work showed that, followed by formation of parent gas slug at T1 the splitting of gas slug occurred at the successive T-junctions (T2, T3 and T4). Three types of splitting mechanisms were observed such as obstructed (Qoil = 3, 6, 8, 10 ml/min; 0.0023 ⩽CaBlock-I⩽0.0050), partially obstructed (Qoil = 12 ml/min; CaBlock-I = 0.00585) and non-obstructed (Qoil = 16 ml/min; CaBlock-I = 0.0074). It was observed that asymmetry of the splitting at the successive T-junctions was reduced for Qoil⩾10 ml/min (CaBlock-I⩾0.0050). No splitting of bubble was observed for Qoil > 16 ml/min. However, it was also found that the proposed design strategy of splitting distributor for parallel micro-channels yields an almost constant relative lengths of bubbles/slugs (flow uniformity) in all the blocks for Qoil⩾8 ml/min.

NUMERICAL INVESTIGATION OF LIQUID CARRYOVER IN T-JUNCTION WITH DIFFERENT DIAMETER RATIOS

T-junction is commonly used in distributing two-phase flow in a piping system, especially in oil and gas industries. In offshore Malaysia, T-junction is installed at the production header as a compact separator to tap produced gas from reservoir as fuel gas for power generation. However, the splitting nature of two-phase flow with signficant differential density at the junction is a major challenge. Excessive liquid carryover in T-junction present a serious operational issue because it trips the whole production platform. The primary objective of the present study is to investigate the liquid carryover due to formation of slug, subsequently its separation eciency at different diameter ratio. The analysis was carried out on a model with 0.0254 m (1 inch) diameter horizontal main arm and a vertically upward side arm using Volume of Fluid Method from Fluent 16.1. Three different side to main arm diameter ratio of 1.0, 0.5 and 0.3 were investigated with different gas and liquid supercial velocities. The inlet boundary is a prescribed mass flow rate and atmospheric pressure is assumed at the two outlets of the T-junction. Pressure velocity coupling was achieved by using the SIMPLE coupling scheme. The results showed that, when the flow regime in the main arm is slug flow, a combination of high liquid supeficial velocity and very small diameter ratio leads to higher liquid holdup frequency or liquid carryover.
 Keywords: T-junction; liquid-gas separation; separation efficiency

Unsupported shock wave induced dynamic fragmentation of matrix in lead with surface grooves

The material damage and fragments features under certain shock waveform are of great significance in the shock compression phenomena. Herein, molecular dynamics simulations were performed to investigate the dynamic fragmentation process under unsupported shock of bulk Pb with sinusoidal surface grooves and the corresponding clusters distribution during the disintegration. Significant subsurface damage was observed when compared with the typical spike-bubble structure under supported shocks. Due to periodic surface grooves, the superposition of the intrinsic decaying shock wave and the reflected waves resulted in the generation of strong tension stress beneath free surface. Consequently, accompanying the evolution of micro-jets, the subsurface area Pb experienced multiple fracture processes, characterized by the lateral stretching damage and the formation of slugs. In the following process, the slugs fragmented in a similar way as the jets, with the volume distribution of clusters changing from power law to exponential function. Thus, the relationship of characteristic volume from steady-state clusters between jets and slugs could be predicted by their strain rate difference based on the Grady’s energy principle.

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.

Numerical simulation of two-phase flow regime in horizontal pipeline and its validation

PurposeThe purpose of this paper is to investigate oil-gas slug formation in horizontal straight pipe and its associated pressure gradient, slug liquid holdup and slug frequency.Design/methodology/approachThe abrupt change in gas/liquid velocities, which causes transition of flow patterns, was analyzed using incompressible volume of fluid method to capture the dynamic gas-liquid interface. The validity of present model and its methodology was validated using Baker’s flow regime chart for 3.15 inches diameter horizontal pipe and with existing experimental data to ensure its correctness.FindingsThe present paper proposes simplified correlations for liquid holdup and slug frequency by comparison with numerous existing models. The paper also identified correlations that can be used in operational oil and gas industry and several outlier models that may not be applicable.Research limitations/implicationsThe correlation may be limited to the range of material properties used in this paper.Practical implicationsNumerically derived liquid holdup and holdup frequency agreed reasonably with the experimentally derived correlations.Social implicationsThe models could be used to design pipeline and piping systems for oil and gas production.Originality/valueThe paper simulated all the seven flow regimes with superior results compared to existing methodology. New correlations derived numerically are compared to published experimental correlations to understand the difference between models.

Abstract 4509: SATB2/β-catenin/TCF-LEF pathway induces cellular transformation by generating cancer stem cells in colorectal cancer

Abstract Recent studies have demonstrated the involvement of colorectal cancer (CRC) stem cells (CSC) in transformation, cancer progression and metastasis. The main goal of this paper was to examine the molecular mechanisms by which SATB2 induced malignant transformation of colorectal epithelial cells. SATB2 induced malignant transformation and these transformed cells gained the characteristics of CSCs by expressing stem cell markers (CD44, CD133, LGR5 and DCLK1) and transcription factors (c-Myc, Nanog and Sox2). Overexpression of SATB2 in normal colorectal epithelial cells increased cell motility, migration and invasion, which were associated with an increase in N-cadherin and Zeb1, and decrease in E-cadherin expression. SATB2 overexpression also upregulated XIAP and cyclin D1, suggesting its role in cell survival and cell cycle. Furthermore, the expression of SATB2 was positively correlated with β-catenin expression in CRC. In contrary, depletion of SATB2 inhibited cell proliferation, colony formation, cell motility and expression of β-catenin, Snail, Slug, Zeb1 and N-cadherin, and upregulated E-cadherin. Furthermore, SATB2 silencing inhibited the expression of stem cell markers, pluripotency maintaining transcription factors, cell cycle and cell proliferation/survival genes and TCF/LEF targets. Finally, β-catenin/TCF-LEF pathway mediated the biological effects of SATB2 in CSCs. These studies support the role of SATB2/β-catenin/TCF-LEF pathway in transformation and carcinogenesis. Citation Format: Yiming Ma, Wei Yu, Sharmila Shankar, Rakesh K. Srivastava. SATB2/β-catenin/TCF-LEF pathway induces cellular transformation by generating cancer stem cells in colorectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4509.

Slug formation mechanism for air–water system in T-junction microchannel: a numerical investigation

In this paper, numerical investigation of slug formation mechanism of gas–liquid two-phase flow in a T-junction microchannel has been carried out. A two-dimensional (2D) model for the microchannel was developed using ANSYS Academic research CFD 18.2 software package, and volume of fluid method was adopted to solve the model numerically. The results obtained are in good agreement with the experimental results. In this work, slug length, pressure drop and velocity variations inside the slugs were captured at several operating conditions. The effects of contact angle (0°–155°), fluid viscosity, and surface tension on two phase flow interaction parameters along with the effect of both gas and liquid superficial velocities are discussed in detail. For gas–liquid two phase flow velocities were ranging from 0.025 to 0.5 m s−1 and capillary number (Ca) from 6.96 × 10−4 to 1.39 × 10−2. One of the key objectives of this investigation was to study the occurrence and behaviour of liquid film around the slug units. In fact, liquid film formed at low capillary number (Ca) was very thin and observed only for fine meshing near wall of the channel.

Quantifying and reducing powder shear sensitivity when manufacturing capsules with lubricants

The purpose of this work was to develop a methodology that quantifies the extent of shear induced during an encapsulation process and show how formulation composition and manufacturing process designs can be changed to reduce the negative impact on drug product quality attributes. The powder feed system used in a dosing disc type pharmaceutical capsule filling machine induced additional shear of the powder prior to slug formation. The shear occurred both in the hopper portion, via the rotation of the feed auger and impeller, and in the powder bowl via the tamping pin agitation and/or shear against the stationary surfaces such as the powder level scraper. The extent of shear was quantified to assess the impact of further dispersing the hydrophobic lubricant, magnesium stearate, in both active and placebo formulations. Stratified samples over the course of the encapsulation run showed suppression in the drug dissolution profiles and decrease in the interparticulate tensile strength of the encapsulated product. The amount of shear (duration and rate) induced during the encapsulation unit operation can be much greater than that from typical bin blending operations and therefore requires consideration during product design and scale-up to ensure product robustness.

Experience coating bubbles with solvent in a downcomer

A downcomer, as employed in the Jameson Cell, was considered for producing solvent-coated bubbles for possible application in a technique referred to as “air-assisted solvent extraction” (AASX). It was found that the bubble bed in the downcomer started to collapse upon introducing solvent, evident from decreasing air holdup, decreasing aspirated air rate, and formation of air slugs. Neither changing solvent introduction method, solvent composition nor addition of frother or salt alleviated the collapse, which could become complete (air rate became zero) at about 900 ppm solvent. To explain, the bubble bed, which in the downcomer can approach 60% air holdup (Atkinson et al., 1993; Jameson and Manlapig, 1991), was likened to a foam that is known to collapse in the presence of hydrophobic liquid (oil) droplets due to a bridging/spreading mechanism that expels water between contacting bubbles. Tests with hydrophobic solids, graphite and talc, gave increased bubble bed stability showing it is the liquid nature of the solvent that is the key factor causing the collapse. One combination, talc and MIBC, did provide stability against solvent addition. Froth collapse/stability mechanisms are discussed. It is noted that the Jameson Cell is successfully employed to remove solvent droplets in SX plants. The solvent levels in those cases appear to be less than 200 ppm where collapse is only partial and the phenomenon may not be noticed.

A slug capturing method in unconventional scenarios: The 5ESCARGOTS code applied to non-Newtonian fluids, high viscous oils and complex geometries

Abstract Previous work showed that a one-dimensional, hyperbolic, transient five-equation two-fluid model can predict automatically the formation, growth, and subsequent development of slugs in horizontal and near-horizontal flow. This method was implemented in a finite volume numerical scheme – called 5ESCARGOTS code. Comparison with experimental data showed that it can be used to predict the flow pattern and statistical characteristics (slug velocity, length, and frequency). However, the capabilities of this approach have been tested only for water-air flows in a straight horizontal pipe. In this work, we validate the application of the code to some unconventional problems. Firstly, we test the possibility of slug capturing approach to describe and predict the relevant features of air/high viscosity oils or air/non-Newtonian fluids flows. Comparisons between some slug characteristics and empirical correlations, available in literature, are discussed. Then, we move from simple geometries toward more complex conditions that may be representative of actual application cases, also employing high viscous oils as liquid phase. Comparison against experimental data shows results in reasonable agreement.

Numerical Investigation of Liquid Carryover in T-Junction with Different Diameter Ratios

In offshore Malaysia, T-junction is installed at the production header as a compact separator to tap produced gas from reservoir as fuel gas for power generation. However, excessive liquid carryover in T-junction presents a serious operational issue because it trips the whole production platform. The primary objective of present study is to numerically investigate the liquid carryover due to formation of slug, subsequently its liquid carryover at different diameter ratio. The analyses were carried out on a model with 0.0254 m (1 inch) diameter horizontal main arm and a vertically upward side arm using Volume of Fluid Method. Three different sides to main arm diameter ratio of 1.0, 0.5 and 0.3 were investigated with different gas and liquid superficial velocities. The results showed that, while the general trend is true that smaller diameter ratio T-junction has lesser liquid take off capacity, it has a very high frequency of low liquid carryover threshold. In other words, under slug flow, smaller diameter ratio T-junction is constantly transporting liquid even though at a lesser volume in comparison to regular T-junction.

Numerical validation of gas-liquid slug flow inside horizontal pipe

The transition of one flow regime into another is a very complex phenomena in pipeline networks, which could be potentially hazardous for the structural integrity of the pipeline. The horizontal air-water two-phase slug flow regimes in the experimental pipe was modeled using Fluent 16.1. The diameter of the pipe and pipe length were 74 mm and 8 m respectively. The transient calculation were accomplished using Volume of Fluid (VOF) model. A comparison between numerical simulations with the hi-speed photographs of slug formation and translational velocities showed a good agreement. The present work was part of a quality control and assurance process to ensure correctness of the methodology and numerical model before deployment

Modeling of complex liquid-solid flow of particle swelling in slurry loop reactors

A swelling-dependent two-fluid model (STFM) is developed for the liquid-solid flows of swelling particles in polyethylene reactors. The model integrates the two-fluid model (TFM) with a species transport equation (STE) to account for the diffusion of alkane molecules from the liquid bulk to the amorphous region of particles, and a population balance equation (PBE) to consider the aggregation of swelling particles. Simulations show that only the TFM fails to capture the main features of swelling systems. By contrast, the STFM captures the gradual increase of power consumption due to particle swelling and aggregation, which agrees with the experiments in a stirred tank. The STFM predicts also the slug formation and a sharp increase of power consumption in a slurry loop reactor as well as the solid accumulation behind pump. The difference of model prediction for stirred tanks and loop reactors suggests the potential of reactor optimization by enhancing local mixing while still keeping high solid concentration for productivity.

Simplified 1D Incompressible Two-Fluid Model with Artificial Diffusion for Slug Flow Capturing in Horizontal and Nearly Horizontal Pipes

This article proposes a numerical resolution of a one-dimensional (1D), transient, simplified two-fluid model regularized with an artificial diffusion term for modeling stratified, wavy and slug flow in horizontal and nearly horizontal pipes. Artificial diffusion is introduced to prevent the unbounded growth of instabilities where the 1D two-fluid model is ill-posed. We propose a method to set the artificial diffusion case by case to obtain the desired cut-off at short wavelengths by combining the choice of the spatial discretisation and the amplification factors obtained by the linear stability analysis of the model. A proper criterion to simulate two-phase to single-phase flow transition, which occurs during slug formation, is also developed. Flow pattern transitions have been numerically computed and compared against theoretical transition boundaries and experimental observations. Moreover, we showed that the developed code computes slug initiation and slug characteristics, in a reasonably accurate way considering the simplicity of the model, comparing numerical results with well-known empirical correlations and experimental data. Furthermore, the model simplicity leads to a computationally-inexpensive numerical resolution; this can be useful in engineering applications where obtaining fast numerical results is fundamental, such as applications involving automated control for two-phase flows.