Slug Frequency Research Articles

An Intermittent Flow Structure in Airlift Pump by Using an Annular Venturi Injector

An annular venturi injector (AVI) was proposed to form an intermittent flow structure in airlift pump for a good pump performance. Experiments were conducted to investigate the performance of the airlift pump with this AVI by comparing with pump performance with traditional injectors, at a series of air flow rates . It was found that airlift pump with AVI had higher flow rates of output liquid and particle, than those with the traditional injectors. This AVI promoted the gas core to collapse and formed an intermittent flow structure in rising pipe. For this intermittent structure, its slug length, firstly increased to a maximal value with increasing gas flow rate and then remained stable even under a high gas flow rate, while its slug frequency decreased with gas flow rate and then remained to a minimal value under a high gas flow rate.

Empirical Modelization of Intermittent Gas/Liquid Flow Hydrodynamic Parameters: The Importance of Distinguishing between Plug and Slug Flows

SummaryIntermittent flow is one of the most complex flow regimes in horizontal pipes. Various studies have classified this regime as two distinct subregimes: plug and slug flow. This classification has been made based on flow observations. In this work, the behavior of several flow parameters that characterize plug and slug flow are presented. Data from eight published works in the open literature were collected and studied to explain the behavior of both regimes. These data include pressure drop, void fraction, and slug frequency, as well as the lengths of liquid slugs and elongated bubbles for slug and plug regimes.It is observed from the evolution and analysis of these parameters that plug and slug flows have several different distinct features and should be considered as two separate regimes for the empirical modelization of the hydrodynamic parameters. The mixture Froude number, and to a lesser extent the liquid superficial velocity to gas superficial velocity ratio, seem to have significant impacts on the plug-to-slug flow transition.

Experimental investigation of sudden expansion's influence on the hydrodynamic behavior of different sub-regimes of intermittent flow

Sub-regimes of intermittent two-phase flows through a sudden expansion have been investigated experimentally. Two pipes of 30 and 40 mm ID connected by a sudden expansion with aspect ratio σ = 0.562 were used. The study was concerned by Plug, Less Aerated Slug (LAS) and Highly Aerated Slug (HAS) flows. The flow visualization showed that the liquid slugs, for Plug and LAS flows, were broken after crossing the singularity which was not the case for HAS flow. However, in general the sub-regime observed on either side of the expansion was the same. Qualitative and quantitative analysis, by means of flow visualizations and standard deviation of pressure drop signals, showed that the flow presents less fluctuations downstream the singularity. Furthermore, the persistence of slug frequency depends on the nature of the sub-regime.

Investigations on the slug two-phase flow in horizontal pipes: Past, presents, and future directives

Precise prediction of the gas–liquid slug flow is crucial for proper design and operation at various industrial processes. This paper provides a comprehensive review of published experimental and numerical works in horizontal pipes, including a critical analysis of state-of-the-art parameters used to characterize the slug flow regime. The discussed slug characteristics are slug length, slug frequency, and slug velocity. Throughout the paper, an endeavor does devote to discerning the areas where modern development work is needed. Accordingly, some observed gaps in the literature, comprising hydrodynamic behavior at the transition zone and the interaction between slug flow and structural pipe. The lack of comprehensive laboratory experimental information for high liquid viscosity and the viscosity impact on the slug characteristic is also observed. On the contrary, robust and less empirically independent CFD models are required for accurately predicting the slug characteristics with a minimal computational cost.

Flow and heat transfer for a two-phase slug flow in horizontal pipes: A mechanistic model

In view of the practical importance of gas–liquid two-phase flow in many applications, such as chemical engineering, petroleum engineering, nuclear engineering, etc., a reliable model of flow and heat transfer for two-phase flow is of practical importance in the two-phase flow analysis. Among various two-phase flow regimes, slug flow is most complicated due to the intrinsic randomness and intermittency. This paper aims at developing a novel mechanistic model of flow and heat transfer for two-phase slug flow in horizontal pipes. First, a hydrodynamic model of two-phase slug flow is developed using the concept of slug unit cell. Then, a heat transfer model is deduced based on the hydrodynamic model. The overall heat transfer coefficient is integrated by the local heat transfer coefficients of liquid slug, liquid film, and elongated bubble. The newly developed mechanistic model is well validated by the experimental results. Finally, the dependence of the heat transfer performance on the overall flow parameters, such as superficial liquid velocity and superficial gas velocity, and the local flow parameters, such as slug frequency, pressure drop, void fraction, and ratio of slug length to unit cell length, is comprehensively investigated. The heat transfer enhancement of two-phase slug flow compared with single-phase flow is mainly attributed to the turbulence increase in liquid by the injection of air and the decrease in thermal boundary layer by the frequent alternation between the liquid slug and the elongated bubble.

Two-phase co-current flow simulations using periodic boundary conditions in horizontal, 4, 10 and 90° inclined eccentric annulus, flow prediction using a modified interFoam solver and comparison with experimental results

Two-phase oil and gas flow were simulated in an entirely eccentric annulus and compared with experimental data at horizontal, 4, 10, and 90° inclination. The gas-phase was sulphur hexafluoride and the liquid phase a mixture of Exxsol D60 and Marcol 82 for the inclined cases (5–16), and pure Exxsol D60 for the horizontal cases (1–4). The diameter of the outer and inner cylinders was 0.1 and 0.04 m, respectively, for the inclined domains and 0.1 and 0.05 m for the horizontal domain. The cases studied consist of liquid phase fractions between 0.3 and 0.65 and mixture velocities from 1.2 to 4.25 m/s. The mean pressure gradient is within 33% of the expected experimental behavior for all inclined cases. In contrast, the low-velocity horizontal domains exhibit significant deviation, with a drastic over-prediction of the mean pressure gradient by as much as 200–335% for cases 1 and 2. The two remaining horizontal cases (3 and 4) are within 22% of the expected mean pressure gradient. Cases 13–16 are a replication of cases 5–8 at an increased inclination; the mean pressure gradient is within 6.5% of the expected increase due to the increase in hydrostatic pressure. By comparing cases 1–4 to previous published simulations at a lower eccentricity, we found a decrease of the mean pressure gradient by 30–40%, which is in line with existing literature, although for single-phase flows. The simulated and experimental liquid holdup profiles are in good agreement when comparing the fractional data; wave and slug frequencies match to within 0.5 Hz; however, at closer inspection, it is apparent that there is a decrease in the amount of phase-mixing of the simulations compared to the experiments. When increasing the mesh density from 115 k cells/m to 2 million cells/m, the simulations exhibit significantly more phase mixing, but are still unable to produce conventional slugs. In a simplified case, conventional slugs are observed at grid sizing of 1 × 1 × 1 mm, whereas the cells of the 2 million cells/m mesh are roughly 1.5 × 1.5 × 1.5 mm.

Flow induced vibration of two-phase flow passing through orifices under slug pattern conditions

Flow restricting orifices are important components for flow and pressure control in the oil, gas, and power generation industries. In many situations, the piping systems are operating under two-phase flow conditions. These orifices affect the redistribution of the two phases and induce vibrations in the piping structures. These vibrations often occur during a slug flow pattern and can severely affect the integrity of a structure; therefore, in the present work, both the vibrations of the piping structure and the characteristics of the two-phase flow dynamics are experimentally investigated in the presence of an orifice. In this study, an air–water​ two-phase flow mixture was utilized at different superficial velocities ranging from 0.12 m/s to 1.7 m/s for air and 0.46 m/s to 1.3 m/s for water. Synchronized flow visualization images, along with local void fraction and vibration measurements, were used to evaluate the response of the structure as the slug flow passed through the orifices. Detailed instantaneous local void fraction measurements were carried out to understand the underlying physics of the phenomenon and their effect on the vibration response of the structure. The results showed that not all slug flow cases excited the piping at the slug frequency. The amplitude of vibration around the slug frequency was found to increase as the superficial velocity of the gas phase increased.

The Numerical modeling of multiphase flow in the oil and gas gathering pipeline from wellhead platform of X field to central processing platform of Y field

The main objective of this research is to investigate the characteristics of slug flow inside the riser which is a 68-meter vertical pipe segment reaching from the seabed to the central processing platform of the Y field. The oil and gas mixture flows into the riser from the wellhead platform of the X field through a 25-km horizontal subsea pipeline. In this study, an integrated modeling approach is used to take into account major physical phenomena associated with the multiphase flow in the gathering system that might have strong influences on slug flow characteristics. The phase behavior model of oil and gas in the gathering pipeline were first built on the basis of Peng-Robinson equation of state to determine the multiphase equilibria and estimate fluid properties at various pressure and temperature conditions. The multiphase flow was then modeled by Beggs & Brill method for pressure drops and Hasan & Kabir for temperature distribution along the pipeline. In particular, the pressure drop calculation is based on empirical correlations from which the flow regimes can be identified and pressure drop is determined accordingly. The heat transfer calculation, on the other hand, is based on the mechanistic approach from which the temperature distribution along the pipeline system can be estimated. Finally, the slug-tracking model was developed to characterize the slug flow with essential properties such as slug frequency, length and surge volume. This helps identify slug flow existence inside the riser segment and predict potential consequences it may cause to surface facilities. The results from this work show that the integrated modeling approach is suitable to the multiphysics nature of the flow assurance problem under consideration. The slug flow might exist in the 68-m riser where more than 20 slugs of 2-m length might occur after every 1.5 hours.

Onset of intermittent flow: Visualization of flow structures

The transition from stratified to intermittent air-water two-phase flow was investigated experimentally, by flow visualization and pressure drop signals analyses, in a 30 mm ID pipe. The intermittent flow’s onset was found to be mainly dependent on the liquid superficial velocity and the pipe diameter. Plug flow, Less Aerated Slug (LAS) or Highly Aerated Slug (HAS) flows could be obtained on the gas superficial velocity grounds. The available models, compared to experiments, could not predict adequately the intermittent flow onset. The appearance of liquid slugs was revealed by peaks in the pressure drop signal. Furthermore, it was shown that the available slug frequency correlations were not valid in the zone of the onset of intermittent flow.

Neural Network Model To Predict Slug Frequency of Low-Viscosity Two-Phase Flow

Summary In this work, the artificial neural network (ANN) is implemented for prediction slug frequency FS for low viscosity μL flow (μL ≤ 1.6 mPa·s) in vertical, horizontal, and inclined pipes. To the authors' knowledge, no ANN model in the literature exists for predicting FS. The input parameters for the suggested ANN are superficial liquid velocity VSL, pipe diameter D, superficial gas velocity VSG, and pipe inclination ϕ. Measured data (450 data points) are gathered from five different resources for developing the ANN model. The ranges of FS, VSL, D, VSG, and ϕ covered by the data set, are (0.03 to 3.167 1/s), (0.05 to 2.073 m/s), (0.01905 to 0.0779 m), (0.133 to 11.84 m/s), and (0 to 90°), respectively. The most popular transfer functions of tangent sigmoid and linear are used in the hidden and output layers, respectively, whereas the Levenberg and Marquardt back propagation algorithm is conducted to train ANN. The experimental data set is divided into 70% for training, 15% for validation, and 15% for testing processes. Due to the absence of a systematic way to find the optimal structure of ANN, an exhaustive search method has been suggested and implemented to find the optimal topology, which is (4-16-1); four neurons in input layer, 16 neurons in the hidden layer, and one neuron in output layer. The proposed ANN predicts correctly the effect of each of the previously mentioned parameters on FS, and it yields a satisfactory prediction and clearly outperforms all the existing models, with a mean square error (MSE) and R2 of 0.0097 and 0.977, respectively.

The extension of the one‐dimensional two‐fluid slug capturing method to simulate slug flow in vertical pipes

AbstractWhile many studies have been conducted on the application of the one‐dimensional (1D) two‐fluid model in horizontal and nearly horizontal pipes under slug flow condition, very few works have been carried out on the mechanistic simulation of the slug flow regime in vertical pipe configurations.This article presents the extension of a novel numerical scheme for slug capturing in pipes in vertical slug flows. The original numerical scheme includes an 1D transient, hyperbolic five‐equation two‐fluid model, and it has been embodied in a computer code called 5ESCARGOTS (5 Equations for Slug Capturing Analysis by a Roe solver in Gas and Oil Transient Simulation).This article demonstrates that the model is able to properly automatically capture and simulate the slug generation even in vertical flow. It shows that when appropriate closure relations are used for interfacial shear forces, the model is able to properly characterize the vertical slug flow.Numerous simulations have been carried out for different pipe configurations and flow conditions leading to slug flow. First, a qualitative discussion of the numerical results is proposed underlining that the model is able to properly simulate the evolution of the slug flow in vertical pipes. Experimental flow pattern maps have also been numerically computed and it has been demonstrated that the model correctly simulates slug conditions. Moreover, slug frequency, velocity, and average liquid holdup, computed numerically, have been compared with experimental data in order to quantitatively validate the model. It is shown that the numerical results agree fairly well with the experimental data. A sensitivity analysis on the mesh size and on the inlet boundary conditions has also been carried out.

Statistical characterization of horizontal slug flow using snapshot proper orthogonal decomposition

Slug flow is an intermittent and complex gas-liquid flow pattern, that can cause severe problems in industrial operations and lead to large uncertainties in multiphase flow metering. These problems are primarily caused by the liquid slugs of the flow. In this context, different cases of slug flow in horizontal pipes are investigated to find a statistical characterization of the slugs in time and space. To achieve this, snapshot proper orthogonal decomposition (snapshot POD) with an additional mode coupling algorithm is employed, which provides an energy-ranked mode basis of the underlying coherent structures. It is shown that the characterization for the considered slug flows can be derived from the temporal and spatial information of the dominant mode pair. In that regard, the slug frequencies, the averaged slug body length and an energy representation of the slugs are obtained from this method. Furthermore, the accuracy of this characterization is shown and the influence of the size of the observed pipe section on its reliability is investigated.

Slug frequency for a gas-liquid plug flow: Review and development of a new correlation

Slug frequency is one of the most important parameters for designing industrial installations where intermittent gas-liquid flows could be found. Thus, the literature is found full of various experimental investigations and developed empirical correlations to predict such parameter. Nevertheless, the effect of intermittent flow sub-regimes on slug frequencies remains an open issue, especially in the case of plug flow. Hence, the present work is devoted to providing an update review and analysis of the slug frequency in plug flow case. For this purpose, data from previous experiments have been collected and deeply examined. It was found that the increase in liquid superficial velocity and the decrease in pipe diameter could induce an increase of plug frequency. The comparison between experimental data and the predictions of twenty available empirical models showed that the correlations based on Strouhal number and input liquid fraction are the best ones that predict well the experimental plug frequency. However, each correlation is found to be valid for a limited range close to the data's conditions that were used for its development. As such, and by considering an extended range of the whole available experimental database, a new empirical correlation has been proposed.

Comparative Study of CFD and LedaFlow Models for Riser-Induced Slug Flow

The goal of this study is to compare mainstream Computational Fluid Dynamics (CFD) with the widely used 1D transient model LedaFlow in their ability to predict riser induced slug flow and to determine if it is relevant for the offshore oil and gas industry to consider making the switch from LedaFlow to CFD. Presently, the industry use relatively simple 1D-models, such as LedaFlow, to predict flow patterns in pipelines. The reduction in cost of computational power in recent years have made it relevant to compare the performance of these codes with high fidelity CFD simulations. A laboratory test facility was used to obtain data for pressure and mass flow rates for the two-phase flow of air and water. A benchmark case of slug flow served for evaluation of the numerical models. A 3D unsteady CFD simulation was performed based on Reynolds-Averaged Navier-Stokes (RANS) formulation and the Volume of Fluid (VOF) model using the open-source CFD code OpenFOAM. Unsteady simulations using the commercial 1D LedaFlow solver were performed using the same boundary conditions and fluid properties as the CFD simulation. Both the CFD and LedaFlow model underpredicted the experimentally determined slug frequency by 22% and 16% respectively. Both models predicted a classical blowout, in which the riser is completely evacuated of water, while only a partial evacuation of the riser was observed experimentally. The CFD model had a runtime of 57 h while the LedaFlow model had a runtime of 13 min. It can be concluded that the prediction capabilities of the CFD and LedaFlow models are similar for riser-induced slug flow while the CFD model is much more computational intensive.

Prediction of Slug Frequency for Medium Liquid Viscosity Two-Phase Flow in Vertical, Horizontal, and Inclined Pipes

Summary Several experimental studies have been conducted to investigate the effect of liquid viscosity on slug frequency in horizontal, vertical, and inclined two-phase flows. Analyses of these studies reveal that the slug frequency is positively related to superficial liquid velocity and liquid viscosity; the superficial gas velocity has a dual minor effect on slug frequency, with an initial increase for low superficial gas velocity and then a decrease for high superficial gas velocity; and the slug frequency increases with increasing flow deviation from horizontal. Also, the analyses reveal that for inclined viscous flow, the slug frequency and slug length follow the same inverse relationship shown in horizontal and vertical slug flows. In the literature, several models have been developed for predicting slug frequency in viscous horizontal flows, whereas only a few models exist for viscous vertical and inclined flows. In this study, we aim to develop models for prediction of slug frequency in two-phase flow of medium liquid viscosity (30 ≤ μL ≤ 250 mPa·s). Dimensional analysis of four published experimental data sets (218 data points) indicates that slug frequency is related to two dimensionless numbers; namely, a modified Froude number and inverse viscosity number. As a result, three slug frequency closure models are proposed for vertical, horizontal, and inclined flows, using a combination of these two numbers. The proposed models are tested against the four data sets, and very good results are obtained, with correlation coefficients ranging from 0.96 to 0.97.

One-way coupled fluid–structure interaction of gas–liquid slug flow in a horizontal pipe: Experiments and simulations

Pipelines conveying a multiphase mixture must withstand the cyclic induced stresses that occur due to the alternating motion of gas pockets and liquid slugs. Few previous studies have considered gas–liquid slug flow and the associated fluid–structure interaction problems. In this study, experimental and numerical techniques were adopted to simulate and analyze the two-phase slug flow and the associated stresses in the pipe structure. In the numerical simulation, a one-way coupled fluid–structure framework was developed to explore the slug flow interaction with a horizontal pipe assembly under various superficial gas and liquid velocities. A modified Volume of Fluid and finite element methods were utilized to model the fluid and structure domains. The file-based coupling technique was adopted to execute the coupling mechanism. By contrast, slug characteristics were measured experimentally, while Bi-axial strain gauges were used to capture time-varying strain signals. Excellent agreements between the predicted and measured stress results were achieved with a maximum error of 10.2 %. It was found that at constant superficial liquid velocity, the maximum induced stresses on the pipe wall increased with increasing the slug length and slug velocity. While for the slug frequency, the maximum principal stresses decreased with increasing the slug frequency.

Statistical features of the flow evolution in horizontal liquid-gas slug flow

The occurrence of the slug flow pattern is frequently detected in the petroleum industry. Theoretical and experimental works on the complex behaviour of this flow pattern are abundantly found, and their aim is to advance the state-of-the-technique on the behaviour of this kind of flow. The goal of the present work is to provide experimental data collected in five probing stations with resistive sensors mounted on a horizontal air–water slug flow in a 25.8-mm ID pipe. A signal processing technique provides the translational velocities, frequencies and void fractions that arise from different gas and liquid volumetric flow rates. Based on the obtained results, the effects of the liquid and gas superficial velocities were correlated with the probability density functions (PDFs) of the parameters variations observed in the slug flow development along the pipe. A quantitative criteria based on maximum likelihood estimates is used to determine the Weibull distribution for the translational velocity and the lognormal distribution type for the slug frequency distribution type.

Slug length and frequency upstream a sudden expansion in gas-liquid intermittent flow

The purpose of this work is to analyze the influence of a sudden expansion on the upstream behavior of the horizontal gas-liquid slug flow. Measurements were made on a 40 mm ID pipe with and without a sudden enlargement of aspect ratio σ = 0.444. The experiments were carried out with two-phase air-water mixture. The slug lengths and frequencies were measured using a non-intrusive video technique. Upstream the sudden enlargement, it was observed that the mixture velocity has no influence on slug length. The variation of slug frequency is found proportional to the liquid superficial velocity for the two cases within this study. It was also observed that the behavior of the slug length and frequency was affected by the presence of the sudden enlargement. The comparison of the results obtained with various empirical correlations available in the literature showed that the latter are not worthwhile in the case where singularity is installed.

Evaluating the two-phase flow development through orifices using a synchronised multi-channel void fraction sensor

Understating two-phase flow through flow restricting orifices is critical in evaluating the piping degradation mechanisms in nuclear power generation systems. Characterizing the instantaneous changes of the two-phase local parameters across flow restricting orifices is critical in evaluating the piping structure dynamics. A multi-channel void fraction sensor was developed to investigate the effect of flow restricting orifices on flow pattern development in a 25.4 mm horizontal pipe. Instantaneous void fraction measurements were obtained at multiple locations upstream and downstream of the flow restricting orifices with area ratios of 0.062, 0.14, 0.25, and 0.56. Additionally, pressure measurements and flow visualizations were carried out to investigate the dynamics flow characteristics through the orifice. A liquid superficial velocity of 0.526, and 1.08 m/s and a gas superficial velocity ranging from 0.164 to 2.795 m/s were selected to represent the intermittent flow pattern. The results revealed that as the gas superficial velocity increased, the flow pattern downstream of the restriction changed to dispersed bubbly, or liquid jet and annular-dispersed liquid for an intermittent flow upstream of the orifice. The flow pattern development along the test section was affected significantly in the region close to the orifice, especially at a lower area ratio. Analysis of statistical characteristics of the slug flow pattern such as slug velocity, elongated bubble length and slug frequency were determined from the void fraction data and presented. The slug velocity upstream of the orifice decreased non-linearly as the area of the flow restriction decreased for the same flow condition upstream. The pressure variations along the pipe for different two-phase flow conditions across the orifices were obtained and different correlations to predict pressure drop across the orifices were evaluated. The Simpson et al. [1] correlation was the best as it predicted 93% of the experimental data with a 25% error.

Fluidization of spherical versus elongated particles - experimental investigation using X-ray tomography

In many industrial applications, particles used in fluidized bed clearly deviate from ideal spheres. This leads to an increasing need for better understanding and developing better simulation models for fluidization of non-spherical particles. So far, the literature is quite scarce when it comes to experimental results which can be used for validation of numerical models. Also, the exact difference in fluidization behavior between spherical and elongated particles in dense fluidizing conditions is not well understood. In this work, we apply X-ray tomography to compare the fluidization behavior of a bed of a Geldart D-type spherical particles of aspect ratio 4 to that of volume equivalent spherocylindrical particles for different gas velocities. Even though the beds of both spherical and elongated particles are operating in the slugging regime, due their size and high bed height to width ratio, we see clear differences in their fluidization behavior. Our results indicate that the bed of elongated particles is slugging less than the one with spherical particles. This is indicated by a lower average bubble size in the case of elongated particles, together with a higher bubble rise velocity. The bed of elongated particles has a considerably higher distribution of small and medium bubbles. The slug waiting time distribution and slug frequency distribution indicate that a bed of elongated particles periodically switches between slugging and turbulent fluidization, unlike the bed of spherical particles which remains in the constant slugging regime.