Slug Frequency Research Articles

Flow Regime, Slug Frequency and Wavelet Analysis of Air/Newtonian and Air/non-Newtonian Two-Phase Flow

This study focused on gas/Newtonian and gas/non-Newtonian two-phase horizontal fluid flow behavior by analyzing their flow regime identification and flow structural analysis on a horizontal flow loop apparatus. This involved the recognition of two-phase flow regimes for this flow loop and validation with existing flow maps in the literature. In addition, the study included flow pattern identification via wavelet analysis for gas/Newtonian and gas/non-Newtonian two-phase fluid flow in a horizontal flow loop apparatus. Furthermore, the study was extended to the detailed examination of slug frequency in the presence of air/Newtonian and air/non-Newtonian fluid flow, and the predicted slug frequency model was applied to the studied systems. The obtained results suggest that the flow regime maps and slug frequency analysis have a significant impact. The obtained pressure sensor results indicate that the experimental setup could not provide high-frequency and high-resolution data; nevertheless, wavelet decomposition and wavelet norm entropy were calculated. It offered recognizable flow characteristics for bubble, bubble-elongated bubble, and slug flow patterns. Therefore, this study can provide deep insight into intricate multiphase flow patterns, and the wavelet could potentially be applied for flow analysis in oil and gas pipelines.

Investigation of fluidized bed behaviour using electrical capacitance tomography

AbstractThe temporal and cross‐sectional distributions of particles in a 127 mm diameter fluidized bed have been obtained using a new generation, high‐speed electrical capacitance tomography. Two planes of eight electrodes were used and mounted at 160 and 660 mm from the gas distributor which was a 3 mm thick porous plastic plate (maximum pore size of 50‐70 μm). 3 mm diameter, nearly‐spherical polyethylene granules made up the bed. Experiments at sampling frequencies of 200‐2000 cross‐sections per second and gas superficial velocities from just below the minimum fluidization to 83% above minimum fluidization velocities were used. The time series of the cross‐sectional average void fractions have been examined both directly and in amplitude and frequency space. The last two used probability density functions and power spectral densities. The information gathered shows that the fluidized bed was operating in the slugging mode, which is not surprising given the size of the particles. It has been found that an increase in the excess gas velocity above the minimum fluidization velocity resulted in an increase in the mean void fraction, an increase in the length and velocity of the slug bubbles as well as the bed height, and a slight decrease in the slug frequency. The results are presented in a level of detail suitable for comparison with later numerical simulation.

Investigation of the Effect of Fluids Superficial Velocity on Initiation and Development of Slug Flow in a Horizontal Hydraulics Pipe

The aim of this study is to investigate the effect of fluids superficial velocity on the characteristics of two-phase slug flow in a horizontal hydraulics pipe covering slug flow initiation and development. Air and water were employed as the working fluids. Slug flow characteristics were visually observed at 25D and 200D in a 19 mm internal diameter of acrylic pipe for the initiation and fully developed slug flow conditions, respectively. Flow visualization was taken using high speed video cameras at 400 fps and then analyzed using image processing method. Based on the flow pattern characteristics, several slug flow initiation mechanisms were obtained. Under the investigated conditions, at low to medium air superficial velocity (JG) with low to high water superficial velocity (JL), slug flow initiation mechanisms consisted of wave growth, hydraulic jump, blockage, and slug decaying. While wave initiation, wave formation, disturbance, wave coalescence, and incompletely developed slug mechanisms were observed at medium to high JG with low to high JL. After slug flow was fully developed, it was found that the higher the water superficial velocity resulted in lower void fraction and higher slug frequency. In the meantime, pressure drop and liquid slug velocity increased considerably with the increase of both JG and JL. Maximum pressure drop of 8.35 kPa/m occurred at JL of 2.00 m/s and JG of 3.76 m/s. The highest slug frequency of 6.67 1/s was observed at JL of 2.00 m/s and JG of 2.12 m/s. This information is important for designing fluid flow in horizontal hydraulics pipes and for consideration in predicting the development of the slug flow in order to avoid the negative impact of slug flow in industrial applications.

Study of intermittent flow characteristics experimentally and numerically in a horizontal pipeline

The gas-liquid intermittent flow regime is the dominant flow pattern, which significantly occurs in the oil and gas industries. The intermittent flow is challenging in the hydrocarbon transportation systems and separators operation. The present investigation aims to comprehend the behavior of intermittent flow characteristics using experimental and numerical methods simultaneously. Superficial velocities of water and air were set in ranges of (0.18–0.84 m/s) and (1.22–4.9 m/s), respectively. The research was conducted in a horizontal pipe with a diameter of 44 mm and 147D long to study slug frequency, slug length, translational slug velocity, pressure drops, as well as slug's initiation location using static pressure data and high-speed camera tools. In this regard, the volume of fluid (VoF) method and SST k-ω model were also applied to specify film thickness, maximum Taylor bubble velocity, and to grasp behaviors of the flow field in more detail. The results revealed that liquid superficial velocity has more effects on slug frequencies than gas superficial velocity. On the other hand, slug initiation location, as well as slug fully developed length, is affected by the gas superficial velocity more than liquid superficial velocity. Moreover, both the gas and liquid superficial velocities have the same effects on translational slug velocities. The obtained experimental results were also employed to define a distinctive inlet boundary condition in the numerical simulation, which leads to a 65 percent reduction in the computational domain.

The effect of surfactants on the initiation and development of air–water slug flow in hilly terrain pipeline

In hilly terrain pipelines, gas–liquid slug flow usually forms in the low-lying elbow and develops in the inclined ascending pipe at a low superficial liquid velocity and influences the effective and safe natural gas transportation. The surfactant additive has a significant effect on this flow and promotes the liquid drainage in hilly terrain pipelines. To further understand the interaction mechanism between surfactants and gas–liquid flow, an experimental investigation of the initiation and development of slug flow with and without surfactants was performed. The acrylic test tubes consisted of an inclined descending pipe, a horizontal pipe and an inclined ascending pipe with a 50 mm ID and inclination angles of ±10°. The 100 parts per million (ppm) of sodium dodecyl sulfate (SDS) and 100 ppm of sodium dodecyl benzoyl sulfate (SDBS) were respectively used as surfactant additives in this work. The flow behavior of slug initiation and development was observed by a visual system and measured using multiple 2 MHz ultrasonic transducers in the elbow and inclined pipeline. The flow morphology of slug flow changed significantly with the foam formation in the air–surfactant solutions flow. The slug frequency, the liquid film and velocity variation and the velocity characteristics of slug flow were obtained and analyzed based on the measured echo and velocity profiles. The addition of surfactants weakened the strong gas–liquid interaction at the front of liquid slug and increased the velocity of backflow in the liquid film. The experiment results make the effect of surfactants on the gas–liquid interface and velocity characteristics of slug flow in hilly terrain pipelines more clear.

Experimental study and modelling of developing intermittent slug flow in inclined annular pipelines

Geometric arrangements known as flow patterns are typically observed when there are multiple immiscible fluids flowing through any control volume. The adequate modelling of the flow characteristic is important, especially in applications where flow visualization is not possible due to field constrains, like, for instance, in oil production. Intermittent slug flow occurs frequently in petroleum production systems and there is a lack of understanding of its characteristics in inclined annular geometries. Even though this is a frequent geometry encountered in the production system, there is lack of in-depth experimental studies about it. Moreover, existing models for calculating the slug frequency in vertical annular duct cannot predict the slug frequency in inclined annular duct. In this work, a dedicated experimental apparatus equipped with flow meters and a high-speed camera was built in order to investigate some characteristics of the slug flow in slightly inclined annulus, mainly the drift-flux parameters and the frequency of the slug. It is shown that modified drift-flux models for vertical annular flow can be used to calculate drift-flux parameters in inclined annular flow. In addition, a new model is proposed for calculating the slug frequency with improved accuracy when compared to the models available in the literature. The proposed model is suitable for industrial applications in the design of equipment in which such duct geometry is present.

Real-Time Detection of Slug Velocity in Microchannels.

Microfluidics processes play a central role in the design of portable devices for biological and chemical samples analysis. The bottleneck in this technological evolution is the lack of low cost detection systems and control strategies easily adaptable in different operative conditions, able to guarantee the processes reproducibility and reliability, and suitable for on-chip applications. In this work, a methodology for velocity detection of two-phase flow is presented in microchannels. The approach presented is based on a low-cost optical signals monitoring setup. The slug flow generated by the interaction of two immiscible fluids {air and water} in two microchannels was investigated. To verify the reliability of the detection systems, the flow nonlinearity was enhanced by using curved geometries and microchannel diameter greater than 100 m. The optical signals were analyzed by using an approach in a time domain, to extract the slug velocity, and one in the frequency domain, to compute the slug frequency. It was possible to distinguish the water and air slugs velocity and frequency. A relation between these two parameters was also numerically established. The results obtained represent an important step in the design of non-invasive, low-cost portable systems for micro-flow analysis, in order to prove that the developed methodology was implemented to realize a platform, easy to be integrated in a System-on-a-Chip, for the real-time slug flow velocity detection. The platform performances were successfully validated in different operative conditions.

Onset of slugging fluidization in supercritical water fluidized bed

The supercritical-water fluidized bed (SCWFB) is a reactor for coal and biomass gasification without pollutant emission. We carried out a series of experiments in a SCWFB, and a dual-capacitance probe measurement system was applied to measure the hydrodynamics of slugs, such as the slug frequency, chord length, and rising velocity. Four groups of Geldart-B particles with different mean diameters were fluidized by supercritical water with a system pressure of 20–27MPa and at 410–570°C. The minimum slugging Reynolds number increases logarithmically with Archimedes number and a predicting correlation of the minimum slugging fluidization velocity in the SCWFB is presented: Rems=32908.84lnAr0.55+260376.65−410361.90. The relative error of the above correlation was within ±15% and the averaged relative error was 7.5%. The effect of operating conditions on the minimum slugging fluidization velocity is discussed. This research provides useful guidance for scaling-up design and for determining the optimum range of operating conditions in the SCWFB.

Slug flow identification using ultrasound Doppler velocimetry

An identification method of slug flow using velocity information was developed in this study. Quantitative analysis of two-phase (gas–liquid) flow in horizontal pipes is essential to ensure the stability of, or to accurately estimate the thermal performance of, various engineering applications. The spatial and temporal distribution of liquid velocity was measured using the ultrasound Doppler velocimetry method under various superficial velocity conditions. From the results, we proposed a novel identification method of the slug flow based on two parameters, the maximum velocity and maximum velocity difference ratio. In addition, the ratio of liquid velocities between the slug body and the bubble area is a reliable parameter for separating the slug flow from the plug flow. Finally, we investigated the changes in moving velocity, slug frequency, and slug length along the pipeline under a wide range of superficial velocities. The results show that the liquid and gas superficial velocity have the opposite effect on the slug frequency, except in the downstream region. It was found that the slug body length increased continually along the pipe and the average slug body length in the downstream region is invariant regardless of the gas superficial velocity.

CFD simulation of two-phase gas/non-Newtonian shear-thinning fluid flow in pipes

The multiphase flow has always been a major concern since it is encountered in many industrial processes that are crucial. Gas/non-Newtonian two-phase flow is found in the upstream of the petroleum industry, where slug flow is the most common flow pattern. This flow pattern has complex hydrodynamics which is crucial to study and evaluate. This study assessed the two-phase gas/non-Newtonian fluid flow in different configurations of pipes. The model was developed using computational fluid dynamics with an orthogonal mesh to evaluate the behavior of the slug dynamics in the different configurations. The model volume of fluid was used to estimate and predict the most important parameters: pressure drop, slug frequency, and length. First, the model was validated with experimental data found in literature in a horizontal 9-m long glass pipe, with an inner diameter of 22.8 mm. The validation was made with carboxymethyl cellulose CMC–water solutions as test fluids. Two concentrations (w/w) of CMC were used: 1% and 6%. The overall average relative error of the model, taking into account the three parameters, was 24.9%. With this result, it was proceeded to evaluate the model and the effect in the slug flow in three different pipe trajectories: toe-down well, one undulation with a hump well and one undulation with a sump well. The comparison was made between results of a gas/Newtonian fluid flow and the gas/non-Newtonian fluid flow. It was found that the slug frequency and length vary in great form. The slug frequency increased in almost all the cases, and the slug length decreased.

On gas-liquid intermittent flow in a horizontal pipe: Influence of sub-regime on slug frequency

Among the intrinsic parameters identified in intermittent gas-liquid flows; slug frequency is known as the most complicated one to model. The present work concerns the investigation of slug frequency for various sub-regimes which might be found in intermittent flow. Experiments, near atmospheric pressure conditions, were carried out in a 13-meter long horizontal pipe with a 30 mm inside diameter. The study involved plug regime, less aerated slug and highly aerated slug flows. Comparison between different methods of slug frequency quantification, from pressure drop signal, was performed with counting, Wilkens & Thomas as well as the Power Spectral Density (PSD) methods. PSD was found to be the best method for slug frequency measurements. A new flow map, using dimensionless numbers as coordinates for the sub-regimes distinction, was established. After performing substantial comparisons between the present results and models available in the literature a new correlation is proposed including the sub-regime types.

Assessment of well trajectory effect on slug flow parameters using CFD tools

Oil and gas industry uses different well trajectories to enhance reservoir production rates. However, two-phase flow characterization can significantly change due to the well configuration. This study assesses this effect on slug flow development using computational fluid dynamics. An orthogonal mesh was used to assure well modeling of the slug flow dynamics. Slug flow parameters such as frequency and length were predicted using the VOF model. Four well trajectories were analyzed: toe-down, toe-up, and one-undulation with a hump or sump. Results revealed a vertical–lateral sections interaction, which increases slug length and decreases slug frequency on the horizontal pipeline. Moreover, using a toe-down well over an only-horizontal trajectory causes an increment in the slug length up to two times. Finally, a severe slugging condition was predicted in the toe-up well configuration.

Holdup and frequency characteristics of slug flow in concentric and fully eccentric annuli pipes

Slug liquid holdup and frequency for gas-liquid flows in horizontal and upward low-inclination annuli are investigated. The annulus consists of an outer pipe of 99 mm inside diameter and an inner pipe of 50 mm outside diameter. Two annulus configurations are studied, namely, concentric and fully eccentric with the inner pipe at the bottom of the outer pipe. The test fluids are water and Exxsol D60 as the liquid phases and sulphur hexafluoride (SF6) as the gas phase. The slug characteristics are obtained from time-traces of the cross-sectional average liquid holdup from broad-beam gamma densitometers. Results show that the eccentricity of the annulus has limited effect on the slug liquid holdup and slug frequency. For a given superficial liquid velocity, the slug frequency has a parabolic profile as function of the mixture velocity with a minimum value at UM ≈ 3 m/s. Once UM is greater than 3 m/s, the slug frequency sharply increases. Experimental results are compared with predictions calculated using existing correlations and models developed for full pipe systems in the literature. These models fail to capture the slug characteristics in annulus configurations. One of the existing models for predictions of slug liquid holdup has been modified in this paper to account for the effect of the annulus geometry. In addition, the paper documents a new correlation development for slug frequency using the Taylor bubble (or translational velocity) and the slug liquid holdup in an annulus geometry. The proposed models, for both slug liquid holdup and frequency, perform better than the existing models in literature when tested against experimentally obtained annulus flow data.

Slug flow characterization in horizontal annulus

AbstractTo characterize slug flows in annuli channels and highlight the effect of the eccentricity on the flow behaviors, experiments were conducted in two horizontal annuli setups (a) concentric and (b) fully eccentric using air and water as the testing fluids. The range of air and water superficial velocities investigated were 0.45–3.49 m/s and 0.15–2.77 m/s, respectively. Slug parameters measured using conductance probes designed for this study include slug length, translational velocity, slug frequency, and slug holdup. It is found that the slug translational velocity is unaffected by the annulus eccentricity; however, parameters including slug frequency, slug holdup, and slug lengths have a higher value in the fully eccentric annulus when compared with the concentric one. We introduced a new definition of hydraulic diameter, which reconciles the correlation between the dimensionless mean slug length and the mixture velocity of the horizontal annuli with different setups.

Gas–liquid slug flow in a horizontal concentric annulus, a comparison of numerical simulations and experimental data

Multiphase flow simulations were run in OpenFOAM utilizing InterFoam, a volume of fluid type solver. A concentric annulus geometry was constructed and periodic boundary conditions were applied to alleviate the requirement for a longer domain. The simulations were run in 3, 5 and 7 m long domains with the annulus dimensioned such that the outer and inner cylinder diameter were 0.1 and 0.05 m respectively. The 4 individual mesh designs were constructed such that the coarsest mesh consists of 102k cells/m while the finest mesh was generated with 400k cells/m. Each mesh was significantly refined within 0.005 m of both the inner and outer cylinder wall. The simulation data was compared with experimental pressure and holdup data collected at Institute for Energy Technology in Norway. The 3 and 7 m domains reproduced slug frequencies to within 9% of the experiment results of 1.43 Hz for all mesh densities. Comparatively, the 5 m domain has larger errors with respect to slugging frequency (22–27%). The 5 m case performs poorly, probably due to an artificial restraint introduced by the limitation of available liquid which is set as αoil=0.53 for all cases. The αoil restriction combined with the domain length determines the amount of liquid in the system. This interaction of factors means that the domain length is an important parameter when preparing the simulation. The pressure data display a stronger dependence on the mesh quality in comparison to the slug frequency analysis. The 3 m domain with a 400k cells/m mesh resulted in a maximum and minimum pressure gradient of 1783.5 and 803.9 Pa/m, compared to the experiment values of 1785 and 822 Pa/m, which are within 3% of the expected results.

Experimental investigation on non-boiling heat transfer of two-component air-oil and air-water slug flow in horizontal pipes

Gas-liquid slug flow in horizontal pipes occurs frequently in industrial applications and its heat transfer characteristics are very important but complicated due to the intermittent flow structures. This paper aims at investigating the heat transfer mechanism of both air-oil and air-water slug flow in horizontal pipes. An experimental setup was fabricated to investigate the flow and heat transfer characteristics of air-oil and air-water slug flow under cooling conditions in horizontal pipes. The test section was made of a copper pipe with I.D. of 26 mm and a polyvinyl chloride pipe with I.D. of 63 mm using concentric geometry. The effect of flow parameters, such as liquid flow rate, gas flow rate, two-phase pressure drop multiplier, void fraction and slug frequency on heat transfer of gas-liquid slug flow was comprehensively investigated. The heat transfer coefficient of two-phase flow was significantly improved by the introduction of air and the maximum two-phase heat transfer enhancement ratio for air-water and air-oil flow were 150% and 300%, respectively. The circumferential temperature distribution and local heat transfer coefficient were also investigated. The local heat transfer coefficient at the bottom was around 25% higher than that at the top due to the uneven distribution of liquid and gas inside the pipe. Finally, a semi-empirical heat transfer correlation for gas-liquid slug flow was developed based on Reynolds and Chilton–Colburn analogies. The newly developed correlation predicts 96% of the present experimental two-phase heat transfer data within ±20% error and the mean relative deviations of the correlations are estimated to be -2.02% and +2.85% for Ref ≤ 2300 and Ref > 2300, respectively. This research helps researchers and engineers to better understand the heat transfer mechanism of gas-liquid slug flow.

Intermittent gas-liquid two-phase flow in helically coiled tubes

This paper experimentally investigated the air-water intermittent flow in four helically coiled tubes with inner diameter of 0.016 m, coil diameters of 0.24, 0.43 and 0.80 m, and inclination angles of 5° and 15°. A double conductivity probe was used to measure the hydrodynamic parameters at 48 points in the outlet cross section. The results showed that centrifugal force and Dean Vortices were the major mechanisms that influenced the void fraction distribution, elongated bubble length, slug length and slug frequency. Because of centrifugal force, the void fraction profile rotated an angle which approached to 90° as liquid superficial velocity increased and coil diameter decreased. Due to the Dean Vortices, the elongated bubble length and slug length decreased as the coil diameter decreased, leading to an increase of slug frequency. A dimensionless parameter Z representing the ratio of the centrifugal force to the gravity was presented, which provides a quantitative criterion to evaluate the influence of centrifugal force. In terms of Z, correlations predicting the deviation angle, elongated bubble length and slug frequency were built, which met the experimental data well when the coil diameter is between 0.24 and 0.80 m and the inclination angle between 5° and 15°. Moreover, a drift flux model for HCTs was presented.

Effect of slug flow frequency on the mechanical stress behavior of pipelines

Slug flow is a prevalent and undesirable multiphase flow regime which occurs in many industrial processes, causing time varying stresses in pipes and supports and consequently causes structural fatigue damage and failure. In this study, a series of experimental tests were conducted to address the effect of the slug frequency on the stresses of structural pipes. The slug frequency was measured using non-intrusive measuring technique through utilizing Phantom 9.1 high speed camera, while the pipe wall strain was captured using bi-axial strain gauges. The effects of the superficial gas and liquid velocities on the slug frequency were investigated. Additionally, the relation between the slug frequency and the induced stresses was examined. Moreover, a Slug Frequency Stress Prediction Model (SFSPM) based on the exerted slug unit forces was developed and validated. The presented results revealed that an excellent correlation with a deviation of 0.7% between the predicted and experimental stresses was achieved.

Experimental analysis of downward liquid-gas slug flow in slightly inclined pipes

Downward gas-liquid slug flow occurs in hilly-terrain pipelines and is frequently observed in oil and gas transportation lines. The onset of this kind of flow owes to instabilities generated by irregular pipe topography. Therefore, a solid understanding on the hydrodynamics of the slug flow is paramount in the design of crude oil production lines as well as in the project of equipment involved in oil and gas operations. The goal of this work is to analyze the mean values of the translational velocity, slug frequency, bubble and slug lengths in ducts with inclination angles from −4° down to −13° on experimental grounds. The analyses were performed at different gas-liquid volumetric flow rates for which the slug flow regime was observed. An existing experimental rig in the NUEM/UTFPR labs was used to collect data. A pair of wire-mesh sensors was used to evaluate the flow structure, and thence void fraction temporal series were obtained. From those temporal series, the experimental data were obtained and comparative analyses could then be made. The mean bubble (C0) and drifty velocity (C∞) coefficients were determined for the translational velocity. The mean bubble and slug lengths were compared for minimum and maximum values. A schematic of the unit cell behavior in downward slug flow is proposed.

Slug generation processes in co-current turbulent-gas/laminar-liquid flows in horizontal channels

We theoretically and computationally investigate the physical processes of slug-flow development in concurrent two-phase turbulent-gas/laminar-liquid flows in horizontal channels. The objective is to understand the fundamental mechanisms governing the initial growth and subsequent nonlinear evolution of interfacial waves, starting from a smooth stratified flow of two fluids with disparity in density and viscosity and ultimately leading to the formation of intermittent slug flow. We numerically simulate the entire slug development by means of a fully coupled immersed flow (FCIF) solver that couples the two disparate flow dynamics through an immersed boundary (IB) method. From the analysis of spatial/temporal interface evolution, we find that slugs develop through three major cascading processes: (I) stratified-to-wavy transition; (II) development and coalescence of long solitary waves; and (III) rapid channel bridging leading to slugging. In Process I, relatively short interfacial waves form on the smooth interface, whose growth is governed by the Orr–Sommerfeld instability. In Process II, interfacial waves evolve into long solitary waves through multiple resonant and near-resonant wave–wave interactions. From instability analysis of periodic solitary waves, we show that these waves are unstable to their subharmonic disturbances and grow in amplitude and primary wavelength through wave coalescence. The interfacial forcing from the turbulent gas–laminar liquid interactions significantly precipitates the growth of instability of solitary waves and enhances coalescence of solitary waves. In Process III, we show by an asymptotic analysis that interfacial waves achieve multiple-exponential growth right before bridging the channel, consistent with observations in existing experiments. The present study provides important insights for effective modelling of slug-flow dynamics and the prediction of slug frequency and length, important for design and operation of (heavy-oil/gas) pipelines and production facilities.