Slug Frequency Research Articles

Numerical Investigation of Slug Characteristics in a Horizontal Air/Water and Air/Oil Pipe Flow

In the present work, the transition from stratified flow to slug flow regime for air-water and air-oil two-phase flow was simulated and analysed numerically. The simulation was carried out by numerically solving a three dimensional (3D) implicit unsteady volume of fluid (VOF) model. Typical slug characteristics, such as slug length, slug translational velocity, and slug frequency were determined. The numerical results were validated by comparison with experimental results and a reasonable agreement with an error less than 8.7% was achieved. Moreover, the results from the proposed model were compared to the results obtained from three empirical correlations for the two-phase slug flow and it demonstrated a good agreement. The simulation results demonstrated that for the same boundary conditions, the characteristics in terms of slug initiation and slug growth were strongly affected by the fluid properties. The simulation results also show that for air-oil flow, the pressure drop, slug translational velocity, and slug frequency values were less than in air-water flow by 2.9%, 14.3%, and 7.9%, respectively.

Experimental investigation on the initiation and flow development of gas-liquid slug two-phase flow in a horizontal pipe

Slug flow is the most prevalent problem of two-phase fluid transportation in the petroleum industries. Here, large amplitude waves lead to the pipe blockage that possible to cause the damage of the mechanical structure. The experimental investigation of gas-liquid slug two-phase flow in a horizontal pipe was carried out to investigate the initiation and flow development mechanisms. Air and water were used as the working fluids. The inner pipe diameter was 26mm. In the present experimental study, the slug initiation mechanisms were explained by visual observation and pressure fluctuations. Moreover, the initiation frequency of slug flow and the evolution of passing slug frequency along the pipeline were also observed by using two high video cameras.As a result, several basic mechanisms of slug flow initiation in a horizontal pipe were clarified. Those were the wave coalescences, the wave growth mechanism, and the large disturbance waves. A flow initiation map was proposed to address those mechanisms. Here, the slug initiation frequency and the passing slug frequency were obtained in relation to the pipe length position. The static pressure measurement was also used to verify the image processing results on the characteristics of slug initiation mechanisms. The obtained qualitative and quantitative results from the investigation can be used to evaluate the recent available models.

Statistical assessment of experimental observation on the slug body length and slug translational velocity in a horizontal pipe

In the present paper, the flow was examined experimentally using air and water as the working fluids. Various sets of experimental data were conducted by varying the superficial velocities within ranges of 0.7–3.84m/s and 0.7–1.33m/s for air and water, respectively. The slug characteristics were acquired and computed using non-intrusive optical based technique. Statistical analysis for the translational slug velocity and slug body length was conducted. It was observed that for fixed water velocity, the slug length was increasing with increasing the superficial air velocity while the slug frequency decreased. However, for fixed superficial air velocity, the slug length was decreasing with increasing the water velocity whiles the slug frequency increased. Also, it was found that when the superficial water velocity increased the slugs were formed and initiated far downstream from the pipe inlet section. Furthermore, the obtained results were found in a good agreement with the previous empirical correlation.

Experimental study of the hydrodynamic behaviour of slug flow in a horizontal pipe

This paper investigates the unsteady hydrodynamic behaviour of slug flow occurring within an air–silicone oil mixture, within a horizontal 67mm internal diameter pipe. A series of slug flow regime experiments were performed for a range of injected air superficial velocities (0.29–1.4ms−1) and for liquid flows with superficial velocities of between 0.05–0.47ms−1. A pair of Electrical Capacitance Tomography (ECT) probes was used to determine: the slug translational velocities of the elongated bubbles and liquid slugs, the slug frequencies, the lengths of elongated bubbles and the liquid slugs, the void fractions within the elongated bubbles and liquid slugs. The pressure drop experienced along the pipe was measured using a differential pressure transducer cell (DP cell). A comparative analysis of the current experimental data and that previously published experimental confirms good agreement.

Two-phase slug flows in helical pipes: Slug frequency alterations and helicity fluctuations

Air-water numerical simulations in the slug flow regime have been performed in horizontal helical pipes and the effects of geometries on the flow regime have been investigated. Depending on the length of the helix, outlet slug frequencies have been reduced with various degrees of efficiency. Correlations between mean tangential velocity and helicity density fluctuations have been identified and investigated qualitatively. These flow fields show smaller time scales than those obtained in volume fractions fluctuations. Shifts observed in the tangential velocity and mean helicity fluctuations to smaller time scales (high frequencies) are associated with regime transitions. For a slug flow undergoing a continuous transition to the annular flow regime, it is shown that the presence of slower (low frequencies) helicity fluctuations is attributed to the variations in the axial velocity. Finally, the analysis of the helicity at gas-liquid interface confirms the presence of the mixing zone at the slug front.

Measurements of pressure drop and particle velocity in a pseudo 2-D rectangular bed with Geldart Group D particles

A small scale pseudo 2-D bed of dimension 0.075m×0.23m×1.22m was utilized to measure the pressure drop and velocity statistics of Geldart Group D nylon beads. The pressure drop was measured using a fast-response pressure transducer and the particle velocities were measured with high speed particle image velocimetry, with particle tracking, in a slugging flow regime for three different superficial gas velocities (2, 3 & 4 times the minimum fluidization velocity (Umf)). At superficial gas velocity of 3 Umf, the measured slugging frequency was about 2.5Hz, while at conditions close to 2 Umf and 4 Umf, the slugging frequency was about 1.5 and 1.7Hz. Additionally, at 3 Umf condition, the bed exhibited a much higher root mean square (RMS) of pressure drop fluctuations than those at 2 Umf and 4 Umf. The varying trend suggested an existence of a complicated regime change for these shallow bed measurements that had not been previously explored. The measurement of millions of individual particle velocities at the vicinity of the wall had enabled accurate measurement of higher order statistics including RMS, granular temperature, skewness, and kurtosis of particle velocity. The measured skewness and kurtosis of the velocity distribution indicated that the particle velocity distribution deviated substantially from a normal distribution as the intersection of walls of the rectangular bed was approached.

Probabilistic modeling of slug frequency in gas/liquid pipe flow using the Poisson probability theory

Abstract Slug frequency is a critical characteristic of two-phase slug flow in pipes that impacts production system operation, design and flow assurance aspects. For example, slug frequency is not only a required input for mechanistic models to predict pressure gradient and liquid holdup, but also is related to pipeline erosion/corrosion rates, pipeline structural integrity and stability and downstream separation and process facilities sizing. Conversely, slug frequency is the least accurately predicted parameter in two-phase flow, due to the high uncertainty of slug frequency predictive empirical correlations and models. Comprehensive studies by Zabaras (2000) and by Al-Safran (2009) showed that the error in existing slug frequency correlations’ predictions averages around 75% for horizontal flow and 115% for horizontal and inclined flows. Therefore, this study, as oppose to previous studies, aims to develop a probabilistic model to predict slug frequency and quantify the associated probability. The proposed model can predict the P10, P50, and P90 of slug frequency predictions for a given flow condition. These probability values can be propagated in a mechanistic model to predict the expected, low- and high-end values of pressure gradient and liquid holdup that can be used for efficient pipeline and downstream facility design and optimum operation. In addition, these probability values are important in predicting the possible maximum and minimum corrosion/erosion rates to efficiently design corrosion inhibitor program, which has a significant impact on project economic. For transient slug tracking model such as OLGA, application of the proposed slug frequency probabilistic model can be to calculate the Delay Constant Parameter, the time interval between two initiated slugs, which requires a probabilistic distribution to quantify its uncertainty and impact on the slug tracking predictions. Poisson probability theory is proposed in this study to model slug frequency predictions in horizontal pipeline. A Poisson probability model is used because of its suitability to predict the probability associated with events occurring during a specific time interval, such as slug frequency in a pipeline. In addition, a new slug frequency empirical correlation is proposed to predict the mean slug frequency in a horizontal pipeline, which is used in the Poisson probability modeling. Validation study of the slug frequency empirical correlation show a very good results, especially for the tuned model. In addition, the slug frequency Poisson probabilistic modeling predicted the probability associated with a specific value or range of slug frequency, slug frequency confidence interval for a given probability value, and the expected range of slug frequency values under normal operation.

Characterization of two-phase slug flow sub-regimes using flow visualization

Abstract Flow visualisation experiments are carried out to analyse the internal structure of different sub-regimes of slug flow. Five distinct slug flow sub-regimes are identified based on the visual observations. These sub-regimes correspond to slug at formation stage (onset of slug); less aerated slug; highly aerated slug; slug and plug; and slug and wavy. While superficial liquid velocity influences the formation of slug, it is observed that superficial gas velocity has a significant influence on the transition of sub-regimes. Maximum and minimum slug frequencies are observed in highly aerated zone and slug formation zone respectively. Highly aerated slug zone is extremely unstable and exhibits complex flow structure. Depending on superficial gas and liquid velocities, maximum value of slug frequency is observed at a distance between 30D and 60D from the pipe inlet. Slug frequency reduces along the length of the pipe due to viscous damping. An empirical correlation is developed for predicting the non-dimensional slug frequency (product of Strouhal number and Froude number) in terms of non-dimensional superficial liquid and gas velocities (represented by superficial Reynolds number of liquid ( Re SL ) and gas ( Re SG )) and length to diameter ratio ( L / D ). Identification of slug flow sub-regimes and their flow behaviour reported in this paper are of significant interest to oil production and transport processes where control of flow regime transitions is important in order to avoid severe unwanted jigging due to the formation and growth of slugs.

Slug Dynamics in a Nearly Horizontal Pipe Using Non-Invasive Measurement Techniques

This paper examines the effect of gas and liquid superficial velocities on the dynamics of the slug flow in anearly horizontal pipeline. Instead of the invasive measurements the experiments were conducted usingnon-invasive techniques to measure slug frequency and bubble length in the slug flows. A simultaneous PIV-LIFtechnique was employed to measure the instantaneous velocity of both liquid and gas phases simultaneouslyusing two CCD cameras. Effect of gas superficial velocity on the instantaneous measurements were studied andanalyzed.

Two-Fluid Model for 1D Gas-Liquid Slug Flows: Realizable Mean Slug Characteristics

A two-fluid model is used to predict hydrodynamic characteristics of gas water two phase slug flows in horizontal pipes. Mathematical models are averaged based forms of balance laws with algebraic source terms. Predictions of transient solutions of such systems rely on accurate evaluations of speeds and scales of discontinuities generated by the flow conditions. Slug flows are characterized through slug frequencies, slug lengths and slug translational velocities. These are the fundamental quantities that one has to estimate in order to assess the validity of the model and the numerical method in use. A two fluid 1D model for slug flows along with an AUSMDV* numerical scheme is able to predict correct translational speeds, lengths scales and frequencies of slugs. An adaptive mesh refinement AMR procedure based on Kelvin-Helmholtz stability condition as an indicator for refinement is used to speed up simulations. The marginal Kelvin-Helmholtz stability condition is also introduced to estimate slug body length.

Experimental study on the characteristics of air–water two-phase flow in vertical helical rectangular channel

Experimental investigations on the characteristics of air–water two-phase flow in the vertical helical rectangular channel are performed using the high speed flow visualization. The flow pattern map and the transition in the helical rectangular channel are presented. The flow pattern evolution in different positions of the helical rectangular channel is illustrated. The discussion on the coalescence of the bubble and slug is presented. The slug velocity, slug length distribution, liquid slug frequency, falling liquid film velocity and falling film thickness along the slug are investigated. The dimensionless liquid film thickness of the annular flow on the outer side of the channel is measured using the digital image processing algorithm.

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

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

Managing Internal Corrosion of Mild Steel Pipelines in CO2‐Enhanced Oil Recovery Multiphase Flow Conditions

AbstractThe “carbon capture, utilization and storage” project has attracted considerable attention in recent years as the demands for reducing CO2 emissions increased. However, the produced fluids during CO2‐enhanced oil recovery (EOR) production are supersaturated with high‐pressure CO2 gas, which leads to serious corrosion damages of the pipelines. To model the corrosive conditions during CO2–EOR production, a self‐designed multiphase flow loop for internal corrosion evaluation was built. The role of flow patterns and emulsification that decisively influencing pipeline corrosion was systematically investigated. It was found that a higher gas‐to‐liquid ratio could be beneficial for reducing pipeline corrosion, which was attributed to a reduction in slug frequency and an enhancement in the stirring effect on the oil–brine mixtures. It was also found that corrosion can be completely inhibited if the transporting liquids were properly controlled under water‐in‐oil emulsion condition. A model considering the transport parameters that ensures the safety of crude‐oil transportation is proposed. It suggests that corrosion control by designing the transport parameters would be a suitable and economic method for tackling the corrosion damages to CO2–EOR pipelines. These ideas have been accepted for guiding the pipeline engineering in the CO2–EOR section of the Shengli oilfield.

Intermittent flow regime of water/gas emissions as sinkhole trigger in alluvial plains: a study case at S. Vittorino plain (Rieti, Italy)

A case of natural intermittent flow regime (slug/churn) of a CO2-enriched spring water is discussed as an example of trigger process for the collapse of sinkholes. The hydrodynamic regime of the spring, situated in the area of S. Vittorino alluvial plain (Rieti, Italy), was investigated by monitoring acoustic emission, carrying out geochemical analyses and installing a sedimentation trap during a long-lasting phase of slug flow regime followed by a churn flow phase. The slug and churn flows can erode and transport upward fine and medium size particles of sand, progressively removing them from some underground shallow sandy levels where the groundwater mixes with gas of deep crustal origin to form the intermittent flow that can be observed at the surface. The slug flow regime was characterized by a stable slugging frequency whose value is in agreement with the estimated superficial velocities of Taylor bubbles and liquid slugs. The transition from the slug to the churn flow regime, the latter characterized by an increase of gas flux and by more severe erosion, was also observed during our study. In a span of 15 years, the transport of sand upward could produce a collapse volume similar to those of the sinkholes that recently originated in the same area. The study of such multiphase flows prolonged over time could help to monitor and prevent the sinkhole formation process.

Development and statistical characterization of slug in two-phase flow along horizontal pipeline

In many industrial processes, the presence of liquid and gas mixtures creates a slug flow. This kind of regime is observed when slug’s liquid blocks the whole pipeline and moves as a coherent mass downstream at a velocity approximately equal to the gas velocity. The aim of this study is to provide statistical information on slug in two-phase flow in horizontal pipe. Experiments were conducted in a pipe of 0.04 m diameter and a length of 14 m. First of all, a flow regime map is compiled for air/water two phase flows. Data on pressure gradient, slug frequency and liquid holdup are presented. It was found that mean slug frequency clearly increases as the superficial liquid velocity increases but it weakly depends on the superficial gas velocity.

Evaluating the effects of high liquid viscosity and flow variables on horizontal oil–gas slug flows by gamma radiation method

Hydrodynamic slug fl ow is the commonest fl ow regime observed in high viscosity liquid–gas horizontal pipelines over a wide range of different fl ow conditions. Hydrodynamic slugging tends to generate large vibrations that may impose structural instability or even damage oil production pipelines. For that reason, there is a need to investigate high viscosity slug fl ow regime to understand its complex characteristics. This is pertinent when considering that existing slug fl ow models used in the petroleum industry to design production pipelines are not suitable for predicting the behaviour of high viscosity oil–gas fl ow. In this study, the effects of liquid viscosity and fl ow variables on slug fl ow regime were investigated experimentally through the analysis of two key parameters—slug frequency and slug body liquid holdup, both measured with a gamma densitometer. Comparison of the measured slug parameters to existing correlations revealed that slug body liquid holdup correlations were in close agreement with high viscosity experimental data. However, none of the existing slug frequency correlations used was able to produce accurate predictions. A new empirical correlation for slug frequency was proposed. Compared with existing correlations, the newly proposed correlation performed much better in predicting slug frequency of high viscosity liquid–gas fl ows.

Study on Severe Slugging in an S-Shaped Riser: Small-Scale Experiments Compared With Simulations

Summary Severe slugging is a transient multiphase-flow phenomenon that can occur in pipeline-riser systems, particularly in offshore production of oil and gas. It is characterized by large pressure fluctuations at the base of the riser and is accompanied by fluctuations in fluid delivery from the top of the riser. This unstable phenomenon is undesirable because production and equipment are affected adversely by the large pressure and flow-rate fluctuations. In this study, air-/water-flow experiments have been carried out at the S-shaped-riser facility in the multiphase-flow laboratory of the Norwegian University of Science and Technology (NTNU) and have been compared with results from a flow simulator (OLGA). The results obtained in the work show that stability maps, pressure amplitudes, and slug frequencies are in acceptable agreement with each other; however, some deviations are seen regarding the slug frequencies at low flow rates.

Flow patterns and bubble departure fundamental characteristics during flow boiling in microscale channels

This paper presents an experimental investigation into the fundamental characteristics of flow boiling in microscale channels based on diabatic high-speed flow visualizations. Experiments were conducted with R134a and R245fa refrigerants flowing in a 0.40mm circular horizontal channel for mass velocities ranging from 100 to 900kg/m2s and heat fluxes of up to 226kW/m2. Flow images were captured at recording speeds of up to 100,000frames/s. Results for bubble departure diameter and frequency, bubble growth ratio, slug frequency and velocity, flow pattern transitions, characteristics of the liquid film and liquid–vapor interface are provided. The experimental data obtained are carefully analyzed, discussed and compared against previous results for tubes of 1.00 and 2.00mm internal diameter. Predictive methods available in the literature for bubble departure diameter and frequency are evaluated by comparing their predictions against the data obtained in the present study. New bubble departure diameter and frequency correlations are proposed for small channels. The following conclusions can be drawn from the present study: (i) bubbles can detach from the wall with diameters much smaller than the tube diameter; (ii) the bubble growth process has a square root time-dependence; (iii) two different methods for estimating the average surface heat flux based on flow boiling videos have been developed; (iv) bubble active nucleation sites are observed for all flow patterns; (v) buoyancy effects are still present for a 0.40mm tube, and (vi) six different sources of vapor–liquid interface oscillations have been identified.

Experimental study of the hydrodynamic behaviour of slug flow in a vertical riser

This paper presents an investigation of the hydrodynamics of slug flow in a vertical 67mm internal diameter riser. The slug flow regime was generated using a multiphase air–silicone oil mixture over a range of gas (0.42<USG<1.35m/s) and liquid (0.05<USL<0.38m/s) superficial velocities. Electrical capacitance tomography (ECT) was used to determine: the velocities of the Taylor bubbles and liquid slugs, the slug frequencies, the lengths of Taylor bubbles and the liquid slugs, the void fractions within the Taylor bubbles and liquid slugs and the liquid film thicknesses. A differential pressure transducer was used to measure the pressure drops along the length of the riser. It was found that the translational velocity of a Taylor bubble (the structure velocity) was strongly dependent on the mixture superficial velocity. As the gas superficial velocity, was increased, the void fraction and the lengths of the liquid slugs and the Taylor bubbles were observed to increase. The increase in gas superficial velocity causes an increase in the frictional pressure drop within the pipe, whilst the total pressure drop (which is a sum of the hydrostatic and frictional pressure drop) along the length of the riser decreases. In addition, the frequencies of the liquid slugs were observed to increase as the liquid superficial velocity increases, but to be weakly dependant on the gas superficial velocity. The manual counting method for the determination of slug frequency was found to be in good agreement with the power spectral density (PSD) computed method.

Slug behavior and pressure drop of adiabatic slug flow in a narrow rectangular duct under inclined conditions

A visualization-based investigation was carried out on slug behavior and pressure drop of air–water slug flow in a narrow rectangular duct with cross section of 43mm×3.25mm under inclined conditions. The velocity and length of slugs were obtained through image processing. Based on the liquid Reynolds number, slug flow was divided into laminar flow region (Rel<3000) and turbulent flow region (Rel⩾3000). Experimental results showed that both the slug velocity and the slug length increased with the inclination angle increasing in laminar flow region, while they were nearly unvaried in turbulent flow region. The slug length and slug frequency decreased with the gas superficial velocity increasing or the liquid superficial velocity decreasing for all cases. For laminar flow, the predictions of frictional pressure drop by Chisholm model, Mishima–Hibiki correlation and Lee–Lee correlation could be significantly improved by replacing the void fraction with the ratio of the slug velocity divided by the gas superficial velocity, us/jg; for turbulent region, all three models could well predict the experimental data if void fraction is calculated by the Jones–Zuber correlation.