Air-water Slug Flow Research Articles

Influence of pipeline diameters and fluid properties on slug frequency in horizontal pipelines

The accurate prediction of slug frequency is critical for the construction of multiphase flow models and the flow assurance of oil and gas pipelines. In this paper, the slug frequency data are obtained by visualization method in short (65 mm ID, 215 D) and long (46 mm ID, 35957 D) pipelines. The slug frequency of horizontal pipeline is inversely proportional to pipeline diameter (26–300 mm) and proportional to the liquid viscosity (1.01–7500 mPa·s). The quasi-developed distance for the air–water slug flow at VSG ≤ 1.00 m/s is about 1200 D, and the frequency of the quasi-developed slug flow is less affected by the pipeline length. Based on the gas Strouhal number and the liquid viscosity ratio, the quantitative relation between the slug frequency data at different viscosities is constructed, and a new correlation is developed to predict both the slug frequency of water and viscous liquids.

Experimental characterization of the dispersed bubbles in the slug of an air–water slug flow in a vertical pipe

Experimental characterization of the dispersed bubbles in the slug of an air–water slug flow in a vertical pipe

Experimental study on sub-regimes of air-water slug flow in a rectangular micro-channel

In mini/micro-channel two-phase flow, slug flow occurs easily because of the significant influence of surface tension and the low gravitational effect on the flow. Although an irregularity of slug flow has been reported, previous studies on mini/micro-channel slug flow have mainly concerned the slug flow with the alternate of film and liquid slug zones regularly. Therefore, the present study investigates sub-regimes of micro-channel slug flow with regular or irregular alternating film and liquid slug zones to understand their principal characteristics. Experiments are performed to visualize and simultaneously measure the transient local pressure of the air-water mixture flow in a horizontal rectangular channel with a 2 mm height and 1 mm width. Superficial Reynolds numbers for liquid and gas phases range from 56 to 1116 and 5.75 to 245, respectively. With the aid of visualization data, slug flow is categorized into two types of stable slug flow and four types of unstable slug flow. Present slug flow data is compared with previous correlations for predicting the regime of bubble formation mechanism, flow regime boundaries, and frictional pressure gradient. Measured temporal pressure records are analyzed using a fast Fourier transform and digital Butterworth filter, while a dominant frequency is identified from the normalized average power spectrum. Then, the local pressure fluctuation at the dominant frequency is examined for different types of slug flow using the visualization results. A morphological repetition is confirmed to be associated with the pressure fluctuation: the repeated film and liquid slug zones in both types of stable slug flow, the alternative between liquid-only flow and a group of gas bubbles and liquid slugs in type 3 unstable slug flow, and the periodic variation of bubble length in type 4 unstable slug flow. However, no correlation between morphological characteristics and pressure fluctuation is found in type 1 and 2 unstable slug flows.

Liquid holdup modeling analysis in horizontal gas–liquid slug flow

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

Void fraction measurement and calculation model of vertical upward co-current air–water slug flow

The focus of this paper is on the measurement and calculation model of void fraction for the vertical upward co-current air–water slug flow in a circular tube of 15 mm inner diameter. High-speed photography and optical probes were utilized, with water superficial velocity ranging from 0.089 to 0.65 m·s −1 and gas superficial velocity ranging from 0.049 to 0.65 m·s −1 . A new void fraction model based on the local parameters was proposed, disposing the slug flow as a combination of Taylor bubbles and liquid slugs. In the Taylor bubble region, correction factors of liquid film thickness C δ and nose shape C Z* were proposed to calculate α TB . In the liquid slug region, the radial void fraction distribution profiles were obtained to calculate α LS , by employing the image processing technique based on supervised machine learning. Results showed that the void fraction proportion in Taylor bubbles occupied crucial contribution to the overall void fraction. Multiple types of void fraction predictive correlations were assessed using the present data. The performance of the Schmidt model was optimal, while some models for slug flow performed not outstanding. Additionally, a predictive correlation was correlated between the central local void fraction and the cross-sectional averaged void fraction, as a straightforward form of the void fraction calculation model. The predictive correlation showed a good agreement with the present experimental data, as well as the data of Olerni et al. , indicating that the new model was effective and applicable under the slug flow conditions.

Sub-Regimes of Air-Water Slug Flow Characteristics in Horizontal Pipes by Liquid Hold-up Model Correlated to Bubble Behaviours (LHmBb)

Sub-regimes of air-water slug flow in horizontal pipes have been characterized by a liquid hold-up model correlated to bubble behaviors (LHmBb). The LHmBB is composed of two sections which are the liquid hold-up model and bubble behavior investigations. The liquid hold-up model is developed based on the statistical analysis of the probability density function (PDF) to quantify the bubble distribution. The bubble behaviors are qualitatively investigated based on the high-speed camera and correlated to the quantified LH for the characterization of sub-regime of air-water slug flow. The LHmBB characterizes the sub-regime of air-water slug flow in a horizontal transparent acrylic pipe with an inner diameter of underwater and air superficial velocities of and, respectively. As a result, four sub-regimes are determined as Initially dispersed Bubbles (IdB), Low dispersed Bubbles (LdB), High dispersed Bubbles (HdB), and Dominantly dispersed Bubbles (DdB). The presence of bubbles determines the type of sub-regime by decreasing the number of bubbles and dispersing the bubbles mechanism. Moreover, the proposed LHmBb includes the correlation function to ease the prediction of the sub-regimes of air-water slug flow characteristics which leads to the enhancements of the two-phase flow pattern map in horizontal pipes.

Numerical investigation and prediction of phase separation in diverging T-junction

Purpose In plethora of petroleum, chemical and heat transfer applications, T-junction is often used to partially separate gas from other fluids, to reduce work burden on other separating equipment. The abundance of liquid carryovers from the T-junction side arm is the cause of production downtime in terms of frequent tripping of downstream equipment train. Literature review revealed that regular and reduced T-junctions either have high peak liquid carryovers (PLCs) or the liquid appears early in the side arm [liquid carryover threshold (LCT)]. The purpose of this study is to harvest the useful features of regular and reduced T-junction and analyze diverging T-junction having upstream and downstream pipes. Design/methodology/approach Volume of fluid as a multiphase model, available in ANSYS Fluent, was used to simulate air–water slug flow in five diverging T-junctions for eight distinct velocity ratios. PLCs and LCT were chosen as key performance indices. Findings The results indicated that T (0.5–1) and (0.8–1) performed better as low liquid carryovers and high LCT were achieved having separation efficiencies of 96% and 94.5%, respectively. These two diverging T-junctions had significantly lower PLCs and high LCT when compared to other three T-junctions. Results showed that the sudden reduction in the side arm diameter results in high liquid carryovers and lower LCT. Low water and air superficial velocities tend to have low PLC and high LCT. Research limitations/implications This study involved working fluids air and water but applies to other types of fluids as well. Practical implications The novel T-junction design introduced in this study has significantly higher LCT and lower PLC. This is an indication of higher phase separation performance as compared to other types of T-junctions. Because of lower liquid take-offs, there will be less frequent downstream equipment tripping resulting in lower maintenance costs. Empirical correlations presented in this study can predict fraction of gas and liquid in the side arm without having to repeat the experiment. Social implications Maintenance costs and production downtime can be significantly reduced with the implication of diverging T-junction design. Originality/value The presented study revealed that the diameter ratio has a significant impact on PLC and LCT. It can be concluded that novel T-junction designs, T2 and T3, achieved high phase separation; therefore, it is favorable to use in the industry. Furthermore, a few limitations in terms of diameter ratio are also discussed in detail.

Experimental investigation on characteristic parameters of air–water slug flow in a vertical tube

Experimental investigation on vertical upward co-current air–water slug flow was carried out at atmospheric pressure, employing optical probes and high-speed photography. Experiments were performed with water superficial velocity ranging from 0.089 to 1.52 m/s and gas superficial velocity ranging from 0.049 to 1.06 m/s in a polymethyl methacrylate tube of 15 mm inner diameter. Taking advantage of the time-domain characteristics of optical probes, slug flow has been disposed as a combination of Taylor bubbles and liquid slugs. Slug flow identification, characteristic parameters measurement and analysis were conducted in the present study. Firstly, combined with subjective observation, a developed methodology of flow pattern identification based on machine learning was proposed, with emphasis on slug flow. Results showed that the proposed method exhibited an outstanding advantage in the identification in the flow pattern transition region. More importantly, extensive data on local void fraction in liquid slugs (αl,LS) and in continuous slug units (αl), gas-phase velocity of Taylor bubble (UTB) and liquid slug (ULS), length of Taylor bubble (LTB) and liquid slug (LLS), were obtained. Through comprehensive analysis, it was found that turbulence intensity and flow pattern transitions were the dominant factors in affecting the characteristics of slug flow parameters.

Experimental investigation on gas-liquid two-phase flow distribution characteristics in parallel multiple channels

The maldistribution in the parallel multiple channels commonly leads to a reduction in the efficiency of heat exchangers and even causes the equipment to suffer the risk of failure. The distribution of gas–liquid two-phase slug flow when moving through the parallel multiple channels is still an unresolved problem and limited by the lack of experimental data. This work presents an experimental study on the air–water two-phase flow in the parallel multiple channels characterized by a horizontal header and 2~4 vertically-upward branches under the slug flow pattern conditions and develops a new predictive model. The effects of inlet superficial velocities and branch number have been investigated in the ranges of inlet gas and liquid superficial velocities of 1.4~25.0 and 0.2~1.7 m·s−1, respectively. Results show that the distribution characteristics of two-phase slug flow highly depend on the inlet superficial velocities which manifest in that the peak value of liquid phase flow ratio shifts to the downstream branches sequentially as the inlet superficial velocities increase. Decreasing the branch number is beneficial for the uniform distribution of each phase. A predictive model is proposed for the distribution of air–water slug flow by considering the effects of superficial velocities and branch number. The new model achieves the agreement of 91.3% for prediction distribution within the range of ±25% when compares to present experimental data and shows good agreement with previously reported experimental results of both T-junction and parallel multiple channels. The acquired experimental data and predictive model are crucial for the development and optimal design of the heat exchangers.

Pressure Drop of Horizontal Air–Water Slug Flow in Different Configurations of Corrugated Pipes

Abstract Corrugated pipes (CP) have regularly shaped and spaced cavities on their internal walls that can induce dynamic changes in the flow, such as increased pressure drops. Offshore petroleum production pipelines are an example of an industrial application of CPs, known as flexible lines. Slug flow is the most challenging flow pattern in those lines due to its complex hydrodynamics. A number of previous studies proposed correlations to predict the two-phase flow pressure drops in smooth pipes (SPs). However, limited researches have evaluated the pressure drops associated with liquid–gas slug flow in CPs. In this work, experiments to analyze the pressure drops in horizontal air–water slug flow under different configurations of CPs were carried out. The tests were performed in three different CP internal diameters (IDs) (26, 40, and 50 mm) with different cavity widths (1.2, 1.6, and 2.0 mm). The effects of the internal diameters and the cavity widths on the pressure drops associated with slug flow were analyzed. Results demonstrated that the pressure drops increase with increasing cavity widths. The experimental data were fitted and a pressure drop correlation using the concept of multiplier factor was proposed. Comparisons between predictions and the experimental data proved to be within ±10% accuracy.

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.

Thermal diffusion response to gas–liquid slug flow and its application in measurement

A novel on-line, contactless and nearly non-intrusive measurement method based on thermal diffusion is proposed for gas–liquid slug flow in this paper. Temperature fluctuations of a pipe wall heated at a constant heat flux are induced by Taylor bubbles and liquid slugs due to their different heat exchange capacities. The slug flow characteristics, including velocity, length, and frequency, in the vertical pipe, can be obtained through temperature fluctuation detection. Correlations between wall temperature fluctuations and slug flow characteristics are developed. Through experimental tests on air–water slug flow in a 25 mm diameter pipe at the atmospheric pressure of 1 bar and room temperature of approximately 20 °C, the average relative errors in average velocity for the Taylor bubbles and liquid slugs are 1.84% and 2.28% with maximum values of 4.67% and 5.79%, respectively. The average relative errors in average length are 4.65% and 3.68% with maximum values of 12.75% and 10.48%, respectively, under the conditions of 0.16–0.65 m/s of gas superficial velocity, 0.22–0.49 m/s of liquid velocity, and 0.34–2.96 of gas–liquid ratio.

Numerical investigation of drop dynamics in presence of wettability gradient inside a serpentine channel of proton exchange membrane fuel cell

Water management significantly affects the performance of a proton exchange membrane fuel cell (PEMFC). Therefore, interest is felt to numerically investigate water droplet movement and slug formation inside the microchannel (gas) of PEMFC. Two important parameters—water coverage ratio and pressure drop have been studied in detail. A U-shaped geometry with a round corner is used for this purpose. 3D unsteady-state models are used to study the drop dynamics using commercial CFD software ANSYS FLUENT 18. For tracking of water drop dynamics, the volume of fluid model is used. Two different situations are simulated. In the first case, the investigation of hydrodynamics of the 0.4 mm drop adhered to the surface of the gas diffusion layer (GDL) has been done. In the second case, simulation of air-water slug flow has been done. GDL surfaces at upstream and downstream of bend are modified using user-defined functions, such that the GDL surface has a dynamic contact angle with respect to the direction of flow. This makes it a continuously hydrophilic surface at upstream and continuously hydrophobic surface at downstream with respect to the direction of flow. The impact of GDL wettability on water retention and removal has been discussed. It is noted that the presence of a gradient facilitates the removal of water drop adhered to the GDL surface. For the case of a suspended drop with an increase of 1°/mm in the magnitude of the gradient, a decrease of 30% is observed in water coverage ratio and pressure drop observed in the channel. Such modified surfaces aids in the conversion of slugs to film at the downstream of bend that reduces maldistribution. The pressure fluctuations and average pressure drop are reduced by 66% when subjected to the aforementioned hybrid gradient.

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.

Gradual contraction of pipe cross-section effects on transient behavior of air–water slug flow

The current experimental investigation was aimed at understanding the influence of reducing sections on the transient behavior of slug flow. Experiments were carried out using two concentric area cross-section reducers with 160 mm lengths; the first one was shaped as a conical frustum with a 44 mm inlet diameter reducing to a 30 mm outlet diameter and the second one as a frustum with the same inlet diameter converting to a rectangular cross-section (34 mm × 21 mm). Four different water superficial velocities (0.43, 0.55, 0.72 & 0.84 m s−1) and three different air superficial velocities (2.5, 3.6 & 4.9 m s−1) were applied in the course of the experiments. Visualization studies, as well as frequency distribution analyses, were used to observe flow field changes. The visualization study showed that the gas slug tail, liquid slug, and gas slug subsequent to passing through the reducers converted to non-homogeneous bubbly flow, several small slugs, and pseudo-slugs, respectively. The obtained frequency distributions, which were calculated using a Fourier transform tool, showed that unlike that of the dominant frequency, the power of other frequencies was amplified in the low-frequency range owing to the high formation rate of pseudo-slugs and small slugs. This change indicates that intermittent air–water slug flow with a specified dominant frequency subsequent to passing reducers varies to an uncertain flow with semi-uniform distribution in the low-frequency range. Estimation of Shannon entropy also showed that entropy increased by almost five times downstream of the reducers, which confirms that energy is distributed uniformly in a broad frequency bandwidth. Analyses of the essential bandwidth showed that the reducing section acted as a frequency distribution modifier whose performance decreased with liquid flow rate increments. Based on this achievement, it was found that both the circular and the rectangular reducers were able to generate the named conditions.

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.

Flow disturbances induced by an orifice plate in a horizontal air-water flow in the slug regime

An experimental study of the flow disturbances induced by an orifice plate in an air-water slug flow is carried on a horizontal test section with 0.026 m and 1009 pipe diameters long. Three orifices with distinct area contraction ratios are tested at three axial positions along the test section. The instantaneous orifice pressure difference as well as the absolute pressure and void fraction along the test section are measured. The data is further processed to get the bubble nose velocities and the slug frequency. The pressure difference at the orifice and the absolute pressure along the test section reveal a pulsatile pressure behavior due the intermittent passage liquid slugs and elongated bubble through the orifice. The spectral analyses disclose a match between the frequency of passage of slugs through the orifice and the frequency of the orifice pressure fluctuations. The orifice disturbances on the void fraction, bubble nose velocity and frequency of passage exhibits a dependency on the orifice size and to the distance from the mixer.

The development of image processing technique to study the interfacial behavior of air-water slug two-phase flow in horizontal pipes

The aim of the present paper is to propose an image processing technique in order to study the interfacial behavior of air-water slug two-phase flow in horizontal pipes. Here, the experiments were performed in two sets of transparent acrylic pipes with inner pipe diameters of 16mm and 50mm. To observe the interfacial behavior of slug flow, the liquid and gas superficial velocities were set to 0.2–0.77m/s and 0.7–2.83m/s, respectively. Visualization studies by using high speed video camera combined with image processing techniques were since it offered numerous advantages. Moreover, a sequential procedure was implemented for processing of each image. This technique has succeeded to enhance the image contrast and remove the uneven background. Next, a number of steps applied to binary image were also performed in order to remove the dispersed bubble attached on bubble tail which becomes one of the biggest challenges for the visualization studies of slug flow. As a result, the developed image processing technique was able to minimize the errors in order to produce the accurate measurement. Moreover, the processed image have been utilized to gather the quantitative information. It reveals the comprehensive characteristics of slug flow in horizontal pipes. Finally, a good agreement between the present and previous works proves the reliability of the developed technique.

Experimental investigation of a vertically downward two-phase air-water slug flow

The present experimental work investigates the vertical downward two-phase slug flow in a transparent vertical pipe of 34 mm internal diameter. The gas and liquid superficial velocities are in the range of 0–3.27 m/s and 0.015–1.4 m/s respectively. The cross-sectional averaged void fraction has been measured at seven positions along the test section by the intermediary of the conductance probe technique.The results of the development of the void fraction time-series and Probability Density Function along the tube are presented. A good accordance was found between the experimental data and the line transitions of the flow pattern map proposed by (Usui, 1989). We also present results of the structure velocity of the slug flow, where the distribution parameter is found to be lower than unity. A new correlation for the prediction of slug frequencies is also proposed. It agrees well with the data collected in this study with ±10% error.

Experimental analysis of horizontal liquid-gas slug flow pressure drop in d-type corrugated pipes

Slug flows are commonly found in oil and gas offshore production operations, where pressure drop estimation is essential for facilities design. Several studies have been made to evaluate pressure drop for slug flows in smooth pipes. However, many offshore oil production lines consist of corrugated pipes, and the drag induced by the discrete cavities is rarely investigated for slug flows. For this purpose, an experimental flowloop was designed to measure pressure drop of air-water slug flows in 26mm-ID corrugated pipes with four different groove geometries, consistent with the groove configurations used in offshore petroleum exploitation. The experimental data is regressed using the concept of pressure drop multipliers. Whilst this procedure is well established in literature for smooth pipes, an additional multiplier associated to the effect of the corrugated pipe on the pressure drop is proposed in this work. With such approach, a correlation for the pressure drop in horizontal liquid-gas slug flows in corrugated pipes is obtained by means of the groove geometry and the flow input data. Comparison of the proposed model predictions with experimental data shows ±7% accuracy.