Liquid Slug Length Research Articles

Experimental investigation on the flow-induced vibration of a free-hanging flexible riser by internal unstable hydrodynamic slug flow

Unstable hydrodynamic slug flow is extensively encountered in marine risers, resulting in internal flow-induced vibration, which is quite different with the vortex-induced vibration caused by a cross flow. Experiments were performed in an air-water test loop to study the internal slug flow-induced vibration of a free-hanging flexible riser with aspect ratio (the ratio of the length to the diameter of a riser) of 158. A non-intrusive technique of high speed imaging method was employed to record the vibration displacements of the riser model and the flow regime of two-phase mixture in the riser simultaneously. The results show that an in-plane vibration is created by the internal slug flow, and the responses along the two directions in the plane are interdependent to each other. In the considered gas-liquid ratio (the ratio of air flow rate to water flow rate), the second-order mode dominates the response. The hydrodynamic behaviors of slug flow, including the gas-liquid ratio, liquid slug length and superficial velocity, affect the local pressure fluctuation and hence influence the vibration amplitude. The fluid-structure interaction is reflected from the same frequencies presenting in the vibration and pressure fluctuation.

Experimental Study on the Velocities, Length and Frequency of Liquid Slug in Horizontal Curved Tube

In order to explore the characteristics of slug flow in horizontal curved tube, four kinds of curved tubes are researched in this paper of which are with 0.5m circumference, 26mm inner diameter. When the superficial liquid velocity or the superficial gas velocity increases, the probability distribution of the average velocity, average length and average frequency of liquid slug conform to the Gaussian distribution at all of the experimental conditions. The average velocity, average length and average frequency of liquid slug increase with the increase of superficial gas velocity. In addition, as the radius of curvature of the pipeline increases, the average velocity, average length, and average frequency of the fluid plug increase.

Three-phase gas-oil-water flow in undulating pipeline

Three-phase gas-oil-water (G-O-W) is common in wells and pipelines producing from mature reservoirs, imposing serious flow assurance problems. Therefore, prediction of flow behavior and characteristics is critical for optimal and sustainable production. In this study, experimental three-phase G-O-W slug flow in undulating pipelines were acquired and analyzed with respect to average liquid holdup, phase distributions and slug characteristics. The slug characteristics considered in this study include, translational velocity, slug length and frequency along the system. The objective of this study is to experimentally investigate the slug flow characteristics to improve the current understanding of G-O-W flow behavior in undulating pipelines and the effect of water cut on slug flow characteristics. Further objective is to modify and validate an in-house two-phase flow simulator and commercial transient multiphase flow simulator OLGA™ with the experimental data. This study contributes to the improvement of the existing slug tracking models and the development of new models for the proper design and safe operation of three-phase flow in pipeline systems.Several results have been found in this study, for example, seven three-phase slug flow patterns were identified based on oil-water mixture in the upstream horizontal section of the undulating pipe section. These flow patterns were analyzed and compared with slug dissipation in the downhill section of the undulating pipe section. When these flow patterns were compared with two-phase slug dissipation behavior, no water cut effect was observed. For moderate and high flow rates, slugs with different oil-water mixing status had differences in slug frequencies and lengths. However, the evolution of liquid slug length distributions for 20% and 80% water cuts in the upstream horizontal section and upward inclined section did not show any significant dependence on water cut.Furthermore, the modified in-house two-phase model predicted the slug flow evolution along the undulated pipeline with a reasonable accuracy. OLGA™ predictions of average total liquid and water holdups, slug frequency and length data showed a fair match especially with the high flow-rate conditions. The models validation study shows that the effect of water cut on slug flow evolution in undulating pipelines was limited to the effect on dispersion characteristics.

Characterisation of gas-liquid two-phase flow in minichannels with co-flowing fluid injection inside the channel, part II: gas bubble and liquid slug lengths, film thickness, and void fraction within Taylor flow

Current research proofs the potential of apparatuses containing minichannel flow structures to intensify gas-liquid-solid contacting processes. The excellent heat and mass transfer in these devices as well as a sharp RTD mainly result from the Taylor flow regime. A proper design of corresponding contactors requires precise information on the provided interfacial areas. However, the characterisation of gas-liquid Taylor flow with industrially relevant fluids at elevated pressure and created by capillary injection devices gained little attention so far.This work analyses adiabatic gas-liquid Taylor flow in a square minichannel of 1.0mm hydraulic diameter using water, water-glycerol, or water-ethanol mixtures as liquid phase and hydrogen or nitrogen as gas phase to cover a broad range of material parameters. In the mixing zone located within the flow channel, gas was injected into the co-flowing liquid by so-called capillary injectors with variable inner diameter (0.184, 0.317, 0.490mm).Two different bubble forming mechanisms were identified leading to a complex interaction between physical properties of the fluids, geometrical parameters and the observed gas bubble and liquid slug lengths. According to the Pi-theorem, these lengths were affected by 6 dimensionless groups, namely (uG,s/ uL,s), ReL, WeL, (dIn,CI/ dh), (dOu,CI / dh), and Θ*. Based on more than 370 experimental data, novel correlations to predict gas bubble and liquid slug lengths were developed.

A CFD study of the parameters influencing heat transfer in microchannel slug flow boiling

Boiling heat transfer in microchannels has been investigated extensively in the last two decades. However, most results from independent experimental studies display a substantial disagreement on the influence of the flow conditions and mechanisms on the heat transfer performance. The objective of the present paper is to clarify by means of CFD simulations the effect of the primary flow parameters on the saturated flow boiling heat transfer performance of a slug flow within a horizontal, circular microchannel. The vapor quality, heat flux, mass flux, channel diameter, bubble frequency, and saturation temperature are the parameters under analysis. The numerical model is based on the volume of fluid method included within ANSYS Fluent v. 14.5, which is here augmented by implementing ad-hoc models to calculate the surface tension force, the mass and energy transfer at the interface due to phase change, and to generate a continuous stream of bubbles with an arbitrary frequency. The high fidelity of the numerical framework provides unprecedented insight on the effect of these flow parameters on the bubble dynamics and heat transfer performance, which are tightly interrelated in microchannel slug flow boiling. In principle, the heat transfer performance is enhanced by flow conditions promoting a thin liquid film surrounding the elongated vapor bubbles and by the flow within short liquid slugs between the bubbles. Furthermore, numerous interesting secondary effects driven by the different flow conditions are observed and described according to the underlying flow dynamics: time-dependent interfacial waves on the bubble surface which significantly reduce the local film thickness; the response time of the channel wall temperature to changes of the flow boundary conditions at different channel sizes; the liquid film thickness dependence on the bubble length and on the liquid slug length; the heat transfer dependence on heat flux via the bubble generation frequency. A dimensional analysis is finally conducted in order to identify the nondimensional groups relevant to the microchannel bubble dynamics and heat transfer in the slug flow regime.

Maldistribution susceptibility of monolith reactors: Case study of glucose hydrogenation performance

In this work an ultrafast electron beam X‐ray modality was applied for the first time to characterize the gas–liquid Taylor flow inside each channel of an opaque honeycomb monolith structure ( ) for and . Significant spatial and temporal deviations in the phase holdup as well as in the gas bubble and liquid slug lengths were found. To evaluate the impact of Taylor flow maldistribution on the reactor performance, the data of more than unit cells were used to simulate the reactor productivity in the hydrogenation of glucose. The results verify that a monolith reactor solely designed by using superficial velocities and empirical correlations for gas bubble and liquid slug lengths fails significantly in achieving high product selectivity and the desired conversion. The developed methods are a solid base to design and select proper distributors ensuring the favorable flow configurations for specific chemical processes. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4346–4364, 2016

Experimental investigations on air–water two-phase flow through a minichannel U-bend

Experimental investigations are reported for air–water two-phase flow through a 2.25mm circular minichannel U-bend. Influence of radius of curvature is established by developing two minichannel U-bends with radius of curvature as 6mm and 12mm. T-junction is used as a mixing geometry for air and water. The superficial gas and liquid velocities are set in the range of 0.0419–0.4192m/s. Total five different flow patterns such as plug, slug, bubbly, bubbly–plug and deformed plug are observed. Flow pattern transition maps are developed as a function of radius of curvature. The positional shift of gas bubbles within the curved portion of minichannel U-bend minichannel for bubbly, plug and deformed plug flow is identified. The influence of radius of curvature on hydrodynamics of Taylor bubble flow such as void fraction, liquid holdup, Taylor bubble velocity, Taylor bubble and liquid slug length is investigated. The radius of curvature with other flow parameters is found to have significant effect on two-phase flow patterns, its transition boundaries and hydrodynamics of Taylor bubble flow.

Comparative analysis of gas–liquid flow in T-junction microchannels with different inlet orientations

In this study, a comparative analysis of two-phase flow in T-junction microchannels with different inlet orientations was carried out. Based on computational fluid dynamics and the volume-of-fluid model, bubble size, bubble velocities, and pressure distributions were analyzed. The numerical algorithm was validated with the experimental observations from former literatures; the results show that when the capillary number, Ca, is low, the length of gas and liquid slug in the symmetric T-junction is higher than that in the cross-flow T-junction. The effect of different forces acting on the slugs during the process of bubble formation was studied by investigating the velocity gradient and pressure distribution in the mixing zone. The result shows that the shear stress in cross-flow T-junction is over two times of that in T-junction with two symmetric inlets under the same operating conditions, which indicates that the size of bubble and liquid slug depends on the shear stress at low Ca number.

Two-Phase Slug Flow Characterization Using Artificial Neural Networks

Gas-liquid two-phase flows are present in nature and in different industrial activities alike, such as the chemical, petroleum, and nuclear industries. In this type of flow, the liquid and gas phases assume different spatial configurations inside the pipe, called flow patterns. The mathematical modeling of slug flow comprises from simple steady-state models to more complex models for transient regimes. Those models require closure relationships, e.g., empirical correlations and statistical distributions of characteristic flow parameters. In this paper, a model based on artificial neural networks (ANNs) for predicting the two-phase slug flow behavior is proposed. With this ANN model, the parameters that characterize the flow are extracted from the time series of void fractions obtained experimentally. The variables of interest are superficial velocities of the fluids, liquid slug and gas bubble lengths, and the bubble translational velocity and their standard deviations. The knowledge and understanding of those parameters will improve the characterization of the intermittent slug flows and will also provide information on the development of physical models that describe this phenomenon, such as the unit cell models, the drift flux model and the slug tracking model. In general, the estimation models based on ANNs showed good results compared with reference values obtained experimentally. The results show that the estimation models present a mean square error below 2%. The methodology presented here, combining experimentally obtained void fraction time series and ANN, is an appropriated method to infer flow parameters and thus to support slug flow characterization.

Modeling and Experiments of Severe Slugging in a Riser System

A transient mathematical model based on continuity equations for liquid and gas phases, with a momentum equation for the mixture, was developed, and numerical solutions and simulations corresponding to severe slugging in pipeline-riser system were presented, and the results were compared with the experimental data to verify the mathematical model. In numerical solutions, backward Euler schemes were adopted as predictors and trapezoidal methods were used as correctors. Variable time steps were employed for higher computational efficiency and accuracy in the integration. Experiments of severe slugging characteristics were performed, and the simulation results of the cycle periods and bottom pressure were compared with experimental values. Finally, the calculation results of detailed characteristics were analyzed thoroughly. The results show that the developed mathematical model can accurately predict the cycle time and the detailed characteristics of severe slugging. Under the experimental conditions, the liquid slug length can reach 1.6 times the height of the riser, and the maximum instantaneous gas velocity of outlet is 50 times the inlet gas velocity, and the maximum instantaneous liquid velocity of outlet is 28 times the inlet liquid velocity, having important implications for the hazard assessment of severe slugging.

Slug flow in a vertical narrow rectangular channel – Laminar and turbulent regimes in the main flow and turbulent regime in the wake region of the Taylor bubble

Using a high-speed video-camera system, the hydrodynamics of gas–liquid two-phase slug flow in a vertical narrow rectangular channel with the cross section of 2.2 mm × 43 mm is investigated for laminar and turbulent regimes in the main flow and turbulent regime in the wake region of the Taylor bubble. The Taylor bubble velocity is the function of the liquid slug length ahead of it, and the interaction of Taylor bubbles for turbulent flow in main flow is weaker than that for laminar flow. Thus, two correlations are proposed for laminar and turbulent flows, respectively. The distributions of Taylor bubble velocity, lengths of liquid slug and Taylor bubble are all positively skewed. The lengths of Taylor bubble and the liquid slug decrease with the liquid flow rate. The Taylor bubble length increases with the gas flow rate, whereas the influence of the gas flow rate on the liquid slug length is insignificant.

Vertical upward intermediate scale Taylor flow: Experiments and kinematic closure

The vertical upward Taylor flow regime has been extensively studied at the capillary and large channel scale limits. However, flow behavior at the intermediate scale (5≲Bo≲40, or 6mm≲D≲17mm for ambient gas–water flows) is comparatively poorly characterized. This regime is fundamentally different because classes of forces conventionally associated with either small or large Bond number flows are all relevant. In this investigation, air–water Taylor-flow experiments are conducted in 6.0, 8.0, and 9.5mm diameter tubes. High-speed video data are collected, and automated image analysis algorithms are developed to measure flow parameters including: bubble rise velocity, liquid film thickness, void fraction, and Taylor bubble and liquid slug lengths. New correlations and flow models are developed to predict these parameters at the intermediate scale. Results from this study enable kinematic closure of intermediate scale Taylor flows.

On the leakage flow around gas bubbles in slug flow in a microchannel

The leakage flow is that liquid does not push gas bubbles and leaks through the channel corners. This leakage flow was confirmed by tracking particles moving in the liquid film with a double light path method and was quantified by tracking the gas–liquid interface movement. The results show that leakage flow varies during bubble formation process. The average net leakage flow Qnet‐leak in a bubble formation cycle at T‐junction can be as large as 62.4% of the feeding liquid flow rate, depending on the liquid properties. Qnet‐leak for regular flow at main channel is much smaller, ranging from about 0 to 30% of the feeding liquid flow rate. The difference between the two leakage flows would lead to an increase in liquid slug length after generation. Finally, the effects of parameters such as phase flow rates, surface tension, and viscosity were investigated. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3964–3972, 2015

Characteristics of gas–liquid two-phase flows through a sudden contraction in rectangular microchannels

A series of experiments were conducted to study the effects of contraction ratio and different liquid properties on gas–liquid two-phase flow through sudden contraction in micro-channel. Two rectangular microchannels with different contraction ratio were used. The widths of the larger channels upstream of the contraction were 0.53 or 0.78mm (0.230mm in height), while those of the smaller ones were fixed at 0.270mm (0.230mm in height). Distilled water, ethanol 49wt% aqueous solution, pure ethanol and hydrofluoroether (HFE)-7200 were used as the test liquids, and nitrogen as the test gas. Flow pattern, bubble length, liquid slug length and bubble velocity were determined by analyzing video images of the flows. In addition, pressure profile upstream and downstream from the contraction was also measured for single-phase and two-phase flows. The pressure loss due to the contraction was determined from the pressure profiles. In the analysis, bubble velocity data and the pressure loss data were compared with calculations by some correlations in literatures. The bubble velocity data both upstream and downstream from the contraction were correlated with Kawahara et al.’s correlation (2010) where the correlation was accounted for aspect ratio of the channel. The correlation of the contraction coefficient for single-phase liquid flows was newly developed with Reynolds number and contraction ratio. The two-phase pressure drop data reasonably agreed with a slip flow model combined with the present contraction coefficient correlation and drift flux model based void fraction correlation. Moreover, numerical simulations were performed using the computational fluid dynamics software FLUENT (Fluent Inc.) and the volume of fluid (VOF) model. The present numerical simulation data agreed well with the experimental data for flow pattern, bubble velocity, bubble length, bubble pitch and pressure changes through the contraction.

Experimental investigation of gas–non-Newtonian liquid two-phase flows from T-junction mixer in rectangular microchannel

In this study, gas–non Newtonian liquid two-phase flows in a horizontal rectangular microchannel have been investigated. The rectangular microchannel has a hydraulic diameter of 0.235mm, and a width and depth of 0.24mm and 0.23mm, respectively. A T-junction-type gas–liquid mixer was used to introduce gas and liquid in the channel. Polyacrylamide aqueous solutions with different mass concentration, which exhibits pseudo plastic with viscoelasticity, were used as non-Newtonian liquids, while the nitrogen gas was used as the test gas. The flow pattern, the bubble length, the liquid slug length, the bubble velocity and frictional pressure drop in two-phase flow were measured. Each experimental data are compared to those for nitrogen gas–water two-phase flow. From the comparison, rheological properties of polyacrylamide solutions are found to significantly affect flow parameters measured. In addition, two-phase frictional pressure drop could be correlated by using Lockhart–Martinelli method with a newly developed two-phase friction multiplier as function of void fraction and rheological flow index.

気液スラグ流を伴う並列T字マイクロリアクタのスラグ長さ推定

気液スラグ流を伴う並列T字マイクロリアクタのスラグ長さ推定

Estimation of gas and liquid slug lengths for T-shaped microreactors

Microreactors with gas–liquid slug flow have an advantage over the conventional batch reactors in mass transfer between mutually immiscible fluids because of the internal circulation flow within the liquid slug. To realize stable long-term operation of microreactors with gas–liquid slug flow, the slug lengths in the microreactors have to be monitored and controlled, because they influence mass transfer performance. In this study, an experimental investigation is carried out to analyze the formation of gas–liquid slug flow in a T-shaped microreactor. The experimental result shows that the pressures in gas and liquid feeding tubes connected to the microreactor oscillate periodically along the formation of a pair of gas and liquid slugs. Based on this result, a method for estimating slug lengths from feed pressure measurements is developed. The method is non-invasive and does not affect slug formation or the manner of gas–liquid slug flow. Its effectiveness was assessed through experimental case studies using T-shaped microreactors having different gas–liquid confluence angles and different gas and liquid flow rates. The relative root mean square errors of estimated slug lengths were within 2% for one minute operation. The results clearly show that the proposed method can be applied to the real time monitoring of slug lengths.

Characteristics of air/water slug flow in an intermediate diameter pipe

Experiments have been carried out in a 67mm diameter pipe with air and deionised water in which gas superficial velocities between 0.04 and 0.55m/s were studied. A capacitance technique, employing electrodes mounted on either side of the outside of the acrylic resin pipe, has been used to provide time varying cross-sectionally averaged void fraction. Probability Density Functions of these data has permitted characterisation of the slug flow. Pressure drop measurements were made using differential pressure transducer. The mean void fraction, the void fraction in the slug region, the slug velocity and the dimensionless length of liquid slug all increase with the superficial gas velocity. On the other hand increasing the superficial gas velocity was shown to decrease the fractional slug length, the frequency and the film thickness around the Taylor bubble as well as the pressure gradient. A new version of the drift flux equation for bubble rise velocity is proposed and found to perform well against the present data and that from the literature.

Heat Transfer Characteristics of Liquid-Gas Taylor Flows incorporating Microencapsulated Phase Change Materials

This paper presents an investigation on the heat transfer characteristics associated with liquid-gas Taylor flows in mini channels incorporating microencapsulated phase change materials (MPCM). Taylor flows have been shown to result in heat transfer enhancements due to the fluid recirculation experienced within liquid slugs which is attributable to the alternating liquid slug and gas bubble flow structure. Microencapsulated phase change materials (MPCM) also offer significant potential with increased thermal capacity due to the latent heat required to cause phase change. The primary aim of this work was to examine the overall heat transfer potential associated with combining these two novel liquid cooling technologies. By investigating the local heat transfer characteristics, the augmentation/degradation over single phase liquid cooling was quantified while examining the effects of dimensionless variables, including Reynolds number, liquid slug length and gas void fraction. An experimental test facility was developed which had a heated test section and allowed MPCM-air Taylor flows to be subjected to a constant heat flux boundary condition. Infrared thermography was used to record high resolution experimental wall temperature measurements and determine local heat transfer coefficients from the thermal entrance point. 30.2% mass particle concentration of the MPCM suspension fluid was examined as it provided the maximum latent heat for absorption. Results demonstrate a significant reduction in experimental wall temperatures associated with MPCM-air Taylor flows when compared with the Graetz solution for conventional single phase coolants. Total enhancement in the thermally developed region is observed to be a combination of the individual contributions due to recirculation within the liquid slugs and also absorption of latent heat. Overall, the study highlights the potential heat transfer enhancements that are attainable within heat exchange devices employing MPCM Taylor flows.

Severe Slugging in Air-Water Hybrid Riser System

In the subsea pipeline gathering system, severe slugging flow is prone to occur. Severe slugging flow brings major threat to production and flow assurance in oil and gas industry due to periodical pressure oscillation and large liquid volume. Currently many researchers pay much more attention on L-shaped riser, catenaries, and S-shaped riser; little research has been made on hybrid riser, which is applied in the Africa West and Gulf of Mexico oil fields. Flow characteristics simulation for hybrid riser is made in this paper, using the one-dimensional and quasi-equilibrium model to simulate not only the riser-base pressure, severe slugging period, and the liquid slug length of the whole system but also base-pressure in the flexible pipe section. The calculated results match well with the experiment data. Besides, the influence of flexible pipe to the severe slugging characteristics of hybrid riser system is analyzed, which are significant for the determination of riser structure.