Gas-liquid Slug Flow Research Articles

Experimental Study on the Slugging Characteristics of Gas-Liquid Slug Flow in Horizontal Pipes.

This study investigates the slugging characteristics of the gas–liquid slug flow interface in horizontal pipes. Using air and water as the experimental media, an experimental system was established using double-parallel conductance probes in a pipe with an inner diameter of 5 cm. By capturing the transient development process of the gas–liquid interface, the slugging characteristics of the gas–liquid two-phase flow interface in different flow regions were revealed. The results show that the value of gas-phase superficial velocity has an important influence on the shape and development of the interface wave during the slugging process. When the gravity wave generated during the slugging process can propagate upstream, the slugging phenomenon is periodic, and when the gravity wave cannot propagate upstream, the slugging phenomenon is random. The experiment verified the correctness of the interface instability theory and the liquid slug stability theory, and clarified the definitions of ho and hs. In addition, the paper analyzed the influence of gas–liquid velocity on slugging distance, ho and hs, and liquid slug frequency.

Characteristics of Gas–Liquid Slug Flow in Honeycomb Microchannel Reactor

The gas–liquid slug flow characteristics in a novel honeycomb microchannel reactor were investigated numerically and experimentally. Computational fluid dynamics (CFD) modeling was carried out with Comsol finite element software using the phase-field method, and the simulation results were verified by micro-particle image velocimetry (micro-PIV) analysis. The breakups of liquid slugs at the bifurcations of current honeycomb microchannel followed a complex behavior, leading to non-uniformity in each branch. The pressure distribution inside the microreactor was closely related to the phase distribution. The increasing inlet gas velocity increased the gas phase volume fraction, as well as the gas slug length. Higher gas velocity resulted in stronger turbulence of the liquid phase flow field and a deviation of residence time distribution from normal distribution, but it was favorable to even more residence time during the liquid phase. There also exists a secondary flow in the gas–liquid interface. This work reveals the intrinsic intensified effect of honeycomb microchannel, and it provides guidance on future microreactor design for chemical energy conversion.

Experiments on liquid film thickness around Taylor finger

Abstract Taylor finger is characterized by a single elongated air bubble that grows upward during the drainage of liquid from closed top vertical tubes. The characteristic of the Taylor finger is similar to the Taylor bubble commonly observed in gas–liquid two-phase flow. During the upward growth of the Taylor finger, liquid from the tube drains as a thin film around it. The exact prediction of film thickness is important in several engineering designs and process calculations such as the design of contacting devices, two-phase flow through porous media, boiling in tubes, and monolith reactors. The present study proposes an experimental technique to estimate the thickness of the draining liquid film. Based on experiments an empirical model has been proposed for non-dimensional film thickness in the inviscid region. The proposed model agrees well with the experimental data and equation proposed in published literatures (Davies, R. M., and G. Taylor. 1950. “The Mechanics of Large Bubbles Rising Through Extended Liquids and Through Liquids in Tubes.” Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 200, 375–90; Dukler, A. E., and J. Fabre. 1994. “Gas-Liquid Slug Flow.” Multiphase Science and Technology 8: 1–4; Fabre, J., and A. Liné. 1992. “Modeling of Two-Phase Slug Flow.” Annual Review of Fluid Mechanics 24: 21–46).

Gas–Liquid Slug Flow Studies in Microreactors: Effect of Nanoparticle Addition on Flow Pattern and Pressure Drop

Colloidal suspensions of nanoparticles (e.g., metals and oxides) have been considered as a promising working fluid in microreactors for achieving significant process intensification. Existing examples include their uses in microflow as catalysts for enhancing the reaction efficiency, or as additives to mix with the base fluid (i.e., to form the so-called nanofluids) for heat/mass transfer intensification. Thus, hydrodynamic characterization of such suspension flow in microreactors is of high importance for a rational design and operation of the system. In this work, experiments have been conducted to investigate the flow pattern and pressure drop characteristics under slug flow between N2 gas and colloidal suspensions in the presence of TiO2 or Al2O3 nanoparticles through polytetrafluoroethylene (PTFE) capillary microreactors. The base fluid consisted of water or its mixture with ethylene glycol. The slug flow pattern with nanoparticle addition was characterized by the presence of a lubricating liquid film around N2 bubbles, in contrast to the absence of liquid film in the case of N2-water slug flow. This shows that the addition of nanoparticles has changed the wall wetting property to be more hydrophilic. Furthermore, the measured pressure drop under N2-nanoparticle suspension slug flow is well described by the model of Kreutzer et al. (AIChE J 51(9):2428–2440, 2005) at the mixture Reynolds numbers ca. above 100 and is better predicted by the model of Warnier et al. (Microfluidics and Nanofluidics 8(1):33–45, 2010) at lower Reynolds numbers given a better consideration of the effect of film thickness and bubble velocity under such conditions in the latter model. Therefore, the employed nanoparticle suspension can be considered as a stable and pseudo single phase with proper fluid properties (e.g., viscosity and density) when it comes to the pressure drop estimation.

Void Fraction Measurement Using the Coaxial Line Phase Sensor in the Vertical Gas-Liquid Slug Flow

Void fraction is one of the most important parameters in gas-liquid two-phase flow, which plays a crucial role in the pressure gradient calculation and the flow pattern analysis. In this paper, a new coaxial line phase sensor based on the electromagnetic wave propagation is designed and phase shift principle is proposed for the void fraction measurement, and the function relationship between the void fraction and phase difference is derived. By the theoretical analysis and 56 static calibration experiments, the measurement model is proposed in this paper. Air-water two-phase flow experiments have been conducted in a vertical tube at 48 flow conditions in a stainless-steel tubular test section which has an inside diameter of 50.0mm. The dimensionless quantities related to the void fraction, e.g. dynamic viscosity ratio, gas-liquid density ratio, gas-phase Froud number (Frg) and Lockhart-Martinelli parameter ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mathrm {X}}_{{\mathrm {LM}}}$ </tex-math></inline-formula> ), are dimensional analyzed. According to the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Pi $ </tex-math></inline-formula> theorem, a new void fraction correlation which could acceptably handle all the available data of different flow conditions (authors’ and literature) is proposed. Laboratory results indicate that the mean absolute error (MAE) of the correlation is 6.46% within an error band of ±10% and 8.03% (183 literature data points) within an error band of ±15%. The void fraction correlation has good prediction accuracy and a certain extrapolation for different flow regimes and fluid characteristic parameters.

Slug Length Estimation for Gas-liquid Slug Flow in T-shaped Microdevices with Liquid Film

To realize stable long-term operation of microdevices with gas-liquid slug flow, the slug lengths have to be monitored and controlled, because they influence mass transfer performance. In this study, an experimental investigation was carried out to analyze the gas-liquid slug flow in a T-shaped microdevice with a liquid film. The experimental result showed that the pressures in gas and liquid feeding tubes oscillate periodically along the formation of a pair of gas and liquid slugs. Then, the correlation equation between the liquid film thickness and the number of capillaries was identified on the basis of the experimental data. Based on these results, a method for estimating slug lengths and liquid film thickness from measurements of feed pressure and feed flowrate was developed. The developed method is non-invasive and does not affect slug formation or the manner of gas-liquid slug flow. Its effectiveness was assessed through an experimental case study, and the relative root mean square errors of estimated slug lengths were within 8.5%. The result show that the developed method can be applied to the monitoring of slug lengths.

Temporal-spatial mode competition in slug-flow induced vibration of catenary flexible riser in both in plane and out of plane

The slug-flow induced vibration (SIV) of a catenary flexible riser with an aspect ratio of 360 is experimentally investigated in this work. Four different flow rates for the internal gas-liquid slug flow are considered in the tests that carried out at a fixed gas-liquid ratio of 3.0 in a small-scale air-water test loop. The temporal-spatial mode competition is discussed in terms of the response amplitudes, frequencies, modal weights, transferred energy between the riser and flow, pressure fluctuations and slug flow characteristics. Experimental results indicate that the modal weights of higher modes increase with the flow rate although the in-plane response is dominated by the fundamental mode, resulting in the intense mode competition. The occurrence of mode transition first emerges at the response trough, presenting the higher second modal weight. The distinct switching between the first and second mode is observed for the pipe section between the response peak and trough, and such mode switching becomes more pronounced as the flow rate grows. The distribution and lengths of liquid slugs passing through the riser affect the mode switching in terms of the distribution and magnitude of fluid forces. The riser storages the energy extracted from the fluid when it approaches to the equilibrium position, while releases the energy as it departs the equilibrium position. The occurrence of out-of-plane response is found when the mixture velocity exceeds 2.0 m/s. The out-of-plane frequencies appear in the in-plane response, and vice versa, manifesting the interaction between the in-plane and out-of-plane responses.

Gas–liquid slug flow in microfluidic heat exchanger: Effect of gas hold-up and bubble size on pressure drop and heat transfer

This work is an extension of our recent article, which was inspired by discussions with the reviewers. The paper presents an experimental study of the gas-liquid segmented flow as a method of intensifying microfluidic heat exchangers (MFHE) with a focus on studying the influence of the gas volume fraction. It was found that the highest heat transfer coefficient is achieved when the bubble size coincides with the channel diameter, which corresponds to a gas hold-up of 12%. Experimental studies have also shown that segmented flow makes it possible to increase the Nu up to 1.54 and reduce thermal resistance up to 1.67 times compared to single-phase flow.

Finite Element Simulation of a Taylor Bubble in Two-Phase Gas-Liquid Slug Flows using Petrov-Galerkin Formulation

Petrov-Galerkin finite element scheme for systematic analysis of the dynamics of a rising Taylor bubble and general free surface flow problems is derived and implemented. The validity of the scheme is confirmed by simulating the buoyancy-driven motion of a Taylor bubble through a stagnant Newtonian liquid in a vertical pipe characterised by dimensionless inverse viscosity number and Eötvös number of magnitude 111 and 189, respectively. Comparison of the numerical results for the steady state features defining the nose, film, and bottom regions around the bubble with the experiment shows a good agreement between the numerical simulation and the experiment. The percentage deviation of the numerical computed rise velocity, equilibrium film thickness, and stabilisation length ahead of the bubble from the experimental determined values are 8.4%, 2.3%, and 9.5%, respectively.

Simulation Study on the Development Process and Phase Interface Structure of Gas-Liquid Slug Flow in a Horizontal Pipe

Here, a unified 3D numerical model of gas-liquid two-phase flow in a horizontal pipe was established using the interface capture method based on the open source software package OpenFOAM. Through numerical simulation of the natural slugging and development process of slug flow under different working conditions, the motion, phase interface structure, pressure and velocity field distributions of the liquid slug were fully developed and analyzed. The simulation results are consistent with the experiment. The results showed that during the movement of the slug head, there is a throwing phenomenon and a wave-like motion of the liquid slug. In addition, the slug tail and body area have very similar velocity profiles, and the overall velocity field distribution becomes more uniform with the development of liquid slug. Moreover, there are sudden pressure fluctuations at the head and tail of the liquid slug.

Numerical and experimental simulation of gas-liquid two-phase flow in 90-degree elbow

In the present research, the effects of four 90-degree horizontal elbows with different curvature radii (17, 34, 51, and 68 cm) and an identical rectangular cross-section (20 * 34 mm) on a gas–liquid slug flow pattern have been studied experimentally and numerically. Gas and liquid superficial velocities in numerical and experimental parts were set in 2.5 m/s and 0.44 m/s, respectively. In the computational fluid dynamics section, the distribution of volume fraction, velocity, pressure, turbulence intensity, and swirling intensity parameters were investigated using the VoF model and the SST k-ω method. Experimental studies were conducted utilizing pressure data and signal processing tools to study the dominant frequency of slug flow alongside bandwidth distribution as well as the Shannon entropy. The results exhibited that although the dominant frequency of slug flow did not change after passing the 90-degree elbow, the frequency signal's bandwidth increased. In this regard, the application of Shannon entropy quantity revealed that the frequency distribution signal experienced a greater increase in bandwidth as the elbows’ curvature radius increases. The turbulent intensity of the elbows’ outlet reveals that by increasing the curvature radius, the flow field behaves smoother. Also, swirling intensity distribution shows that decreasing the elbow radius leads to increment in the swirling intensity.

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

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

Measurement of Three-Dimensional Characteristics of Slug Flow

Gas-liquid slug flow in horizontal pipes occurs frequently in industrial applications and is harmful to the piping systems owing to its intermittent structures. Therefore, it is essential to accurately describe the flow structure and investigate the flow mechanism. A non-intrusive fluorescence imaging method is employed to quantitatively reconstruct the slug bubble in a 15mm ID horizontal pipe in this article. Optical distortion correction method is firstly developed to improve the measurement accuracy of the instantaneous gas holdup, and then the time-average axial bubble velocity is simultaneously measured according to the feature matching of cross-sectional images. Additionally, the influence of phase superficial velocity on the interface stability is investigated through the phase holdup and the circumferential curvature distribution analysis. The typical three-dimensional structures of slug bubble are finally reconstructed through the velocity-based bicubic interpolation and median fusion algorithm. Previous correlations and reference instrumentation are used to evaluate the proposed method. Comparison results show that the method can effectively measure the flow parameters and reconstruct slug bubbles.

Development of Gas-Liquid Slug Flow Measurement Using Continuous-Wave Doppler Ultrasound and Bandpass Power Spectral Density

This paper addresses the issues of slug detection and characterization in air-water two-phase flow in a vertical pipeline. A novel non-invasive measurement technique using continuous-wave Doppler ultrasound (CWDU) and bandpass power spectral density (BPSD) is proposed for multiphase flow applications and compared with the more established gamma-ray densitometry measurement. In this work, analysis using time-frequency analysis of the CWDU is performed to infer the applicability of the BPSD method for observing the slug front and trailing bubbles in a multiphase flow. The CWDU used a piezo transmitter/receiver pair with an ultrasonic frequency of 500 kHz. Signal processing on the demodulated signal of Doppler frequency was done using the Butterworth bandpass filter on the power spectral density which reveals slugs from background bubbles. The experiments were carried out in the 2” vertical pipeline-riser at the process system engineering laboratory at Cranfield University. The 2-inch test facility used in this experiment is made up of a 54.8 mm internal diameter and 10.5 m high vertical riser connected to a 40 m long horizontal pipeline. Taylor bubbles were generated using a quick-closing air valve placed at the bottom of the riser underwater flow, with rates of 0.5 litres/s, 2 litres/s, and 4 litres/s. The CWDU spectrum of the measured signal along with the BPSD method is shown to describe the distinctive nature of the slugs.

Heat transfer modeling of non-boiling gas-liquid slug flow using a slug tracking approach

This study proposes a new set of equations for the energy balance for predicting the temperature distribution of gas-liquid slug flows using a slug tracking approach. The model proves able to work around non-physical oscillations of the temperature distribution found with a previously published set of equations for the energy balance. The main difference here is that rather than being applied to each structure (namely the liquid slug, the liquid film and the elongated bubble) individually, the temperature is assigned to the whole unit cell. Central Differencing Schemes were avoided since the problem is convection-dominant, thus preference to Upwind Differencing Schemes was given. Comparison of the new model with an experimental dataset for air-water flow in a 52-mm ID pipeline showed deviations of ±25% for the temperature and pressure gradients. An extra head loss term caused by the recirculation of liquid in the elongated bubble wake was introduced into the momentum balance, correcting an existing –20% systematic deviation found in the pressure gradient as computed by the former model. Simulations for an extended, 200-m long pipeline comparing isothermal, cooling and heating flows are also presented. These simulations demonstrate the competition of gas expansion caused by pressure drop and of gas expansion/contraction caused by mixture heating/cooling, and their consequences on the elongated bubble length increase/decrease, on the mixture acceleration/deceleration, on the coalescence rate of elongated bubbles, and on their statistic distributions.

Mass transfer and reaction characteristics of homogeneously catalyzed aerobic oxidation of 5-hydroxymethylfurfural in slug flow microreactors

Oxidation of 5-hydroxymethylfurfural (HMF) using air or pure oxygen was performed in polytetrafluoroethylene capillary microreactors under gas–liquid slug flow, with Co/Mn/Br as the homogeneous catalyst in the acetic acid solvent. The temperature was varied from 90 to 165 °C at a pressure of 1 or 5 bar. At atmospheric pressure conditions (and 90 °C), acetaldehyde was further added as a co-oxidant to accelerate the reaction. At 150 °C, 5 bar oxygen and a residence time of 2.73 min, an HMF conversion of 99.2% was obtained, with the yields of 2,5-diformylfuran (DFF), 5-formylfurancarboxylic acid (FFCA) and 2,5-furandicarboxylic acid (FDCA) being 22.9%, 46.7%, and 23.8%, respectively. By operation under wetted slug flows and elevated partial oxygen pressures, mass transfer limitations and oxygen depletion in the microreactor could be eliminated. This allowed to run the microreactor under kinetically controlled conditions, where both the HMF consumption and DFF formation were found zero order in partial oxygen pressure and roughly first order in HMF. The total selectivity towards DFF/FFCA/FDCA was ca. 40% at low partial oxygen pressures due to the dominant occurrence of side reactions. By using pure oxygen at 5 bar the total selectivity was improved to 60–94%. The space time yields of DFF and FFCA in the microreactor exceeded those obtained in conventional (semi-)batch reactors at slightly elevated temperatures and pressures, due to the superior mass transfer and higher initial HMF concentrations in the microreactor. For highly efficient FDCA synthesis, more dedicated microreactor operations are needed to tackle its precipitation.

Effect of swirl on fluctuation of pressure drop in a gas–liquid slug flow

Slug flow as one of the typical two-phase flow patterns occurs widely in industry. The fluctuating phenomenon in slug flow is harmful to the pipeline system and should be suppressed. Flow-pattern modulation by swirl has been widely used to satisfy practical application. In this work, influences of swirl on pressure gradient and interface morphology in a gas–liquid slug flow were studied by visualization experiments. The results showed that slug flow will transform to swirling intermittent flow when the gas and liquid velocity is relatively high. In this case, pressure drop in the slug flow is higher and its fluctuation within a relatively narrow range compared with those in a non-swirl slug flow; the probability density function distribution is more concentrated in the swirling intermittent flow; the slug flow transforms to the flow pattern altering between gas column and gas slug. The slug flow was kept downstream of the swirler when the gas and liquid velocity is relatively low. In this case, effects of swirl on both pressure drop and interface morphology are not obvious. Along the streamwise direction, the morphology interface, fluctuation of pressure drop and pressure signals showed that the swirling intermittent flow gradually transforms to non-swirl slug flow. Finally, the transition boundaries between these flow patterns were proposed and compared with other published transition boundaries in swirling and non-swirling flows.

Reconstruction of Taylor Bubbles in Slug Flow Using a Direct-Image Multielectrode Conductance Sensor

Gas-liquid two-phase flows are widely encountered in industrial processes. To realize the dynamic detection of gas-liquid slug flows, a direct-image multi-electrode conductance sensor (DMCS) is optimized using the finite element method to access high measurement sensitivity. Through carrying out an experiment of vertical gas-liquid two-phase flows, the gas-liquid interfaces in slug flows are detected by the DMCS and compared to the results from a planar laser-induced fluorescence (PLIF) system. The interfaces detected by the DMCS and the PLIF system indicate a good agreement. By selecting an optimal threshold, the liquid film thicknesses around Taylor bubbles (TBs) are derived, and the asymmetry of circumferential film thickness distribution is investigated. Besides, the local cross-correlation velocities at circumferential pipe wall are measured by the DMCS. Finally, the TBs in slug flows are reconstructed based on the thickness and the velocity of the TBs. The reconstructed TBs allow for a clear indication of gas-liquid slug flow structures.

Structure Detection of Horizontal Gas–Liquid Slug Flow Using Ultrasonic Transducer and Conductance Sensor

Slug flow is one of the most common flow patterns in horizontal gas-liquid two-phase flow. Structure detection of slug flow is of great significance for understanding the heat and mass transfer between phases and uncovering the mechanism of flow pattern transition. However, it is remarkably difficult to accurately measure the interface structures of slug flow with a single-sensing mode due to its multiscale characteristics. In this article, we design a combined ultrasonic and conductance sensing system that consists of a transmission ultrasonic transducer (TUT), a reflection ultrasonic transducer (RUT) and two parallel-wire conductance sensors (PWCSs). The response signals of the ultrasonic transducers and the conductance sensor for slug flows were collected in an experiment of horizontal gas-liquid flow. Based on the different measurement mechanisms of the ultrasonic transducers and the conductance sensor, the 2-D gas-liquid interfaces in slug flows were reconstructed. The experimental results demonstrate that, compared with the TUT and the PWCS, the RUT has obvious advantages in the detection of the local gas-liquid interface structure. In addition, based on the cross-correlation velocity measured by the PWCSs, the slug lengths in the gas-liquid flow were accessed. It was found that the slug lengths derived from the ultrasonic transducers and the conductance sensor indicate a good agreement and present complicated evolution characteristics due to the coalescence and crushing of the slugs.

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

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