Liquid Slug Length Research Articles

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.

Regulation of gas-liquid Taylor flow by pulsating gas intake in micro-channel

Bubble formation mechanism and slug flow characteristics have been studied in a T-junction rectangular micro-channel under pulsating gas intake conditions. A high-speed camera is used to track the trajectory of gas-liquid interface by the Lagrangian method, while the numerical simulation is used to acquire the flow field distribution at different moments using the Euler method. The dynamic pressure, shear stress and pressure difference across the forming bubble at the T-junction increase with the pulsation frequency and liquid velocity. Under pulsating gas intake conditions, none of these three types of force can be neglected, indicating a transition regime between squeezing and dripping regime. The evolution of bubble length under pulsating conditions is similar to the general gas-liquid flow, but with a shorter value, while the liquid slug length is longer than the latter. The slug flow is transformed into wave flow when the pulsation energy inputted to the system reaches a critical value. The bubble velocity varies periodically over time with a main fluctuation frequency equal to that of pulsation. The present work proves that the technique of pulsating gas intake enables precise control the gas-liquid Taylor flow pattern, which would be useful for future application in gas-liquid reactions.

Cu-catalyzed aerobic oxidation of diphenyl sulfide to diphenyl sulfoxide within a segmented flow regime: Modeling of a consecutive reaction network and reactor characterization

A continuous flow homogeneous Cu-catalyzed aerobic oxidation of diphenyl sulfide to diphenyl sulfoxide was investigated. The protocol utilized copper sulfate pentahydrate as catalyst, pyridine as ligand, molecular oxygen as oxidant, 2,2,6,6-tetramethyl-1-piperidinyl oxidanyl (TEMPO) as co-oxidant, and methanol as solvent. The continuous flow reactor system was characterized through a residence time distribution (RTD) study, showing that the flow reactor could be modeled as a plug flow reactor. The hydrodynamics of the system were simulated using computational fluid dynamics (CFD). The liquid slug length, bubble length and bubble frequency from the simulation closely corresponded to the experimentally-observed values. The volumetric mass transfer coefficient (kLa) was estimated by applying Higbie’s penetration model for micro-scale diffusion. A reaction profiling approach was applied within continuous flow to generate experimental data to fit a consecutive reaction network for the monooxidation of diphenyl sulfide to the diphenyl sulfoxide and then the subsequent overoxidation to the diphenyl sulfone. The reaction network comprised of four kinetic parameters (A1, Ea1, A2 and Ea2). Three out of four of the fitting parameters could be estimated with less than 7% uncertainty and the fitted model closely correspond to the experimental data, with R2 = 0.994. The fit-for-purpose model was then used to explore the experimental design space in silico. The models were successfully validated in a scale-out experiment which was operated over 4 h collection time to give 72% conversion and 65% sulfoxide yield, corresponding to a selectivity of 91% and a throughput of 0.61 g·h−1.

Experimental investigation of the characteristics of the transition from spherical cap bubble to slug flow in a vertical pipe

Reliable and accurate prediction of the transition from spherical cap bubble to slug flow is crucial not only to the operation of industrial facilities such as the crude oil pipelines, bubble column, and nuclear reactors but also for model development for computational fluid dynamics (CFD) studies. The present paper presents a review of the transition mechanics from spherical cap bubble flow to slug flow in vertical pipes. The bubble flow was split into sub-regions, bubbles and spherical cap bubbles and the mechanisms to classify them (i.e., bubble terminal velocity and cap bubble velocity) was analysed. For now, the literature review does not present some important previous works. This paper presents an original data set of gas–silicone oil in vertical pipes to support the new findings. The experimental two-phase data classifies the flow patterns using the probability density function (PDF) and shows the important flow variables such as average void fraction, pressure gradient, slug body void fraction, liquid slug, Taylor bubble and slug unit lengths, structural velocity and frequency obtained by electrical capacitance tomography (ECT) and a wire mesh sensor (WMS).

Pengaruh Liquid Hold-Up terhadap Transisi Pola Aliran 2 Fasa Air-Udara dari Stratified Ke Slug pada Pipa 50 mm

The slug flow pattern is one of the most avoiding factors in piping systems due to its destructive characteristics. Research on the criteria of the slug formation is still the concern. The present study focused on finding out the influence of liquid hold-up toward the transition of the flow pattern from stratified to slug pattern. It was conducted on a 50 mm-diameter horizontal acrylic pipe. The visual data was obtained from a fully developed area (180-210 D) by variations of liquid superficial rate (JL) of 0,03-0,3 m/s and gas superficial rate (JG) from 0,7 to 3,77 m/s. The high-speed camera with 2000 fps recording speed was the instrument to obtain the visual data. The measurement of the liquid hold-up is made by Constant Electric Current Method (CECM). The drastic decrement of liquid hold-up value on the wave growth mechanism (JG ≤ 1,88 m/s) indicated the changes in flow patterns from the stratified to the slug pattern, while on the wave coalescence mechanism (JG > 1,88 m/s) the liquid hold-up remains increases. The pattern change from pseudo slug to the slug pattern was not decreasing on its film thickness, yet, it tends to increase the frequency and the liquid slug length.

Hydrodynamics and gas-liquid mass transfer in a cross-flow T-junction microchannel: Comparison of two operation modes

Hydrodynamics and gas-liquid mass transfer of CO2-monoethanolamine (MEA) aqueous solution were investigated experimentally in a cross-flow T-junction microchannel. Two operation modes were compared, namely OM-1 (gas entering the branch channel, liquid flowing into the main channel) and the opposite feed mode OM-2 (gas entering the main channel, liquid flowing into the branch channel). The effects of gas and liquid rates and MEA concentrations in the two operation modes were studied in terms of bubble and liquid slug lengths, specific interfacial area, liquid-side mass transfer coefficient (kL), liquid-side volumetric mass transfer coefficient (kLa) and pressure drop (Δp). The results show that the variations of kLa and Δp with the total rate of two phases present a turning point in both two operation modes. Before the turning point, the gas rate is the key factor increasing the kLa and Δp, after that, the liquid rate would predominate. Notably, kL is more likely to be affected by the liquid rate. Comparing with OM-2, more dispersed gas-liquid distribution in OM-1 can be observed, which enhances the mass transfer but simultaneously increases the pressure drop. However, OM-2 is beneficial to a narrower residence time distribution in the liquid phase and better mass transfer performance under a strict energy requirement. This work could provide guidance for the operation of a cross-flow T-junction microreactor based on specific product requirements and energy consumption restrictions.

Modeling and prediction of slug characteristics utilizing data-driven machine-learning methodology

Slug liquid holdup, translational velocity, and slug length are representative slug characteristics. The precise prediction of these parameters is essential for the accurate calculation of the average liquid holdup and pressure gradient in pipes. Nevertheless, existing correlations have mainly been developed by an empirical approach. Each correlation can only be applied to a restricted fluid property and geometrical condition, preventing people from selecting a suitable one without professional knowledge. In this study, slug characteristics were predicted by several machine learning methodologies with 2590 experimental data to overcome the limitation of selecting the proper model or correlation. The experimental dataset includes −10° ≤ θ (pipe inclination angle) ≤ 10°, 0.0258 m ≤ I.D. (pipe inner diameter) ≤ 0.0780 m, 0.01 m/s ≤ vSL (superficial liquid velocity) ≤ 3.4 m/s, 0.03 m/s ≤ vSg (superficial gas velocity) ≤ 20 m/s, 769 kg/m3 ≤ ρL ≤ 998 kg/m3, 1.12 kg/m3 ≤ ρG ≤ 7.36 kg/m3, 0.001 Pa s ≤ μL ≤ 4.65 Pa s, and 6 ≤ ReM (mixture Reynolds number) ≤ 863,920. The existing dataset has been preprocessed to pursue better machine learning performance. The random forest results reveal a better training performance and prediction than that of a deep neural network and present a competitive prediction compared with the existing correlations, indicating a great potential of utilizing the data-driven machine learning methodology.

Gas-liquid mass transfer around Taylor bubbles flowing in a long, in-plane, spiral-shaped milli-reactor

Gas–liquid mass transfer was investigated around Taylor bubbles flowing in a long, in-plane, spiral-shaped milli-reactor involving various configurations. Using a colorimetric technique and image post-treatment, the variation of the equivalent O2 concentration inside the liquid slugs was measured. The coloration positions, corresponding to 99% of the maximum concentration reached in the liquid slug, changed significantly under various operating conditions. Overall volumetric mass transfer coefficients were evaluated on the basis of these coloration positions, without considering the hydrodynamics change. The intensification factor of two configurations was found to be proportional to the increase in Dean number. Considering the changes in pressure drop, bubble length and velocity, the axial variation of cumulated mass flux could be measured. It decreased along tube and finally tended towards a constant value. A scaling law for the Sherwood number was proposed by introducing a normalized time, dimensionless liquid slug length, and the Péclet and Dean numbers.

Non-Newtonian Effects on Gas-Liquid Slug Flow in Micro-channel with T-junction Mixer

The length of liquid slug and gas bubble, gas bubble velocity and pressure drop of gas-liquid two-phase flow in a rectangular channel were measured using a high speed camera and a different pressure transducer. Water and Glycerol-solution as Newtonian liquids and Sodium carboxymethyl cellulose, Xanthan gum and Polyacrylamide-water solution as non-Newtonian liquids were used as the liquid phase, and N2 gas was used as the gas phase. The equivalent hydraulic diameter of the rectangular channel was 0.51 mm. The measured pressure drop data were compared with unit cell model, which applied to gas bubble pressure drop prediction proposed by Kurimoto et al. (2017). The following conclusions were obtained: (1) Different flow pattern was observed only in Polyacrylamide-water solution. (2) Frictional pressure drop on two-phase flow can associate with an effective viscosity of liquid. (3) Effects of elasticity of liquid on non-Newtonian two-phase two-phase flow should be accounted to predict accurately pressure drop.

Comprehensive experimental study of liquid-slug length and Taylor-bubble velocity in slug flow

Slug flow is an intermittent two-phase flow pattern that provokes undesirable pressure variations in pipes. Mathematical models are commonly used to study these variations; so that it is necessary to know the experimental liquid-slug length, Taylor-bubble length, and pressure drop to validate such mathematical models.In this work, we experimentally studied the water-air slug flow through an acrylic pipe loop 6 m long and 0.01905 m internal diameter. We assembled infrared sensors on the acrylic pipe to get voltage signals accordingly to the presence of liquid-slugs or Taylor-bubbles. We applied Fourier transform on the voltage signals to obtain dominant frequencies to determining the liquid-slug length.Moreover, we obtained the cross-correlation function to get the delay time between two groups of the voltage signals to determine the velocity of Taylor-bubbles. Additionally, we measured the liquid-slug length by video technique and pressure drop with a digital manometer. The liquid-slug lengths obtained by using dominant frequencies are in agreement with the ones measured by video technique.On the other hand, Taylor-bubbles could touch or not the wall pipe at different inclination pipe angles; this affects pressure drop. Then, we observed the inclination angle when the Taylor-bubble detaches from the wall of the pipe, under different flow conditions. We found that the Taylor-bubble detaching angle is 45°, and as the inclination angle is higher, the slug-liquid and Taylor-bubble lengths are smaller. The detaching angle can be used as a criterion to neglect the gas shear-stress into mathematical models to improve predictions of the hydrodynamic behavior of slug flow.

Hydrodynamics of Gas–Liquid Slug Flows in a Long In-Plane Spiral Shaped Milli-Reactor

An experimental investigation of gas–liquid Taylor flows in a millimetric in-plane spiral shaped reactor with various tube curvature ratios (52 < λ < 166) is reported. Thanks to the compactness of the reactor and the use of an ad hoc imaging system and processing, the axial evolution of bubble lengths and velocities could be recorded and extracted along the whole reactor length (~3 m). The experimental results showed a significant linear increase of bubble length and velocity with axial position. Very long, stable Taylor bubbles (LB/dit up to 40) and liquid slugs were generated, in particular due to the poor wettability of the surface and the important role it played in bubble formation. At identical inertial force (i.e., identical Reynolds number), a higher centrifugal force (i.e., lower tube curvature ratio) likely led to shorter Taylor bubble lengths while only slightly affecting the liquid slug lengths. The axial pressure drop could be estimated from the axial increase in bubble volume, and compared with the measured pressure drop and that predicted by the correlations from literatures. By considering both the friction and capillary pressure drops, it was observed that the predicted two-phase pressure drop was slightly dependent on the centrifugal force and that the capillary pressure drop, determined from the unit cell number, capillary number and static contact angle, was dominant.

Investigation of Hydrodynamic and Heat Transfer Characteristics of Gas-liquid Taylor flow in Square Microchannel

Abstract Heat transfer and flow characteristics under air-water Taylor flow in a square microchannel with T-junction were investigated in this work. Different hydraulic diameters of models were discussed numerically by VOF method. Flow patterns such as bubbly flow, slug flow, annular flow and churn flow were identified by both numerical simulation and experimental methods. Simulation results including bubble formation process, bubble length, bubble velocity, void fraction and heat transfer fit well with literature data. The pressure differential of two sides in gas phase played an important role in bubble development. The gas and liquid superficial velocities were found to have a significant impact on bubble behavior. And the higher liquid viscosity would promote higher bubble velocity, also enhance heat transfer, but weaken the void fraction. The results showed a tiny but not ignorable effect of geometric dimensioning on bubble and liquid slug lengths. An appropriate correlation was proposed to estimate bubble length, and the deviation was −10 ~ + 15 %. By using moving frame of reference technique, the internal circulations inside the moving slugs were displayed more clearly.

Coupling vibration response of a curved flexible riser under the combination of internal slug flow and external shear current

Apart from the vortex-induced vibration (VIV) caused by flow over flexible risers, internal slug flow-induced vibration (FIV) is extensively encountered in practice as risers are conveying oil–gas two-phase flow. This paper presents an experimental study of the coupling response of a flexible catenary riser under the combination of internal slug flow and external shear current. The vibration displacement and the internal flow regime are synchronously captured using the non-intrusive technique of high speed imaging method. The experimental results illustrate that the contribution of internal FIV and external VIV depends on the phase difference and the water depth. For the in-plane response, the contribution of internal slug flow is greater than the external flow at the upper part, while the out-of-plane response is dominated by the external current. Unlike the standing wave behavior in the internal FIV case, a traveling wave occurs with the participation of external flow, associated with the multimodal response. Additionally, the response frequencies vary over time, possessing oscillating characteristics. Thus unstable response and mode switching are observed in the deflected shapes of the flexible riser. Due to the vigorous out-of-plane vibration, the longer liquid slugs are split into shorter segments. As the gas–liquid ratio increases, the liquid slug length is decreased gradually, resulting in the reduction of system mass. Both the in-plane and out-of-plane responses are enhanced accordingly.

Statistical Moments Transport Model for the prediction of slug flow properties

The present work introduces the Statistical Moments Transport (SMT) Model for a description of the mean and standard deviation values of bubble and liquid slug lengths in horizontal, inclined and vertical flows. The model considers gas depressurization and the interaction (coalescence) between long bubbles. Results are compared to three other theoretical approaches – unit cell, slug tracking and slug capturing models – and six different experimental data sets. The gain in computing time as compared to the slug tracking model even in relatively short pipes is of two orders of magnitude.

Hydrodynamic Characterization of Phase Separation in Devices with Microfabricated Capillaries.

Capillary microseparators have been gaining interest in downstream unit operations, especially for pharmaceutical, space, and nuclear applications, offering efficient separation of two-phase flows. In this work, a detailed analysis of the dynamics of gas–liquid separation at the single meniscus level helped to formulate a model to map the operability region of microseparation devices. A water–nitrogen segmented flow was separated in a microfabricated silicon-glass device, with a main channel (width, W = 600 μm; height, H = 120 μm) leading into an array of 276 capillaries (100 μm long; width = 5 μm facing the main channel and 25 μm facing the liquid outlet), on both sides of the channel. At optimal pressure differences, the wetting phase (water) flowed through the capillaries into the liquid outlet, whereas the nonwetting phase (nitrogen) flowed past the capillaries into the gas outlet. A high-speed imaging methodology aided by computational analysis was used to quantify the length of the liquid slugs and their positions in the separation zone. It was observed that during stable separation, the position of the leading edge of the liquid slugs (advancing meniscus), which became stationary in the separation zone, was dependent only on the outlet pressure difference. The trailing edge of the liquid slugs (receding meniscus) approached the advancing meniscus at a constant speed, thus leading to a linear decrease of the liquid slug length. Close to the liquid-to-gas breakthrough point, that is, when water exited through the gas outlet, the advancing meniscus was no longer stationary, and the slug lengths decreased exponentially. The rates of decrease of the liquid slug length during separation were accurately estimated by the model, and the calculated liquid-to-gas breakthrough pressures agreed with experimental measurements.

Characteristics of gas-liquid Taylor flow with different liquid viscosities in a rectangular microchannel

Characteristics of gas-liquid Taylor flow with different liquid viscosities are investigated in a rectangular microchannel, aiming at providing knowledge and aid in the design of processes involving viscous fluid, such as polymers and ionic liquids. The effect of liquid viscosity on the bubble formation dynamic, film thickness, bubble velocity, and pressure drop is investigated. The results reveal a specific viscous effect compared to those in square or circular channels. For the same capillary number, both the liquid film thickness at the corners and at the short planes are much larger than in square channels. New correlations are proposed for predicting the film thickness in the rectangular channel. The bubble shape sheared by the liquid phase is also distinct from literature observations that a smaller radius occurs at the rear cap. For the conditions studied (0.00065 < Ca < 0.0525), increasing the viscosity leads to an increase in the instantaneous flow rate, and also an later shift from filling stage to squeezing stage. As the bubble formation is driven by both squeezing pressure and shearing force, the bubble/slug length is affected by both capillary number and flow rate ratio. The recirculation inside liquid slugs is found to play an important role in the pressure drop, which can be well described by an empirical correlation including dimensionless liquid slug length and capillary number.

Flow patterns and plug/slug flow characteristic of R134a in a 0.643 mm microchannel tube

This paper presents visualization of the two-phase flow of R134a in a 24-port microchannel tube with the average hydraulic diameter of 0.643 mm. The experiment is conducted on the same facility in the previous work for R32 (Li and Hrnjak, 2019). Mass flux covers the range from 50 to 250 kg-m−2s−1. The two-phase flow is generated by adding heat in several steps to originally subcooled refrigerant at the very entrance. When mass flux is 50 kg-m−2s−1, no annular flow is observed in the tube. The annular flow starts at x = 0.9 when mass flux is 100 kg-m−2s−1 and x = 0.6 when 250 kg-m−2s−1. The transitional flow starts at x = 0.25 (G = 50 kg-m−2s−1) and x = 0.8 (250 kg-m−2s−1). Revellin and Thome (2007), Ong and Thome (2011), and Zhao and Hu (2000) correlations are plotted against the measurements. None of them fit our data since the experiment is in lower mass flux than the database of their correlations’. The interface velocity and vapor plug length fraction (close to void fraction) are measured from the high speed videos. The velocity and vapor fraction agree to results based on the homogeneous assumption. In plug/slug flow, both the vapor plug and liquid slug lengths are uneven. The length distribution of plug and slug seems to follow Beta distribution. When the quality is low, the quantity of short vapor plugs is larger than long plugs. At fixed mass flux, when quality increases, length of vapor plugs increases and length of liquid slugs decreases.

Effect of wall contact angle and carrier phase velocity on the flow physics of gas–liquid Taylor flows inside microchannels

This paper investigates numerically the characteristics of gas–liquid sliding Taylor flow in a two-dimensional (2D) T-junction rectangular microchannel having both inlets perpendicular to the horizontal mixing chamber. The governing equations describing the flow are discretized and solved by employing finite volume method based computational tool ANSYS Fluent 15.0. The volume of fluid (VOF) multiphase method is used for capturing the gas–liquid interface. A dynamic mesh, based on the adaption of gradients of volume fraction, is used for grid refinement to tackle sharp gradients during the two-phase flow. A focus is laid on exploring the influence of wall contact angle on the flow physics of sliding Taylor flows inside the microchannel in which the inlet liquid velocity is varied in the range 0.05 < uL < 0.25 m/s) (or inlet Reynolds number, 4.97 < Re < 248.75), and the wall contact angle is varied in the range 0° < Θo < 170°. The effect of contact angle hysteresis on the overall pressure drop, bubble and liquid slug lengths, and the dispersed phase volume fraction is reported. The bubble length and the overall pressure drop obtained from the simulations are compared with the benchmark correlations available in the literature. It is found that the trends of the variation of bubble length with liquid velocity are in reasonable match with the correlations. In addition, the pressure drop and slug length decrease in hydrophilic channels unlike in hydrophobic channels, which signifies the importance of contact angle in two-phase sliding Taylor flows.

Core design and performance of the FASt TEst reactor (FASTER)

Characteristics of the slug flow, mean void fraction, liquid slug and Taylor bubble lengths; and structure frequency were all extracted from the void fraction time series. Average void fractions at nine axial positions have been measured on a 6 m long (and 34 mm diameter) pipe test section by using conductance probes.For the flow conditions used, bubbly (with spherical cap bubble), slug/plug and churn/semi-annular flow patterns were observed. These observations were also confirmed by using statistical numbers.Time series analysis of the mean void fraction and its corresponding PDFs show that the effect of the mixer on the flow becomes, practically, negligible from a distance of 95 pipe diameter.It was found that, generally, the structure velocity can be well predicted from other available correlations in the literature.The theoretical model of Brauner and Ullmann was used to estimate the liquid in slug void fraction. It was inferred that this model is able to predict well the liquid slug void fraction; thus, it can be considered as one of the useful methods for predicting such parameter and to ascertain slug flow regime.The total pressure gradient was found to decrease with increasing gas superficial velocity.

Experimental study of the characteristics of an upward two-phase slug flow in a vertical pipe

Characteristics of the slug flow, mean void fraction, liquid slug and Taylor bubble lengths; and structure frequency were all extracted from the void fraction time series. Average void fractions at nine axial positions have been measured on a 6 m long (and 34 mm diameter) pipe test section by using conductance probes.For the flow conditions used, bubbly (with spherical cap bubble), slug/plug and churn/semi-annular flow patterns were observed. These observations were also confirmed by using statistical numbers.Time series analysis of the mean void fraction and its corresponding PDFs show that the effect of the mixer on the flow becomes, practically, negligible from a distance of 95 pipe diameter.It was found that, generally, the structure velocity can be well predicted from other available correlations in the literature.The theoretical model of Brauner and Ullmann was used to estimate the liquid in slug void fraction. It was inferred that this model is able to predict well the liquid slug void fraction; thus, it can be considered as one of the useful methods for predicting such parameter and to ascertain slug flow regime.The total pressure gradient was found to decrease with increasing gas superficial velocity.