Slug Length Research Articles

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.

Two-phase flow behaviour and performance of polymer electrolyte membrane electrolysers: Electrochemical and optical characterisation

Understanding gas evolution and two-phase flow behaviour are critical for performance optimization of polymer electrolyte membrane water electrolysers (PEMWEs), particularly at high current densities. This study investigates the gas-bubble dynamics and two-phase flow behaviour in the anode flow-field of a PEMWE under different operating conditions using high-speed optical imaging and relates the results to the electrochemical performance. Two types of anode flow-field designs were investigated, the single serpentine flow-field (SSFF) and parallel flow-field (PFF). The results show that the PFF design yielded a higher cell performance than the SSFF design at identical operating conditions. Optical visualization shows a strong relationship between the flow path length and the length of gas slugs produced, which in turn influences the flow regime of operation. Longer flow path length in the SSFF results in annular flow regime at a high current density which degrades cell performance. The annular flow regime was absent in the PFF design. It was found the effect of flow rate on performance depends strongly on operating temperature in both flow patterns. Results of this study indicate that long channel length promotes gas accumulation and channel-blocking which degrades performance in PEMWEs.

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.

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.

Study and characterization of gas-liquid slug flow in an annular duct, using high speed video camera, wire-mesh sensor and PIV

An experimental study is presented on air-water two-phase flow in a 10.5-m-long annular duct with an external diameter of 155 mm and an inner diameter of 60 mm. Particle image velocimetry (PIV) is applied to obtain instantaneous velocity measurements of the flow field. The annular duct inclination is of 5° from the horizontal. A CCD camera (2448 pixel × 2050 pixel, 5 Mpixel,12-bit) was positioned in the test section to record the seeding particles. The illumination was provided by a double pulsed PIV laser (Nd:YAG, frequency doubled to 532 nm) with a measured pulse intensity of 70 mJ/pulse. It was used at 15 Hz (resulting in the independence of the velocity samples). Based on the instantaneous local velocities, Probability Density Functions (PDF) and mean velocities are calculated. Two-phase flow arranged in the slug-flow pattern is observed, at superficial velocities of jw = 0.154 m/s and ja = 0.044 m/s. 3000 samples per case are processed using cross-correlation procedure, for the PIV analysis. A home-made Annular Wire-Mesh Sensor (AWMS) was applied to obtain time-signal-measured void-fraction data as a function of the electrical permittivity. The average bubble velocity is estimated by two techniques, (i) High-Speed Video Recording and (ii) Particle Image Velocimetry (PIV) together with the AWMS. A comparison of the two techniques is presented. A new technique based on AWMS for the measurement of bubble-passage frequency, bubble length and slug length is proposed. It was observed: (i) deceleration of the water phase beneath the bubble as it passes, shown by velocity profiles at different bubble locations, (ii) an increase in bubble velocity as air superficial velocity is increased and (iii) the complexity of the flow pattern, shown in details by AWMS cross-sectional images. The new experimental results are of great value for comparison with CFD models and for the development of more refined pressure-drop prediction tools in two-phase annular-duct flows.

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.

Vortex ring propagation and interactions studies

This paper experimentally investigates the vortex ring propagation and interactions with thin cylindrical and flat surfaces. Dye-based visualization technique is adopted for the interaction studies. Vortex rings are generated from a circular nozzle of 19 mm diameter with the stroke length ratios (length of the fluid slug to nozzle diameter, LN/DN) of 1 to 5, and ejection velocities in the range of 0.05 to 0.2 m/s. Vortex interaction studies are carried out with two different bodies; firstly, with the circular cylinders having the diameters of 0.2, 0.6, 1.5 and 2.5 mm, and secondly with a flat solid surface. Results indicate that the trails in the vortex ring start following at LN/DN = 4. The influence of the initial velocity is found to be insignificant on the vortex ring diameter, however, found to depend on stroke length ratio. Vortex-cylinder interaction studies indicated that vortex velocity decreases with increase in cylinder diameter after the interaction. Reconnections of vortex rings are observed in lower cylinder diameter cases. In case of vortex ring interaction with the flat surface, stretching of the vortex core is observed leading to a considerable increase in the vortex ring diameter.

Extraction of samarium using a serpentine Y-junction microreactor with 2-ethylhexyl phosphonic acid mono-2-ethylhexyl

In the modern industrialized rare earths production, solvent extraction facilities possess many problems such as long extraction time, large occupation area, high processing cost and so on. The present work attempts to extract samarium (III) from hydrochloric acid solution in a rectangular cross-section Y-junction microchannel, using 2-ethylhexyl phosphonic acid mono-2-ethylhexyl (EHEHPA or P507). The effects of flow rate, channel length and width on the extraction efficiency, slug length and mass behavior of a liquid-liquid extraction system are assessed. The slug length decreased with increase in the flow rate, increased in channel length and channel width. The maximum extraction efficiency of 75.1% could be achieved at the longer channel length (pH = 1.5, channel width = 300 μm, flow rate = 10 μL/min). The volumetric mass transfer coefficients (kLa) of the microfluidic extractor was compared with the conventional extractor and found to be several orders of magnitude higher, authenticating the beneficial effects such as: Higher specific interfacial area and shorter diffusion length.

Mini-channel Two Phase Flow of Hydrogen Peroxide: Decomposition with Silver Catalyst

Hydrogen peroxide is a chemical compound which is commonly used as a disinfectant at lower concentrations. The chemical is called as a green propellant for space applications. Water and oxygen are the products after self-decomposition of the compound at higher concentrations. The reaction rate at lower concentrations can be enhanced by using catalysts for faster reaction rates. In the present study, an experimental analysis of the reaction rate of hydrogen peroxide (6% by volume) with silver catalyst for varying flow rates is detailed. Reaction takes place in a long cylindrical borosilicate glass tube of diameter 2.50 mm. Separation of product mixture takes place in an enlarged tube of diameter 5.50 mm. Various two phase flow patterns formed after decomposition reactions are identified by images captured using high speed camera. Length of oxygen slug formed after catalytic reaction is measured by employing Image processing techniques.

Slug formation mechanism for air–water system in T-junction microchannel: a numerical investigation

In this paper, numerical investigation of slug formation mechanism of gas–liquid two-phase flow in a T-junction microchannel has been carried out. A two-dimensional (2D) model for the microchannel was developed using ANSYS Academic research CFD 18.2 software package, and volume of fluid method was adopted to solve the model numerically. The results obtained are in good agreement with the experimental results. In this work, slug length, pressure drop and velocity variations inside the slugs were captured at several operating conditions. The effects of contact angle (0°–155°), fluid viscosity, and surface tension on two phase flow interaction parameters along with the effect of both gas and liquid superficial velocities are discussed in detail. For gas–liquid two phase flow velocities were ranging from 0.025 to 0.5 m s−1 and capillary number (Ca) from 6.96 × 10−4 to 1.39 × 10−2. One of the key objectives of this investigation was to study the occurrence and behaviour of liquid film around the slug units. In fact, liquid film formed at low capillary number (Ca) was very thin and observed only for fine meshing near wall of the channel.

Effect of geometrical parameters on slug behaviour and two phase pressure drop in microchannel T-junctions

Geometric parameters of a microchannel such as cross-sectional area, shape and aspect ratio are important factors in determining the two phase flow characteristics. Two phase flow analyses were carried out experimentally for microchannel T-junction geometries at low values of capillary number. Three hydraulic diameters were used, namely 250, 400 and 550 μm, while the aspect ratio was varied between 0.1 to 1. Experiments were performed for the gas volume flow rate ranging from 2 to 30 ml/min while the liquid volume flow rate ranged between 0.1 to 10 ml/min. Slug length was estimated from the processing of high speed images of slug flow and pressure drop was simultaneously recorded. For constant mass flux in rectangular channels, an increase in the aspect ratio leads to an entrapment of continuous phase at the corner. This influences the transport characteristic of such channels. The effects of aspect ratio on slug length and pressure drop are reported. A simplified scaling model based on the geometric parameters of microchannels as well as gas and liquid flux is proposed which agrees reasonably well (within ±15%) with the present experimental data as well as data available in literature. A flow based pressure drop prediction model was developed which agrees well (within ±10%) with the experimentally measured two phase pressure drop. It is believed that a priori information on slug length and pressure drop in a given flow through a microchannel will help to design the flowing system better.

Slug flow models: Feasible domain and sensitivity to input distributions

The present work discusses the feasible and validity domains of unit-cell slug models. The models of Dukler and Hubbard (Ind. Eng. Chem. Fund. 14 (4): 337–347, 1975) and Orell (Chem. Eng. Sci. 60 (5):1371–1381, 2005) are particularly considered as reference models, for they are based on distinct formulation hypotheses. A parameter sensitivity analysis is also carried out to assess the uncertainty in predicted slug length distributions. The work further discusses a kinematic methodology for the estimation of slug length distributions, a problem of great interest in the design and determination of the operational conditions of gas-liquid separators. The results are validated through new experiments and the data of Ujang et al. (Int. J. Multiphase Flow 32 (5): 527–552, 2006). The new experiments introduce data (including the pdf distributions) on pressure gradients, bubble lengths, passage frequency and translational velocity of bubbles. Particle image velocimetry measurements furnish the local mean velocity profiles in the continuous phase of the liquid slug and film.

Fluid structure behaviour in gas-oil two-phase flow in a moderately large diameter vertical pipe

Intermittent flows in vertical pipes occur in many industrial settings including power generation and downstream oil-and gas production. This type of flows include cap bubble, slug and churn flow regimes. These regimes are of interest as downstream processes and control may heavily depend on the intermittency of the inflow. There are a number of correlations that predicts the features in such flows in vertical pipes. Most of the correlations were developed for air and water fluid pair for slug flow regime in vertical pipes with 25 to 50 mm inner diameter. In this paper, an attempt has been made to assess the suitability of several of these correlations specific to slug flow regime for a fluid pair that is different to air-water system. In this work, air-silicone oil flow development was experimentally investigated in a vertical pipe with an inner diameter of 68 mm. A Wire Mesh Sensor (WMS) and an Electrical Capacitance Tomography (ECT) sensor were installed in series at four locations (15D, 30D, 45D and 65D) downstream of the mixing section. The flow was visually observed using a high speed camera. The void fraction time series obtained from the WMS and the ECT were used to establish the flow characteristics such as slug length, slug frequency, void fraction in liquid slugs and Taylor bubble velocity. A comparison showed that the void fraction measurements using ECT and WMS are in good agreement. Axial measurements shows that the flow development beyond 45D is minimal. Change in physical properties of the liquid phase is responsible for the deviation associated with the existing slug flow models, particularly those developed to predict the gas holdup in liquid slugs.

Unsteady two-dimensional jet with flexible flaps at the channel exit

The present work studies the effect of passive exit flexibility on a two-dimensional starting jet. The exit flexibility is introduced by attaching two flexible (deformable) flaps at the jet exit of a high aspect ratio rectangular duct with the flaps initially being parallel to the channel walls. A controlled piston motion is used to generate the starting jet, which is composed of a rapid acceleration to a constant velocity ($U_{p}$) that is maintained for a given duration of time, after which it is brought to rest impulsively. The parameters which are varied include the flexural rigidity ($EI$) of the flaps, flap length ($L_{f}$) and piston speed ($U_{p}$), with measurements of the flap kinematics and flow field in each case. The flaps initially bulge due to the acceleration of the piston from rest, with this bulge growing in size and moving downstream as the flow develops, culminating in a large opening at the flap exit. Subsequently, the flaps return to their initial parallel position and remain there as long as the piston is in motion. Once the piston stops, the flaps collapse inwards due to fluid deceleration causing additional flow out of the flap region in the form of a jet that adds to the net amount of fluid pushed by the piston. We find that the flap kinematics is affected by the flap$EI$and$L_{f}$besides$U_{p}$. We define a non-dimensional flexural rigidity$EI^{\ast }=EI_{eq}/(1/2\unicode[STIX]{x1D70C}U_{p}^{2}L_{f}^{2}d)$, where$EI_{eq}$is an equivalent flexural rigidity which takes the self-weight of the flaps into account ($d=\text{channel width}$;$\unicode[STIX]{x1D70C}=$fluid density). We find that across different$EI_{eq}$,$L_{f}$, and piston speeds, the maximum opening of the flap tip and the time taken to reach this maximum opening in terms of$L/L_{f}$(where$L=\text{fluid slug length}$) fall on a single curve for all the cases studied, when plotted with$EI^{\ast }$. Particle image velocimetry measurements show that the motion of the flaps results in the formation of additional pairs of vortices when compared to the single vortex pair formed in the absence of flaps. The total final circulation coming out of the flap region remains nearly the same as that of the rigid exit case. However, the final fluid impulse is always found to be higher in the flap cases, with the fluid impulse in most flap cases being approximately two times the fluid impulse of the rigid exit case. This increase in impulse is shown to be linked to the fact that the centroids of vorticity get spread out more in the lateral direction due to the opening of the flaps. The increased impulse and the higher time rate of change of impulse, which is linked with force, suggest that introduction of flexible flaps can help improve thrust performance when looked at from a propulsion point of view.

Prediction of Pressure Variation at an Elbow Subsequent to a Liquid Slug Impact by Using Smoothed Particle Hydrodynamics

Liquid slug flow driven by pressurized air in an inclined pipe with a downstream elbow is investigated numerically. As the liquid slug hits the elbow, the impact pressure and the associated force generated at the elbow may damage pipe supports as well as the pipe itself. It is essential for the design engineers of pipeline systems to accurately predict the pressure trace during the impact for safe operation. The slug arrival velocity and slug length (i.e., mass) at the elbow directly affect that pressure. In order to calculate these slug parameters just before the impact, an improved one-dimensional (1D) model proposed in the literature is used. At the elbow, pressure variation with respect to time is calculated by a recently developed computer code which uses a two-dimensional (2D) smoothed particle hydrodynamics (SPH) method. In the numerical setup, two representative initial slug lengths, one for short slugs and one for long slugs, and three different initial air tank pressures are used. The obtained numerical data are validated with available experimental results. For both short and long slugs, calculated peak pressures show great agreement with measured peak pressures.

Investigation of the Effect of Geometric Parameters on EWOD Actuation in Rectangular Microchannels

Efficient actuation of liquid slugs in microfluidic circuits is a matter of interest in droplet-based microfluidic (DMF) applications. In this paper, the electrowetting on dielectric (EWOD) actuation of a liquid slug fully confined in a microchannel is studied. A set of experiments are conducted in which the mean transport velocity of a liquid slug enclosed in a microchannel of rectangular cross section and actuated by EWOD method is measured. A printed circuit board-based (PCB-based) microfluidic chip is used as the platform, and the transport velocity of the slug is measured by processing the images recorded by a high-speed camera while the slug moves in the channel. To investigate the effect of microchannel geometry on the mean transport velocity of the slugs, different channel heights and widths (ranging between 250−440μm and 1–2 mm, respectively) as well as different liquid volumes (ranging between 2.94and5.15μL) are tested and slug velocities up to 14.9 mm/s are achieved. A theoretical model is also developed to analyze the effect of involved parameters on the transport velocity. The results show that, within the range of design parameters considered in this study, for a constant slug volume and channel width, increasing the channel height enhances the velocity. Moreover, keeping the slug volume and channel height fixed, the transport velocity is increased by enlarging the channel width. An inverse proportionality between the slug length and velocity is also observed. These results are also shown to agree with the theoretical model developed.

Dimensionless analysis on liquid-liquid flow patterns and scaling law on slug hydrodynamics in cross-junction microchannels

Liquid-liquid flow patterns and slug hydrodynamics were experimentally studied in non-circular glass microchannels with water-butanol, water-toluene, water–oil and water-hexane systems, considering various hydraulic diameters (600 µm, 400 µm, 200 µm), inlet junctions (crossed, T) and aspect ratios (0.5, 1). The aqueous phase was the continuous phase due to the hydrophilic microchannel walls and the organic phase was the dispersed phase. Three main flow patterns were observed, i.e., annular flow, slug flow and droplet flow. The mechanism of flow pattern formation was explained by a force analysis, based on which, a dimensionless analysis regarding Weber number and Reynolds number was performed to develop general flow pattern transition criteria. Additionally, slug velocities and slug length were investigated. A new scaling law was proposed to predict the slug length and it showed a good agreement with the experimental results. A linear relationship between the slug velocity and the bulk velocity of the two phases was obtained.

Experimental investigation on particle transport of coal fines in unsteady terrain slug flow

The coal fine output from coalbed methane (CBM) formation during the production of CBM wells seems to be a common occurrence. At the lower section of wellbore, the aggregation of coal fine particles may occur, that lead to the increase of local coal fine concentration and the negative effect on the work of downhole equipment. To investigate the migration of coal fine particles in gas-liquid flow, the gas-liquid-solid three-phase flow experiments were carried out. The experiments were conducted in a downbent V-shaped pipe with different incline angles (5°, 15° and 25°), and recorded the real-time liquid holdup at certain positions of the lower elbow with the conductivity method. The terrain-induced slugs can be identified on the time series curves of liquid holdup. And the characteristic parameters of each slug, such as slug length and translational velocity, were calculated. By injecting marked coal fine particles into the multiphase flow loop and observing whether they can be discharged from the V-shaped pipe, the lower limits of gas and liquid velocities to void the particle retention at the lower elbow were collected. According to the experimental observation of the coal fine particle movement in one slug unit, we defined dimensionless particle velocity function FD to analyze the particle transport mechanism in the terrain slug flow.The FD value distribution of terrain slugs, calculated by the use of slug parameters, changed with the slug dissipation and coalescence in the experimental incline pipeline. Detail analyses of FD statistical parameters show that the mean values of FD increase with the rise of mixture flow velocity, and the decrease of inclination angle. The mean values of FD at the critical particle discharge conditions are positively correlated to particle concentration. And a linear relationship between the critical means of FD and the particle concentrations is presented.

Intermittent flow initiation in a horizontal tube: quantitative visualization and CFD analysis

© 2018, The Brazilian Society of Mechanical Sciences and Engineering. The onset of slugging in stratified gas–liquid flow in a 26.4-mm ID horizontal pipe was investigated experimentally using a high-speed digital video processing technique. Spatio-temporal displacement maps were produced to show the evolution of two-phase flow structures (waves, slug precursors and liquid slugs) in the flow entrance region of the air–water mixture flow. The two-phase distribution derived from the visualization technique was in good agreement with measurements performed using non-intrusive capacitance sensors. Numerical results from a hybrid CFD approach that combined the two-fluid model and volume-of-fluid method to track the position of the large-scale interface in time and space also agreed well with the quantitative flow visualization data for phase distribution, slug length and slug frequency. The relative errors between the flow structure velocities calculated by cross-correlating the numerical and experimental (image analysis) phase distribution signals were 6.8 and − 7.4% for the slug and plug flow cases, respectively.

An experimental investigation of the motion of long bubbles in high viscosity slug flow in horizontal pipes

The slug bubble velocity is one of the most important closure relations in multiphase flow models, and has been studied extensively for decades. Available models and correlations may provide quite reliable predictions for low viscosity liquids, but this is not always the case for high viscosities, mainly due to a lack of detailed data. An investigation of viscous slug flow was carried out, based on 241 experiments in a 15 m long horizontal pipe of 57 mm inner diameter, applying three different liquids with viscosities in the range of 240–730 cP. Measured parameters include slug bubble velocities, slug lengths, liquid holdup and pressure drop. A main objective has been to quantify the effect of viscosity on slug bubble velocities, in order to improve predictive models of slug flow in the laminar region. Slug front, tail and pattern velocities were obtained by cross-correlating holdup time series recorded at two locations at the end of the test section. The dependency of individual slug bubble velocities on slug lengths and bubble nose shape was investigated in detail for selected experiments. A new slug bubble velocity model has been developed, incorporating viscosity, which compares quite well with available high viscosity slug flow data.